Peavey Backstage Chorus 208 No Dirty Channel

[DrGonz78]

Well, I have a Peavey Chorus 208 that cannot switch from clean to the dirty channel. In fact, the channel switch button on the amp is shunting the dirty channel altogether. I do not have the foot switch to try changing channels a different way. As I was looking over the schematic and the circuit board layout I could not determine what component is involved as the relay for switching channels. The K1 part looks like it might play a part or it could be a bad transistor somewhere. There are 8 transistors on the board that I am not sure if they play a role in switching channels. The channel switch jack is a stacked component w/ the chorus on/off switch on top of the channel switch button.

I took that switch off and cleaned it. The chorus when I got the amp was working then I did some soldering and it did not work. But after cleaning the channel and chorus switch it came back to life.

Any ideas as to where to troubleshoot for a solution would be greatly appreciated, thanks. I am gonna do some more test on the channel switch to see if it is bad.

peavey_backstage_chorus-208_sch_[ET][1].pdf

[DrGonz78]

I just did some test on the switch and it seems to be working fine. So I am now looking at some resistors that leads to K1.

[Enzo]

K1 relay contacts are switching in the R36 and R41 family of parts.

That relay is controlled by panel switch S1C. See the relay coil drawn just under the contacts? then follow over to the switch.

Now for the switch to work, the bottom end of it must get to ground. Q2 on the breakaway board does that.

WHEN NO FOOTSWITCH IS USED, the base of Q2 is pulled up by R49 and R123. That turns the transistor on, completing the relay circuit if the switch is closed.

Does the switch make the relay click? What voltage is at the collector of Q2 or at pin 5 of the ribbon to the footswitch jack board?

There is a jumper on the footswitch to turn off Q2 so the footswitch itself does the work. But without that, that transistor has to be working.

[DrGonz78]

Measuring to ground on the chasis I get about 4-4.3v on Q2. However, when I measure from the ground of the power source I get infinite on both sides. It must be bad. Another thing to point out is when I ground out the gate w/ the meter I hear click of the relay. So, now I am gonna look in my spare parts to find another transistor that might work. Thanks so much for the help Enzo.

[Enzo]

Q2 has three terminals. Telling me it has 4v doesn;t tell me the story. And it is also confusing to hear that reading one place gets 4v and another place gets infinite. Infinite volts? We need to specify units of measure as well as specific points measures are taken.


I'd be looking for cracked solder on the ribbon headers and even on Q2.

If you have 4v on the collector of Q2, then it isn;t turning on or is ungrounded. If we have 4v on the emitter of Q2 then the ground connection is open. The base of Q2 ought to sit at about half a volt. If you get 4v there, then there is no path through it to ground.


If S1C is closed, then grounding the collector of Q2 should click in the relay.

[DrGonz78]

Okay sorry for lame post earlier it was late last night and I rushed my process a little too much. Now I have the right measurements to assess.

With channel switch set to clean I get 120Vac/-6.5Vdc on pin1 on the ribbon and on pin5 I get 75Vac/38Vdc. I then measured the collector of Q2 and it read 75Vac/38Vdc while the Emitter side was getting 120Vac/1Vdc.

I then pushed in the channel switch to engage the dirty channel and pin1 measures 120Vac/-1Vdc while pin5 measures 120Vac/15Vdc. Q2 read 120Vac/15Vdc on the collector and 120Vac/1Vdc on the emitter.

I have another transistor to put into the place of the original transistor and the measurements are...

Set switch to clean pin1 120Vac/-7Vdc, pin5 68Vac/45Vdc the collector = 68Vac/45Vdc and emitter = 120Vac/1Vdc.

Set switch to dirt pin1 120Vac/-7.2Vdc, pin5 120Vac/15Vdc the collector = 120Vac/15Vdc and emitter = 120Vac/1Vdc.

I do not hear the switch click of the relay when switching between channels still and only hear it when I ground out collector & base together. Also, I checked the transistors w/ hfe setting on multimeter and they all read 004. One of the new ones I got read 006 so did not use that one.

I also checked the ribbon and all components for continuity... I soldered it all to make sure it was not the weak link.

[Enzo]

Wait, where are you grounding your meter? I have a hard time believing you have 120VAC on those switching circuits. At least not and only having a switching problem to show for it.

Remember earlier I mentioned R123 and R49? They are responsible for turning on Q2. There should be about +15v on both ends of R123. Then that same voltage should be on one end of R49, and the other end of R49 should be the base of Q2 and that ought to be about a half a volt.

[DrGonz78]

I was measuring from green wire from the wall plug that connects to the chassis. But I must have measured from black wire incorrectly. Still learning and will be forever.

I measured one end of R123 tonight and it was 15v. I will check voltages on the ends of the resistors and double check my measurements. Thanks for setting me straight on that one.

[Jazz P Bass]

I was measuring from green wire from the wall plug that connects to the chassis. But I must have measured from black wire incorrectly.

That is some scary sh*t.

[DrGonz78]

Well I feel pretty stupid about this point in the game, but I am not gonna let it stop me from learning the correct methods. I just need to ask more questions and not worry about if the question might seem stupid. I won't make that mistake ever again.

At least I wasn't testing a tube in a tube amp for microphonics by tapping it w/ a metal screwdriver!! I remember reading some guy do that a while back and that makes me feel better. I think the guy was hopefully joking that he had actually done that... Then one day he wasn't sending any posts anymore and have not heard from him again.

[DrGonz78]

I am getting 15v from one side of R123 but only 9.5v from the other side of the resistor. I am now gonna lift another resistor off another board I have that matches and test that too.

[DrGonz78]

Now w/ the new resistor in place I am getting the 15v on both sides of R123. Furthermore, I am getting 15v on one side of R49 and w/ a .5v reading at the emitter side of Q2.

So, I then could tell that the switch was making a connection and put it back together... And it fixed the problem!!

Thanks so much Enzo!

[Playervin]

Back to the Peavey Backstage Chorus 208

Enzo, where is the jumper you refer to? What jumper is required to turn Q2 off?
Thank you in advance.
Vin

[Enzo]

In the footswitch.

Look at the schematic. To turn off Q2, we ground its base. That base is wired to pin 3 of the FS jack. Grounding that pin disables Q2.

[Playervin]

Thank you. Just before I viewed the message saying you responded, I arrived home from work I poured a glass of wine and studied the scheme for a while. It all made sense when I realized I needed to ground pin 3. I placed a jumper between pin 4 & 3 and the distortion channel switched in and out like a charm. Guess I jumped the gun contacting you. Know I appreciate your correct answer!