[SOLVED] Troubleshooting a TRS-80 switch mode power supply. - Page 1
Forum section: electronics / beginners

author: PoDuck
author_rank: Regular Contributor
timestamp: February 06, 2018, 01:13:14 am
content: Hello everyone,This is my first post here, although I have been using this forum as a resource for a bunch of information over the last couple years. I have just never had an issue I couldn't figure out on my own yet.I am trying to resurrect an old TRS-80 model 4, and I have been having nothing but trouble with this power supply. I have run into a couple of issues following the schematics, and I am completely unsure of whether what I am thinking should be the case or not.Here is the service manual I have been using, which goes into amazing detail about everything:https://ia600901.us.archive.org/14/items/TRS80Model4PServiceManual/TRS80%20Model%204P%20Service%20Manual.pdfThe schematic for the power supply is on page 161, which is pdf page 164.When I got this computer, the power was cycling on and off pretty quickly, and the screen just displayed garbage for the short time it would come on. I thought it might be the power supply, so I disconnected all the internal connections to the power PCB except for the mains input. I then applied power, with the intent to test the voltage on each of the outputs. At this point, I oddly got no voltage, so I connected everything back up to see if I could get it to produce voltage again. It started working as before, but at this point, the SCR on the board released the smoke, but I could still hear the switching of the floppy drives as I reached to shut the power down, meaning it was still passing power. Knowing that it is likely that something else caused the SCR to go bad, I tested everything that was anywhere close to the SCR on the schematics to make sure everything worked. When it all seemed proper, I replaced the SCR, repaired the blown out trace, and touched up a good number of solder joints that appeared to have broken, and a bunch that looked fine. At this time, I applied power, expecting to see voltage on the outputs, but again no voltage. Crossing my fingers, and assuming I might blow the SCR again, I plugged all the outputs together again, thinking that I should at least see the problem I was having previously, but I wasn't even getting that out of it. Needless to say, at this point, I just started tracing the power from mains through the circuit, because I wasn't sure where things went off.I have gotten to the larger of the two transformers with 168 volts across pins 4 and (5,6), but it appears to be DC voltage. My meter at least registers it as DC voltage, and doesn't see any AC voltage. Here is where my question comes in. First, why is it DC voltage? How is voltage supposed to pass through the transformer if it is DC voltage? Being fairly new to attempting electronics repair, I only have a vague understanding of how switch mode power supplies work, and I don't quite understand why I am seeing what I am seeing. Beyond any of this though, I don't see any voltage coming out of the transformer at all. Am I correct in assuming that I may have a problem with the PWM? Is there a way to test the PWM without a scope? The only scope I have at the moment is a broken old TEK 464, so I'm attempting to fix this without one.Further, I thought I might just get a new PWM and replace it and see if that fixes things, but I am having difficulty finding a MC33060 or compatible chip in a DIP14 package from a US supplier. All the old standbys only have the SOT package listed as a replacement, and I have only found DIP14 replacements on Ebay so far, and only from China. I really hate waiting 2 weeks to 3 months for stuff to be delivered from China. Is there a place in the US that can supply these discontinued parts?Other things I have done to this board, while waiting for parts, is to replace all the electrolytic caps. A couple of them tested pretty low, and it being a ~35 year old machine, I felt it best not to tempt the gods. I tested every single diode, the rectifier, the transistors, and capacitors. I tested the replacement SCR to see if it had blown like the first one did. I tested all resistors in circuit to ensure there were no shorts. I tested continuity on the T1 transformer on all its connections showing on the schematic. At this point, I am somewhat convinced that the problem is the PWM. Considering I'm not 100% up on what role the opto-isolator plays, I guess that could be an issue as well, since I didn't test it either (not knowing how), but I'm not sure.Anyway, sorry this was so long, but the NEWBIES PLEASE READ BEFORE POSTING post said I should detail what I have done so far. Any help anyone could give would be greatly appreciated.

author: Cliff Matthews
author_rank: Unknown
timestamp: February 06, 2018, 01:59:57 am
content: Welcome! Possibly this may a better candidate for the Vintage Computer section. Eg: for schematics there is this:https://www.eevblog.com/forum/vintage-computing/vintage-computer-schematics-are-these-available-in-the-public-domain/From personal experience, I've found PCB's from that era can require a lot of care. I aquired an original b/w MAC where the solder mask was so thin the traces had oxidized and simply faded away. I pre-date JFK's grassy knoll and may have stopped drinking sometime after that..

author: PoDuck
author_rank: Regular Contributor
timestamp: February 06, 2018, 04:06:47 am
content: Thanks. I don't think switch mode power supplies have changed all that much over time, they are used in everything, and in reality, that is what this question really is about.As for the board, it is in very good shape. I had a couple pads try lifting on me on one capacitor, but otherwise, everything is in great shape.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: February 06, 2018, 04:35:29 am
content: Let's look through the schematic a bit.Starting at the AC input, the impedance of transformer T2 somewhat limits current to the BR1 bridge rectifier. However, it will not provide isolation. This will charge C26 to 120 * 1.414 = 170V. Note that your 168V DC is therefore correct. With the switcher circuit not operating, there will only be some small AC ripple at that node.When the switcher circuit is conducting, the 168V DC applied to pin 4 of T1 flows through Q7 and through current sense resistor R37. The control circuit will turn Q7 on and off. In that transition, current will circulate from the coil at pins 4 and (5,6) into capacitor C37. According to the oscilloscope traces this oscillation happens at 40 kHz. This induces a current into the secondary coils on transformer T1.To debug, use the multimeter to probe both AC and DC voltages at key points on the control side. Note that you have 168V DC around there, along with AC without isolation. That power supply looks like it was inspired by television designs from the '50's. Be very careful and ideally run it through an isolation transformer, keep well away from things, etc. In fact, you may also want to run it using the "dim bulb" method. Here is a good explanation by Mr. Carlson:https://youtu.be/51mjt9nFoeATo start with perhaps measure the oscilloscope points A, B, C, D, E, F and G, along with U1 VIN.As for the SCR, do you mean Q6?

author: PoDuck
author_rank: Regular Contributor
timestamp: February 06, 2018, 09:40:04 pm
content: Thanks Tomorokoshi,I got readings from all the test points with my multimeter. The only thing that gives any power is test point C on Q7. I read 168 VDC there. No voltage anywhere else. I thought I might be able to read the frequency from the PWM, but the only frequency I see is ~60 Hz. I see 60 Hz almost everywhere. Nowhere do I read anything that would suggest it is coming from U1.Also, yes, the SCR is Q6.

author: PoDuck
author_rank: Regular Contributor
timestamp: February 06, 2018, 10:18:41 pm
content: Strangely, I just found two dead diodes, and I don't have replacements for them, so I'll have to wait for those to be delivered I guess. I thought I had checked them all.

author: PoDuck
author_rank: Regular Contributor
timestamp: February 14, 2018, 03:32:06 pm
content: Okay, I'm back to being perplexed about this board.I fixed what turned out to only be one diode. The other was a zero ohm resistor that I mistook for a diode. The faulty diode was the VR1 30V zener. I'm still at exactly the place I was though, other than I know some things that haven't caused the problem.First of all, I wanted to do the tests that the manual says to do before connecting to AC, but I don't have a DC power supply that goes to 35 volts DC, so I'm not sure how I can run those. Therefore, I just connected to AC, and hoped for the best. I got excited for a moment when I was actually reading DC power from the outputs, although the voltage was low. That was short lived though, as the zener blew again. I have replaced the zener again, but I'm not sure what to do next to track down the problem.So, I guess I need to understand what could be causing the zener to blow. I did notice that there are some resistors that are not the same value on the board as they are in the schematic. Pretty much every 10Kohm resistor in that circuit is actually a 4.7Kohm resistor on the board, but since it has run in the past, I assumed it might be one of those changes that the real world demanded, and it just wasn't reflected in the schematic.I know the problem is in the bottom left quadrant of the schematic, but I as I am pretty new to this stuff, I am quite lost trying to decide what and where to test things.As I was waiting for my zeners, I was thinking that since the part of the circuit that is having the problem seems pretty reliant on whatever mains voltage is, as it has not really gone through any active components yet, it might benefit me to slowly ramp up the mains voltage and run voltage tests at voltages that might not blow my zener, so I ordered a variable autotransformer that should arrive tomorrow. Is my thinking possibly correct on this? Even if it is, I really wouldn't know what to look for, even if I were able to get a lower voltage to not kill things.

author: floobydust
author_rank: Super Contributor
timestamp: February 14, 2018, 10:14:21 pm
content: The 30V 1/2W zener in the start-up circuit, I can't see a reason for it to cook unless the power supply has lost regulation- but then the SCR crowbar should activate. R26 does not supply enough current to cook the zener.With this age of SMPS, I would automatically replace the primary-side electrolytics esp. C14; C10, C8, C1 as well.

author: PoDuck
author_rank: Regular Contributor
timestamp: February 14, 2018, 10:50:00 pm
content: I replaced every electrolytic on the board with brand new Nichicons that all tested correctly before I replaced them. I even replaced the C26 and C29 bulk caps, even though they almost certainly weren't bad. As for the SCR, I had to replace that. I have checked the SCR, and it is okay.Is it possible for a little step by step walkthrough of how this circuit should work? I'm getting lost in the schematic with my limited understanding of the circuits involved, and it keeps me from knowing what to look for. I'm not normally into repairing stuff. I've mostly been combining circuits I find from other people to create things, and with those little bits of things I can fairly easily troubleshoot, because it is easy to know what each individual thing is doing.For instance, I have a basic idea of what an scr crowbar does, but in this, I'm not sure how it is triggered, and how to test that it would be working.I have done some research on how switch mode power supplies work, and I have a fairly basic understanding of the concept, but this has some differences that I haven't seen.Thanks

author: floobydust
author_rank: Super Contributor
timestamp: February 15, 2018, 06:48:08 am
content: The service manual has a good explanation of each section of the power supply. I can explain some of it but don't want to write a bookIf VR1 keeps failing and the crowbar SCR had also failed, then I would say the PSU has lost regulation and is basically running full-blast, as if it had full load. So all the output voltages are too high. This would keep tripping the SCR crowbar and explain the 30V zener getting too much current. It's best to have some dummy load on the PSU- resistors or car 1157 light bulbs even.It's difficult to troubleshoot a feedback or "control chain" pg. 153,157 problem.There are many different approaches to repairing SMPS, from blindly replacing parts to following the signal in every portion of the circuit.The service manual approach to apply 35VDC to the primary-side is not going to help us troubleshoot our problem.With the PSU off I would reverse-feed the +5.1V output from a bench power supply, limited to 0.1A or so in case the SCR crowbar trips, to not damage it.So injecting 4-6V at the PSU's output and then measure the voltages on U3 TL431 and U2A optocoupler 4N28, to see if they are working. Do you have gear to do this? Or look for trouble with these parts, or just replace them.The main PWM controller IC MC34060 14-pin DIP, I notice is also compatible with a TL494 which is common, but that is a 16-pin DIP. The end-two pins simply not used here. You can see the service manual has both pinouts and both part numbers listed. Possibly a 16-pin DIP PC board footprint, and a 14 or 16-pin IC used?

author: PoDuck
author_rank: Regular Contributor
timestamp: February 15, 2018, 07:40:35 pm
content: LOL, I didn't intend for anyone to write a book, but I don't see a way of making what I need clear. What you are doing is okay.First things first. Why would Q6 (SCR) have anything to do with VR1 (zener) blowing? Aren't those two things isolated from each other through the transformer? From the docs, it says that Q6 is to keep the 5 volt rail from getting out of hand. VR1, I assumed was helping to regulate power to the PWM, which makes it so power can go through the transformer, which creates the 5 volts the SCR is meant to manage. Am I wrong in my thinking?As for equipment, don't yet have a proper bench power supply that allows current to be limited. All I have at the moment is a cheap variable DC power supply that goes from 4-16 VDC, but no current limiting.As for the PSU resistors, which resistors are you talking about?So far, I have somewhat blindly replacing parts, but for prevention, not cure. I replaced the electrolytics, just because I don't trust 34+ year old electrolytics. I replaced the two x-caps as a preventative measure, because I know those regularly go out on this board. Besides that though, I have replaced the SCR, because I saw it smoke. I replaced the zener, because it was shorted, which I looked at because all the voltages I tested were at a full 168 volts DC. I have even been testing resistors, often removing them to test them out of circuit, to make sure their values are correct, which is how I found that the schematics and what is on the board are two different things in regard to the supposed 10Kohm resistors. Doing that, I found that R34 was shorted and replaced that. I am just about to the point of not trusting anything on this board and replacing it all. lolAnyway, let me give you a rundown of the lab equipment I have available. I now have an oscilloscope, where I didn't when I first started this. I have the power supply I spoke of. I have a good Fluke 87V DMM. I have a frequency counter, which I doubt will be useful. I have an arbitrary waveform generator, which will not help for this at all. I have an isolation transformer, and a variac.Anyway, if I need to limit the current and try what you suggested, it will be a few days, because I don't have the parts to build any current limiting device.Thanks

author: floobydust
author_rank: Super Contributor
timestamp: February 16, 2018, 03:17:36 am
content: Think of the transformer putting out too much voltage, because the controller IC is setting the pulse-width large as if a heavy load exists i.e. both floppy drives rollin' Pacman jumpin'But in reality there is no load at all.So all output rails are too high in voltage. The +/-12V, +5.1V, and the 30V.The +5.1V rail goes up to say +5.8-6.5V and eventually trips the SCR crowbar Q6.The aux. winding powering the primary-side control IC also provides too much voltage. Normally a resistor would limit current through the VR1 zener. But here we have no resistor. If you have say 33V (at C14) it roasts the 30V zener and stresses Q4 E-B junction. The PWM IC is good to over 40V.This is the link between the two circuit sections, that I can come up with as a theory.If the PSU worked, at say +5.2V U3 is supposed to turn on U2A LED and then U2B tells the PWM IC to back off.But it seems to not be happening. It's complicated because U3 also monitors the two +12V rails. So a missing 12V rail might also cause this problem.You can use a resistor to limit current for the U3 TL431 test I am proposing. The 4-16V adjustable power supply sounds good, add a series-resistor any junkbox part around 50-100 ohms. Or even a small 6V flashlight bulb will work. Do you have anything to use for current-limiting during a 6V test.Earlier I was suggesting dummy load resistors, to connect to the PSU output as we troubleshoot.Some SMPS do not like running with no load at all, they go unstable. Not sure if this PSU minds or not.For the component values not matching the schematic, the Engineering Changes are in the sch top right corner.Depending on the production year, there were different values for some parts. Revision C,D,E etc.

author: PoDuck
author_rank: Regular Contributor
timestamp: February 16, 2018, 10:26:25 pm
content: That all made sense, and I decided to forego backfeeding, and just put dummy loads on the rails. I decided to do all of them just for good measure. When I turned on the power supply, it came right up. I had been wary to plug anything into the power supply previously, in case the voltages weren't correct, and I guess that killed the zener. That may have been what killed the zener in the first place, since I didn't plug it into anything after replacing the SCR.Anyway, all is good now. Thanks for all the help.