Tektronix PS2521G output is unstable at startup - Page 1
Forum section: electronics / repair

author: Fluvance
author_rank: Contributor
timestamp: May 06, 2025, 04:07:17 am
content: Hey guys, I bought a Tek PS2521G triple output linear PSU recently, which works nearly perfectly. After performing calibration, all voltages and currents are exactly as set, within a few millis. Regulation under load and load changes is tack-sharp with basically no overshoot or ringing. All functions, such as OCP and OVP, serial, parallel, and independent configuration, or saving and recalling the memory slots all work perfectly. Eventually I'll have to replace the Dallas NVRAM module, but even that's holding calibration values just fine.However, channel 1 has an issue. When outputs have been disabled for a time, and then switched on, channel 1 exhibits significant instability for up to 2 seconds. It will ramp up at the same rate as the other channels, then hit some point where it suddenly starts dropping - though at a slow rate, indicating it is still trying to maintain the voltage even as it drops - then after 50-80ms it ramps back up again, etc. This will happen several times. During this time, its relays are clicking very rapidly. After a second or two, the instability will end and channel 1 will remain perfectly accurate at its set voltage without any further issues. If I turn the outputs off and then back on within a minute, channel 1 will have no instability.This also only happens when the voltage is set above ~8.8V, which I assume means that level is using the default transformer taps, as it can switch between 0V and 4V on the low side of the channel, and 17V and 25V on the high side. The relays that control the tap selection are what get triggered many times during this instability.The power supply itself being on or off has no effect, just whether the outputs have been enabled recently. The longer they've been off, the worse the instability.I have inspected the boards and found no visible signs of defect. No leaking caps or cracked components. Some in-situ testing show capacitors, diodes, resistors, transistors all have expected values. Any values that didn't match the label, I checked the same measurement on the identical channel 2, which is isolated from 1 but uses the same layout. I'd assume it could be a leaky cap somewhere that's creating instability in the feedback and voltage control circuit.I spent some time looking at the DAC for channel 1. VREF is stable, as are all the voltage rails. The DAC output wasn't as I'd expect, but it was identical to the waveform on channel 2's DAC, so I assume it's fine. If anything, channel 1 is a little more noisy with thicker traces, but the voltage rails look clean so it could just be environmental noise? I am not super confident where to start probing in the voltage control and feedback circuitry to determine what's the cause.If anyone has any insight to where I should look, I'd much appreciate it.The original schematic from Instek: (page 9 contains the control and feedback circuitry for channel 1 - It is mistakenly labelled CH2)https://drive.google.com/file/d/1ctUQelKXaDMosl9bCg6s_btpugdDhV10/viewA redrawn schematic from CuriousMarc: (page 6 contains all the control and feedback circuitry for channel 1, much cleaner)https://drive.google.com/file/d/11hd8jXzhW4CFqOC0aOXFa9o9yihaTAbV/viewImages:All Outputs compared.png - Three channels are present. Channel 1 is the pink waveform. Output 1 and 2 are set to 18V, Output 3 is 6V and scaled 3x. All have identical rise performance.Output 1 Voltage Control.png - Both outputs from the channel 1 DAC, as well as the subsequent buffer opamp output. And fourthly, PDrv in green is the final signal that controls the transistor driving the power BJT. These signals seem incredibly unstable but I captured identical waveforms when looking at output 2's circuitry.Output 2 voltage control.png - The same capture but on channel 2, using the common point for channel 2.Output 1 DAC zoomed.png - Looking at a single spike on the DAC output, which typically sits around 0V, but then spikes over a voltEDIT: One more image: Output voltage compared to drive signal.png - A small snapshot of channel 1's output compared to the signal being fed to the power BJT. You can see some noise and instability in the drive signal more clearly right before it starts to rise again, as well as the fact that the drive signal climbs to compensate for the sagging output, but doesn't stabilize at a level that maintains the set voltage.

author: jwet
author_rank: Frequent Contributor
timestamp: May 07, 2025, 05:03:35 pm
content: I banged my head for quite a while repairing an Instek PST3202, a cousin of yours most likely- circuit looks similar, mine has a rotary encoder and keyboard input but similar. I never got really good schematics for mine- thanks for that- that CuriousMarc must not sleep- beautiful work. My unit had an isolated logic supply go overvoltage (5 became 12) and it took out tons of stuff- mostly that logic that drives the dacs through the shift registers and optos. I got it working up to a point. Someone had been in mine and done some "repairs" in that parallel/series relay stuff and it kind of lost me. The curiousMarc schematic makes this clearer- I might have to dig it back out of storage- I got tired of it and put it away.Considering your symptoms, I would look at the transformer ranging and switching stuff. CuriousMarc's schematic makes this circuit much clearer. Since you have a possible temperature related problem, a can of freeze spray could help. The two big channels are identical so you can A/B between them. The x01 and x02 relays that pick the transformer windings use an elaborate discrete circuit that I guess is sending the drop on the pass transistors. My guess would be a weak transistor, etc in that section for Chan 1.I think in the first scope shot that channel 1's relay is in wrong postion and feedback doesn't know what to do- probably hitting CM voltage limits on FB amp, etc.I'll study these schematic some more and your scope traces. Good luck.

author: Fluvance
author_rank: Contributor
timestamp: May 08, 2025, 03:16:17 am
content: Quote from: jwet on May 07, 2025, 05:03:35 pmThanks for the advice! I appreciate the reply. It's so fascinating digging through older circuits with all this discrete logic. It's also very lucky when you can find resources like CuriousMarc who simplify a large part of the process.Until today, I also suspected the transformer ranging circuit. But after doing some probing of the collectors that drive the relays, I am confident that everything in this circuit is working as intended. The relays are only responding to the output voltage sagging during instability, and trigger each different tap as the voltage drops lower and lower. The only part that confuses me is that section of the green Q618 trace around -420ms. I believe this is a digitally controlled line to force the relays into the max voltage. But it almost seems like the processor detected the voltage drop and tried to reset the circuit by triggering this line. But not in the rest of the instability.Next up is the voltage control and feedback circuit, but this is on a board that's sandwiched under another, so it'll take time to disassemble, attach probes, test, repeat. Freezing spray would be great but I don't have any and cans around here are quite expensive.EDIT: I made a mistake on the voltage ranges. The redrawn schematic has the relays reversed for NC/NO, so I was under the assumption the ranges went: 16V supply for 0-4V output, 12V for 4-9V, 25V for 9-14V, and 21V for 14-21V. Which seemed odd to me but that's what the schematic says lol. In actuality, following the original schematic, the range is a more rational 12V, 16V, 21V, 25V. So some of my labels in the image I attached are incorrect. But the function is still correct.

author: jwet
author_rank: Frequent Contributor
timestamp: May 08, 2025, 04:22:04 am
content: Ok- I follow your logic. When you get into the voltage FB circuits, etc, things are pretty weird. You have the schematics so you can follow.Each of the 3 channels has a daughter board that takes in opto signals and runs them into 4x 4094 shift registers. This to make up to 32 of isolated control lines for that channel. The DACs are each a 12 bit 7541's with an OP-07 on the output and a 741 to buffer the REF. After the DAC are a couple of analog muxes- this acts as a sample and hold for the DAC so it can do quadruple duty. In operation, the DAC is set, mux enabled, .47 uF on them that charge up, they then open the mux and this value sits there- like a sample and hold. These things get refreshed before they droop too much. This is clever. Its also used to make a poor man's ADC to read back values. You'll see several amps with no feedback- these are used as comparator and the processor does some kind of successive approximation to read back values. After this its just power driving and another pile of relays for the output control and series/parallel and rear panel stuff. That stuff is confusing but you can just go up stream of it and ignore it mostly.I'll look at your waveforms in the morning and let you know if I see anything.Good luck- I got my Instek out of storage. I still have 4 jobs in front of it but will get back into it.

author: Fluvance
author_rank: Contributor
timestamp: May 08, 2025, 09:29:54 pm
content: Quote from: jwet on May 08, 2025, 04:22:04 amThanks! It's been a bit tricky to understand the fundamentals of what's going on after the DAC, so that's definitely helpful to know.I did notice in my two captures of the DAC outputs, channel 1's opamp output doesn't have the same voltage swing as channel 2. Channel 2 goes about a volt lower. The voltage rails are fine, and the DAC input waveform has very similar amplitudes when overlayed in Photoshop. So there may be some issue with the opamp or feedback cap. Possibly that would prevent the rest of the voltage set/fb circuit from fully activating when cold? The DAC is also outputting more frequently on channel 2, despite having identical loads attached, even after the instability on 1. Need more testing there.

author: jwet
author_rank: Frequent Contributor
timestamp: May 15, 2025, 02:29:50 pm
content: Poke- just thought I'd see how things were progressing...

author: Fluvance
author_rank: Contributor
timestamp: May 16, 2025, 09:54:22 am
content: Quote from: jwet on May 15, 2025, 02:29:50 pmCouple updates! So I swapped channel 1 and 2's OP07CP, the integrating opamp for the DAC, but that didn't change anything. So whatever reason there is that channel 2's opamp has a larger voltage swing is irrelevant to this issue.After more probing, I found that the voltage comparator opamp, U621, has some fault. Its output on channel 1 goes high, however looking at channel 2, this output should always be negative. The period it goes high is the exact length of the instability, meaning that the output transistor loses its control signal during this time. I was certain that meant the opamp itself was defective, however I swapped the opamps between channels and the issue remains. It must be a connected leaky capacitor or other component nearby. When channel 1 has no instability, then the opamp output doesn't go high. Something in this surrounding circuit is causing the opamp to misbehave when first enabling the output. I will desolder caps C620 and C622 to test or just replace to see if that remedies the issue. I'm not sure what else could be causing this. There are only a couple devices in that immediate circuit that are even connected to 15V, namely U621 itself, U619A (but its output has no incorrect deviations), and the 15V pullup at the base of Q604. Both inputs to the opamp (set voltage and feedback voltage) seem to be correct.With U621, the signal goes high on both the output, and pin 8, the comparison input. Pin 1, the Balance/Comparison line also goes slightly high but I'd assume this is coming through the cap connecting it to 8.Images:Channel 1 U621 Instability: Yellow = The set voltage input. Pink = Feedback voltage, inverted. Blue: The opamp output. You'll see that the feedback voltage (inverted) only loses its stable ramp once the output line goes high, during which the feedback is unstable. Once the instability ends, the output settles at the set voltage.Channel 1 Voltage and Current Comparator: The channel labels are accidentally reversed, however pink = set voltage input, green = voltage comparator output, yellow = set current input, blue = current comparator output. The current signals are stable. However, comparing to the next image, the voltage comparator output is incorrect.Channel 2 Voltage and Current Comparator: Same channel colours. We see that green, the voltage comparator output, does not go high like channel 1. Channel 2 is fully functional, so this means channel 1 should not be going high.Channel 1 U621 Pin 8 and connected components: Yellow = U621 pin 8, the compensation pin. Should not be going high like this. Pink = The positive end of a 10V zener attached to pin 8. Pink is 10V below yellow. Blue = Q601 base, which is connected to a status line on another board. Green = Q619 base, which is connected to Power_Good.Channel 2 U521 Pin 8 and connected components= Same probe points but on the functional channel 2