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Resistor in series

Chuck Harris
 

Just because they documented their fix for a mistake
doesn't mean it isn't a mistake.

You can gain more wattage in a pair of resistors than
you can in an individual resistor in the same board space...
If, you can use a little extra altitude to hold the pair
of resistors.

I have seen way too many brown burned tektronix boards
to ever believe they didn't make mistakes with heat.

-Chuck Harris

Jeff Dutky wrote:

Chuck Harris wrote:

So, I would guess that a smaller resistor was specified when the
board was designed, and it was found to get too hot
but this resistor pair is present in the schematics, not just for the 475A, but also in the early 475 service manual schematics. I know that's not really a refutation of your point, but it sure looks like they meant to do this from really early on.

The on-end resistors are clearly visible in the PCB images. I haven't opened up my oldest 475 to check the physical board, but I was planning to do that, and will report back what I find.

Also, I know that schematics do not necessarily precede the physical objects they represent, so they may not accurately reflect original intent.

I once worked at an engineering company where, as we were packing a large machine to be shipped to the client the lead engineer was taking each part and comparing it to the existing drawings, in order to find parts that had been modified (or completely fabricated) during testing and development. When he would find a part that didn't have a drawing he would quickly gin one up in AutoCAD before the part was packed and shipped. My impression, at the time, was that this was part of our contractual obligations to the client, but it occurs to me now that it may have been entirely internal; so that our people would be able to correctly reassemble the machine at the client site based on the engineering drawings.
 

Again, I am not claiming that they were perfect (7A12 disaster is one example). What I am claiming is that the likelihood of a resistor power miscalculation is small, especially in the A version of a scope model whose original had the same arrangement. Folks are much too quick to attribute to incompetence something which was done out of deliberate intent. I've learned not to jump to such conclusions in Tek designs.

I will acknowledge that, as with any population, there was a distribution of talent. Not all Tek engineers were equally stellar. John Addis (of 485 and 7104 fame) has written of one instance where he was invited to a design review of a proposed amplifier IC. The thing was a beast and would've been the most complex, largest die that they'd ever attempted. Yields were going to be low. Addis realized that the amplifier would perform about the same if most of the transistors were simply cut out.

I won't name names, but the designer of that chip was also the designer of the U800.

-- Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070

On 11/25/2020 18:23, Chuck Harris wrote:
Perhaps Tom, but I have seen many instances of them doing
just that. U800 as installed on the 2465 was built to handle
much higher wattage, and a heat sink it didn't need. When they
decided not to spend the money on a heat sink it didn't need,
they used 4 star washers to make a kludge fix that created a
problem with U800 case cracking...

Bad epoxy in the 547 transformers... something I know a little
bit about, as I have built hundreds to replace their mistake.

They weren't perfect; they were engineers.

They fixed their mistakes, often with what they called "tents"
made of suspended parts and black wires. The rest of us called
it gumball construction... best left for prototypes.

If you haven't seen any tektronix mistakes, you haven't been
looking very hard.

-Chuck Harris

Tom Lee wrote:
In a word, no.

Anything is possible, I suppose, but asking us to believe that Tek's design team carelessly forgot to check power dissipation is a lot to swallow. This wasn't their first rodeo -- they'd already done the 465, 465B (it was pretty much the same team) and the 475. If you look at the 465, there is a similar series combo in the same general part of the blanking circuit. It is obviously done to reduce capacitance. There is a tiny trimmer of about 1pF across the combo, so clearly they were carefully accounting for fractions of a pF.

I'm sticking with parasitic capacitance concerns as the most likely reason for the choice in the 475A.

The team did make some mistakes (in the 465 in particular; they fixed those in the B version), but forgetting to calculate power dissipation here was not one of them.

Cheers,
Tom

Sent from my iThing, so please forgive the terseness and typos.

On Nov 25, 2020, at 14:54, "Chuck Harris" <cfharris@...> wrote:

I've done this for many, many years, and usually my reason is
I cannot fit a higher wattage resistor in the same space as two
smaller resistors sitting upright like this.

So, I would guess that a smaller resistor was specified when the
board was designed, and it was found to get too hot, so two resistors
of the same wattage, in series were fitted... with no changes in
the circuit board.

-Chuck Harris.

Jeff Dutky wrote:
Chuck Harris wrote:

How do you know tektronix did this? Most of the stuff we have hasn't
been at the factory for 20 or 30 years.
The main reasons that I think Tek did this are 1) that the two resistors appear in the schematic in the service manual, and 2) the resistors look like matches for resistors found elsewhere on the unit. Also, the method of vertically mounting the resistors and soldering their upward pointing leads together is found elsewhere on the unit, specifically on the vertical preamp board in the 3rd stage amplifier.

-- Jeff Dutky
 

Well, I've presented quantitative calculations explaining why capacitance matters in this circuit, and thus why series combinations are helpful. In rebuttal, you provide "but they might still have made a mistake." That's moving the goal posts, Chuck.

I am happy to concede (for the nth time) that Tek's engineers were not flawless. But the series resistors in the 475A are that way because of capacitance, not incompetence.

--Tom
--

Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070

On 11/25/2020 18:27, Chuck Harris wrote:
Just because they documented their fix for a mistake
doesn't mean it isn't a mistake.

You can gain more wattage in a pair of resistors than
you can in an individual resistor in the same board space...
If, you can use a little extra altitude to hold the pair
of resistors.

I have seen way too many brown burned tektronix boards
to ever believe they didn't make mistakes with heat.

-Chuck Harris

Jeff Dutky wrote:
Chuck Harris wrote:
So, I would guess that a smaller resistor was specified when the
board was designed, and it was found to get too hot
but this resistor pair is present in the schematics, not just for the 475A, but also in the early 475 service manual schematics. I know that's not really a refutation of your point, but it sure looks like they meant to do this from really early on.

The on-end resistors are clearly visible in the PCB images. I haven't opened up my oldest 475 to check the physical board, but I was planning to do that, and will report back what I find.

Also, I know that schematics do not necessarily precede the physical objects they represent, so they may not accurately reflect original intent.

I once worked at an engineering company where, as we were packing a large machine to be shipped to the client the lead engineer was taking each part and comparing it to the existing drawings, in order to find parts that had been modified (or completely fabricated) during testing and development. When he would find a part that didn't have a drawing he would quickly gin one up in AutoCAD before the part was packed and shipped. My impression, at the time, was that this was part of our contractual obligations to the client, but it occurs to me now that it may have been entirely internal; so that our people would be able to correctly reassemble the machine at the client site based on the engineering drawings.
 

I've seen plenty of design errors in all makes of equipment - nobody's perfect. When it comes to high bandwidth circuits like in scope amplifier chains, you may have to cut them some slack. It is a technical compromise between electrical performance (usually speed and fidelity) and temperature rise. For high performance, you want all the parts to be tiny and with minimal parasitics, and jammed together as closely as possible. For running cooler and longer life, you want stuff bigger and spread out - especially if you're relying on convection cooling. I've seen plenty of discolored and even charred boards in nearly every Tek scope or plug-in - usually in the vertical amplifier section, and usually from hot resistors jammed together and right on the board. Everybody knows that you can lift them up some and drop the temperature and reduce board damage - the same back when these were designed. But, the particular arrangements were what worked, and often ran hotter than expected or wanted - a trade-off. If you raise the parts and spread them out, you get different parasitics that may spoil the performance. If you add forced air cooling, you get different mechanical issues, cost, and noise. Over the years, I think I've heard a lot more complaints and discussion about fan noise than hot resistors.

I doubt that the designers would have wanted the boards to char, but maybe didn't realize just how hot some of these things would actually get, and the long term result. Some of these probably showed up obviously earlier in product life, and resulted in design or part changes or mods. Tek knew very well how to make stuff work well and cool enough, but didn't always get it right. If you want to see a nice RF resistor, take a close look at the dual one on a ceramic substrate, which is used to terminate the CRT's transmission line deflection plates in most of the high BW scopes. Ultra-low intrinsic C and L, high power dissipation, and high isolation (low CM C). Tek made.

So anyway, you can always make thermal improvements in weak spots, but don't assume it's easy to do in high BW,, critical designs. BTW be wary of substituting or making composite values of metal film resistors. In any given family and power rating, there may be a point where the film structure necessarily changes from a cylindrical tube to a helix, as the resistance values go up, so you get some extra inductance for free. There's a practical limit to sheet resistance (film thickness) range, so higher resistances need to stretch it out. Special "non-inductive" types are available, with different patterns to reduce or cancel the effective turns. For the lower R values used in our amplifiers, regular types should be fine, but I think somewhere above the few kilohm neighborhood, they may start getting helical. This could also be a reason to use two or more lower R parts in series, in place of one higher one - more power dissipation surface area, and less likely to be helical.

A word about resistor power ratings. I think many actual applications don't provide enough clearance for free air convection cooling, but fortunately, most aren't operated anywhere near their limits. It's the bigger, hotter ones that get the attention - you know it right away when you touch one. The parts are usually rated in particular model circumstances with convection cooling, or certain air flow forced. The way these high BW circuits are jammed together, the individual resistor convection power ratings are kind of meaningless. You'd really have to take them as a whole unit, and measure the temperature rise. Remember, anything can dissipate any amount of power - it's just a question of how hot it needs to be.

Ed
 

So I feel like a superstitious/nostalgic idiot, but I had second thoughts about replacing the out-of-spec resistor pair with modern metal film resistors. I went to eBay and bought a dozen (allegedly) genuine Allen Bradley carbon composition 7.5 K Ohm 1/2 Watt resistors for what is probably a completely unjustifiable price. I also looked to see if anyone was selling FDH2161 diodes, but no luck on that score.

My logic, such as it is, is pretty simple: a carbon composition resistor is going to have basically zero inductance, and I wasn't sure that a metal film resistor would NOT have some inductance (my understanding is that the film resistors have a spiral cut through the film to increase the effective length of the resistive element; that should create some significant inductance, right?). My illogic is that I will be replacing the existing resistors with cosmetically identical parts (yes, in a place where nobody will ever see them, but I will know, and that will be enough. A person should take pride in their work even if nobody else ever sees it).

Anyhow, is this pure foolishness, or is this foolishness justifiable?

Also, I went over the parts of the board that looked to have been reworked and cleaned things up with a cotton swab and IPA. It looks a lot better now, and I was also able to determine that there were NOT any solder bridges shorting things that shouldn't be shorted.

-- Jeff Dutky
 

Hi Jeff,

You are correct that, depending on value, metal film resistors can have significant inductance (relative to the carbon comp version). In this particular circuit, though, you are looking at 7.5 kilohms of resistance in series with a negligible inductive reactance (I'd guess an inductance well below 100nH, so an inductive reactance of tens of ohms or so at the top end of the scope's bandwidth). So here, the choice is driven by power and capacitance (and maybe aesthetics). Inductance of the resistor is totally irrelevant.

I confess to a general prejudice against carbon comps (they're horribly noisy under bias and don't age gracefully), but they will work here, of course. Just pre-measure the resistance before installation. You might find that even NOS resistors have drifted significantly upward in value just sitting on a shelf.

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070

On 11/25/2020 20:09, Jeff Dutky wrote:
So I feel like a superstitious/nostalgic idiot, but I had second thoughts about replacing the out-of-spec resistor pair with modern metal film resistors. I went to eBay and bought a dozen (allegedly) genuine Allen Bradley carbon composition 7.5 K Ohm 1/2 Watt resistors for what is probably a completely unjustifiable price. I also looked to see if anyone was selling FDH2161 diodes, but no luck on that score.

My logic, such as it is, is pretty simple: a carbon composition resistor is going to have basically zero inductance, and I wasn't sure that a metal film resistor would NOT have some inductance (my understanding is that the film resistors have a spiral cut through the film to increase the effective length of the resistive element; that should create some significant inductance, right?). My illogic is that I will be replacing the existing resistors with cosmetically identical parts (yes, in a place where nobody will ever see them, but I will know, and that will be enough. A person should take pride in their work even if nobody else ever sees it).

Anyhow, is this pure foolishness, or is this foolishness justifiable?

Also, I went over the parts of the board that looked to have been reworked and cleaned things up with a cotton swab and IPA. It looks a lot better now, and I was also able to determine that there were NOT any solder bridges shorting things that shouldn't be shorted.

-- Jeff Dutky
 

Jeff, I don't know what the circuit context is here, but if carbon comp types were originally used, then the exact value isn't that critical. If they have drifted badly (typical after many years), and there are two in series, then you can trim it back to normal with slightly different values of CC - like make one of them lower nominal value so they add up about right, presuming they've drifted upward (typical).

As Tom mentioned dislike for CC resistors, I feel the same mostly, but there are some spots where they are the best choice.

Ed
 

Tom,

Yes, I had anticipated that these NOS components might not have retained their specified values. I've been going through my father's effects while working on his scopes and finding all sorts of now-50-year-old components, some of which were expensive when purchased new, and thinking that some fair fraction of them are probably now terribly out of spec, if not completely destroyed (there are a number of big metal can capacitors that I know were stored for at least 20 years in a hot attic, and I don't have much hope that they are still in good condition). I purchased a dozen resistors in the hope that I could find two that were within spec.

Back in college I worked in receiving QC for a NASA contractor that built flight-rated systems. We would first receive a lot of 10 or 20 components of some description, which we would label and ship over to Goddard. They would eventually ship us back 2 or 3 components with paint marks and certification sheets, and we were required to use only those marked components in the parts we built for them. The other components we had shipped to them had been tested to destruction, and only the surviving 10% had been returned to us as flight qualified components. Careful selection of parts is something I have thought about ever since.

But what you're saying is that while my intuition was correct, the actual math makes a fool of me. I should really just use the metal film resistors when they arrive because they are lower noise and the inductance is a non-issue.

I can always gaze lovingly at the old Allen Bradleys. I'm quite sad that nobody seems to make resistors that have those nice sharp edges anymore (or, at least, I haven't found them for sale).

-- Jeff Dutky
 

Hi Jeff,

I've only taken a quick look at the circuit, but it doesn't look overly fussy about the exact resistance value, so the carbon comps you bought are likely to be fine if they are still within tolerance. So don't rule them out just yet.

The noise of a carbon comp is indeed truly wondrous (and it is proportional to dc current; film resistors do not exhibit this effect to any significant degree), but in this particular circuit, the noise isn't of consequence. The signal amplitudes there are so much larger than the noise that you'll never see any effect. So, if you find that the AB resistors you just purchased are within tolerance, I think there's no harm at all in using them. I have to admit to sharing the same nostalgia that you have for these retro resistors. But beyond that, there is also one sound engineering reason to keep some around: Better surge tolerance, owing to the current being distributed over a larger cross section than is the case with film resistors. I'll occasionally specify CCs for circuits that may experience transient overloads, such as some HV circuits which may arc occasionally. Metal film resistors don't tolerate transient overloads as well.

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070

On 11/25/2020 21:18, Jeff Dutky wrote:
Tom,

Yes, I had anticipated that these NOS components might not have retained their specified values. I've been going through my father's effects while working on his scopes and finding all sorts of now-50-year-old components, some of which were expensive when purchased new, and thinking that some fair fraction of them are probably now terribly out of spec, if not completely destroyed (there are a number of big metal can capacitors that I know were stored for at least 20 years in a hot attic, and I don't have much hope that they are still in good condition). I purchased a dozen resistors in the hope that I could find two that were within spec.

Back in college I worked in receiving QC for a NASA contractor that built flight-rated systems. We would first receive a lot of 10 or 20 components of some description, which we would label and ship over to Goddard. They would eventually ship us back 2 or 3 components with paint marks and certification sheets, and we were required to use only those marked components in the parts we built for them. The other components we had shipped to them had been tested to destruction, and only the surviving 10% had been returned to us as flight qualified components. Careful selection of parts is something I have thought about ever since.

But what you're saying is that while my intuition was correct, the actual math makes a fool of me. I should really just use the metal film resistors when they arrive because they are lower noise and the inductance is a non-issue.

I can always gaze lovingly at the old Allen Bradleys. I'm quite sad that nobody seems to make resistors that have those nice sharp edges anymore (or, at least, I haven't found them for sale).

-- Jeff Dutky
 

OK, yuzz made me look. I found the schematic for the 475A, and my quick assessment is that there's nothing special about low capacitance or inductance or precise value of the R1356/R1544 combo. I also looked at the regular 475 schematic, which appears about the same. Without seeing the actual physical layout, I'd guess that the circuitry is quite crammed in there for space reasons, and two 1/2W resistors "fit" better than a single 1W piece - enough so that it was worth continuing the tradition over modifying the board layout.

Q1344 and Q1352 form a shunt-feedback linear amplifier, and the rest of the circuitry handles all the various conditions the Z-axis must take, which are plentiful. The collector of Q1352 is a low impedance node by virtue of the amplifier power gain, with DC feedback from R1369, while C1352 provides AC feedback and compensation. The output has to drive the DC restorer, a significant capacitive and nonlinear load. R1368 isolates the DC restorer load, and C1352 stabilizes the amplifier - it would likely oscillate otherwise. The presence of all the diode clamps and blocks in the amplifier stages indicate a number of operational conditions are needed, depending on what's going on in the horizontal system.Q1352 provides strong pull in the down direction, while Q1358 and Q1354 form a cascode amp for the up direction, but only during transient conditions - at DC, it's a constant current source. It looks like the two Rs in question supply collector load current for Q1352 whenever Q1358 is in cutoff, to keep the loop closed and ready for fast response to the next event, or from overdrive recovery. Z-axis amplifiers don't need high fidelity, but do need fast response in one direction at least, to properly unblank at the beginning of the trace. The tail end (blank) can be slower since the trace is usually off screen by then.

So anyway, I wouldn't worry too much about these resistors. Yes, there must be a reason they went this way, but whatever it is, I don't think there's anything critical about these parts. The maximum DC in them should be about 7 mA, so about 700 mW, when the output is held near zero. Two 1/2W resistors standing upright should handle it OK - better than a single 1W part laying on the board.

Happy Thanksgiving all. Enjoy.

Ed
 

Carbon Composition resistors absorb moisture. Bake them for a while before
tossing them out. Also, high value resistors require voltage across them to
work at their marked value This used to be plainly marked on reels of CC
resistors. I posted a scan once. I caught hell for 'making up that
bullshit'. They also don't realize the first resistor series was 50%
tolerance. yet they were working on pre-war radios. Once again I was told
that I was 'making up that shit'.

On Thu, Nov 26, 2020 at 12:18 AM Jeff Dutky <jeff.dutky@...> wrote:

Tom,

Yes, I had anticipated that these NOS components might not have retained
their specified values. I've been going through my father's effects while
working on his scopes and finding all sorts of now-50-year-old components,
some of which were expensive when purchased new, and thinking that some
fair fraction of them are probably now terribly out of spec, if not
completely destroyed (there are a number of big metal can capacitors that I
know were stored for at least 20 years in a hot attic, and I don't have
much hope that they are still in good condition). I purchased a dozen
resistors in the hope that I could find two that were within spec.

Back in college I worked in receiving QC for a NASA contractor that built
flight-rated systems. We would first receive a lot of 10 or 20 components
of some description, which we would label and ship over to Goddard. They
would eventually ship us back 2 or 3 components with paint marks and
certification sheets, and we were required to use only those marked
components in the parts we built for them. The other components we had
shipped to them had been tested to destruction, and only the surviving 10%
had been returned to us as flight qualified components. Careful selection
of parts is something I have thought about ever since.

But what you're saying is that while my intuition was correct, the actual
math makes a fool of me. I should really just use the metal film resistors
when they arrive because they are lower noise and the inductance is a
non-issue.

I can always gaze lovingly at the old Allen Bradleys. I'm quite sad that
nobody seems to make resistors that have those nice sharp edges anymore
(or, at least, I haven't found them for sale).

-- Jeff Dutky
Chuck Harris
 

Hi Tom,

You must have done that calculation before the thread caught my
attention.

I've checked everything I wrote, and I don't see anywhere that I
have said, "but they might still have made a mistake" as you appear
to be attributing to me.

When did I write that, or are you simply making up quotes for effect?

Tektronix had excellent engineers. Way better than most, but in
terms of reliability, they very often compromised the quality of
their designs in production by using poorly specified parts, and
cheese ball fixes for their mistakes. Their "tented" parts are
legion, and some are very embarrassing to see as an engineer.
(See 2465 A1 board for plenty of examples...)

That said, it is a fool who changes the arrangement or parts
selection, of an RF circuit he does not understand.

-Chuck Harris

Tom Lee wrote:

Well, I've presented quantitative calculations explaining why capacitance matters in
this circuit, and thus why series combinations are helpful. In rebuttal, you provide
"but they might still have made a mistake." That's moving the goal posts, Chuck.

I am happy to concede (for the nth time) that Tek's engineers were not flawless. But
the series resistors in the 475A are that way because of capacitance, not incompetence.

--Tom
 

Baking the resistor will only fix it for the short term.
It will absorb moisture again as you have not fixed the problem, only the symptom.
There is a crack or 0r other point of moisture incursion in the resistor case.

The resistor needs to be replaced.

Glenn

On 11/26/2020 6:59 AM, Michael A. Terrell wrote:
Carbon Composition resistors absorb moisture. Bake them for a while before
tossing them out. Also, high value resistors require voltage across them to
work at their marked value This used to be plainly marked on reels of CC
resistors. I posted a scan once. I caught hell for 'making up that
bullshit'. They also don't realize the first resistor series was 50%
tolerance. yet they were working on pre-war radios. Once again I was told
that I was 'making up that shit'.

On Thu, Nov 26, 2020 at 12:18 AM Jeff Dutky <jeff.dutky@...> wrote:

Tom,

Yes, I had anticipated that these NOS components might not have retained
their specified values. I've been going through my father's effects while
working on his scopes and finding all sorts of now-50-year-old components,
some of which were expensive when purchased new, and thinking that some
fair fraction of them are probably now terribly out of spec, if not
completely destroyed (there are a number of big metal can capacitors that I
know were stored for at least 20 years in a hot attic, and I don't have
much hope that they are still in good condition). I purchased a dozen
resistors in the hope that I could find two that were within spec.

Back in college I worked in receiving QC for a NASA contractor that built
flight-rated systems. We would first receive a lot of 10 or 20 components
of some description, which we would label and ship over to Goddard. They
would eventually ship us back 2 or 3 components with paint marks and
certification sheets, and we were required to use only those marked
components in the parts we built for them. The other components we had
shipped to them had been tested to destruction, and only the surviving 10%
had been returned to us as flight qualified components. Careful selection
of parts is something I have thought about ever since.

But what you're saying is that while my intuition was correct, the actual
math makes a fool of me. I should really just use the metal film resistors
when they arrive because they are lower noise and the inductance is a
non-issue.

I can always gaze lovingly at the old Allen Bradleys. I'm quite sad that
nobody seems to make resistors that have those nice sharp edges anymore
(or, at least, I haven't found them for sale).

-- Jeff Dutky






--
-----------------------------------------------------------------------
Glenn Little ARRL Technical Specialist QCWA LM 28417
Amateur Callsign: WB4UIV wb4uiv@... AMSAT LM 2178
QTH: Goose Creek, SC USA (EM92xx) USSVI LM NRA LM SBE ARRL TAPR
"It is not the class of license that the Amateur holds but the class
of the Amateur that holds the license"
 

That might be true, for something that is rarely used. If used at least
weekly, it will be fine. If it is a shelf queen, all bets are off.

On Thu, Nov 26, 2020 at 5:00 PM Glenn Little <glennmaillist@...>
wrote:

Baking the resistor will only fix it for the short term.
It will absorb moisture again as you have not fixed the problem, only
the symptom.
There is a crack or 0r other point of moisture incursion in the resistor
case.

The resistor needs to be replaced.

Glenn

On 11/26/2020 6:59 AM, Michael A. Terrell wrote:
Carbon Composition resistors absorb moisture. Bake them for a while
before
tossing them out. Also, high value resistors require voltage across them
to
work at their marked value This used to be plainly marked on reels of CC
resistors. I posted a scan once. I caught hell for 'making up that
bullshit'. They also don't realize the first resistor series was 50%
tolerance. yet they were working on pre-war radios. Once again I was told
that I was 'making up that shit'.

On Thu, Nov 26, 2020 at 12:18 AM Jeff Dutky <jeff.dutky@...>
wrote:

Tom,

Yes, I had anticipated that these NOS components might not have retained
their specified values. I've been going through my father's effects
while
working on his scopes and finding all sorts of now-50-year-old
components,
some of which were expensive when purchased new, and thinking that some
fair fraction of them are probably now terribly out of spec, if not
completely destroyed (there are a number of big metal can capacitors
that I
know were stored for at least 20 years in a hot attic, and I don't have
much hope that they are still in good condition). I purchased a dozen
resistors in the hope that I could find two that were within spec.

Back in college I worked in receiving QC for a NASA contractor that
built
flight-rated systems. We would first receive a lot of 10 or 20
components
of some description, which we would label and ship over to Goddard. They
would eventually ship us back 2 or 3 components with paint marks and
certification sheets, and we were required to use only those marked
components in the parts we built for them. The other components we had
shipped to them had been tested to destruction, and only the surviving
10%
had been returned to us as flight qualified components. Careful
selection
of parts is something I have thought about ever since.

But what you're saying is that while my intuition was correct, the
actual
math makes a fool of me. I should really just use the metal film
resistors
when they arrive because they are lower noise and the inductance is a
non-issue.

I can always gaze lovingly at the old Allen Bradleys. I'm quite sad that
nobody seems to make resistors that have those nice sharp edges anymore
(or, at least, I haven't found them for sale).

-- Jeff Dutky







--
-----------------------------------------------------------------------
Glenn Little ARRL Technical Specialist QCWA LM 28417
Amateur Callsign: WB4UIV wb4uiv@... AMSAT LM 2178
QTH: Goose Creek, SC USA (EM92xx) USSVI LM NRA LM SBE ARRL TAPR
"It is not the class of license that the Amateur holds but the class
of the Amateur that holds the license"
 

There is another explanation for doing something like this which is not as obvious.
If you look at the schematic for the 067-0681-01 Tunnel Diode Pulse Generator designed by John Addis at Tektronix


There are three 1/8W 1Kohm resistors (R4, R5, and R6) connected in series going to the tunnel diode. Several years ago I asked him why he did not use one 3Kohm resistor instead. What he knew and I learned was that the body capacitance of each of these 3 resistors in series was far less than the body capacitance of one 3Kohm resistor. The TD is being driven by the collector of the transistor which is high impedance, and the three 1Kohm resistors in series reduces the time constant of the pulse going to the Tunnel Diode without adding any additional capacitance across the Tunnel Diode that would slow it down.

I doubt this had anything to do with Tek portable scopes since their frequency response is almost two orders of magnitude slower than the fast edge of the Tunnel Diode Pulse Generator but sometimes there are other explanations for why you might encounter things that are not obvious.

Dennis Tillman W7pF

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Milan Trcka
Sent: Tuesday, November 24, 2020 9:09 PM
To: [email protected]
Subject: Re: [TekScopes] Resistor in series

Jeff, the resistors connected end to end are an attempt to make a required resistance that was not available as a standard resistor value. Shortage of parts? Engineering change? Select in test? Who knows. I have a few of those in my 453 scope.







--
Dennis Tillman W7pF
TekScopes Moderator
Bob Albert
 

Les obvius is that the engineer had stock in a resistor company/? Sure, a resistor is only 2? but multiply that by 1000 and you get into some real money,

On Friday, November 27, 2020, 02:50:13 PM PST, Dennis Tillman W7pF <dennis@...> wrote:

There is another explanation for doing something like this which is not as obvious.
If you look at the schematic for the 067-0681-01 Tunnel Diode Pulse Generator designed by John Addis at Tektronix


There are three 1/8W 1Kohm resistors (R4, R5, and R6) connected in series going to the tunnel diode. Several years ago I asked him why he did not use one 3Kohm resistor instead. What he knew and I learned was that the body capacitance of each of these 3 resistors in series was far less than the body capacitance of one 3Kohm resistor. The TD is being driven by the collector of the transistor which is high impedance, and the three 1Kohm resistors in series reduces the time constant of the pulse going to the Tunnel Diode without adding any additional capacitance across the Tunnel Diode that would slow it down.

I doubt this had anything to do with Tek portable scopes since their frequency response is almost two orders of magnitude slower than the fast edge of the Tunnel Diode Pulse Generator but sometimes there are other explanations for why you might encounter things that are not obvious.

Dennis Tillman W7pF


-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Milan Trcka
Sent: Tuesday, November 24, 2020 9:09 PM
To: [email protected]
Subject: Re: [TekScopes] Resistor in series

Jeff, the resistors connected end to end are an attempt to make a required resistance that was not available as a standard resistor value. Shortage of parts? Engineering change? Select in test? Who knows. I have a few of those in my 453 scope.







--
Dennis Tillman W7pF
TekScopes Moderator
 

Bob Albert wrote:

Sure, a resistor is only 2? but multiply that by 1000 and you get into some real money
Here's an oral history video discussing the history of industrial design at Tek:

The speaker is the former head of industrial design at Tek (Gale Morris) and he mentions (possibly hyperbolically) that the 453 sold 70,000 units in a single shot to IBM, so the pennies can really add up.

-- Jeff Dutky
 

Dennis,

I saw similar things when I was working on Silicon Valley 700W FM amplifiers. The physical orientation of the ATC caps was critical. We noted that sometimes two or three caps needed to be placed on edge or even stacked on top of each other.

RF is always fun but this added a new twist.

Regards,

Stephen Hanselman
Datagate Systems, LLC

On Nov 27, 2020, at 14:50, Dennis Tillman W7pF <dennis@...> wrote:

?There is another explanation for doing something like this which is not as obvious.
If you look at the schematic for the 067-0681-01 Tunnel Diode Pulse Generator designed by John Addis at Tektronix


There are three 1/8W 1Kohm resistors (R4, R5, and R6) connected in series going to the tunnel diode. Several years ago I asked him why he did not use one 3Kohm resistor instead. What he knew and I learned was that the body capacitance of each of these 3 resistors in series was far less than the body capacitance of one 3Kohm resistor. The TD is being driven by the collector of the transistor which is high impedance, and the three 1Kohm resistors in series reduces the time constant of the pulse going to the Tunnel Diode without adding any additional capacitance across the Tunnel Diode that would slow it down.

I doubt this had anything to do with Tek portable scopes since their frequency response is almost two orders of magnitude slower than the fast edge of the Tunnel Diode Pulse Generator but sometimes there are other explanations for why you might encounter things that are not obvious.

Dennis Tillman W7pF


-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Milan Trcka
Sent: Tuesday, November 24, 2020 9:09 PM
To: [email protected]
Subject: Re: [TekScopes] Resistor in series

Jeff, the resistors connected end to end are an attempt to make a required resistance that was not available as a standard resistor value. Shortage of parts? Engineering change? Select in test? Who knows. I have a few of those in my 453 scope.







--
Dennis Tillman W7pF
TekScopes Moderator
 

VIAsMicrodyne built their business on equipment built around their low
phase noise Synthesizer. It had undergone many minor changes over several
decades, until our source for some bare ceramic disc capacitors dried up.
They were installed over a 1/8" VIA, to solder the back side to the boards.
One of the newer 'know it all engineers didn't even bother to see what they
were used. He simply changed the BOM to a SMD cap, on edge, and marked a
spot on the top side of the VIA to mount each of them. This caused many
problems. He chose the cheapest cap available. The new mounting
repositioned the flying leads that were soldered to the other end, and the
open VIAs caused impedance bumps in the board's ground plane. Even worse,
some of these spots were part of the band switching for the 350 to 510 MHz
VCO board. It was split into three bands, to divide the total range into
tree smaller, overlapping ranges. The switching was done electronically.
The VCO's inductor was switchable at two points, to ground. The one end was
permanently grounded. The poor tech that had been doing them for years
suddenly had 100% failure rate. The only reply from the Engineer was 'My
ECO can't be the cause, just do your damned job!" He was about to quit,
because of frustration. I was pulled off my work. I noticed the difference
between the old and new design right away. Sure enough, the problems had
started with the first new unit. That 'Engineer' didn't follow procedure to
have a sample tested by production before issuing the ECO. It took me a
little while to absorb the total design. It looked like it should work, but
in the back of my mind, I was thinking about lead length inductance causing
problems at 500 MHz. So, I filled the empty VIAs, one at a time as I
watched the Phase Noise drop. The last were the two band switching
circuits, which simply turned on one or two forward biased diodes, hard.
They were very close to the worst of the original design, which could be
brought into spec by repositioning some flying leads. Moving the new caps
to the center of the filled VIAs made that unit pass. Slight realignment of
the flying leads made it cleaner than any of the gold standards. I
documented the modifications needed to make them pass. He raised hell with
me, in front of my supervisor who was the head of the Production Test
Department. He refused to admit to his mistakes, and to issue a new ECO.
You didn't do that to me, and get away with it. When I was pulled from my
regular work to troubleshoot production problems, I found the problems, and
I either got results, or heads would roll. I already had a reputation,
after dragging the Division VP out of his office onto the production floor
to demand action on bad lighting. I had also complained about the poor
copies of copies of copies schematics the print room gave out, rather than
doing a new reduction from a master. Another time, they refused to release
the preliminary documentation for a critical test for a design
modification. Needless to say, I got some people fired. Office politics be
damned, we needed documents, ECOs and competent engineers. It was taken out
of his hands. Someone else did the ECO, as I wrote it. He was transferred
to sales, but he didn't last. I had to take one ECO to one of the semi
retired engineers who founded Microdyne to get approval for a new test
procedure. It had two tests that were interactive with a MC1496. The MC1596
had just been discontinued, which could barely pass both. Only one was
important to the board, and any attempt to improve the other severely
affected it. No one, including the director of Engineering had the guts to
challenge any of HIS designs. I caught him during his next visit. I
explained the problem. I pointed out in an old Motorola data sheet that
that test did nothing good for us. I informed him that particular parameter
was no longer guaranteed on a current Datasheet, and simply told him, We
waste hours per board to pass them for something we don't need. It also
lowers the overall quality of the products using this board. He was a
little red faced. It was one of his first designs, but as we discussed it,
he finally agreed to delete it. Quality went back up, and production time
went down. Then I redesigned the test fixture. You had to set test voltages
accurately, but Engineering had simply given Production the proof of
performance/first article jig. The most critical voltage was 0.400 Volts.
We had no precision Lv references in house so I used a forward biased
1N4001, and a series resistor to allow the 0-10V pot to become a
0-0.425Volt pot. This further reduced test time, and improved quality. It
also eliminated the frequent replacement of the 1/8" shaft, panel mounted
pots.

I realize that this is long, and some may view it as off topic, but many
engineering problems are not obvious without taking the time to really
understand the functions. I hated drudge work, and I hated careless design
or spec changes, even more. Like a 5% spec on a circuit wit two 10%
resistors, and a 20% pot in series. That was my first encounter with a bad
document. I was brushed off with a 'We don't do ECOs on older designs, so
forget it!'. That wasn't an answer, it was a challenge! Then there was the
odious ISO9001 moron contractor brought in to set up our system...

My job title was 'Production Test Tech', but I worked jobs in most
departments because they wanted someone to fix a problem, simply and at low
cost without upsetting everyone in the process. What the heck? I loved
challenges!








On Fri, Nov 27, 2020 at 10:23 PM Stephen Hanselman <kc4sw.io@...>
wrote:

Dennis,

I saw similar things when I was working on Silicon Valley 700W FM
amplifiers. The physical orientation of the ATC caps was critical. We
noted that sometimes two or three caps needed to be placed on edge or even
stacked on top of each other.

RF is always fun but this added a new twist.

Regards,

Stephen Hanselman
Datagate Systems, LLC
On Nov 27, 2020, at 14:50, Dennis Tillman W7pF <dennis@...>
wrote:

?There is another explanation for doing something like this which is not
as obvious.
If you look at the schematic for the 067-0681-01 Tunnel Diode Pulse
Generator designed by John Addis at Tektronix


There are three 1/8W 1Kohm resistors (R4, R5, and R6) connected in
series going to the tunnel diode. Several years ago I asked him why he did
not use one 3Kohm resistor instead. What he knew and I learned was that the
body capacitance of each of these 3 resistors in series was far less than
the body capacitance of one 3Kohm resistor. The TD is being driven by the
collector of the transistor which is high impedance, and the three 1Kohm
resistors in series reduces the time constant of the pulse going to the
Tunnel Diode without adding any additional capacitance across the Tunnel
Diode that would slow it down.

I doubt this had anything to do with Tek portable scopes since their
frequency response is almost two orders of magnitude slower than the fast
edge of the Tunnel Diode Pulse Generator but sometimes there are other
explanations for why you might encounter things that are not obvious.

Dennis Tillman W7pF


-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of
Milan Trcka
Sent: Tuesday, November 24, 2020 9:09 PM
To: [email protected]
Subject: Re: [TekScopes] Resistor in series

Jeff, the resistors connected end to end are an attempt to make a
required resistance that was not available as a standard resistor value.
Shortage of parts? Engineering change? Select in test? Who knows. I have a
few of those in my 453 scope.







--
Dennis Tillman W7pF
TekScopes Moderator
 

Jeff
Choosing a substitute resistor type, as others mentioned here, is not a simple matter.
You really need to know what its function is in the circuit.

Carbon Comp resistors are the only absolutely suitable choice, if the circuit has current surges.
Film resistors of all types simply cannot handle the range of current surges.

If you really cannot find a Carbon Comp (and the circuit requires it), then a Ceramic MAY be a good substitute, and if you are still really stuck, then the next possible substitute is a Metal Oxide type. BUT, Metal Oxide starts to introduce inductance into the circuit.

Carbon Comps also have no inductance (inductance kills - as in obliterates - performance in RF circuits).

Now, Ceramics come in two types - Inductive, and non-Inductive. Again, For an RF circuit, ONLY a NON-Inductive is suitable.

Notice that I haven't even mentioned film resistors.
On these vintage machines, you really want to investigate using Carbon Comp or Ceramics first, to solve the problems, without introducing new problems!

Your choice was correct to use Carbon Comps, but I would recommend buying new production pieces from Mouser or Digikey. Highly unlikely that you'll find significant deviations due to moisture in cracks, or whatever..

Menahem Yachad
CondorAudio

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