Tektronix ps503a trouble shoot question. - Page 1
Forum section: electronics / projects

author: ahnuts72
author_rank: Regular Contributor
timestamp: August 14, 2013, 11:03:30 pm
content: This is one of the power supplies that came with the tm500 module i got off ebay.I knew when i bought it one of the supplies had a low output but i wanted to seeif i could trouble shoot it.Here are symptomsPositive side a little over 2 volts maxNegative side 0-20 like its supposed to be.Here is what i have done so far.Check power rails all within a volt of schematic test points.Check reference for correct output a little low but close enough to workCheck all zener diodes for correct voltage all fine.I think i have it narrowed down to u45 or Q85When i check the base of Q85 i get the .5 volts it shows in the schematic,collector is almost at full output of bridge and emitter is just a hundred mV or soWhen i check the base for voltage as i ramp up the output it goes up to about 1.4 volts or so and stops.So I thought maybe something wrong with voltage control op-amp U45 check power rails positive side good negative side good.Check at inputs seem to be right if my calculations are good.here's where I'm at now.Check output of op-amp as i adjust the output voltage the output of U45 goes up to about 1.4 volts then jumps straight to the power rail voltage at about 26.7 volts butI only get a little over 1 volt out at Q85.(Q195 base tracks U145 output is why im wondering if U45 is bad but this is my first time trying to trouble shoot so i may be wrong in my assumption)Is U45 the culprit or do i need to keep looking(am I on the right track as it were).

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 15, 2013, 12:11:52 am
content: Hi,I believe that U45 is a 741 op-amp nothing too special.You can see which part of the circuit is controlling the output voltage determine if CR46 or CR48 is conducting.You could also have the over voltage protection circuit Q65 pulling down on the output.Are you able to probe the circuit while it is running? I have done one of two things:1) remove the covers on the TM500 series main frame to allow access to the boards in the module.2) Temporarily attach wires to key points in the circuit and bring the wires out of the mainframe for measurement.Jay_Diddy_B

author: ahnuts72
author_rank: Regular Contributor
timestamp: August 15, 2013, 01:54:13 am
content: Thanks for the responseU45 shows as a MC1741CP1Haven't checked the OVP yetIll look at it now.I purchased a extender cablekit from jammaboards so I couldwork on it outside the power module.

author: ahnuts72
author_rank: Regular Contributor
timestamp: August 15, 2013, 04:28:20 am
content: Ok before I check the over voltage protection can someone tell me if i understand it right.Under normal operating conditions Q65 is off.The scr is constantly monitoring the output through R66 and VR64.So if the output goes above 24 Volts Q65 turns on and pulls the base of Q85 to groundshutting off the series pass transistor.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 15, 2013, 12:11:41 pm
content: Hi,Your understanding of the OVP circuit is correct.MC1741CP1 was Motorola's, now ON Semi, name for the 741 op-amp.Jay_Diddy_B

author: ahnuts72
author_rank: Regular Contributor
timestamp: August 15, 2013, 11:19:46 pm
content: Ok check me on the constant current sideSay I'm set for .5amps i should have about .300mV across R67Now the load tries to draw 1amp so the voltage drop increases to .600mV which raises the inverting input above the non-inverting input so the op-amppulls down on the output forward biasing cr48 pulling the base current from Q85 and at the same time reverse biasing cr45 effectively shutting U45 output off.This will continue till the load is removed.Constant voltage side.On the voltage control side U45 you have the reference voltage into the inverting input which controls the output based on feedback from the adjustment pots.If you turn down the adjustment pot the resistance goes down letting more voltage get to the inverting input than the non-inverting so the output goes low toequalize the two inputs forward biasing cr45 and pulling base current from Q85Or am I totally wrong?

author: ahnuts72
author_rank: Regular Contributor
timestamp: August 16, 2013, 06:00:43 pm
content: No one?Sent from my Milestone X using Tapatalk 2

author: Unknown
author_rank: Unknown
timestamp: August 16, 2013, 07:01:03 pm
content: Quote from: ahnuts72 on August 15, 2013, 11:19:46 pmSounds good to me, although I don't see why CR45 would have to be reverse biased. It may also be forward-biased depending on the voltage setting and the voltage drop across your load, it's not going to supply current to the base of Q85 either way.Quote from: ahnuts72 on August 15, 2013, 11:19:46 pmAgreed.Quote from: ahnuts72 on August 16, 2013, 06:00:43 pmI didn't understand this post until I scrolled up. Punctuation is important.

author: ahnuts72
author_rank: Regular Contributor
timestamp: August 16, 2013, 07:15:29 pm
content: Thank you for the reply I see what you mean about cr45.It would depend on the voltage setting when it went into cc modeSent from my Milestone X using Tapatalk 2

author: ahnuts72
author_rank: Regular Contributor
timestamp: August 17, 2013, 12:11:43 am
content: Ok after looking at the schematic a little more I see what you are saying about U45 and U55.All the base current going to Q85 comes from R87 the op-amps just control how much it is receiving.Also what is the purpose of Q15 in the schematic?It shows -2.5 on the base and .48 on the collector butwhen I probe it I only get around 10 or 12 mV.

author: Unknown
author_rank: Unknown
timestamp: August 17, 2013, 03:46:15 pm
content: Looks like Q15/Q115 are related to power on/off. Say you power on the power supply with 1A load on the positive output and no load on the negative output. This means that the voltage across C110 will increase much faster than across C10. Q115 will then pull up the base of Q195 to prevent the negative output voltage from overshooting.Edit: I just found the theory of operations in the manual, it's actually for power off with unbalanced loads. Close enough. Did you read the theory of operations?

author: ahnuts72
author_rank: Regular Contributor
timestamp: August 17, 2013, 04:58:27 pm
content: I'm part way through it don't remember that part when I read the theory of operation part though.Have to reread it when I get home.Every ones help plus going throughthis circuit has helped me understand this stuff alot more .I really appreciate every ones help.Sent from my Milestone X using Tapatalk 2

author: ahnuts72
author_rank: Regular Contributor
timestamp: August 17, 2013, 11:07:54 pm
content: I found it some how I skipped that page when I was reading the pdf.Gonna have to get the manual printed I prefer reading this type of materialon paper instead of a computer screen anyways.I think I have enough info to finish trying to trouble shoot it now.

author: ahnuts72
author_rank: Regular Contributor
timestamp: August 18, 2013, 03:20:37 am
content: Ok after trying to check all the diodes I'm kinda stumped.Should a 1n4152 conduct both ways I always thought upto a certain voltage no diodes will conduct when reverse biased.I'm getting around .5 volts when forward and any where from1.2 to 1.8 volts when reversed.This is while they are still on the board if it makes any difference.

author: Unknown
author_rank: Unknown
timestamp: August 18, 2013, 01:15:46 pm
content: That will likely make a difference. Desolder one of their legs and try again.

author: ahnuts72
author_rank: Regular Contributor
timestamp: August 25, 2013, 09:28:58 pm
content: Thanks for all the replies everyone.I pulled the scr and now the supply is working.Would the scr being shorted have made the voltage control opamp go full rail on the output if it was bad?Sent from my Milestone X using Tapatalk 2

author: Unknown
author_rank: Unknown
timestamp: August 25, 2013, 09:53:40 pm
content: Probably, since the op-amp is essentially operating open loop as a comparator then.