Diode Failure - Page 1
Forum section: electronics / repair

author: Excavatoree
author_rank: Frequent Contributor
timestamp: March 10, 2015, 03:29:14 pm
content: I'm repairing a Fluke 412a power supply. While troubleshooting problem 1, it developed problem 2 with the warm-up delay circuit. This is an earlier model with a thyratron tube driving a relay. I believe that as a capacitor is charged, the voltage rises and triggers the tube, which turns on the relay.The problem is that a resistor is smoking. I'll spare everyone my false trails and say that I discovered the diode in the circuit (half wave rectifier from one transformer winding) is bad. I got fooled because my Fluke meter on diode check said it was OK. I checked the diode the poor man way, by using my power supply and seeing that it was not acting like a diode.In my previous life as a repair person, diodes would fail so that they were almost completely ohmic, or the knee would get soft. (straight line on the curve tracer, or soft, lazy knee) I put the diode on my curve tracer, and the forward characteristic looked good. However, the reverse was terrible. I've never seen that. Is my exposure limited, or is this a rare event?If anyone is interested, I'll post the curves.I have no idea what to replace it with, but I'll have to fire up Dave Cad and draw a schematic and get some information together before I can ask for advice.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 10, 2015, 05:45:23 pm
content: Yeah, post it.Diodes can be incrementally damaged, so that there's a short in one part of the junction (usually resulting in the ohmic case), or the diffusion changed so the breakdown is reduced or something, or usually they just explode when there's too much surge or overvoltage. I guess we're talking vintage diodes (but still silicon?) too?Tim

author: Excavatoree
author_rank: Frequent Contributor
timestamp: March 10, 2015, 06:07:47 pm
content: Quote from: T3sl4co1l on March 10, 2015, 05:45:23 pmI can't find any information about the diode. It's marked SP 230, and is from a Fluke 412a, from the 1960s. I know I stink at internet searches but I can't come up with anything. I have no reason to believe it's anything other than a GP rectifier.

author: tautech
author_rank: Super Contributor
timestamp: March 10, 2015, 07:02:33 pm
content: Quote from: Excavatoree on March 10, 2015, 06:07:47 pmPackage?

author: Seekonk
author_rank: Super Contributor
timestamp: March 10, 2015, 08:30:53 pm
content: SP230.....Are those the ones in the fuse package? When I was just a sprout I built a breakdown tester. I was repairing lots of amplifiers with the TO220 package and car radios. I bought the lowest voltage transistor in the series by the hundred and sorted them out by breakdown voltage. 90% tested at the highest voltage in the series, the rest went in the car radios. Most of them had textbook curves but every once in a while the strangest things showed up. I tossed those in the can. Couldn't afford things to come back.

author: Excavatoree
author_rank: Frequent Contributor
timestamp: March 10, 2015, 11:34:34 pm
content: First a picture of the diode. I'm not sure what this case is called. My memory was good, it is marked SP 230.

author: Excavatoree
author_rank: Frequent Contributor
timestamp: March 10, 2015, 11:35:30 pm
content: Forward curve:Vertical (current) is 1 ma per divisionHorizontal (voltage) is .2 volts per divisionI'm sorry about the poor quality. I couldn't get the USB interface to work with the 575. I had to take a crappy picture, in bad conditions. Values are approximate, the curve tracer needs adjusting.This curve looks good to me. Seems like a silicon diode.

author: Excavatoree
author_rank: Frequent Contributor
timestamp: March 10, 2015, 11:36:23 pm
content: Reverse:Vertical (current) 1 ma per divHorizontal (voltage) 10v per divisionI think this is my problem.

author: cosmicray
author_rank: Frequent Contributor
timestamp: March 11, 2015, 12:47:37 am
content: The unofficial name for the case is "Top Hat". I think it is no longer being used tho. For an example of vintage diodes in that case, search for "1N91 Top Hat". There are similar pictures. But if you go look up 1N91 at the part sales sites, none of the current versions are made with that case. If you can figure out the device specs, there may be an equivalent in a more modern case.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 11, 2015, 12:49:48 am
content: Given the purpose, I'd guess a 1N4007 or something like that would be more than good enough. If you can tell (or measure) more about the surrounding circuit, a perfect pick should be easy.Tim

author: vk6zgo
author_rank: Super Contributor
timestamp: March 11, 2015, 03:49:15 am
content: Quote from: cosmicray on March 11, 2015, 12:47:37 amAs far as I remember,the main manufacturers who liked painting their diodes black were RCA & STC,both of which are defunct.There were two versions of "top hat",the better ones were used by Philips,while the lousy one was used very early on by RCA.The one in the pix,although painted black,is of the later,better version.I very much doubt whether ,in the application described,any part is played by the diode characteristics.You should be able to replace it with a modern device with superior specs,such as the 1N4007 suggested byT3sl4co1l.

author: Excavatoree
author_rank: Frequent Contributor
timestamp: March 11, 2015, 12:17:02 pm
content: I seem to have made an error on my notes, so I've got to take the thing apart and re-trace some items before I can finish the schematic. Nothing I've seen indicates I need anything other than a garden variety rectifier like a 1N4007 or so. Voltage is less than 300V, even in the fault condition, current was 340 ma or so.D'oh! The diode in question isn't shown in the schematic of the manual I have, but I didn't think that the same diode must be used elsewhere. It's a 600 PIV, .5 amp rectifier - just as we all suspected.I started another thread for discussing the rest of the repair of this supply.