Bench power supply (old) resistor value question??? - Page 1
Forum section: electronics / repair

author: Daybear63
author_rank: Newbie
timestamp: March 16, 2022, 02:08:57 pm
content: I found a old bench power supply that works (sort of). It powers up but i can not adjust the voltage. I then took it apart and discovered a burnt non working resistor in the circuit. But my knowledge regarding electronics know how are limited,so could anyone help me in this matter? It looks like the first color ring is brown but the rest are burnt beyond recognition. I made a sort of schematic that i hope would help. Help would be very much appreciated.Oooops, i was clearly screwing the schematic up, but here is the new one, i hope this is correct at least it now works.

author: Andy Watson
author_rank: Super Contributor
timestamp: March 16, 2022, 03:13:50 pm
content: Some thing around 100should work. However, if there has been enough voltage across that resistor to fry it, the BC108 will also have been fried. There may be other dead components too. Do you know what caused it, or did it "just happen", (there is little protection in the circuit).

author: Kleinstein
author_rank: Super Contributor
timestamp: March 16, 2022, 03:17:55 pm
content: It looks like with most of the small transistor base and emitter are interchanged.The broken resistor schould be something in the 100-500 ohms range, depending on the maximum voltage.Chances are that there are more parts bad, especially shorted transitors.

author: Hamelec
author_rank: Regular Contributor
timestamp: March 16, 2022, 03:20:06 pm
content: for me the circuit looks like that it NEVER worked before.BEC leads of BC108 and 2N3053 not having the proper orientation...tbc2N3053 right side:1 => E2 => B3 => C2N3053 in the middle:1 => C2 => B3 => EBC108:1 => E2 => B3 => C

author: Daybear63
author_rank: Newbie
timestamp: March 16, 2022, 04:02:34 pm
content: I clearly was not concentrated with the schematic, so here is the new one added to the main post and i hope this one is correct?

author: Andy Watson
author_rank: Super Contributor
timestamp: March 16, 2022, 04:32:56 pm
content: I had assumed the C-E swap was a transcription error, however, the current limit 2N3053 still appears to be swapped - both on your diagram and on the actual PCB.

author: Hamelec
author_rank: Regular Contributor
timestamp: March 16, 2022, 05:42:55 pm
content: 2N3055 B-C is swapped

author: Daybear63
author_rank: Newbie
timestamp: March 20, 2022, 07:29:31 pm
content: Quote from: Kleinstein on March 16, 2022, 03:17:55 pmQuote from: Andy Watson on March 16, 2022, 03:13:50 pmI finally came around to test the transistors and you where right, BC108 was dead but the rest was ok. The only equivalent i had laying around was 548B (i hope it is good enough)? I also followed your suggestion on the resistor that was burned and used a 100Ohm. So it finally came alive and it worked fine. Adjustment of voltage works, no components get hot except 2N3055 which was expected. But you mention protection or lack of it, is it good enough with a fuse of som sort on the output or should it be more advanced than that?Also this power supply (the pcb part) is quite old and simple so i wondered if someone could point me in the right direction to build a variable one myself (preferably with schematics and even better a pcb layout and parts list)? The transformer Voc is measured to 21.8v.I also thought of maybe bying a variable power supply like this onehttps://www.ebay.com/itm/224794921517or is that not recommended?Regarding the confusion on my part for swopping CBE on the transistors, i am really sorry for that but now i hope it is ok and has been edited in the first post

author: Kleinstein
author_rank: Super Contributor
timestamp: March 20, 2022, 07:52:03 pm
content: A fuse would not protect the circuit from a transistor damage. Usually a transistor burns much faster. With some luck a fast fuse may protect a rectifier oder large diode, but usually not a transistor. The fuse would be more protection from fire after the circuit has failed.For a little more protection one could consider some 100 Ohms in sereis with the base of the BC108 or the replacement.The BC548 should be OK as a replacement.A moder modern next step up would be a LM317 based power supply. The maximum power and thus current would be a bit lower however.The kit from the EBAY link is not so bad and has some weak points. With a 21 V transformer it could be just OK, though still stretching the limits - more voltage can cause trouble in that circuit. There are quite a few threads about this type of kit and how to improve on it. A main point is to not use a transformer of much more then 20 V.Also don't expect to get the full claimed 30 V : it would naturally be more like 20 V max and preferrably only 2 A.