REPAIRED!     Need help with BK Precision 1745A power supply - Please Help - Page 1
Forum section: electronics / repair

author: Bargainhunter
author_rank: Regular Contributor
timestamp: January 28, 2017, 10:02:11 pm
content: Hi,I am going round and round with this power supply. It was not putting out any voltage when I received it. I found a burned trace between J2 and J7 (schematic for 1746 attached). I also replaced LM723 and LM324. Everything was working fine. I was in the middle of calibrating the unit when the relays started clicking on and off and the voltage was going up and down. I found out a couple of the push on wire going to relays K1 and K2 from the transformer were loose. When I turned the power supply back on the voltage was staying at 50-60 volts! I called it a day. Next day I tore it apart and checked the semiconductors. Everything checked - still 50-60 volts. I then replaced LM723 wind LM324 again (I always buy two of every part I order). I put it back together and it worked! After calibrating it again, I find I cannot adjust the power supply below 5volts. I can go up to 35v and the relays kick in and out depending on the range, but it will not go below 5v.The schematic supplied to me by BK Precision is for a 1746 even though I asked for a 1745A. They said they don't have any but use the 1746 schematic to repair 1745A's. It is similar except for analog meters vs. Digital display.Any help would be appreciated. Also when the say to measure pin 14 with respect to the Red + terminal, I assume the black (-) lead of the multimeter goes on the Red + terminal of the power supply - is this correct?UPDATE!!!I just turned it on and it is back up to 50volts. I put a 10ohm load on it and I was able to adjust the voltage from 3v to 36v but then the relays started clicking and now it stays at 59volts.Thank you,Chuck

author: Kleinstein
author_rank: Super Contributor
timestamp: January 28, 2017, 11:16:19 pm
content: The control circuit is relative to the red output (positive) terminal. So the description of voltage measurement is correctI would be surprised if the 723 gets damaged. This chip is only used for the reference voltage and internal supply at a few points.It is rather unusual to break the LM324 too - normally the more failure prone parts are the power transistors and the relays. With such an old supply, there is also a chance the electrolytic caps are bad. Especially C3,C4 and C14 (output cap).I can't find anything odd in the circuit, except not having an capacitor at the supply of the LM324 and not having an output enable switch / protection against a spike on turn on.Not going below a 5 V voltage is strange. It could be a sign of something oscillating or maybe leakage trough one of the output transistors.

author: Bargainhunter
author_rank: Regular Contributor
timestamp: January 29, 2017, 12:15:28 am
content: Thank you, KleinsteinI originally suspected C14. The traced was burned open at that point. The schematic (1743 is closer to my 1745A) shows a value of 220mf 100v. The cap in my 1745A is a 470mv 100v. My LC102 show a reading of 720mf. I think the tolerance is supposed to be +/- 20%. The LC102 report it as bad unless I entered -20 / +80%. I wasn't sure that would affect the performance. Maybe I will just re-cap the supply. It is a current model. I hesitate to replace c2 it is a large 22,000 35v and expensive. It did check good.Any suggestions on this?Chuck

author: Armadillo
author_rank: Super Contributor
timestamp: January 29, 2017, 08:03:52 am
content: I would suggest that you replace the potentiometers [VR1 to VR4, R21 etc] or at least clean the contacts [use DeOxit or equal]. Then try again.Next measure voltage at TP23, black probe to Red output, and red probe to TP23. Adjust R21 [contract cleaned], confirm -5v.Then update.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 29, 2017, 10:16:20 am
content: For the output cap, the capacitance is not that important. The problem is that with old caps the ESR often goes up, and this could lead to instabilities. The circuit might need some ESR for stability, but not too much. To be on the safe side I would replace the output cap. My choice would be something like a normal (that is higher ESR) 220 µF in parallel with a low ESR 220 µF.For the large main filter cap, I would keep it - the large ones tend to age less, as the liquid to seal area ratio is higher. At least this would not be the first priority.With 35 V main filter cap, there should not be 50-60 V at the output !What is the rating on that supply ?The first test should be done with no load or a small load (e.g. 1 K), to reduce the risk of further damage.

author: Bargainhunter
author_rank: Regular Contributor
timestamp: February 01, 2017, 09:16:42 pm
content: Many thanks for all the help.The power supply is working - thanks to you.There were multiple problems.. Bad capacitor across the RED & Black outputs. Bad diode across the relay coil. Wire off one of the power transistors. Several wires loose (spade connectors) from pulling off and sliding back on.Thank you,Chuck / bargainhunter