9090 volume control cleaning

Magnus Back

Magnus Back

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After some troubleshooting of hum and noise I'm pretty sure my volume control is a major source of the noise.
I put in short on the AUX input so I have no input signal and I have noise with zero volume.
As I turn up the volume the noise first rises and then goes down very low and then goes up again and then goes down again to almost nothing.
So I removed the volume control pot for cleaning ...

Anyone have any tips on how to disassemble so I don't destroy it?
 
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I would try to clean up the volume pots with Deoxit first. I have cleaned up the volume/balance pots with Deoxit many times successfully. If you have removed the volume/balance pot from the unit it will be very easy to give it a thorough cleaning. After each treatment check with a multimeter set on ohms to check for "dead" spots as you sweep the pot. Turn the shaft slowly and give the multimeter time to stabilize. I would venture that if the pots do not clean up after a couple of treatments then it most likely is beyond repair. At this point you have a defective potentiometer and it might be worth a shot to open it up and see what you can do if anything. The vol/balance control is a "stack" of four pots with the volume pots on the end so complete disassembly would be necessary. Getting it back together correctly is going to be a challange.
 
There are no dead spot, it works fine across.
I took off the rearmost cover.
It seems the outer (rear) two volume pots have two wipers, one across the resistive layer and one across metal for connection I assume. I suspect that's where I have most corrosion.
 
Magnus Back said:
After some troubleshooting of hum and noise I'm pretty sure my volume control is a major source of the noise.
I put in short on the AUX input so I have no input signal and I have noise with zero volume.
As I turn up the volume the noise first rises and then goes down very low and then goes up again and then goes down again to almost nothing.
So I removed the volume control pot for cleaning ...

Anyone have any tips on how to disassemble so I don't destroy it?
Click to expand...
Is the hum shield in place? The 9090 (non-DB) is supposed to have a metal hum shield over the volume control. No amount of cleaning will stop a hum.
 
Skywatcher said:
Is the hum shield in place? The 9090 (non-DB) is supposed to have a metal hum shield over the volume control. No amount of cleaning will stop a hum.
Click to expand...

Yes the hum shield is in place. What I'm after is not hum, it's hiss or "white" noise.
I think I'll shelve the plans to take it apart, I just don't see how I can do it without breaking it.
The rear plastic plate is crimped on and there is no way I can take it off and get it back on again.
 
Magnus Back said:
Yes the hum shield is in place. What I'm after is not hum, it's hiss or "white" noise.
I think I'll shelve the plans to take it apart, I just don't see how I can do it without breaking it.
The rear plastic plate is crimped on and there is no way I can take it off and get it back on again.
Click to expand...
The 9090 is well known for having white noise hiss. Most of it comes from the half-wave rectifier D01 (10DC2 common kathode) on the F-2546. I change it out with a MUR TO-220 (8A) common kathode to help reduce the switching noise. There's not much else you can do besides using low noise/high gain transistors in the audio path for a better signal to noise ratio. And changing out any green mylar caps in the tone board to better poly caps. High efficiency tweeters really bring out the hiss too.
It's just the nature of the beast in it's design, unfortunately. The DB versions use a full wave rectifier so they are naturally more quiet.
 
Yes I have read a lot of the posts related to noise to see what I possibly could do to improve the noise level :-)
The amp sounds absolutely fantastic as is, much better than I expected. So of course I wanted to make sure I do everything I can to reduce the only downside I really see.

Related to your recommendations I have replaced D01 to a MUR1620 and replaced TR-01&02 with KSC1845 but haven't replaced those green mylar caps on the tone board yet. Will try that and see what happens.
I'm also trying out the BA312 replacement boards bubhardy made and the Sparko Labs BA312 modules and will compare.

However, what baffles me is that the noise actually goes up and down several times as I increase the volume (with zero signal in). At mid level volume and full volume the noise level is quite low, MUCH lower than at say 10-35% volume.
 
Magnus Back said:
Yes I have read a lot of the posts related to noise to see what I possibly could do to improve the noise level :)
The amp sounds absolutely fantastic as is, much better than I expected. So of course I wanted to make sure I do everything I can to reduce the only downside I really see.

Related to your recommendations I have replaced D01 to a MUR1620 and replaced TR-01&02 with KSC1845 but haven't replaced those green mylar caps on the tone board yet. Will try that and see what happens.
I'm also trying out the BA312 replacement boards bubhardy made and the Sparko Labs BA312 modules and will compare.

However, what baffles me is that the noise actually goes up and down several times as I increase the volume (with zero signal in). At mid level volume and full volume the noise level is quite low, MUCH lower than at say 10-35% volume.
Click to expand...
That is kind of strange. I've never come across this scenario. Loudness on or off? Be sure to check the ground(s) on the tone board. It almost seems like it is getting a (bleeding) ground through the volume control. Turning the volume down seems to increase noise from the tone, loudness, or filter board(s), but the audio signal attenuated. Things like this are a "hair puller" for sure. :confused:
 
I have gone through all the grounds and re-flowed them but no change. None of the controls affect the noise level in any meaningful way except the Mid Range and Treble. Increasing the Mid Range increases noise more than Treble.
I suspect the issue is the volume control and there's nothing I can do about that except swap it, which is unlikely to happen.
I still need to replace C15 and C16 hoping to get that done later today.
 
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