I usually start from the rear (i.e., the transformer end) of a power supply and work forward, checking voltages and ripple. Since you have no control over the voltage, apart from a bad pot, my first reaction is usually the pass transistor(s). The voltage doesn't shoot up, like a shorted transistor, so it might be something else or the transistor isn't completely fried. It could also be a combination of things (e.g., electrically leaky filter cap that slowly charges and a shorted transistor). Can it sustain a small load (e.g., 100mA)?
Also, you might want to temporarily swap out the broken locking toggle switch for a normal one so that you don't have the switch in some funky state causing weird behavior.
Although I haven't seen a PD with Helipot multi-turns, they're good stuff, too. So, it's plausible that it could be original. PD did have more variations in their BOM during the later years of the company's history. Even the brushed faceplate wasn't oriented in the same direction all the time (my TP340A being one such example).
Edit: Actually, looking more carefully at the photo, there's a high probability that the switch is the problem. If you turn down the voltage knob on supply B, does A follow it? When in tracking mode, A does not operate. B is the master.
Exterior:
I will polish the meter windows with plastic polish for hobby cars. I cleaned some sticker/marker residue from the faceplate with IPA...but it left the aluminium a bit cloudy in areas, and I'm still able to see some discoloration from where stickers have been. How to clean the aluminium faceplate properly?
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2. Broken locking toggle switch for Source A / Source B tracking mode ON/OFF.
REPLACEMENT: bitseeker indicated possible replacement is Gemini/Alco DPDT Locking Toggle Switch (125V, 5A), part #A201KZQ. This part a little hard to find in single quantity, but I found comparable item that should work: NKK Switches M2022LL1W01 from Mouser.
Operationally:
I have found a problem with Source A: The corresponding pot does not control the voltage output. Instead, when the unit is turned on the voltage slowly increases starting from zero. After a few minutes it has gone from 0V to 21V...it would probably go higher but I turned the unit off
Thanks for the guidance bitseeker! I printed the schematics and PCB component layout on multiple sheets so it's easy read and get started. Attached is rotated schematics/component layout pages in PDF for anyone following along
The top PCB is entirely for Source C, whereas Source A and B are on the side PCB.
Definitely I have to replace the toggle switch ASAP...who knows how the contacts are being connected internally... Internally the switch seems in one position as no matter what way I move the little bit of lever that remains there's no "latching" into any other position and no change in the behavior of Source A/B voltage meters.
Load Test
All channels can support a 0.5ohm (50W) load up to max current limit.
Source A: Amp meter showed increased from 0A up to 1A and then the FAULT light came on. The current increased automatically as the voltage is not being controlled by the pot.
Source B: Voltage/Current as expected, with FAULT lighting when I got to 2V for the 0.5ohm resistor.
Source C: Voltage/Current as expected. FAULT light coming on at 3A but it's just the current limit setting. I tweaked that and went to 5A.
I've measured about 120mV ripple at the output terminals of Source B, C. My unit has PCB revision B from 1975 (according to schematics) so the electrolytic caps are about 43 years old. Perhaps I can proceed to replace the output filter capacitors and switch, and then see where we are at?
The good thing is Source B seems to be functioning as expected so that can be a good reference for Source A.
Many of us here have had excellent results with melamine sponges (also called Magic sponges). Dip it in isopropyl alcohol and rub firmly with the grain of the metal. It does work wonders!
As bitseeker has suggested, you really need to address the broken switch first. The feedback loop that sets the voltage goes through that switch. In one position both A and B have their own feedback network. In the other position B shares A's network. That's how the tracking works.
Yeah, just a regular DPDT toggle will be fine for the interim. Since the handle broke off the original one, there's nothing to lock it into position.
Cool. Thanks. That's handy.
I wouldn't just yet. This is why I usually work from the back of the suppy, forward. Seeing ripple on the front end doesn't necessarily mean that the output caps are bad. They could be overwhelmed by heavy ripple from the back end.
Yep, you're off to a good start. Get that switch replaced, double-check voltage control of A with the tracking switch on and off, then check for problems starting from the output of the rectifier, CR202, and the ripple on the main caps, C203 & C204. If all looks OK, continue forward.
Many of us here have had excellent results with melamine sponges (also called Magic sponges). Dip it in isopropyl alcohol and rub firmly with the grain of the metal. It does work wonders!
Thanks for the tip! I have tried it but without success. My faceplate does not have the brushed/grain finish --- it is flat without texture. The IPA evaporates and leaves a cloudy residue. I will have to keep trying something else.
I will update here once I get the switch in. Going to be a few days until I get it...
I've measured about 120mV ripple at the output terminals of Source B, C.
Ah, yes. I have some precision supplies that have a satin finish. Also, it seems that some panels had a lacquer or other clear coat over the aluminum. That could be what's going cloudy. One of mine has some yellowing of that finish coating (you can see it in contrast to the clear spots that were under stickers).
I will update here once I get the switch in. Going to be a few days until I get it...No worries. We're not on a schedule.
That's great news, Sparky. The NKK switch looks like a perfect substitute. Looking at my TP340A, TP343B and 2020B, they're all locking types, but the handles are different. The NKK M2022LL1W01 is just like the one on my TP340A (provides some grip), whereas the handles on my TP343B and 2020B are smooth.
Anyway, as I mentioned, PD had variations in the parts they used, even for the same model. So, you've got a fair bit of flexibility.
That said, if you can't find an exact replacement for that broken binding post, you could replace them all so that they're at least consistent with one another. Across all the PD supplies I have are three different binding post styles.
alternatively I was thinking there must be another "for parts" TPxxx out there with perfect binding posts
Take care in how you measure the output ripple and noise. If you haven’t seen it already, watch Dave’s video on the subject. Even doing it as Dave demonstrates will tend to overestimate ripple. I recently discovered this on my PD and HP precision supplies. I was getting about 2 mV of ripple using the best methods Dave demonstrates. I knew they should do better than that and sure enough, once I took the time to do a proper setup with a Tek AM502 differential amp, the numbers dropped significantly - down to the few hundered microvolt range. But even if you don’t have a differential amp or probe, following Daves recommendations will get you close.