B-52 AT112 Repair Help

[o2bthecream]

I just bought a used B-52 AT-112 that has a non-functional channel switching circuit. Stuck in the Overdrive channel and when the channel switch is pushed neither channel LED lights up. I believe I've traced the problem back to the 4070 cmos XOR chip.

Now here's my problem. I got a copy of the schematic from B-52 and from what I can tell the 4070 chip appears to be completely superfluous. I would really appreciate it if someone could help me with the following:

A.) Confirm that the 4070 chip does nothing critical to the operation of the circuit.

B.) Confirm that if I remove the chip and short the contacts for pin 1 to pin 4 and pin 10 to pin 13 this will not cause the amp to explode and should restore channel switching functionality.

I have attached the schematic for reference. Thanks.

[Amp Kat]

If the chip is working correctly the outputs will show it. Use the truth table for an Xor gate
0&0= 0
0&1= 1
1&0= 1
1&1= 0

pins 1 & 2 = output 3
pins 5 & 6 = output 4
pins 8 & 9 = output 10
pins 12 & 13 = output 11
7 & 14 gnd and VCC

So if pin 1 = 0volts & pin 2 = 5volts (ie) then pin 3 should be 5 volts.
If any don't jive then the chip is probably bad.
Bad connections and the footswitch jack are main concerns for this amp problem as the connectors don't always make good contact.

[techineer]

I have seen cases where the relays in these units weld their contacts into the active condition. Just something to be aware of....

[o2bthecream]

Thanks for the suggestions. The 4070 tests ok. Looks like the problem is in the +12v power supply. It's a voltage tripler on the 5v AC secondary of the power transformer fed into a 7812 regulator. Incidentally this regulator is incorrectly marked on the schematic as it +15v brother, the 7815.

I'm only getting about 3v out of the regulator and 6v into it. I'm going to check the caps and diodes in the voltage tripler; It shouldn't be too hard to locate the misbehaving component.

[Amp Kat]

Nice job and that is one of the first things to check. That 7812 should have oh maybe 18 to 24 vdc on pin 1 or at triple the 5v, 15vdc but that may be to low of a drop in voltage for that regulator. Should be around at least 20 volts. That sounds like you're problem though so do a diode check and the 7812 could be gone too.

[o2bthecream]

Fixed! First diode in the tripler (D16 on the schematic) tested infinite in both directions on my DMM. Replaced it and channel switching came back immediately. Thanks for all your help. Now to bias this puppy up...

[olddawg]

Originally Posted by o2bthecream

Fixed! First diode in the tripler (D16 on the schematic) tested infinite in both directions on my DMM. Replaced it and channel switching came back immediately. Thanks for all your help. Now to bias this puppy up...

While you have it apart replace all of the diodes in that tripler circuit. It will save you pulling it apart again to save a few pennies.