Fender pro reverb AA1069 nasty noise on normal channel - diyAudio
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Old 23rd May 2013, 12:35 AM   #1
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Default Fender pro reverb AA1069 nasty noise on normal channel

A short story about this amp.
Was in a bad shape when I got it didn't work.
Replaced caps and valves, removed old signal cables and a few pots.
Plug the guitar on normal channel and thats the beginning of the problem.
The volume starts to crackling and popping, checked all solder joints but they are ok, put new pot on the normal channel nothing changes.
Move the guitar to the vibrato channel it plays fine, no noise.

The noise sounds like when you got a bad pot, that scratches all over.
The noise only happens when I turn the volume up or down, once I stop turning the pot the scratch sound disappears.
I chance some of the B+ 100K res. problem remains.
I also change the 1st stage components totally but no difference.

If someone as been trough a problem like that and don't mind to share some thoughts.
Its a great sounding amp and I cant afford to take it to a specialist right now.
Your input its very important to me.
Thanks all.
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Old 23rd May 2013, 02:02 AM   #2
Enzo is offline Enzo  United States
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Get out your meter and see if that noisy control has any DC voltage across it. DC on a pot will make it noisy when turned. If you do find some DC voltage there, the first suspect is a leaky cap in the tone stack for that channel.
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Old 23rd May 2013, 03:23 PM   #3
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Check the plate voltage of the triode immediately after the volume pot. If the plate voltage is too low, around 100V, that stage will have grid current. The usual cause would be an off value resistor or bad cathode bypass cap.
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Old 23rd May 2013, 05:34 PM   #4
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Hi
I did a chart that may help with voltages.
I cant get it sorted, poking around with a wooden stick did not found any problems, new volume pot nah...clean tube sockets, fit a new 5U4GB rect, swap pre amp tubes again, nothing good.
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Old 23rd May 2013, 11:22 PM   #5
Enzo is offline Enzo  United States
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Go right to the noisy control, is there any DC voltage right across its terminals?
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Old 24th May 2013, 04:41 PM   #6
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The symptoms you have described point to some DC voltage across the volume pot. I don't think it's the problem I described above, the voltages look good. It has to be either a bad cap as Enzo suggests or it could be leakage on the black eyelet board or tube socket. Leakages can be caused by moisture or solder flux. Sometimes leaving the amp out is the sun for a couple of hours on a dry day will solve the problem.

Connect your voltmeter set to DC volts across the pot's outside terminals. The reading should be very close to zero. Disconnect the wire that goes to the center of the Treble control. If the DC voltage is zero now, check the caps one at a time by unsoldering the end that goes to the pots and recheck the DC voltage. If you still have DC volts, look at the tube socket or try a different tube.
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Old 24th May 2013, 11:04 PM   #7
Enzo is offline Enzo  United States
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Yes, stray voltage can come from several things, but we need to determine that it is there first.

The leaky eyelet board is really mysterious to someone the first time they encounter it. I hadn't even thought of it until LT mentioned it.
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Old 25th May 2013, 10:32 AM   #8
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I agree with all the rest, it's almost certainly DC on the volume pot - there are three capacitors (rather bizarrely) that could cause it, the 250pF, 0.1uF and 0.022uF.

As others have said, simply check for voltage on the top of the volume control (or the slider of the treble control) - it's fairly certain you will have a DC voltage there. Then simply disconnect the capacitors one by one, and see which one makes the DC voltage disappear, then replace that cap.

Without seeing the make of caps, I would suggest the likelihood (and the order I would disconnect them in) is the 0.1uF, the 0.022uF and finally the 250pF. If you disconnect all three and the voltage is still there, then (as also suggested) it could be a leak in the board.

Historically leaky anode/grid coupling capacitors was VERY, VERY common. The most common caps to fail back then were made by Wima - and a standard repair technique was to change all the Wima caps before you even bothered fault finding.
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Old 28th May 2013, 10:31 PM   #9
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Thanks everyone.
Someone mention wima pots, no they are the original fender dark blue "orange drops " style.
I measure the voltages across the -Normal channel volume pot with multimeter set to VDC and it measures (-).397vdc, then removed the meadle tag wire on the treble pot and it measures .224vdc.

Also someone mention that the eyelet could be the cause of such noise, and that could be right.
When I got the amp wile a go I remove every single component of the board, because it was full of white moisture marks there was high humidity where the amp was stored, then I clean it and baked it... so no moisture on the board...but then, as someone mention FLUX may it be the culprit. I use it randomly across the board. I tough about that before but then I just forgot to mention it.


Really appreciate your input.

Last edited by rocaat; 28th May 2013 at 10:43 PM.
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Old 28th May 2013, 10:46 PM   #10
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Nigel, I did some time ago swap the first stage coupling caps.
Thanks may
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