Peavey 6505+ Lead channel....tone controls not working?

[acorkos]

No tone control functions on the Lead channel. The Post Vol pot is acting strange, too. From 0-2 the volume increases, then drops off around 3-4, then up again 5+. Not much gain with the Pre Vol control cranked all the way also. New tubes all around...no change. I checked the ground connection at the mid pot...it's ok. Not sure where to look now. Any suggestions? Thanks...

[Enzo]

SOunds like the signal path is interrupted, and that volume control is controlling only crosstalk.

If the ground was off the mids, then the tones would have no effect, but the volume wouldn't crash.

Trace the signal through the amp.

[acorkos]

i've traced the signal and i find no interruptions

ok, the tone controls do work. if i put the post vol up to 5, and the pre vol just past 0, i can hear the tone controls doing what they're supposed to do.

it seems the problem is from the post vol control....regardless of the post vol setting, the pre volume is very loud, even just past 0. it's like the post volume is up full. with the pre control down to zero, the overall volume increases slightly as the post vol is raised, with a slight decrease around the 4-5 positions.

[Enzo]

The control could be faulty. You'd have to remove it to test it.

[acorkos]

i replaced both pre and post pots on the lead channel....no change.

what else could it be? I have traced the signal from the input jack to the post vol pot and can't find any break in the signal. what about the switching relay

[Enzo]

Try this. Measure resistance from the bottom of the mids pot or the bottom of the Lead Post pot to chassis. SHOuld be about zero ohms. If not zero, then 47 ohms. On the schematic, that is the ground with the circle around it. As opposed to the ground without a circle, like in the first half of the preamps. That ground is connected to the chassis at the loop jacks and preamp out jack. Iy has to go through wires from the main board over to the little jack board to do this. If that connection fails, then those parts are not grounded.

There is a backup, R76, 47 ohms. Bottom center of the schematic, there are two little resistors to nowhere - R76,77. They seem to be grounded at both ends. In case the chassis ground connection fails, this resistor will keep the two ground systems still close together electrically. But if it opens, then that protection is lost. R76 is right behind the clean channel bass control. Both ends of it should connect to chasis. measure across it, you should get zero or maybe an ohm or two. If the chasis connections are bad, it should then read 47 ohms.

If you want to test the relay contact, turn the amp on to the lead channel and turn the lead post control to zero. Then measure resistance to ground from the other side of the relay at R81. If the relay contact is OK, the zeroed wiper on the control will be reflected in a zero reading at that resistor.

You didn't mention, how is the clean channel working? it shares this ground. There is a ncie wide ground trace running almost the length of hte main board right behind the controls. There is only one heavy trace that does that. It then narrows and curls around under the larger push switch to find the rear facing end of R76. Follow that trace all along and make sure it is not cracked anywhere

[acorkos]

thanks enzo

i will check those things out today

the clean channel works normally.

[acorkos]

R76 and R77 are ok and both reading zero

the relay is ok

resistance from bottom of mid pot to chassis is zero. connections at effects loop board are good, too.

i guess i now need to check the traces for cracks.....


here's what the lead channel post pot is doing.....with the pre pot set at 1-2, the post volume is very loud at zero, with a heavy midrange emphasis, and the tone controls don't affect the tone. from zero to 3 on the post pot, the volume decreases to where it is the lowest, but still loud enough to bother my kids. from 3 up to max, the volume gradually increases....at 3, the tone sounds pretty scooped in the mids, and all the tone controls work normally.

[acorkos]

Enzo...i'm having at this amp again. R82 (470K) connects to pin 2 of V1....very, very noisy when i tap on the tube side lead of R82. is this normal? the other side of R82 connects to relay K2, which in turn connects to ground.

[acorkos]

well....i have encountered another 6505+ with the same problem as the one i wrote about in the original post 18 months ago. anyone with any new ideas, or anyone that may have discovered a fix for this?

[TopOfTheMountain]

Is there any update on this? I have one on my bench now. It's driving me crazy.

[TopOfTheMountain]

Well, I figured out the first problem. C37 was bad. All controls and channels work properly. However, after playing for a few minutes it slowly lost power (volume) like a preamp tube went bad. I have since tried everything. Still no full volume. I'm not a fan of this amp