Tek PS503A - Page 1
Forum section: electronics / repair

author: kevbry
author_rank: Contributor
timestamp: February 03, 2015, 05:34:44 pm
content: I picked up a Tektronix TM504 chassis a year or two ago with a full set of modules, including a PS503A power supply. The power supply behaves in a way that I'm not sure is quite right, but I don't have anything to check it against so I can't be sure.When I have the chassis powered, but the output enable switch on the PS503 set to off, I see around -0.015v appearing between the positive and common terminals. If I turn the voltage control pot all the way up, that climbs (drops?) to -0.038v. The same is true for the negative output, with around 0.011v appearing with voltage control at 0, around 0.035v appearing with it set all the way up. The stranger bit is that there will never be more than -14ma driven between +V and Common with the output set to off, regardless of where I set the voltage control, with a similar positive amount on the -V side as well.When I turn the output enable on, a setting of 0 on the +V control pot gets me the same -0.015v, but turning it up passes through zero and up to the expected output voltage, all the way up to 20v. Same sort of thing for the negative side. Essentially, a setting of zero gets me a slight negative voltage on the positive rail, and a positive voltage on the negative rail. If the output is enabled, I can dial in a correct voltage, but as soon as I turn it off the positive rail drops back down to slightly negative. Neither the current set pot or the tracking pot settings seem to have any effect on the output voltage when the output is disabled, but work as expected when the output is on and I have in-range voltages set for each output. The trimmers for the +/- rail seem to work fine for adjusting the maximum output, but have no effect on the output off/0v negative voltages.I've also noticed that as soon as I shut off the chassis the outputs slowly oscillates between about -0.030v and +0.3v between +V and common, with a similar opposite amount between -V and common, until it settles down to 0 after a few minutes.When I first opened it up I noticed that someone had replaced U55 opamp with a 741 compatible, so I just replaced all of the Tek 741s with a set of closely matched new ones. Since then I've replaced all of the small electrolytics and tantalum caps with new ones, with no effect. Everything I've taken out has tested good. I've started testing reference voltages from the manual (http://bama.edebris.com/download/tek/ps503a/tek-ps503a.pdf), and most have been off slightly (+-0.02v), but I'm not sure if that's due to me testing with the voltage trim at 0, which is giving a non-zero output voltage. I noticed that the manual states that a zero setting should result in "0v +/-100mv", so I might be completely out to lunch here with even thinking anything is wrong.If I ground reference the common terminal I don't see any drops below 0 on the positive rail, or above 0 on the negative, so that's how I've been using it so far. Besides this strangeness with inverted voltages when off/set to 0 it seems to work well. If anyone has any ideas, I'm all ears and would really appreciate it.

author: tautech
author_rank: Super Contributor
timestamp: February 03, 2015, 06:29:24 pm
content: Could the mainframe PSU if out of spec possibly affect the PS503A power supply ?

author: VintageTekFan
author_rank: Regular Contributor
timestamp: February 03, 2015, 06:49:32 pm
content: According to the performance check - Spec with the voltage control knobs fully CCW, should be 0V +/- 100mV. So that sounds like it is fine.I've got a couple that are working - I'll double check tonight if I see similar behavior.

author: kevbry
author_rank: Contributor
timestamp: February 03, 2015, 06:57:38 pm
content: Thank you very much! I'm hoping to have some time this weekend to test the chassis itself... I didn't figure there would be anything wrong with it, since everything else (FG503, DC504, DM502) seems to work just fine.For the 0v+/-100mv, I wasn't sure if that was 0v+100mv for the positive rail, 0v-100mv for the negative rail, or if it was +/- for either. I don't have any other linear supplies to check against to see if that's common behaviour, so thank you very much for checking yours.

author: timb
author_rank: Super Contributor
timestamp: February 04, 2015, 05:10:36 pm
content: Yes, it's common. They have to drive the voltage down to zero, so they're biasing to the opposite rail to get there. That -15mV is most likely the op-amp's offset voltage coming into play. On precision power supplies, there's generally a "zero adjust" pot inside you can set to compensate for said offset.The designers at Tek most likely didn't think it was needed for a general purpose supply so didn't bother with the added complexity.And yes, it's +-100mV on each rail. That is the positive output can be between -100mV and 100mV at zero, same with the negative output.Keep in mind that the cables connecting the PSU to DUT can produce tens of mA by themselves just from RF pickup, being moved and thermal gradients, so a 15mA offset isn't going to hurt anything. The dropout voltage for most transistors and diodes is generally above 200mV, so there's no risk of turning anything on for a positive offset. There's also no way a hundred mV negative offset can damage anything.TL;DR: It's normal and well within spec. It ain't broke, don't fix it.Sent from my Smartphone

author: kevbry
author_rank: Contributor
timestamp: February 05, 2015, 07:00:49 pm
content: Sounds good to me, thanks for the info. I actually picked up an old Lambda supply yesterday that appears to do pretty much exactly the same thing. I can probably add in a trimmer to zero things out if it's really bugging me... the circuit seems fairly straightforward.