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Author Topic: Ashdown MAG200 - was blowing fuses - now loud hum  (Read 524 times)

stuckinstandby

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Ashdown MAG200 - was blowing fuses - now loud hum
« on: June 18, 2017, 05:22:02 PM »

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!
« Last Edit: June 18, 2017, 05:23:26 PM by stuckinstandby »
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stuckinstandby

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Re: Ashdown MAG200 - was blowing fuses - now loud hum
« Reply #1 on: June 19, 2017, 03:47:00 PM »

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  8)
« Last Edit: June 19, 2017, 04:02:22 PM by stuckinstandby »
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phatt

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Re: Ashdown MAG200 - was blowing fuses - now loud hum
« Reply #2 on: June 20, 2017, 06:12:57 AM »

Hi stuckin,, and welcome :tu:

Sounds like you may have a grounding issue which is often overlooked as it's nearly always assumed to be there.
With power OFF take the circuit Common (usually the point on the PCB where the two main filter caps join, the center tap/com of main supply) set your meter to resistance. Check for continuity from that CT to the chassis of the amp.
While you are clipped to the CT also check continuity to the ground of the input socket.
The ground path is sometimes hard to track down as every manufacturer has different ways to create that path.

With some gear the PCB has grounding pads that only make contact with the chassis when the amp is fully assembled causing a whole lot of weird outcomes.

I had issues with one rig that used those plastic sockets which have a tiny metal pin protrusion which only makes contacts with the chassis when the nut is tightened resulting in,,, you guess? bad hum. Sent me on a wild chase until I worked out how the ground connection worked. :duh

As you have noted,,Similar problems arise when you put the PCB back in the chassis but don't put the nuts on the pots (so you can test before you close the lid). If the pot case is not grounded on a sensitive part of a circuit you will often get hum.
Short jumper wire with alligator clips come in handy for temporary connections. :tu:
Hope it Helps a bit, Phil.
« Last Edit: June 20, 2017, 06:15:28 AM by phatt »
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aaron

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Re: Ashdown MAG200 - was blowing fuses - now loud hum
« Reply #3 on: June 20, 2017, 03:20:13 PM »

am I the only one who thinks that the large cap (to the right of C10) has a huge dome on top?

Does the white stuff on TR5 look suspicious to anyone?
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stuckinstandby

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Re: Ashdown MAG200 - was blowing fuses - now loud hum
« Reply #4 on: June 20, 2017, 04:39:10 PM »

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.
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phatt

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Re: Ashdown MAG200 - was blowing fuses - now loud hum
« Reply #5 on: June 21, 2017, 07:37:36 AM »

I notice you have *Send/Return* sockets on the front so try plugging a signal into the *Return* and that will (Should) bypass the preamp circuit.
This will allow you to test the power amp ONLY. master Should still work but all the rest will be bypassed.
Any line level audio output device will do such as an MP3 player or use the line out of another amp. I just use my desk radio line out.

If the Power amp works with that connection and has close to zero VDC on the speaker terminals and no bad hum then at least you know the power stage is ok.

Yes fluro's in close proximity can be a source of hum with the amp opened up.
If the preamp still hums then you may have to track down just which way the ground path to the preamp takes. You may have disconnected a ground wire while working on the open pcb and not realized,, heck I've done that a few times over the years.
Phil.
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stuckinstandby

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Re: Ashdown MAG200 - was blowing fuses - now loud hum
« Reply #6 on: June 22, 2017, 03:18:43 AM »

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
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phatt

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Re: Ashdown MAG200 - was blowing fuses - now loud hum
« Reply #7 on: June 22, 2017, 08:52:51 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.

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

Yes it makes a big difference. ;)
Usually FX loops are *in series* with the signal path so when you insert a plug into the return socket an internal switch in that socket  completely disconnects the preamp from the power amp section.
This allows you to establish the source of the hum. My guess is the power stage is working ok and you have an issue with grounding in the preamp.
If the power amp is working from a signal into the return socket then the hum is in the preamp.
The fact that your finger changes the hum tells me you have a grounding issue. Circuits working outside of a case can often pickup hum.

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. 
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stuckinstandby

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Re: Ashdown MAG200 - was blowing fuses - now loud hum
« Reply #8 on: June 22, 2017, 02:05:31 PM »

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.

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?
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Enzo

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Re: Ashdown MAG200 - was blowing fuses - now loud hum
« Reply #9 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.
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stuckinstandby

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Re: Ashdown MAG200 - was blowing fuses - now loud hum
« Reply #10 on: June 22, 2017, 07:00:18 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?
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Enzo

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Re: Ashdown MAG200 - was blowing fuses - now loud hum
« Reply #11 on: June 22, 2017, 10:39:23 PM »

Sure is.  Is the DI hummy when played into a balanced channel somewhere?  That side of IC2 might be causing grief.   Somehow the line out jack might be an issue.  Plug the guitar into FX return and zero its volume - hum continues or goes down?  Hell, plug the guitar into the line out, what happens?
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