Music Man 1650 RD channel switching problem?

[pontiacpete]

This amp doesn't seem to be changing channels. Since the ICs are socketed I tried swapping out IC2 and IC1 which did nothing.
The 'limit' channel is the only side that's working with or without use of the foot switch. The panel switch is working according to the ohm meter
All the ICs have the right supply voltages.
Is there anyone here that knows what to look for next?

[Jazz P Bass]

Have you verified that the switch circuit functions correctly?

Q3 & Q4.
They are the inputs to IC2.
IC2 is a quad switch.
When the input pin goes hi or low the outputs open or close.
https://www.rapidonline.com/pdf/83-0338_v1.pdf

[pontiacpete]

The voltages on the collectors of Q3 and Q4 are not changing when pressing the switch on the front panel.
Q3 has 5vdc on the collector
Q4 has -8vdc on the collector

Does this mean Q3 and 4 are no good?

[J M Fahey]

I´d rather suspect switch/wiring/tracks/solder.

What voltages do you have at each end of D5?
Do they change when you press S2?

With S2 open so voltage present at D5 Cathode and Anode, does the Led light when you press the footswitch?

What Vbe does Q3 have at each S2 position?
Same for Q4.

[pontiacpete]

Just went over all the solder connections in the area of the switching circuit. Still not switching

Here are the voltages on Q3 and Q4

Q3
C 4.9vdc
B -7v
E -7.6v

Q4
C -7,6v
B -7.1v
E -7.7v

The LED on the footswitch does go on and off when pressing footswitch.

Voltages on D5
Anode has in 'clean' , switch open, 0v
When closed it's about 4vdc

cathode - switch open .7vdc
switch closed 4.8vdc

[Enzo]

Q3,4 are switching circuits. DO the collector voltages change when you push the panel switch in and out?

One collector should be positive and one negative. When you push the switch, the polarities should go opposite.

[pontiacpete]

DO the collector voltages change when you push the panel switch in and out?

The voltages do not change on collectors. They don't go to the opposite polarities. They change only by a fraction of a volt when switching.

[Enzo]

OK, Q4 is just an inverter, its collector will be opposite whatever Q3 collector is. SO go to base of Q3, is that toggling when you push the switch?

By the way, we are doing this without a footswitch, right?

[pontiacpete]

The voltage at the base of Q3 is moving between -7.2v and -8.8v when switching

Yes, this is without the footswitch

[52 Bill]

Earlier you said you swapped IC1 and IC2 was this correct?

Try removing IC2 the 4016B from its socket and test the voltages at Q3 and Q4.

[pontiacpete]

Earlier you said you swapped IC1 and IC2 was this correct?

yes I swapped out both.

Without IC2 here are the voltages
Q3
C 4.8v
B -8.7v
E -7.4v

Q4
C -7.4v
B -6.9v
E -7.5v

edit: and when switching the only voltage that changes is again the base of Q3, it goes from -8.7 to -7.4

[Enzo]

So test Q3.

[pontiacpete]

Pulled Q3 and tested with one of the peak DCA55 transistor testers and it tests fine. Put in a new 2n4401 for a try and still nothing.

[pontiacpete]

Ok, now changed out Q4 also and it's working. Are the 2n4401 a good sub? The collector voltages change from 1.7v to 2.2v when switch on both Q3 and Q4. Not a jump from +8v to -8v like the schematic shows us.

Thanks for the help!!!

[nevetslab]

The parts for Q3 and Q4 are 2N3391, a 25V rated NPN, 500mA, though much higher hfe than the 2N4401, a 40V NPN, 600mA part, but, in this application, it's a suitable sub. As it is now working.