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Re: Resistor in series
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 |
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Re: Resistor in series
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 |
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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 |
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Re: 2710 Spectrum Analyser 'Cannot count VCO, IF' error
Martin,
One other thought is compare the cabling to the service manual. I have often seen 271x's that don't work and when someone got into them and found a bigger problem they just closed up the unit and connected the cables to anything that looked close or fit. Lot more than once! |
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Re: Choosing the right replacement cap
Scroll down this thread a bit. Good picture of the cap sitting in front of the 1/4" nuts holding A1 (& A2, CH2) to the face. But this is for a 475 and those lucky bastards have a plug in cap. But that's the big cap that's gotta come out.
And it's the whole assembly of A1, A2 (attenuators), A3 (preamp), and A4 that come out together. So look for EVERYthing that's connected to all of these boards. Dave |
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Re: Anyone willing to make 3D printing of multiplier boxes
Greetings,
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A close friend of mine, who is an electronics engineering tech, owns some nice 3D printers as a side business. If there is interest I will give him a ¡°heads up¡± and offer his contact information to the group. I have no financial interest, but he does nice work. Regards, Ken On 25Nov, 2020, at 5:24 PM, Tam Hanna <tamhan@...> wrote: |
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Re: Choosing the right replacement cap
Yeah, the delay line definitely needs disconnecting, among a couple of other odds-n-ends. I don't have my notes in front of me. One of the gotcha's that almost got me is the ground strap that goes to A9 (on the bottom). It's hiding under the invert and beam find pushrods.
Careful disconnecting those pushrods. I removed one to get at the ground strap, and glad I did. The ground is a big heat sink and it took a bit (of heat) to get it to release. Having to thread a hot iron between those two plastic pushrods would have not gone well. I've encountered a broken push switch trying to get another one of those pushrods off. It's a whole other story that I'll be posting later. But pulling strongly but steady longitudinally on the pushrod while gently using a screw driver to open the ends enough to get it to release does the trick. Those push switches have a nasty weakness in them that I'm going to share later. The grey plastic push button that goes into the switch has a tiny cross section inside that can break. Then you're buying a whole switch. Anyone have a good trick for getting those pushrods off without stressing the plastic bits? To remove the delay line I first used a wick to remove the solder from the shield connection. Then I was able to use narrow needle nose pliers on the shield pulling 90 degrees away from the board while heating the shield connection allowing it to come away. Took a little wiggling and a few goes at it. Slow and steady does the trick. Taking a few shots at it keeps the heat going into the shield down. Hope that helps. Good luck! Dave |
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A Couple Restorations
My main pleasure with equipment is finding inoperative or basket cases and restoring them. I just finished a 2230 and an R-631B. This is an account of the projects, skip now if you aren't interested.
The R361B was obtained from a member of this group, and I was quite pleased with it when it arrived. I replaced the power cord, and tried it out. It came alive, had a trace, then a loud snap was heard and it went dead. I feared the worst, but continued with the work. It needed a bath, and got it, followed by about ten hours in a hot box to dry it out. I wanted to see if I could get the power supply working before I put the jug back in after the cleanup. First tests were aweful. None of the voltages were correct. The negative 100 volt supply is the reference for everything else, and it was producing about 115 volts. All of the others were very low. After a few hours of round and round circuit tracing, I found the key problem- a partially shorted ceramic condenser. It was one of the square red Erie brand ones, rated at 200 volts. There were two on the board, I replaced them with 600 volt ceramic discs. There were two other old electrolytics, I changed them out as well. The fuse protecting the high voltage supply was gently blown, not fried and flashed all over the glass tube. After finding and replacing two fried transistors in the positive 125 volt supply, everything came alive and right on what they should have been. I put a fuse in the high voltage supply, it came alive with an output nearly 4KV, it should have been 3300. I turned the HV adjustment all the way down, but could not get it below 3500. Both of the transistors in the HV regulator are "special selected", and tested good. After studying the manual, and not having exact original transistors, I added a 12 meg resistor in parallel with the 2.2 meg resisator coming from the moving contavt on the HV adjust pot. That allowed me to adjust down to 3300 volts. I cleaned all the switches and pots in both the mainframe and plug ins, re-installed the jug, and tried it out. With adjustment of the sweep and position controls, I was rewarded with a good bright trace. External signals and the built in calibrator were displayed quite well. The vertical plug in I have is a single trace differential, not especially useful to me, so I have a 72 dual trace on order. I will let the scope cook on the bench for another day or so before I put it aside waiting for the dual trace preamp to arrive. It will probably be used in my RTTY setup, the vertical response is only about 600 KC. The 2230 was a genuine basket case, almost entirely dismantled and filthy with carbon dust. It was a freebee with another purchase at the Rochester hamfest. I was quite skeptical until I got it home and found that except for many screws, it was complete. It wasn't stored in the dirt, it was operated in the filth because the knobs and front panel were almost unreadable, and the focus pot was completely covered in fuzz. Getting the front panel and control label plastic sheet off took a bit of exploration, but WD-40 and a blow dryer softened up the adhesive on the sheet, and it came off with very little damage. Now I could remove the front panel to give it the cleaning it needed. Now fully dismantled, everything got a bath. The cleaner and rinse water were black as night, but everything cleaned up very well. After re-assembly, cleaning and lubing switches and pots, it was time to see if it would come alive. Turning the power on did nothing for a second or so, but the pilot lamp blinked and the cooling fan moved. I remembered a similar problem with broadcast waveform monitors and vector scopes, and replaced all of the smaller low voltage electrolytic condensers in the circuit that drives the inverter. Now, the same delay but the fan started and the pilot lamp lit. There were some bright green blobs on the screen, adjustement of the focus control produced brilliant menu text. After exploring the controls, I found that one of them had to be pushed in to display waveforms. I spent an hour or so exploring the locking action of the pushbuttons, and there was no was to get two of the switches to latch in, even if I borrowed the latch pins from other switches. The others that were supposed to latch did. Because the buttons are plastic stems on the switches, I filed a notch in the bottoms of the two that would not latch. This allowed them to latch in, a gentle lift released them. Everything seemed to work fine, the scope has many features I will probably never use, but it displayed waveforms and would store them. After several hours of cooking, I put the shield cover over the power supply section and the brace that supports the top circuit board back in. I turned it on, and was met with a total disaster. The only thing I could get on the screen was blobs bouncing around. I shut it off instantly, and started looking for bad solder joint on the bottom board where the power supply is located. A solder splat was found and removed, the scope came on and performed fine. After a few more hours of cooking, I put the case and back panel on and ran it on test for a few more hours. It performs very well, it will probably be used mostly in my shack to troubleshoot RF circuits and monitor transmitter outputs because it has a 100 MC. vertical bandwidth. These are examples of the kind of projects I really like. Now, does anyone have a 224 handheld scope project they would like to find a home for? 73. Bruce Gentry, KA2IVY |
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Re: Choosing the right replacement cap
Dave,
Thanks for the write up. Did you disconnect the delay line as mentioned in the manual? I may have to pull the board after all. One of those capacitor blocks that are plugged in seems to have a bad contact. I have cleaned the delicate switch contacts and they seem ok. The test signal looks funky on CH2 and if I put slight pressure on one corner of the 10x block, it becomes normal. I tried removing it and cleaning the 6 pins but it does not seem to be fixing the issue. It goes back to the weird wave form if you tap a corner with anything. If I try to wiggle it, it comes back ok. It may be the tiny socket that holds one of the pins is loose. Or it may be the inside of it is tarnished. I remember someone on EEVblog mentioning that he had to slightly flatten the pins for a better contact. He even mentioned how he cleaned the inside but I cannot remember what he had written and now I cannot find the thread. I will post there and see how it goes. Ananda |
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Re: Resistor in series
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.
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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 |
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Re: Resistor in series
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.
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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 |
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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 |
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Re: Resistor in series
Chuck Harris
Just because they documented their fix for a mistake
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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: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. |
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Re: Resistor in series
Chuck Harris
Perhaps Tom, but I have seen many instances of them doing
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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. |
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Re: Resistor in series
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.
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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 On 11/25/2020 16:59, David C. Partridge wrote:
sadly they did make that very mistake quite a few times! Witness the 7904 |
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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, |
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Re: Tek 3A1 Module
"If not for FleaBay, then where else can one obtain these old scope
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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. 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? |
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Re: Resistor in series
David Partridge wrote:
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 |
Dave Peterson
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