So while working on lowering hum in my HB I noticed some strange behavior out of one channel of my amp. All measurements are taken with both inputs shorted and a speaker connected to the offending channel. While listening to hum I noticed this periodic issue. It happens like clockwork about every 10 seconds. On the scope it is getting a burst of DC from normal being 4-6mV dc to over 300mv when it occurs. It also looks like some oscillation on recovery. I'll attach scope shots. The shots are in order at a faster timebase each pic. Not sure where to start troubleshooting?? Maybe just walk through each gain stage until I see the behavior?
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Can you hear it in the speaker? If not, then I'd look at how you're measuring it. Maybe your scope isn't grounded in the right place.
I would guess that it’s environmental. Some appliance somewhere that turns on and off and sends out an inductive spike across the mains wires. As far as I know, there’s nothing in a linear amplifier that could plausibly generate that.
If you have a small mains transformer lying around then have a look at the secondary voltage on the scope and see if you see anything appear on that. Could it be some kind of mains signalling being used somewhere. Is it a 24/7 thing or only at certain times of the day?
Does it always appear in the same place on the waveform or is it random? If random it could be WIFI or phone, if it is not random it's likely to be internally generated. Check AC line as suggested. Check the other channel as well, does it behave the same way? Testing into a resistive load might provide some additional clues.
Ok I'll check. But this is what I know. It can be heard in the speaker but have to have your ear right on it. This started as me listening to hum. The hum goes away when the above trace occurs then comes back until it does it again. It is not random which you can see in traces, every 10 seconds. It only happens in one channel, the other has no issues. Both channels share a common power supply. The amp is plugged into a Isobar filter. I do not think any appliance is turning on. It happens continuously when powered on.
I will scope the power feeding each ch/board to see if anything is going on there. it really seems like it is happening on the board.
I will scope the power feeding each ch/board to see if anything is going on there. it really seems like it is happening on the board.
If checking the "mains" you have to do it with an isolation transformer, elsewise you'll blow the ground clip off your oscilloscope probe.
I never suggested looking at the mains directly 🙂 I said use a small transformer and look at the secondaries.
(the reason I didn't suggest looking at the transformer in the amp is because you would not be able to clip a scope directly across the secondaries without causing a short)
Strongly recommend reviewing what you did while working on the unit, and seeing if it was equally performed in both channels. Backwards component, solder crack, forgot to solder, etc...
Yes of course on backtrack but I made no changes to the board. They have been the same for many years. Was just measuring hum levels when I noticed it. I was wondering the same thing as far as a capacitor on board maybe going bad
Just to go over what I tried. Wanted to verify nothing I was connecting for measurement was causing. Disconnected scope, ADC for REW and bench meter. Only thing connected is speaker and I can still hear it doing it. Guess I should verify that it does it without a speaker connected. Also change the speaker maybe. Seems like something on the board though.
Problem solved. With the way my boards are laid out one ch sets above the other on the back wall. I can't see much of the bottom ch. With a mirror I could see that only one LED for power rails was lit. The negative fuse was blown. The small 1/8W resistors are still on board and allowing enough to power but DC offset was acting up not stable. Replaced fuse and tweaked DC offset and no more problems. Thanks for help guys