Quote from: Enzo on June 22, 2017, 04:08:15 PM
Nope.

If master volume controls the level of some hum, then the source of that hum is before the control.

Sorry if I'm being slow, but if that happens when plugged in to the return socket, does that mean, the issue is between the return socket and the master volume? And that's a relatively small number of components?

Quote from: stuckinstandby on June 22, 2017, 03:18:43 AM
I don't see much difference in this when playing via another amp into the return socket, so it sounds like the issues are in the power amp.

I should have said, "I observed no audible difference when playing via another amp into the return socket"

Does that not suggest it's the power amp? I should add that my bass is coming through in both cases, just with hum/buzz.

Quote from: phatt on June 22, 2017, 08:52:51 AM
BTW, I just looked at the schematic and it actually states on the Schematic to *Ground input and output pots* I can only assume they have a fly lead which maybe missing or lost from a previous repair effort.
Alligator clips back to circuit ground will test that,, if so then you will have to make that connection with some wires. :tu:
Phil.

Good spot! Like I said, I tried I tried clipping the master to the (grounded) treble control and that eliminated the hum caused by touching the pot. It's good to know the design intended it to be connected. I can't see evidence of a connection so a fly lead with a lug over the pot, secured with a nut, with the other end secured to the chassis would make sense. As if by coincidence, there are two unused nuts on the chassis which could be connected to. Not sure where to find a lug of the right size, but I'll work something out, even if it's just soldered or wrapped.

So with that corrected I'm still left with a quieter hum at no volume which increases with the master volume.

Is the speaker thump on power off, anything to be concerned about?

I reassembled everything last night but still having hum/buzz/grounding issues. I tested without the limiter in place and with the fluorescent lights off but the noise remains.

With both volumes down there is a quiet buzz. When I turn up the master volume, while touching the metal of the pot the noise increases. When I let go of the pot, the level of noise reduces significantly but it's still increased from when the volume was at zero. Turning up the input control has a similar effect.

I don't see much difference in this when playing via another amp into the return socket, so it sounds like the issues are in the power amp.

Seems like 2 issues:
- increased buzz from touching the volume control pots
- residual hum which increases with master volume

I did find that if I use a croc clip to attach the master pot to the treble control the noise from touching the master goes away but residual hum still increases. Like I said, it makes sense that this control should be grounded to the chassis like the EQ controls but I don't see a designed way for it to do so. I suppose normally you'd have a plastic knob so might not notice but I doubt it would be designed like that. It's not just one control either, there are three like this.

I checked the grounding as mentioned, phil, and didn't find any issues.

Another thing I've noticed is that when powering off, the speaker makes a "thump" noise and I see it move.

Thanks

Thanks guys.

aaron, you're spot on about the filter cap. C9 does appear domed but it's only the plastic cover. I took it off to check and the metal can is flat. TR5 looks okay in real life, I think it's just a reflection of the flash.

phatt, the odd thing about this master pot (and it's the same for a couple of others which don't seem to have a grounding problem) is that it's not actaully screwed to the front panel. The rest of pots are but these three aren't. The other pots are on a raised PCB, but these three sit further back so the threads don't come out the front. There was a plastic insert securing it, but I can't see anything that would have connected it to the chassis.

I've taken a couple of photos of the front panel just to show how they sit. I've unscrewed everything else so I can take the board out. Shaft of the mid pot has snapped off too.

I was running through the light bulb limiter so I guess that might have had an impact. Also have a fluorescent tube lighting in the room. I'll test again if I don't see anything obvious on the board.

A breakthrough!

I'd been a bit timid about leaving it connected to the speaker after hearing the loud hum. While re-taking measurements earlier today, I noticed that this time, the hum was much quieter to begin with. After 10 seconds or so, there was some crackling and then it reverted to loud hum.

I just tried again and it seems like the hum was due to crackly pots. This doesn't quite make sense though as I'm sure I played around with them while connected before and noticed no change. Anyway, I attacked them with contact cleaner and for now the hum is gone

Still a few issues to sort:
- input control is still a bit crackly - it's difficult to access from the front so I'll have to take the board out
- seems to be a grounding problem with the master control - hums when I touch the metal of the pot
- quiet hum audible as you turn up the master control

Getting there 

Hello.

Last August I acquired an Ashdown MAG200 combo (C410T-200) which was blowing its main fuse. With a light bulb limiter, the 100W bulb was glowing brightly. I posted on another forum and have had some help from Enzo.

I checked for shorts and cold solder joints on the power board but didn't find any. A couple of the power transistors had burn marks so I replaced with the BUZ900P and BUZ905P with ECX10N20 and ECX10P20. Now the bulb is bright initially and then fades to off. The VU meter on the amp is illuminated now.

With the limiter in place, I checked the voltage speaker out and it was 54mV, so I plugged in a speaker but got a loud hum (no bass connected, volumes at zero).

I re-flowed the solder on the larger components including the filter capacitors but it's made no difference.

I'm seeing 46V AC from the transformer and +61.4V DC for V+, -61.4V DC for V-.

Can anyone advise what to try next? This is my first real attempt at repairing an amp other than fixing input jacks.

Photo of board (before replacing transistors) and schematic attached (power board is APC008 on page 4).

Thanks!