Advice for Lodestar Bench Power Supply Repair - Page 1
Forum section: electronics / repair

author: Domokelly
author_rank: Contributor
timestamp: July 22, 2020, 10:08:48 am
content: Hi guys,I'm trying to repair a broken Lodestar 8203 Bench Power supply and I was hoping to ask the hive mind for advice for my next step.So here's the summary:It's a Lodestar 8203. It's old-ish... I believe early 00's. However the good news is that I have an operating manual complete with circuit diagram!The Slave channel failed, so I opened it up and found a vaporised trace on the main slave PCB. I suspected the bridge rectifier so I removed it (and it was shorted closed) so I replaced it, repaired the PCB trace and replaced the fuse.Now it powers up but the slave channel reads -0.5V and the voltage adjustment has no effect.So I'm guessing that the shorted bridge rectifier has damaged some components downline. I suspect the 2N3055 power transistors might have been cooked, so I took one out and tested it and it appears ok- but I might replace the others anyway.So I was wondering if someone might be able to suggest a likely culprit or at least the next component to test?FYI the failed bridge rectifier is D1-4 on the main circuit diagram.Any suggestions would be very much appreciated - it's a good unit and I'd love to repair it rather than tossing it out.Many thanks, Dom.

author: coromonadalix
author_rank: Super Contributor
timestamp: July 22, 2020, 10:18:41 am
content: wow that trace busted pretty goodwhat voltages do you have on c3 c4 the main capacitors ? you have to follow all the path / parts for the busted pcb traceThis psu use a floating regulator topology for the ua723 and lm723 regulators ic's, you have to be careful where you put your ground probe, if the other chanel is okay or seems okay in independent mode you can do some comparaisons between them

author: Domokelly
author_rank: Contributor
timestamp: August 09, 2020, 04:41:02 am
content: Hi @coromonadalix,Thanks for your reply and sorry for the lateness of mine- work has been flat out lately so I had to put the repair on hold.But the caps were reading 0v so I traced it back and it turned out that an output pin of the relay at K1 had been burnt off during the overload and that was causing the open circuit. I sourced a replacement, replaced all the C2 and C3 caps for both rails as well as the power transistors (one was faulty) and both bridge recifiers.Now it performs beautifully! It's great to get it working again.So thanks for your help- it's very much appreciated.Cheers Dom.

author: awais7910
author_rank: Newbie
timestamp: May 28, 2023, 05:03:42 pm
content: I have a Lodestar 8301. The problem is it gives output 47V DC and does not control from voltage variable. I checked the voltages across two capacitors, they provide 47V. I test power transistors but they are fine. I also checked all the rectifiers, they are fine too. I almost check all the components except display ICs, I don't have these. Anyone can guide me on this. Thanks

author: Swainster
author_rank: Frequent Contributor
timestamp: May 31, 2023, 03:31:21 am
content: Not specific to this power supply, but one of a similar design; when I had this problem it was caused by a broken feedback path. In my particular case it was due a dry solder joint on a jumper link, which must have been there from the factory but just deteriorated further over the years, however similar symptoms could be caused by any failed component in the feeback path e.g. resistor/pot/op-amp or whatever.