leaky zener diode I-V curve - Page 1
Forum section: electronics / repair

author: Anks
author_rank: Frequent Contributor
timestamp: July 24, 2014, 01:23:49 pm
content: Ive been fixing a power amplifier that has had multiple faults due to some one messing with the soft start and connecting the mute circuit up to the bridge rectifier. This by looking at it is easier done than one might think as it is a matter of mixing the mute wire and the soft start charge wire up. After tidying the aftermath of that up (lots of burnt traces and shorted transistors diodes). I found the issue with the soft start to be a bad ground that comes from the chassis screw.After all this the amplifier still had a bad 500mv offset on one channel so I replace the diff pair and even swapped it with the other side to rule out any chance of it causing the offset but still the offset was there and couldn't be adjusted out. Ive checked all resistors and changed any that were on the dubious side and still it remained.At this point I decided as I had what now looked to be a good channel to work with Id try my old cure tracer out (clutching at straws) and see if I could find a different signature. Everything was pretty much the same but one 7.5v zener that seems to be conducting before 7.5v and also ramped to the point were it turned fully on. Ive not yet replaced this diode but its the only thing I can find and Ive even swapped the output modules so the fets are not causing the offset. Also Ive added the schematic and was wondering if my hunch on this being the cause of the offset is correct as it is in the drive circuit to the outputs Z14.So I was wondering if this is a leaky zener I-V curve and if there is a better way of finding these components.The other pic is of the good diode for comparison.

author: Anks
author_rank: Frequent Contributor
timestamp: July 24, 2014, 04:55:28 pm
content: Well after replacing that zener the offset is now back down to 4.2mv so I was on the right track. Dose anyone have any other ways to find leaky diodes in circuit that works well.

author: mij59
author_rank: Frequent Contributor
timestamp: July 24, 2014, 05:50:16 pm
content: Hi,Very nice find, not at all obvious fault.Since the fault was in the feedback loop you can't measure it in circuit. (without removing the feedback loop)Wondering why the zener diode failed.

author: Anks
author_rank: Frequent Contributor
timestamp: July 24, 2014, 06:02:45 pm
content: I'm not sure but this fault did show up in circuit using the curve tracer but one leg was then pulled to make sure it was the diode and not another component.Id almost given up on this one has it had got to a point were id even tryed it with the offset but its started to pull to much current and blew a rail fuse. Not sure why this diode would cause it to pull to much current unless the fault was getting worse with a signal and load then clamping one side of the output to the said rail.Anyway seems happy now just biased it for 250ma that is a little cold at 50ma per pair but it seems happy and as this is a power amplifier used for large speakers in pubs and for bands I think running cold is better.

author: zaoka
author_rank: Frequent Contributor
timestamp: July 25, 2014, 09:31:12 pm
content: You can find bad transistors as well with it. It test similar when bad.

author: mzacharias
author_rank: Frequent Contributor
timestamp: July 26, 2014, 11:57:39 am
content: Quote from: Anks on July 24, 2014, 06:02:45 pmNice thing about working in audio is that there is usually another like channel to compare to.I see zeners in these circuits leaky or shorted all the time. Often they are used to protect the bias transistor during overload events; at least they usually fail before the bias transistor.Good job!