Gibson GA15RV no reverb

[pontiacpete]

So the reverb isn't working in the amp. It uses an 8CA3B1B. I'm measuring 28 ohms at the input jack and 42 on the output. They are not open but are these the correct readings for this type of tank?

Thinking that I might save some time I went ahead and changed the the 2 4558s and no change.

I'm wondering if the JFET could cause the reverb to not work? but I can't remember how to check if it's
working or not.

When tapping the tank the reverb can be heard.
Any ideas?

[MarkusBass]

You may check it here: https://www.amplifiedparts.com/tech_...d_and_compared
It seems that the output coil resistance is not correct. But if you hear reverb when tapping the tank, I would say that the problem is rather with the input signal/circuit. Can you check with oscilloscope whether you have signal on the input?

Mark

[g1]

Mark, the page you linked shows that his resistance readings are correct (should be approx. 30 and 40 ohms).
Is the sound from tapping the tank loud or weak? If it is quite weak the fet could be an issue. The fet is how the reverb is turned off. It should read high resistance with amp on and reverb pot up. You can also just remove it to see if the circuit wakes up.
Like Mark said, the signal may not be getting to the input. Is the cable good?

[pontiacpete]

Thanks for the input Mark and G-one.
When I tap the tank it sounds normal with a crashing reverb sound.
I will put the scope on the input and see what's there and then pull the JFET.

[Jazz P Bass]

When I tap the tank it sounds normal with a crashing reverb sound.
I will put the scope on the input and see what's there and then pull the JFET.

The 'crashing reverb' pretty much rules out the JFet, doesn't it?.

Using your scope, walk through the input circuit looking at each drive IC's output pin.

As an extra thought, at this age, C11 may very well be dried up & killing the signal.

[pontiacpete]

Yes the JFET removed did nothing.
As far as a signal with a scope on the "return" connections for the reverb cables there's something of a signal.
Very small.
When I measure to ground on each of these connection I'm getting 12vdc on the 'return' and 'send' connections.

The cables themselves seem to be good according to resistance and continuity checks.

[pontiacpete]

Thanks JP for pointing me to C11.
It's a redial leaded cap. When I pulled it off the board leaking is clearly visible.
Replaced with a new one and it works!!
Why did you zero in on C11?

[Jazz P Bass]

Why did you zero in on C11?

Electrolytic 47uf capacitor, correct?

How old is it?

And they are good for how many years? (roughly)

Actually, it was a stab in the dark.

[52 Bill]

Electrolytic 47uf capacitor, correct?

How old is it?

And they are good for how many years? (roughly)

Actually, it was a stab in the dark.

C11 and the resistor R47 set the gain and frequency cutoff of that stage. So if it goes bad the gain is reduced causing less drive on the springs.

The transducer coils are both wired in as active elements in the circuit. Look at the schematic, neither coil is connected to ground. So finding 12 volts on either coil I think would be normal.