Ampeg G110 Hum / no signal

[Mr_bibbles]

Hey Everyone-

Got a Ampeg g110 on the bench, does not pass signal but it turns on and produces a hum. The hum is controllable via the volume pot, so I checked the circuit before with a signal scope (poor man's Oscilliscope). Signal is lost at IC1, a vintage looking 8-pin circular piece. Voltage at pins 2 & 3 are 26v instead of 13.5v I'm guessing this is the issue because reverb works fine, treble and bass controls change the tone as well.

HOWEEEEVER the rail voltage is off- Coming from A is listed as "48v" but I'm getting "-45v" on a limiter bulb. That might be a typo, but after R44 dropping resistor the voltage only drops to -36v instead of "24v". R44, R43, and R52 all check out.

Any guesses here? Thanks in advance.

Attached a schematic.

[The Dude]

You say, "Signal is lost at IC1". Is it lost at pin 3 input or pin 6 output?

[Mr_bibbles]

It is very faint at pin 3, put pin 6 is just a hum. Shot IC? Which begs the question where can I find a replacement? It's not the usual black rectangle, but a metallic, 8 legged, raised cylinder. Printed on the top is ca3060 RCA 723, but the '6' is scratched and may be an '8' or an 'S

[The Dude]

I might try injecting a signal at pin 6 (cap isolated) first to verify a bad IC.

Edit: Verify first that pin 6 is not shorted to ground. If it is, injecting signal there will prove nothing.

[Mr_bibbles]

Thanks Dude, gunna check this on Tuesday and let you know.

[The Dude]

If the IC is bad, looking at the datasheet and comparing to the schematic, I believe it's a CA3080 and I believe this will work providing lead spacing isn't an issue. It's looks to be a different case style, but the same part.

CA3080EZ Original New Intel Integrated Circuit | eBay

If you want to spend a bit more and get the same case style:

CA3080A Op Transconductance Amps OTA IC CHIP | eBay

[Enzo]

CA3080 was not uncommon. The old metal ones had the same leg count, same pin assignments, just in a circle instead of two rows. A tiny piece of perf board and some bare wires and you can make one up from a DIP.

[52 Bill]

For me the usual tell tale sign of a 3080 is the control pin 5, which in this case is used to create the tremolo effect.

Based on the schematic, there is no negative voltage source in this amp. It is a single sided power supply with all of the op amps biased to 1/2 the supply voltage. So I can't see where a negative voltage reading would come from.

[Jazz P Bass]

I took notice in Post #1 that you have the amp on a limiter.

If the amp itself is stable, please remove the limiter.

As to 52 Bill's post, pin #5 turns on the internal amplifier of the CA3080.
So you will need at least +0.7 Vdc on the pin for the IC to function.
What voltage do you measure at pin #5?

ca3080-a.pdf

[Mr_bibbles]

Thank you everyone, let me answer all your questions in one post-


Dude: Injecting a signal at pin 6 does indeed produce a signal, so we know that the issue is either the IC or it's control circuit, yes?

Enzo- good tip! Probably going to replace with same style casing but I will definitely use that hack in the future.

52 Bill / Jazz P Bass: Voltage at Pin 5 is just a hair under .7 at .670


Since a few of these voltages are off compared to the schematic, I've included all the measurements of IC 1.

1: hovers around .0 mv
2: 23 v
3: 25.7 v
4: 0.1 mv
5: 0.67 mv
6. 24 V
7: 37V
8: 0.02 MV

With these measurements and with the injected signal success, can we effectively diagnose this IC as defective?

Thanks as always for your wisdom.

[The Dude]

Some of those voltages are a bit off, but I suspect it's because the IC is bad. I'd say time to replace the IC.

[52 Bill]

Back in the days when the metal can package was fairly common, we would use cheap wire wrap sockets to convert the circular patterned pc boards over to the DIP packages. The wire wrap sockets had long legs that you could bent enough to fit them into the pc boards. But a lot of the pc boards already have the typical DIP pattern traces and have the circular leads bent to match that.

Back to the problem, if you fiddle with the tremolo controls, does the voltage at pin 5 fluctuate?

[J M Fahey]

At mere $5 a pop, IŽd just get the proper metal can one and call it a day.

FWIW my first Op Amp based preamps (early 70`s) were built on eyeletted boards simply because I had been building Fender clones on same kind of boards, was properly equipped, experienced and very comfortable around that construction technology.

No big deal on transistor amps where you just spread legs a little more than usual (eyelets can only be *that* small and so *that* close and not less) but impossible on DIP ICs.
But I standardized on 8 pin round metallic 741s, then the standard workhorse.

[Mr_bibbles]

OK so the new chip is in and it works.... kind of..... not really.

Amp produces a sound, but there is signal being passed even with the volume at 0, but it is distorted and about the level of a bedroom practice session. volume control does work turning it up. The tremolo is always on even when the intensity is set to 0. The speed control affects it. No signal on pin 3 or 6 now. Injected signal at 6 produces working amplifier.

Voltage:

Pin 1. 1.8mv
2. 31v
3. 3.4v
4. 0mv
5. 10.8v
6.1.2v
7. 40v
8. 1mv

[Mr_bibbles]

Bump! Anyone have an idea what this issue might be? I lose signal right after the .022 from buffer stage, replaced it with a known good one and still no dice... poked around for a short to ground from around there too but not finding much....