Low volume on Ampeg VL-1002

[goldstache]

I'm wrenching on my friends Ampeg VL-1002, it has low volume.  The amp has been passed around to every tech in town from the look inside.  I've been combing the internet for info, and read up on some of the issues these amps experience.  Especially associated with low volume output. I checked operating voltages and everything checks out from a DC perspective. Once I hooked the sig gen to it I reached a dead end pretty quick.  As i traced the clean channel to the Master volume, and then on to the vactrol.  I got what I think are bum voltages.  On lug 3 of the master volume pot, I have about 26volts AC with a 250mv/1K test signal. then on the wiper I have about 1VAC with the master cranked. 


I tried to short the resistive legs of the vactrol to see if It would pass the voltage.  Nope.   


Got any pointers or insight?  Would be much appreciated.  Happy weekend


Schemo:
http://www.4tubes.com/SCHEMATICS/BY-BRAND/Ampeg/VL%20502/

[John]

On lug 3 of the master volume pot, I have about 26volts AC with a 250mv/1K test signal. then on the wiper I have about 1VAC with the master cranked. 

Bad pot?

[goldstache]

Unfortunately, same scenario on channel 2's Master volume.  So I'm kinda thinking it has to do with the vactrols.  Could be volume pot(s), but I want to be sure before I start throwing parts at it, especially the vactrols.  I've got some VTL5C1's in stock but they are pricey, so I'm hesitant to just start swapping out. 


I'll take the dreaded circuit board out later today and remove the pot for testing.  Thanks for posting!

[goldstache]

Man, the separate schematic pages are frying my brain


Well, I took the signal off of lug 3 of the clean channel and inserted it in the FX return jack.  Boom! Full swing signal on the power tubes.  Cool.


Is that starting to sound like a Vactrol issue, or other, to anyone?

[shooter]

the separate schematic pages

most of the systems I worked on had Volumes, 3-6 2" binders was normal 
you have it narrowed down, either keep scopin til you're happy swapping X, or save time and swap "the most likely"   til it's gone. 

[sluckey]

On lug 3 of the master volume pot, I have about 26volts AC with a 250mv/1K test signal. then on the wiper I have about 1VAC with the master cranked.

When the pot is cranked (max CW) the wiper is touching lug 3. They must read the same unless the wiper is not actually touching. That indicates a bad pot.

Try this... Set the master fully CW. Check the voltage on lug 3. Then check the voltage on the wiper. If they are not the same then short between the wiper and lug 3. Any joy?

I don't see how a vactrol can possibly cause this problem.

[shooter]

this is what I'm using to follow along, after you check out Sluckey's suggestion look at J11, pre out might be all gummy/crappy

[goldstache]

Thanks!  Well I tried the wiper with it full CW, it's losing half the AC voltage.  Looks like I'm gonna have to cut one of the vactrols out. 

[PRR]

Is the "+14V" and the voltage on C13 *solid*?

In particular, if R6 has charred and gone high resistance, the 14V will be low and the C13 voltage will almost vanish. This means real-low current in the opto LEDs. Which because the way they do the audio switching, will mean low audio.