Colour coded diode ID? - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 28, 2012, 03:00:29 pm
content: I am trying to repair a Farnell H60/50 60 volt, 50 amp linear power supply unit I bought as a "none runner" I have found one of the three control boards has blown, probably because a cap or rectifier has gone on the main chassis but I am guessing at the moment. Due to someone's immense kindness in dismantling his shack to gain access to his own unit and take photos I have detailed images of components that are not pieces of charcoal, but now recognizableOn the attached photo I need to ID a diode from its colour coding, can anyone verify the correct replacement please? it's D1, but it appears the same as the diode under R7, and this diode is OK on my board, so I could lift a leg and take some measurements if required. Sadly iI have no schematic, so am working solely off the actual boards and chassis. Thanks

author: SeanB
author_rank: Super Contributor
timestamp: August 28, 2012, 03:47:14 pm
content: I would guess they are IN914 diodes.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 28, 2012, 07:55:41 pm
content: Thanks Sean. Sadly this repair has taken a major step backwards. The guy who kindly photographed one of his control boards has given me the bad news that my unit should have 3 control boards, populating all 3 PCB board sockets. I had wrongly assumed one socket was for an optional board. As all efforts at getting a schematic for my version of the control boards has been fruitless I am now stuck againMy only idea, and probably beyond my capability, is to abandon the variable voltage and current and use the transformer, rectifiers, smoothing caps and thyristors and pass transistors, fans etcetera and build some sort of very basic control board to give 50 volts at max current with some over current protection, even if just a fuse. This would be a shame, but I got the thing as I wanted to be able to power a high output semiconductor linear amp that needs 50 volts at a serious current. Not sure how feasible a home brew control circuit would be. I have screenshots of the transformer / rectifier / thyristor / pass transistor end of the thing (which pretty much matches my circuitry, it's the control boards that differ) athttp://www.gatesgarth.com/farnell/farnell.html

author: SeanB
author_rank: Super Contributor
timestamp: August 28, 2012, 08:28:10 pm
content: It is a simple regulator using a 741 opamp, should be easy to make a clone by reverse engineering the photos and by using the 2 boards you have. May not look the sdame, but just needs to do the same function, and from the diagrams it is doable, as the boards have well defined functions. Which board is crispy beyond belief, and which is missing. One drives the SCR preregulators, one is the actual pass transistor drive and the third is power for the other 2 boards.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 28, 2012, 08:44:45 pm
content: The problem is my two boards resemble none of those shown in the schematics I have. I don't know if mine are a newer type, or older, but the component count is a lot less, which makes me think my supply is a later revision.The two boards I have are shown athttp://www.gatesgarth.com/boards/boards.htmlFrom what I can tell the missing board is the auxiliary supplies board, which, if the correct circuit diagram has a similar component count to the board I do have the diagram for, it should, as you say, be not too complex to copy. All the diagrams I have, but which differ from my particular two boards, are athttp://www.gatesgarth.com/diagrams.zipThanks to the owner of its twin (and I daren't impose on him further for a detailed photo of the missing board, tempting as it is, he had to half dismantle his shack to get the photos I have, his PSU is part of the structure of his benching....), I have component values for the obviously damaged devices on the one board of mine. I have re capped both with new electrolytics, and tested all the diodes. I have no means of testing the voltage regulators, (that I am aware of). I am able to, in due course, test the ten pass transistors, the rectifiers, smoothing electrolytics and the thryristors on the main chassis. The transformer seems OK with no earth leakage with a megger of a friend, and gives sensible secondary voltage disconnected from the bridge. Thanks

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 29, 2012, 10:20:49 am
content: Ah, so the diode was simply color coded after all: blue-green-white: 1N659. I assumed that the big (blue) ring was the last one...

author: T4P
author_rank: Super Contributor
timestamp: August 29, 2012, 10:34:19 am
content: Hi chris, i decided to look up the diode for ya,you can use a 1N914/1N4148 to replace the diode.