Trem not working on Ampeg Gemini I

[beedoola]

I get no term sound at all. I got this amp not too long ago as is with the term non working. The reverb works and I repaired/modded the footswitch by adding a 1/4" TRS plug and a TRS jack on the rear of the chassis so it can be disconnected. The switch activates the reverb but not the trem - I've unplugged the footswitch to make sure it just wasn't a wiring problem. The Reverb still works without the footswitch plugged in, the Trem does not.

How common is it for the TM-1 Optoisolator to go out? I know Fliptops carries a replacement but I'd like to make sure its not something else before I drop almost $40 for a replacement. I replaced the 3 25uf 25v Electrolytic caps - the Blue "Ducati." I was hoping the one near the trem was bad and that was causing the trem to fail but still no trem. What troubleshooting methods/tests should I conduct to either rule out other parts failure and or determine for sure its the TM-1? I don't have a scope so I can't test the Opto.

Thanks in advance.

[52 Bill]

Besides replacing the cathode caps what have you done to the amp? The optoisolator can fail but check the other parts before you order a new one.

Is the oscillator running? If it is, you should see a rising and falling dc voltage in the circuit. If it's not there, then try replacing the phase shift caps, and if it wasn't one of the ones that you already replaced the cathode bypass cap on the oscillator tube.

[Enzo]

How common an opto failure is really doesn't matter. If 90% of them fail, and yours did, then you need a new opto. If only one in 10,000 fail, and yours was that one, then you need a new opto. See what I mean?

[beedoola]

Besides replacing the cathode caps what have you done to the amp? The optoisolator can fail but check the other parts before you order a new one.

Is the oscillator running? If it is, you should see a rising and falling dc voltage in the circuit. If it's not there, then try replacing the phase shift caps, and if it wasn't one of the ones that you already replaced the cathode bypass cap on the oscillator tube.

I added a three prong cord, thats it thus far. I ordered a Can 50x50uf and 80uf axial electro cap to do the filtering caps. I haven't tested to see if the oscillator is running - thats why I asked here; what methods to use to determine if it's running. So if I measure for DC voltage say on the Depth Pot Lugs or somewhere like that, that should serve as a point to observe the DC fluctuation?

How common an opto failure is really doesn't matter. If 90% of them fail, and yours did, then you need a new opto. If only one in 10,000 fail, and yours was that one, then you need a new opto. See what I mean?

Ehh, I mean if a tech said - "yeah I've seen a lot of the terms go out on these old Geminis due to the optoisolator," that would serve as a pretty good indicator that that was the reason for the trem not working on mine as well.

Thanks.

[52 Bill]

I added a three prong cord, thats it thus far. I ordered a Can 50x50uf and 80uf axial electro cap to do the filtering caps. I haven't tested to see if the oscillator is running - thats why I asked here; what methods to use to determine if it's running. So if I measure for DC voltage say on the Depth Pot Lugs or somewhere like that, that should serve as a point to observe the DC fluctuation?

I'll be honest, I haven't looked at the schematic for your amp, so I can't give you exact directions on where to look for the fluctuating voltage. I've repaired hundreds of these amps and Ampeg always used a basic phase shift oscillator circuit with three caps. These are the typical three caps that you replace to rebuild the oscillator. Look at the schematic and find the output from the oscillator to the depth control. Now try and measure for the fluctuating voltage there.

Ehh, I mean if a tech said - "yeah I've seen a lot of the terms go out on these old Geminis due to the optoisolator," that would serve as a pretty good indicator that that was the reason for the trem not working on mine as well.

What tech? There has been no mention here of that statement. I've seen more working trem modules than dead ones in these old Ampegs. But that's my experience. It could be some other tech has seen more dead ones than working ones. Fix the amp by diagnosing the problem that your amp has, not by some parenthetical statistical number that can lead you down the wrong path.

[Enzo]

The problem is when we start trying to fix it from a list of what most people think is common, we can waste time trying things that are not the answer rather than just finding the problem in the first place.


I once had a customer with a pair of synthy keyboards, both the same model whatever they were. He brought one in "makes no sound." I fixed it. he wanted to know which parts I replaced. I showed him. he asked to buy more of those same parts off me. Sure, why not, got him a new "set" of parts. Next day he brought in his other board. It also made no sound. Totally different cause, but he has assumed that since replacing that list of parts fixed the first one, it would fix any others with no sound. But that was not the case. If he had brought both boards in the first trip, he would have had them both running two days earlier.

[beedoola]

The problem is when we start trying to fix it from a list of what most people think is common, we can waste time trying things that are not the answer rather than just finding the problem in the first place.


I once had a customer with a pair of synthy keyboards, both the same model whatever they were. He brought one in "makes no sound." I fixed it. he wanted to know which parts I replaced. I showed him. he asked to buy more of those same parts off me. Sure, why not, got him a new "set" of parts. Next day he brought in his other board. It also made no sound. Totally different cause, but he has assumed that since replacing that list of parts fixed the first one, it would fix any others with no sound. But that was not the case. If he had brought both boards in the first trip, he would have had them both running two days earlier.

I get what you're saying but a device making no sound is a little different than the trem of an amp not working. With the former, the is a whole other host of problems that could cause that. With the trem, there is less. I guess what I wanted to know was if trem optoisolators fail often, or in this cause - in old Ampeg Geminis. If they're known to be "notorious" for failure, that would be a pretty good indicator that its bad.

But Yes, you are right: better to find out exactly what the cause is instead of just looking for a high percentage of consensus and just taking a leap of faith that they were/are right. I'll try what Bill suggested.

schematic:

[beedoola]

I'll be honest, I haven't looked at the schematic for your amp, so I can't give you exact directions on where to look for the fluctuating voltage. I've repaired hundreds of these amps and Ampeg always used a basic phase shift oscillator circuit with three caps. These are the typical three caps that you replace to rebuild the oscillator. Look at the schematic and find the output from the oscillator to the depth control. Now try and measure for the fluctuating voltage there.


What tech? There has been no mention here of that statement. I've seen more working trem modules than dead ones in these old Ampegs. But that's my experience. It could be some other tech has seen more dead ones than working ones. Fix the amp by diagnosing the problem that your amp has, not by some parenthetical statistical number that can lead you down the wrong path.

I measured the voltage at the depth or "intensity" pot and got no fluctuation with regard to voltage. There is a slight ticking of the LFO that is audible when I engage the Trem with the foot switch - adjust the speed slows/speeds the tick. Again, this tick is very quite/barley audible.

Did you see the schematic I linked in my last post? Could one of the caps be failing and causing the term not to be intense? It seems the LFO is at least working though. Thoughts?

[52 Bill]

I measured the voltage at the depth or "intensity" pot and got no fluctuation with regard to voltage. There is a slight ticking of the LFO that is audible when I engage the Trem with the foot switch - adjust the speed slows/speeds the tick. Again, this tick is very quite/barley audible.

The foot switch must be turned on for the trem to work.

With this amp's design, there will be no fluctuating voltage at the depth control. The ticking that changes rate with the speed control is a good sign. Try reading the voltage at the plate of the oscillator tube. Look for that fluctuating voltage there.

If the oscillator is weak it could be caused by a bad cathode cap or an off value cap in the feedback circuit. Because it's a simple test, I'd temporarily parallel another cap across the cathode cap to see what happens. If you do this, be sure to watch the polarity.

[beedoola]

The foot switch must be turned on for the trem to work.

With this amp's design, there will be no fluctuating voltage at the depth control. The ticking that changes rate with the speed control is a good sign. Try reading the voltage at the plate of the oscillator tube. Look for that fluctuating voltage there.

If the oscillator is weak it could be caused by a bad cathode cap or an off value cap in the feedback circuit. Because it's a simple test, I'd temporarily parallel another cap across the cathode cap to see what happens. If you do this, be sure to watch the polarity.

Did you see the schematic I linked above? What are the cathode caps in the circuit? The footswitch was ON and thats how I got the ticking sound.

[pontiacpete]

What are the cathode caps in the circuit?

It's the 25uf/25v cap

[52 Bill]

It's the 25uf/25v cap

My bad, as Pete has told you it's the 25uF cap that connects the cathode of the oscillator tube to ground. It is referred to as the cathode bypass cap.

Its' function is basically to raise the gain of the stage to help the tube oscillate. Because it is an electrolytic cap that is prone to drying out, it will sometimes go off value and will reduce the gain of the stage and will prevent the tube from oscillating.

When they go bad, they usually decrease in value so adding a second cap in parallel (wired positive to positive and negative to negative) will act as a simple test to see if the cap has gone off value.

My comment about the foot switch was just to make sure certain that you kept it on while testing the circuit. Some circuit designs have an oscillator that runs all of the time and will use the switch to stop the output from the oscillator from getting to the rest of the circuit. Your amp uses the switch to stop the oscillator from running at all. So yes, I know that you had it on.

[beedoola]

My bad, as Pete has told you it's the 25uF cap that connects the cathode of the oscillator tube to ground. It is referred to as the cathode bypass cap.

Its' function is basically to raise the gain of the stage to help the tube oscillate. Because it is an electrolytic cap that is prone to drying out, it will sometimes go off value and will reduce the gain of the stage and will prevent the tube from oscillating.

When they go bad, they usually decrease in value so adding a second cap in parallel (wired positive to positive and negative to negative) will act as a simple test to see if the cap has gone off value.

My comment about the foot switch was just to make sure certain that you kept it on while testing the circuit. Some circuit designs have an oscillator that runs all of the time and will use the switch to stop the output from the oscillator from getting to the rest of the circuit. Your amp uses the switch to stop the oscillator from running at all. So yes, I know that you had it on.

I replaced the 3 25uf/25v electros already. And the ticking was there prior to my changing the caps.

[52 Bill]

I replaced the 3 25uf/25v electros already. And the ticking was there prior to my changing the caps.

Well then, nevermind. Have you read the voltages on the tube?

[audiopete]

Sounds like the opto. If the LDR side is dead and the neon is working - no trem but you get the click. If your trem tube is good and all caps are good then I would change this. Whoa Fliptops has a very nice new website. Classy.