Hot Rod Deville - No reverb, no drive channel, no master volume

[Fuzzy Slippers]

Hey All!

I've read through all the threads multiple times and have picked up a few ways to figure this one out.

The amp switching does not work. LED does not light at all. Master volume, reverb and channel switch all have no effect when knobs are turned or button is pressed.

The previous owner swapped R78 and R79 for 5W 300ohm resistors (as indicated on the schematic I found). This did not fix the issue. These test as 330ohms with my mulitmeter. A lot of other posts mention -16.6v and -16.6v is the voltages you want to see when testing the resistors. With the amp on I get -15.7v on r79. nothing on r78.

I checked for continuity to make sure the resistors are making connection to the board and everything tests OK. even though the trace to D13 from r78 looks scratched.

On the other side of the resistors where the connect to the caps
-32.6v R79
+30v R78

I checked the dual LED and both sides light up.

What's next?

Thanks in advance. This forum is an amazing asset and I have learned so much.

EDIT: Here's a pic of the resistors. Could the color mismatch on the ink suggest anything?

[lowell]

My guess is the the 16v zener diode is a short. Power down and check resistance across it.

[Fuzzy Slippers]

My guess is the the 16v zener diode is a short. Power down and check resistance across it.

It measures .7 ohms. Is that a yes?

I see that it's a 1n5353B. Do I need that exact number to replace it or can I use another 5w 16v diode?

[52 Bill]

It measures .7 ohms. Is that a yes?

I see that it's a 1n5353B. Do I need that exact number to replace it or can I use another 5w 16v diode?

The diode could be bad, but the filter cap could also be the cause of the short. Remove one end of the diode from the pc board and test it with your meter to see if the diode or the circuit is the problem.

Any 16 volt 5 watt diode will work.

[Fuzzy Slippers]

The diode could be bad, but the filter cap could also be the cause of the short. Remove one end of the diode from the pc board and test it with your meter to see if the diode or the circuit is the problem.

Any 16 volt 5 watt diode will work.

Pulled one end off. and I'm getting .7ohms. Do we have a bingo?!?!??

[Enzo]

Replace the shorted diode.

[Fuzzy Slippers]

Purchased a replacement diode. Will update when it is delivered and installed. Thank you guys so much!

[lowell]

Sorry I've been absent from this thread. Sounds like you're on track.

[Fuzzy Slippers]

Replaced diode. I am now getting +16 and -16 on r79 and r78 but still the channel LED does not light and reverb/master volume is still dead.

What's next?

@52 bill: should I check caps next? if so how do i do that?

[xtian]

Red/green channel LED must also be good in order for switching system to work.

[Fuzzy Slippers]

Red/green channel LED must also be good in order for switching system to work.

as stated above LED tests good.

[nevetslab]

Have you looked at the list of voltage measurements shown in the long chart in the schematic on this amp? It is very thorough and shows the voltages you need to have in order to get the two relays K1 & K2 to pull in which selecting Drive Mode from Normal mode. Drive and More Drive only use the Master Volume. U3, Q3 and Q4 along with some zener diodes all need to be working for the channel switching to take place, then the two JFETS QQ1 and Q2 work to put the amp into More Drive mode.

I've posted the Hot Rod DeVille Service Manual below:

Hot Rod DeVille Svc Manual.pdf

[lowell]

The caps aren't the issue if you have +/-16v.

Check voltages on the reverb opamp pins 1-3 and 5-8 and list them here. I bet it's bad.

Your bipolar power supply is good. So the reverb doesn't work because the circuit has a fault. First suspect is that opamp.
Start there.

[Fuzzy Slippers]

The caps aren't the issue if you have +/-16v.

Check voltages on the reverb opamp pins 1-3 and 5-8 and list them here. I bet it's bad.

Your bipolar power supply is good. So the reverb doesn't work because the circuit has a fault. First suspect is that opamp.
Start there.

I think the reverb opamp is u2: BA4560

1 .02v
2 .02v
3 .01v
5 .002v
5 after r46 as indicated on the schematic .002v
6 .003v
7 -.001v
8 16.6v

what does TP on the schemtic mean? I googled and got nothing.

[The Dude]

TP = Test Point