Fender Concert II problem ( preamp)

[Jazzman2727]

Hello to all. This is my first post so bare with me. I have a Fender Concert II that works fine in the clean channel but has very little power in the second channel. I can barely hear it on the same settings as the first. When I turn it up it is still low lifeless and distored. This is what I have done...

Changed tubes
Cleaned tube sockets,pots,effects/send/return,all input jacks ect...
It sounds good when playing into the power amp input as well. Now here is the strange part, it will come on for a short time (not very often though) then quietly,with no popping or crackling, go out again. Any help would be appreciated. Thanks

[Jazzman2727]

I love a challenge but this amp is ptp. I have no idea how to remove the preamp.

[oc disorder]

Welcome to the forum Jazzman2727.
I presume its this amp ?
http://www.schematicheaven.com/fende...cert_ii_83.pdf
although I doubt its point to point..I thought these were on a pcb.
I would check the anode resistors going to pin 1 and 6 of the preamp valves.
For example relating to the circuit above the first tube (which must be ok as it
passes signal for both channels) it has a 47K resistor which comes from point "E" in the power supply 300volts so one would expect to measure just under 300v on pin 1.
V4A has a 10k,22k and a 82k which connect it to point D 345volts.
With the amp off and unplugged (give it a little while to discharge there are lethal voltages in there) initially measure the resistors and see if they are ok.
100K is the most common value.
Resistors in this position sometimes go "high" ie virtually open circuit and provide the symptoms you describe.
If it is on the circuit board it may be hard to see as components may be mounted on the other side or maybe I'm confusing this with another model.
At least you can check all the pin 1's are over 200volts.

[txstrat]

They are PTP. Have a look at the pic (found this somewhere on the net). It's a Twin Rev. II which has the same layout.
If you whack the top of the amp with your fist (Whacking the amp won't hurt it) while the issue occurs, does it change anything?
I'm asking cause it might be a bad solder joint.

[Jazz P Bass]

Fender Concert

Use your DVM, set on Vac, one DVM lead on chassis ground, with the other DVM lead check the signal level at Ch 2 Vol Master wiper(middle leg).
Then check the signal on the other side of the optocoupler 013556.
If the level after the opto is lower, the opto is bad.

[Jazzman2727]

This is more than iI expected. Thanks for all of the replies. I cant wate to get home to start checking this out. I will give a report as to my progress.

[Jazzman2727]

Use your DVM, set on Vac, one DVM lead on chassis ground, with the other DVM lead check the signal level at Ch 2 Vol Master wiper(middle leg).
Then check the signal on the other side of the optocoupler 013556.
If the level after the opto is lower, the opto is bad.

Thats what it is alright. It is not completely gone but it is too weak. It will work only when I touch the (-) lead with a small metal probe. It is not shorted but it seems the probe causes some type of electric jump start. I am in the process of hunting one down now. Thanks so much. I am proud to be a member.