peavey 6505 strange pot sweep

[matoni]

Hey All,
Someone asked me to fix their peavey 6505 amp. THeir complaint was that the volume was stuck on "high". So to fix this problem, I bought a new peavey brand pot from TAD (A 1meg pot with grounding harness that fit perfectly into the board. Just powered it on for the first time, and the sweep on the pot is super weird. when the pot is at 0, there is volume, when the pot is at around 4, there is less volume but super crunchy, and when the pot is at 10 it is too loud for me to play. WHat do you guys think could be the cause of this. Please note that this is the "post" knob on the lead channel. When I measured the pot, the sweep was extremely strange too. it went from 0 - about 300K, and then down to about 26K... on a 1M pot (please note that this is when the pot was in the board, not alone. I forgot to measure it alone. I doubt it is as simple as a bad pot. Please let me know what you guys think the problem is.


THanks so much in advance for the help

[Stevem]

Sounds like you got shipped the totally wrong pot.
Sounds like a reverse taper sometime or other value.

Time to check it out of circuit!

[matoni]

You think the problem is as simple as that??

[pdf64]

…When I measured the pot, the sweep was extremely strange too. it went from 0 - about 300K, and then down to about 26K...

At high settings, you’ll be measuring the tone stack resistance. As it’s across the volume pot track. I guess the bass control may be set low? Try turning it up.

https://www.thetubestore.com/lib/thetub ... ematic.pdf

[matoni]

I tried turning the bass all the way up and the pot still made the same problem. Here is the pot I bought. It is possible that it is the wrong pot, however, it had the same model number as the pot I removed from the amp, so I doubt that this is the wrong pot. I have also included the link to the service manual I found.

https://www.thetubestore.com/lib/thetub ... ematic.pdf

https://www.tubeampdoctor.com/en/peavey ... cket?c=337

Please let me know if anything looks out of place.

[sluckey]

The only way to get a meaningful resistance reading on that pot is to remove it from the circuit.

[roberto]

When I measured the pot, the sweep was extremely strange too. it went from 0 - about 300K, and then down to about 26K... on a 1M pot (please note that this is when the pot was in the board, not alone. I forgot to measure it alone.

With the pot in the circuit, the circuit itself will fake your measures. Take it out and check it.

[matoni]

Taking the pot out of the board is a huge pain in the butt because of the bracket. Is there anything else I can do to clarify the pot is the correct one?

I really don't want to have to take the board and pot out again and risk breaking the pot. is there any other possible thing that could be causing this?

[matoni]

so i took the pot out (again) and measured it not in the board and it read about 1M on one side and 0 on the other with a pretty even sweep, so it seems like there is no problem with the pot. What do you think I should try next (what are the next steps) to figure out what is wrong with this amp. Again, the volume is about 50% when the post volume knob is all the way down and around 5 it starts to get louder. I at first thought that it was the sweep of the pot as well, however, now that I have measured the pot, I can be certain that it is not the pot. Let me know what I need to do next.

[Stevem]

Please confirm that you are indeed getting a solid 000 ohm reading on the grounded leg of that pot.

[matoni]

it reads 0.33 ohms. That should be close enough to 0.

[Littlewyan]

Sounds like you've got a Linear pot there if it's an even sweep. With the pot at halfway what do you measure from the centre lug to either side? If it's around 500K you've got a Linear pot and it should be a Log pot. That'll explain the weird sweep you're getting.

[matoni]

it is a log pot, I didn't mean it swept evenly. What I meant was that the sweep from the pot wasn't the sweep I was hearing in the amp. it's really weird in that the volume doesn't turn all the way down. Is it supposed to do that? if it is then I guess there is no problem.

[johnnyreece]

Steve touched on this earlier, but it really sounds like a ground issue. If you jumper the ground leg direct to ground, is there any difference? I don't know this circuit, so it's hard to really see what's going on, but, again, it sounds like a grounding issue to me.