Requesting help with lab power supply troubleshooting.. - Page 1
Forum section: electronics / beginners

author: Flávio V
author_rank: Regular Contributor
timestamp: November 14, 2012, 10:35:22 pm
content: I recently got a broken power supply from college, a univolt DT305DD dual 30-3.And i am looking for some help because of that lack of it and i don't know much about the frequent failure in these type of linear power supplies.I am looking for possible service manuals(if there is any, i can't see if they have a website)Type of failure:-Output 1 wrong voltage min-max(0.7V up to 6V)-Output 2 wrong voltages also but difference voltages(min:3.3V max 32V(fine the max)-At the last moments of the testing time i made both outputs had a output voltage of +-52V even with turning the output 2 knob and being in "indep" mode(i got scared with those voltages because the main cap is rated 50V and the output one was larger and both outputs had that voltage)Type of power supply:Op-amp linearMeters than i have available:-UNI-T UT50A 2k count multimeter volt, 20 amps, ohms-Fluke 8060A 20k count multimeter volt, 2 amps, ohms-Fluke 97 60Mhz 3k count scopemeter volt, scope, ohmsThinks than were checked:-Fuse housing(replaced because it was stolen(go figure)-Current and voltage control pots(all 4),they are 6.8k but read 3.8-4k max-Any visual damage in caps(they are DH caps so seem bad and the main 4700uF 50V are vent-less)-Construction of the unit-Output voltages and meter accuracy(fluke 8060A used)-Output transistorsComponents replaced for now:-Fuse housing and placed a 2A ceramic fuse(the recommended one is 4A but that is too high for my taste, the fuse is on 230V phase)-The meters power supply(KA7805 based) has now terminal blocks for easy removal(also planing to supply those meters with 2 mc34063s)Thinks than i am interested in knowing:-Is it right to replace the pots with 4.7k ones or is it better 10k ones?-What can cause the wrong voltages?(i don't know about op-amp still but i think than it is time to get ahead my class again)Pics:Power board(equal for both outputs,this board is for the output 2)

author: Shuggsy
author_rank: Regular Contributor
timestamp: November 15, 2012, 07:08:02 am
content: That looks quite similar to a Mastech branded supply that I have, the HY3003D-3. Here's a schematic for the 5A model of an extremely similar supply (HY3005F-3, I think the main difference is in the front panel meters). Note that there is a fixed 5V output that doesn't apply to your model. I got this from the distributor, but after looking over the schematic a bit I'm not sure if all the connections are listed on there... hopefully it may provide you at least some guidance.Unfortunately I don't have a full service manual for it.

author: alanb
author_rank: Frequent Contributor
timestamp: November 15, 2012, 01:18:45 pm
content: It may be worth checking the output transistors first. On most linear PSUs that I have encountered the output transistors were the cause of the failure. Simple way to test them is to desolder them then use your multimeter on them.

author: Flávio V
author_rank: Regular Contributor
timestamp: November 17, 2012, 06:45:52 pm
content: All the 6 output transistors were fine...tomorrow i will check if there is any output current, that way i can know if the only problem is jumping voltages(with seems to me to be a ua741 op-amp problem)

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 17, 2012, 07:38:57 pm
content: Looks like a rebranded Matrix, at least they look the same and there is much more of them on the net. Unfortunately I could not find the exact model.First make sure that the power supply feeding the 'logic' in there is ok, maybe there's even a +/- supply, look at the V+ and V- pins of the LM324.QuoteWhy do you want to replace them, because they measure a bit low? There's probably something in parallel, if you want to be sure disconnect them first and measure again.

author: Flávio V
author_rank: Regular Contributor
timestamp: November 17, 2012, 07:50:14 pm
content: Quote from: PA0PBZ on November 17, 2012, 07:38:57 pmTomorrow i will check it...after i solder back the wires and the heatsinks to the supply...I want to replace the pots because i want to replace then for 10 turn ones(for voltage)and 2 pots don't stop turning when they should...

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 17, 2012, 07:58:14 pm
content: QuoteI see, well it's hard to say, it depends on how they are used. Best is to check with a pot you have laying around before you buy anything expensive.QuoteThere you go, infinite turns for free

author: Flávio V
author_rank: Regular Contributor
timestamp: November 18, 2012, 01:45:11 am
content: It goes to the max 4k after the turn(+-300º)..so...well...it would be good if was infinite...

author: ptricks
author_rank: Frequent Contributor
timestamp: November 18, 2012, 01:02:35 pm
content: Troubleshoot by following the power.Check all power supply voltages.Next follow the power as it goes through the circuit, schematic makes it easy but not required. Get some graph paper and start drawing the schematic as you go along.You can try to solve the problem by trying this part or that part that you think could be the problem, but you are wasting your time using that method because the fault could be in something unexpected . I have repaired probably several thousand devices in my career , majority without schematics and while it is tedious, following the power always works. Even with a schematic don't do the test here and test there approach because things often fail that are not easy to locate like bad board traces.

author: Flávio V
author_rank: Regular Contributor
timestamp: November 18, 2012, 03:46:20 pm
content: I popped those pots off and i got what i was thinking....2 of then seem faulty...one of then reads above 120k but the valor jumps around to 0nS, the other one reads normal(7,2k)but after some turning near 7k area it gets open circuit and then to 0....Other gives open circuit in the 0k area but i think than that won't be a problem....I will replace for now then for some pots i have, one if 5k precision one and a 100k audio/tuning think one....since according to a similar supply they act was a voltage divider so it might not be a problem....