Re: (OT) Where to go for 70s IBM hardware? I'm looking for a terminal.
I have a pile of old IBM and Telex terminals from a local school system when the state moved away from old IBM mainframes. They are stored in my garage, but I am unable to get to them without help. The IBM terminals are color, but I don't rember the model number. I believe they are RS232. Most of the Telex were dead, but I had swapped enough bords to get some of them to wake up. I am in North Central Florida. On Tue, Nov 24, 2020 at 2:03 PM cheater cheater <cheater00social@...> wrote: Thanks everyone. Not really bothered about the dependency on a control
unit for two reasons. First of all, finding /anything/ of this era will be
difficult, whether it's the rarer RS232 kind, or less rare control unit
based type. Second of all, emulating the bus will likely be part of an
interesting challenge and a good way of figuring out the inner workings of
the display and input device, which is what I want to do, so rather than a
problem it's a positive thing for me. A $10 computer nowadays will be able
to do the job. My current biggest problem is locating those older terminals
in the first place. My assumption is they're mostly in defunct computer
graveyards, warehouses, university store rooms, or bank or utilities store
rooms, dusting and decaying away.
|
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'.
toggle quoted message
Show quoted text
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
|
I have just checked input resistances on a working-to-spec 7A26, the results are quite unexpected!. If I use an analogue meter and have the 7A26 powered off then in the 10mV position the DC 'resistance' from each BNC centre pin to its outer is about 350k measured in both polarities. For 20mV and higher the resistance is 1M. If I use a DMM then with either power on or power off the 'resistance' is 1M as it should be. Clearly Q150 needs to be powered in order to have its gate reverse biassed.
Even if Q150 is faulty, its output, together with the output from the 2X balance pot R160 form the two inputs to the differential amplifier U1350, so if U1350 onwards is all OK, the trace should move.
I have just noticed that there is another DC balance pot (R1353) between U1350 and U1450 so that can also be used to narrow down where the fault is. Since the two beams do move in response to the vertical position front panel control it looks like U1550 in each channel is OK so you can swap the good IC from one channel to the suspect position in the other channel and see what changes.
Regards,
Roger
|
Re: Why you MUST USE the PayPal "FRIENDS and FAMILY" option for Peter's book
On Thu, Nov 26, 2020 at 03:01 PM, Jeff Dutky wrote: ....
My only other complaints about PayPal are mere hearsay, but I've heard too
many stories of sellers having their funds frozen over the years to think
that maybe there isn't at least a little bit of fire behind that smoke.
....
-- Jeff Dutky
I thought about this a bit, with the huge volume of paypal transactions every day across the world, the actual number of horror stories would have to be a fraction of a fraction of a percent.
|
Re: Delay Time Position Vernier
I don't recall seeing it in any Service Manual, but our good friend H?kan has produced a document on the topic:
Colin.
toggle quoted message
Show quoted text
-----Original Message----- From: [email protected] [mailto: [email protected]] On Behalf Of Jeff Dutky Sent: 26 November 2020 00:24 To: [email protected]Subject: Re: [TekScopes] 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: Tektronix 492A input attenuator repair ????
Once I had the same issue with the 492 attenuator. Taking it carefully apart, one of the wafers was burned. I was able to manufacture a wafer myself, using very thin teflon laminate and 0402 SMD resistors. The result was acceptable up to about 8 GHz.
Cheers
Ulf Kylenfall
SM6GXV
|
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
|
Colin and Juoko,
If the 7A26 is arranged anything like the 475 then cleaning the AC/GND/DC coupling switches is almost exactly the same as cleaning the rest of the attenuator switches: soak a narrow strip of 100% cotton bond paper in IPA, slide it between the open switch leaves and the contact plate on the PCB, close the switch trapping the strip of paper, gently pull the paper out, and repeat several times for each contact point. I found the process more difficult for the attenuator switches on the 475 because you have to remove each attenuator block to get access to the switch contacts, and because there are contacts on both sides of the PCB, the ones on the bottom side being significantly more difficult to access. That said, with patience and diligence I was able to clean all the switch contacts, so I think you shouldn't have any trouble.
-- Jeff Dutky
|
Re: (OT) Where to go for 70s IBM hardware? I'm looking for a terminal.
Wish I had kept my Commodore PET/CBM systems...
-Dave
toggle quoted message
Show quoted text
On Wednesday, November 25, 2020, 05:59:43 PM PST, - <rrrr6789@...> wrote: 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.
|
Re: Why you MUST USE the PayPal "FRIENDS and FAMILY" option for Peter's book
John Goller wrote:
I'd like to know what your issues are because I have a
LOT of stuff that I'd like to list here for sale and if there
is some issues that are making things difficult to pay
for things, I'd like to know about it
I don't have particular issues, other than complaints about this particular process (not being offered the options that would have made this process easy), and about the quality of their buyer interface (every time I have tried to log into their web interface I have had to do the "I'm not a robot" dance, and every time the site has responded with "something went wrong on our end" throwing me back to the "I'm not a robot" screen, so that I get to do the dance again, and maybe a third time. That's shoddy UX on PayPal's part, and I haven't got any patience for that sort of thing. Also there's no indication of how far along in the buying process you are, so you just have to keep clicking Continue until suddenly the process is over. That's also shoddy UX). My only other complaints about PayPal are mere hearsay, but I've heard too many stories of sellers having their funds frozen over the years to think that maybe there isn't at least a little bit of fire behind that smoke. That said, I understand why people want to use PayPal: they fill a niche that, even today, has very few players. If you want to accept payment over the web you just don't have many choices, and the established choices (credit card merchant accounts) are not well suited to small players. When PayPal got started they were basically the only game in town for people who couldn't afford to set up a merchant account, and that benefit can not be overestimated. Still, I'm a grumpy sort, and this is one of the things I'm grumpy about. -- 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
toggle quoted message
Show quoted text
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
|
Re: Choosing the right replacement cap
Dave,
Thanks. I saved the URL.
Ananda
|
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
|
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
|
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
toggle quoted message
Show quoted text
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
|
Re: Delay Time Position Vernier
Colin,
I've looked at five different scopes with a delay vernier and only one of them (a 2215) seems to be set at something other than 0.0 at the extreme CCW rotation. All three 475s and the 475A that I'm currently working on start the vernier at 0.0 when rotated fully counter clockwise. Now I certainly don't know about the two 475 parts scopes, or the 475A, but I'm certain that my father never disassembled the vernier or otherwise interfered with it (and, even if he had, he was the sort of person who would have restored it to exactly the condition he found it in). So now I'm doubly curious about how these things are set.
-- Jeff Dutky
|
Re: Anyone willing to make 3D printing of multiplier boxes
Mark,
Call me Michael, I have not designed or printed anything other that these two, mainly because I have not had the need for anything else. I would be glad to try to help with other enclosure boxes. All I would need are some detailed pictures and some general dimensions of the original enclosures. As Walter mentioned, these 3D printed boxes are not very sturdy as printed. However, after the components are installed and they are potted, then they are perfect for the job. All the enclosure must do is hold the components and the potting materials until the latter sets up. I have 2 of the 465/475 HV multipliers running as well as the 455 model. Some of the other HV multipliers are more complex, but I would be willing to try to help with designing the STL or 3D models. I am not in this as a commercial business, just a dedicated hobbyist. I have printed some of these for others, not charging for the 3D print. All I ask is that they cover postage and, in the case of the 465/475 enclosure, the cost of the new mounting hardware. Please let me know if I may be of any assistance.
Sincerely,
--
Michael Lynch
Dardanelle, AR
|
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
|
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
|
Re: Anyone willing to make 3D printing of multiplier boxes
Mr. Lynch,
The photos of your enclosures are nice. I am glad to see a supplier that has these cases for those that have bad multipliers. I brought it up since some have said their multiplier is bad and wanting a replacement. I know the 7000 series has problems with multipliers. I do not know if the 2000 series has that problem also. I know the 2000 series has more parts in them making them larger in size. I do not have an STL file to send anyone. Thank you and the others who replied.
Mark
|
Michael A. Terrell
Jared Cabot
Michael W. Lynch