Ampeg G110 Hum / no signal

[MarkusBass]

When you fix an amp it's good to understand its schematic. And understand how the amp works. In your case you need to understand at least the power supply in this amp:
- the power amp is supplied with +48V,
- then you have R44 resistor and D2 diode (this is most probably 27V Zener diode) - they provide voltage for the preamp,
- +27V supplies IC1 and IC3,
- R20 and R14 resistors form a "virtual ground" for IC1 and IC3 (they call it "COMMON BIAS LINE" on the schematic) and this virtual ground voltage is 13,5V (half of 27V),
- on inputs 2 and 3 of the IC1 you should have 13,5V (on the output - pin 6 there should be also 13,5V),
- pins 1 and 8 are not used in this amp so there is no point listing them.

As you can easily see, almost all voltages on IC1 are incorrect - starting from +27V power supply. It means that, most probably, there is a problem with D2 Zener diode. Also there is a problem with "virtual ground" divider - R20 and R14 resistors.
This is how I see it - I assume that the measurements that you made are correct.
Please fix the problems with DC voltages and then you may start looking for the signal. Check D2 diode and R14, R20 resistors.

Mark

[Mr_bibbles]

Mark, thanks for taking the time to help me understand. I'm self taught so I have a lot of gaps in my education. I really appreciate this forum because I have no other direct human resources.

-I see the 48v, I had tracked it across the R44 beforehand and watched it drop only to 40v instead of 27v. So this means somehow the common bias line, which is a "virtual ground circuit" is not functioning properly (or something downstream...) R44 measures healthily out of circuit, so the draw must be somewhere else??

-Both of the virtual dividers (R14 / R20) and the Zener measure fine out of circuit, but we can assume that the issue is in the power supply. Would you say the next step is to work backwards from the virtual ground circuit and see if I can find something amiss?? Perhaps c15, or some other reference to ground after R44 in the supply?

If I understand correctly, our goal here is to drop the B+ to 27 for the common bias line and then divide that properly to sueach IC.

[g1]

No. The 27V should be at one side of R44. Once you get that right, the common bias line should be correct (13.5V).
If D2 is installed with correct polarity and it's connections are good, it must be bad, as you say the 27V line is 40V if I'm understanding correctly. Anode side of D2 should go to ground.

[MarkusBass]

The issue with the Zener diode is really simple. Either it's bad (open) - needs to be replaced, or its connections (e.g. to the ground) is broken - fix connections.
Another possible problem is that the integrated circuits could have failed because they were getting 40V power supply. I would remove both ICs and fix the Zener diode and virtual ground. Only, when you have 27V and 13,5V you can put back the ICs into the circuit (assuming they are OK).

Mark

[Enzo]

"zener measures fine out of circuit" Well, just how did you measure it? If it measures like a simple diode, at least it isn;t shorted, but a zener is used in breakdown mode with reverse voltage across it, no way to test that with your hand meter, unless you create additional circuits for it to put voltage across it. A 15v zener might check OK as a diode, but in a circuit, it might not "zene" at all or might zene at 6 volts.

[Mr_bibbles]

Hey G1- thanks for the input. The diode in question is a IN4148, polarity correct. It measured good out of circuit with a diode check function on the multi meter. Replaced with a new one just to be sure since that is the consensus on here, but same results.

[Mr_bibbles]

Mark! great advice about the IC's. I will do this presently. The Zener is a second brand new one in case of the first being defective, connection to ground at anode is 0.0.

[Mr_bibbles]

Thanks for pointing this out Enzo! I will do some research on reverse voltage, I've only heard a little bit about it. I replaced the new zener with another new zener just to be sure as I've got abtu 100 IN4818s lying around.

[g1]

The diode in question (D2) is supposed to be a 27V zener, not a 1N4148.

[Enzo]

This^^^

1N4148 is not a zener, and will do nothing to regulate voltage down.

1N4148 is a very common part, but it is only a plain old signal diode.

[Mr_bibbles]

Wow. I'd like to take this time to apologize to my friends, my family, and my lovely wife for the pain I have caused them by being an idiot.

So I realize now how little I understood Zeners. I put two 15v zeners in series to get it ballpark enough to test- Power supply sitting at a 30v until I can get a 27v Zener in the mail. Beginning to understand why they are instrumental in supply voltage.

Amp sounds pretty good, now the only remaining issues are...

A: The volume pot at 0 still produces a good amount of sound. Gets very loud very quickly, but sounds amazing at breakup levels. Check around the volume pot, yes?

B: Reverb does not work. Pot produces hiss when turned up, you can hear the springs when tapping on the can.

[nosaj]

Wow. I'd like to take this time to apologize to my friends, my family, and my lovely wife for the pain I have caused them by being an idiot.

So I realize now how little I understood Zeners. I put two 15v zeners in series to get it ballpark enough to test- Power supply sitting at a 30v until I can get a 27v Zener in the mail. Beginning to understand why they are instrumental in supply voltage.

Amp sounds pretty good, now the only remaining issues are...

A: The volume pot at 0 still produces a good amount of sound. Gets very loud very quickly, but sounds amazing at breakup levels. Check around the volume pot, yes?

B: Reverb does not work. Pot produces hiss when turned up, you can hear the springs when tapping on the can.

Any chance reversing reverb cables works?

nosaj

[Mr_bibbles]

reversing the cables does not work, Gunna start with the volume issues.

Replaced the pot with a 250 just to eliminate that from the suspects. removed and check the variable resistor, measures at 1.3m out of circuit. It looks like the volume control here is not just your everyday voltage divider.... maybe a feedback controller for the IC's?? I'm really at a loss for most SS devices...

Again the symptoms are a huge jump in volume from 0-1, not in the way a linear pot behaves but in a "bedroom quiet signal at 0 ---> LOUUUD ASS SIGNAL at 1"

[52 Bill]

measures at 1.3m out of circuit. It looks like the volume control here is not just your everyday voltage divider....

The schematic show a gain control that is a 1M5 pot and a volume control that is a 200K pot. Which one are you talking about?

Have you tested the inside tank wiring? If the tank makes noise through the speaker when you rattle the tank, then you know that the reverb tank output is okay and the amp's recovery circuit is working. If the tank's input transducer is open circuit or the driver chip is bad, then the reverb will work the way you describe.

Read the resistance across the tank's input jack.

[Mr_bibbles]

The "gain control" mini-variable resistor measures 1.3m out of circuit, but while measuring the the voltage on either side of it it stays constant 16v across the entire range of the resistor. Am I correct in thinking that this controls functions of the reverb as well?