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
toggle quoted message
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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
|
Re: Anyone willing to make 3D printing of multiplier boxes
OK. I have enough trouble already.
If you want me to print something, send an email to tamhan aeht tamoggemon point com. With an included STL file.
Then I check if I can do it and will report back.
I cannot cackle and levitate across the Gobi desert, looking for human meat. Keep in mind that I print all of these things FOR FREE, and usually also eat the postage costs.
Tam
--
With best regards
Tam HANNA
Enjoy electronics? Join 15k7 other followers by visiting the Crazy Electronics Lab at
|
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:
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Show quoted text
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.
|
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:
toggle quoted message
Show quoted text
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
|
Ah, not so fast, David! You happened to have chosen an example that illustrates a larger point, but not that there was an oversight in a power calculation.
First, in my sm, R724 and R725 are not 499 ohms, but 332, but no matter. The designer of the main vert. amp, Thor Hallen, had a tough decision: Use a bigger resistor and suffer the parasitics, or use a resistor on the edge of the power spec? He went for the latter. The resistor runs hot, but it is, on paper, within spec. The scope met the bandwidth and operational lifetime targets.
This was one of several instances of compromises being forced on them because they were pushing technology to the edge. The 7904 was an extremely important product for Tek; HP was winning the bandwidth wars and mocking Tek for bells and whistles (e.g., on-screen display). The 7904 was given the goal of "bandwidth uber alles". Trading off other parameters was ok, but not risking 500MHz bandwidths. So the 7A19 is a delicate beast, the 7B92 is a bad design, and you couldn't swap plug ins freely with the mainframes without going through a cal as an ensemble. That was permissible, but risking a failure to hit 500MHz was not.
If you elect to put in different resistors in the vertical gain path, you will see their effects. The scope will still "work" but you might find it challenging to keep the aberrations within spec, or you might find that the bandwidth doesn't quite hit 500MHz with every plug in.
I'm not saying that Tek never made a mistake. But I am saying that one shouldn't be so quick to indict them, especially in the specific case that Jeff's post is about. Again, the team had already had ample experience with the 475 (and the 465B and 465 before that), so if there had been a dissipation problem, it would have been caught and fixed long before the 475A came along. And, as I've explained, this particular circuit is extremely sensitive to parasitic capacitance.
While we're on this subject, anyone who cares about this more deeply, take a look at the corresponding circuit in the 465. The two series-connected resistors there are not equal in value. A clue as to why is that these resistors are in a negative feedback path. Consider the effect of parasitic capacitance from the common point to ground. By ratioing the resistors in the way they have, that common point turns out not to vary in voltage, making any capacitance there irrelevant.
These engineers were thoughtful. Not perfect, but thoughtful. Any time I think I've spotted a mistake, I have to be very careful, because additional thinking has almost always proven that I was too quick on the draw.
We can discuss their legitimate mistakes in the 465, 7B92 and some other circuits/products in some other thread. I've devoted a good portion of an upcoming book chapter to them.
--Cheers,
Tom
--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
toggle quoted message
Show quoted text
On 11/25/2020 16:59, David C. Partridge wrote: sadly they did make that very mistake quite a few times! Witness the 7904
vertical board ... R724 and R725 (499 Ohm 1/2W) always run way too hot
(burnt fingers hot) and are often well off value from overcooking ... the
PCB gets well toasted too! I normally replace them with two 1kOhm 2W metal
film in parallel and they still run pretty warm.
David
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Tom Lee
Sent: 25 November 2020 23:20
To: [email protected]
Subject: Re: [TekScopes] Resistor in series
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.
|
Re: (OT) Where to go for 70s IBM hardware? I'm looking for a terminal.
Actually $1200 is completely reasonable IMO. I sold mine for almost that much 15 years ago and i've seen at least one that sold for almost double that. Like the prices or not but the really unique and/or first of it's kind vintage computers are bringing serious money. Even good, clean and complete Commodore 64s and the Radio Shack computers are bringing hundreds of dollars and they were sold by the millions. On Wed, Nov 25, 2020 at 2:00 PM cheater cheater <cheater00social@...> wrote: Those prices are not reasonable, and the sellers do not accept offers,
contrary to what's advertised on eBay.
On Wed, Nov 25, 2020 at 4:57 PM Jeff Dutky <jeff.dutky@...> wrote:
A quick look at ePay reveals 3 IBM 5110s, and two 5120s all asking in
excess of $1200 (all but one are OBO, so you might be able to get a
discount). Shipping on these things is going to be bear, however. There are
lots of watchers on each one.
The 5120 is especially interesting looking, with a larger display, two
vertically mounted 8 inch floppies, and a generally "rounder" look to the
case: the thing just screams 1975. If I had any interest in these machines
I might be tempted to pick one up.
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"If not for FleaBay, then where else can one obtain these old scope
modules?"
They show up pretty regularly at the surplus stores around here. I just
bought two TM500 (DM502 and FG502) plugins for $5 at a store near here and
they still have three plugins for the Tek 7000 scopes there (don't recall
what they were now.) Also on Craigslist. Also lots of the guys that
collect vintage computers or other types of electronics come across them
regularly and most have some laying around that they got with other
equipment. Also hamfests. You just have to get out and beat the bushes,
or pay the inflated E-Greed prices.
I watch Fleabay pretty closely for anything of interest that's nearby
and that I can go and pick up in person and save the shipping costs and
damage.
Put a want ad on Craigslist. It's free.
toggle quoted message
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On Wed, Nov 25, 2020 at 2:38 PM David Kuhn <Daveyk021@...> wrote: If not for FleaBay, then where else can one obtain these old scope modules?
On Wed, Nov 25, 2020 at 1:19 PM - <rrrr6789@...> wrote:
My rule of thumb when looking at electronic equipment on E-bay or
anywhere else is to look *closely* for signs that anyone has been into it.
If it isn't *sealed* and it's not guaranteed to work then expect the
worst. I wouldn't pay more than a *minimal* price for anything that has
been opened and that isn't guaranteed to operate properly. I've seen far
too much equipment for sale by big time used equipment dealers such as
Tucker Electronics being sold in "as-is" condition that I'm sure have
already been stripped of anything useful.
On Wed, Nov 25, 2020 at 10:08 AM David Holland < david.w.holland@...>
wrote:
Yeah, as Brenda said....
The manuals detail what vacuum tubes are in the plugins. I'd pay close
attention to any photos as I've gotten burnt once or twice by not
paying
attention to them. I would presume if there's no photos, and no statement
one way or the other, that the tubes are missing.
If you start getting into the advanced (sampling) plugins, I would pay
closer attention to the photos for the presence of the transistors, as most
of them are socketed, long obsolete, and made from 99.95% pure unobtanium.
A few plugins (ex: 3B3) use tunnel diodes, but as far as I've seen,
they're
generally soldered to the board. Tunnel diodes are also mostly made
of
unobtanium, and may be dead, but at least they'll likely still be there...
:-)
Again, the various plugin manuals all have a relatively complete parts list
in the back, if you wonder what should be in a given plugin.
David
On Tue, Nov 24, 2020 at 12:05 PM Brenda via groups.io <brendda75=
[email protected]> wrote:
Hello Dave, The 3A1 plugins that I have come across ends up not only the
6DJ8 tubes missing, there are 2 8233 tubes that end up missing as
well.
There are 4 7586 nuvistor tubes in the front end, but seems like the
tube
pickers pass over these from my experience. I am in the market for a
few
3A1 and 3A6 plugins myself...at a reasonable price, $400 is just way
too
much!!
|
David Partridge wrote:
sadly they did make that very mistake quite a few times! Witness the 7904
vertical board ... R724 and R725 (499 Ohm 1/2W) always run way too hot
(burnt fingers hot) and are often well off value from overcooking ... the
PCB gets well toasted too! I normally replace them with two 1kOhm 2W metal
film in parallel and they still run pretty warm.
So, while I'm in here, and planning to replace these two resistors, should I increase the wattage? I expect that these are carbon composite resistors (but certainly don't know that for sure) and what I have ordered are supposed to be metal film 1/2 W resistors. Maybe I should order a kit of 1 W metal film resistors as well and use them instead? Part of me thinks that it won't make much difference: these are underneath the metal shield over the HV section, so they have restricted air-flow under any circumstance. Will larger components really make much difference in their heat dissipation if they are trapped in a hot box? Also, these resistors have likely been cooking for over 30 years, and they've only drifted by about 20%, so either they're not getting too badly cooked, or they're made of sterner stuff. -- Jeff Dutky
|
Re: Anyone willing to make 3D printing of multiplier boxes
|
sadly they did make that very mistake quite a few times! Witness the 7904
vertical board ... R724 and R725 (499 Ohm 1/2W) always run way too hot
(burnt fingers hot) and are often well off value from overcooking ... the
PCB gets well toasted too! I normally replace them with two 1kOhm 2W metal
film in parallel and they still run pretty warm.
David
toggle quoted message
Show quoted text
-----Original Message----- From: [email protected] [mailto: [email protected]] On Behalf Of Tom Lee Sent: 25 November 2020 23:20 To: [email protected]Subject: Re: [TekScopes] Resistor in series 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.
|
Re: Anyone willing to make 3D printing of multiplier boxes
The 465/475 box is an almost exact replica of the original. Modified the mounting to make it easier to install.
--
Michael Lynch
Dardanelle, AR
|
Re: Anyone willing to make 3D printing of multiplier boxes
I have designed and printed several for the 465-475 etc. I also have printed a stand alone box for 455/475M as well. I can supply an STL for either one.
--
Michael Lynch
Dardanelle, AR
|
Re: Anyone willing to make 3D printing of multiplier boxes
encapsulating the HV multipliers is easily done using a "Potting Shell".
they are available cheaply off-the-shelf from many vendors, here's two examples:
a thin-shelled 3D printed box would be pretty fragile, I think.
all the best,
walter
--
Walter Shawlee 2
Sphere Research Corp. 3394 Sunnyside Rd.
West Kelowna, BC, V1Z 2V4 CANADA
Phone: +1 (250-769-1834 -:-
+We're all in one boat, no matter how it looks to you. (WS2)
+All you need is love. (John Lennon)
+But, that doesn't mean other things don't come in handy. (WS2)
+Nature is trying very hard to make us succeed, but nature does not depend on us.
We are not the only experiment. (R. Buckminster Fuller)
|
Re: Anyone willing to make 3D printing of multiplier boxes
Hello,
how big are they.
Tam
--
With best regards
Tam HANNA
Enjoy electronics? Join 15k7 other followers by visiting the Crazy Electronics Lab at
|
Re: Delay Time Position Vernier
Colin Herbert wrote:
Your other problem is setting the Vernier where it is supposed to sit when you put it all
together again! They don't all sit at 0.00, There is data about where they are supposed
to read when the pot is at the extreme anticlockwise. If you have problems, just make
them known here and you should get the right advice. Don't just adjust it to 0.00 - it is
likely to be wrong.
Is this covered in the service manual? I would be interested in knowing how this needs to be set, as I am planning to move the guts of one 475 into the frame of another, and that would obviously involve disassembling and reassembling the delay vernier. -- Jeff Dutky
|
Re: Fix or Part Out a Tek 475A
David Collier wrote:
Yes the trigger slope shafts were bent as well! Now straightened.
How did you straighten the shafts? I mean, other than bending them, did you do anything special? I've got two parts scopes that I'd like to make into one working scope and one parts scope, where the main damage is to the knobs and shafts on the front panels of both scopes (and, like yours, the scopes themselves seem to be in almost perfect working order but for the front panel damage), and I would appreciate any pointers you might have in how to repair the trigger slope switch shafts (I may just have to accept broken knobs, since I haven't found any good source for the trigger level knob, which is what appears to be the common victim of this mistreatment). The 475 seems to be a remarkably durable instrument, even in the face of obvious physical and electrical abuse. I had thought that it was amazing that my father's scope was in such good working order, but it had been treated with kid gloves and stored in a dehumidified basement for most of its life. One of my parts scopes is clogged with black, sooty dust and covered in something like grey paint, but seems to be in even better working order than my father's scope. And the 475A that I'm currently working on clearly had something happen in the HV section, but the rest of the scope appears to be in very good shape (meaning that other functions seem to be working even though the beam intensity amplifier is busted). I was expecting all kinds of things to be damaged: blown diodes, transistors, and ICs, dried out and shorted caps, resistors that had drifted out of spec, dim or busted CRTs, etc., but, aside from the 475A (which is certainly delivering on some of those expectations) things have been quite functional, and I deliberately bought the cheapest, most likely to be busted scopes I could find on eBay! It's a testament to the engineering prowess of the folks who designed and built these machines. -- Jeff Dutky
|
Anyone willing to make 3D printing of multiplier boxes
I was wondering if anyone was willing to make the boxes for the multipliers
so ones that go bad can be made. The box would need the holes for the wires
and mounting threads as original. The old epoxy will break down like the
transformers do. Anyone willing to rewind/build replacement transformers
would be welcome. Buying new diodes, condensers, etc. to put in the box
then use silicon sealant will give enough insulation. I know the sealant
works on 21kV. I had to put some on the PDA where the white was no longer
sealing. The clear type I used stopped the arcing and the scope works fine.
The diodes that can be used are 2CL2FM. They are 20kV and heavier capacity
than originals. I know the diode and condenser from ground to the current
limiter/sensing is low voltage. The diode should be very fast with low
reverse current at 600-1000V, e.g. 1N4937, and the condenser is ,01mfd. I
see comments about multipliers going bad. This would be a good way to get
the scope back working.
Mark
|
OS-34/URM-32 and OS-4B/AP schematics wanted
I am looking for the schematics of the two military scopes. I prefer paper
or print-out. Thanks.
Mark
|
Re: Delay Time Position Vernier
Ah, thank you Colin. I was wondering about that. My working scope isn't at 0.0, and this one wasn't either. And I have no recollection of what the Army scopes were set to. I can get close 'cuz I do remember that it was very close to 0 and like 1/4 turn ccw on the fine knob.
Long way to go before I'm back to setting this!
Dave
toggle quoted message
Show quoted text
On Wednesday, November 25, 2020, 03:17:39 PM PST, Colin Herbert via groups.io <colingherbert@...> wrote: Your other problem is setting the Vernier where it is supposed to sit when you put it all together again! They don't all sit at 0.00, There is data about where they are supposed to read when the pot is at the extreme anticlockwise. If you have problems, just make them known here and you should get the right advice. Don't just adjust it to 0.00 - it is likely to be wrong. Colin. -----Original Message----- From: [email protected] [mailto: [email protected]] On Behalf Of Dave Peterson via groups.io Sent: 25 November 2020 21:13 To: [email protected]Subject: [TekScopes] Delay Time Position Vernier Anyone know the trick to removing the Delay Time Position Vernier pot? I have the fine adjustment knob off. Now what!? Befuddled.
|
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.
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Mlynch001
David C. Partridge
Dave Peterson