Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jonathan_kea on January 01, 2024, 10:04:46 am
-
I’m working on a 1967 Ampeg Reverbojet j-12-r amp and the tremolo is not working. I replaced the .05 caps and electrolytic in the tremolo section to no avail.
There is a Clairex Photomod Opto device in the tremolo circuit that may be defective, but I’m not sure how to test it.
I placed a multimeter across one of the legs of the trem pot and can see DC oscillating between approximately +150v and -150v when the pot is halfway, which appears to create the widest swing in voltages. There is no voltage when the pot is dialed all the way. This behavior is perplexing to me!
Any ideas on how I can troubleshoot this?
-
> multimeter across one of the legs
"Across one leg" isn't clear to me.
Have you observed the voltages on the schematic? Exact value not critical, but they should be near, and should change for trem on/off. The lower cathode voltage in ON suggests the oscillator is running. (The plate voltages given seem wrong to me; tell us what you see.)
There's multiple ways to light the lamp and measure the resistance. A one-wire-snip way is to break the circuit as shown, use clip-lead ot tack-solder to connect the tube end of the 470k to either B+ or Ground, while signal passes.
-
I misspoke this morning about testing the lugs on the trem pot - that was pre coffee I guess.
However, I must have tested the plate voltages because there is a swing in voltages on pin 1.
At pin 1, with trem switch on, I am getting 115-160v. When off, I am getting 201v. These voltages are also negative on the multimeter.
Pin 3 is almost dead on with the values in the schematic.
-
@PRR,
So Ampeg is creating tremolo by varying the signal coupling between the preamp triode (upper left) and the tone network to the right?
The opto (vactrol?) is bypassing the 5M6 resistor, shunting more or less signal around the 5M6?
This is a different technique than the Fender circuit I'm working with, where the trem voltage modulates the opt tube pair G1 bias.
-
Often to fix any tremolo issue ;
1- You need to replace the 3 .05 caps ; you do it
2- the opto coupler
To check it :
1- disconnect the resistor at X Y
2- put voltage on the (neon ) bulb ; + and - ( ground)
3- Read resistor at X Y
4- voltage on = low value on resistor
5- Voltage off = high value
I build few photo coupler for Fender amps for less than $3
I never build one for Ampeg
Here a DIY Photomod Opto
https://www.electroschematics.com/photomod-optical-coupler/
-
At pin 1, with trem switch on, I am getting 115-160v. When off, I am getting 201v. These voltages are also negative on the multimeter.
How can that be? There is no negative voltage supply in that amp. I suspect operator error or cheap meter that can't measure properly when there is a big AC voltage present on a positive DC voltage.
-
The opto (vactrol?) is bypassing the 5M6 resistor, shunting more or less signal around the 5M6?
Correct. And the downside is if the optocoupler neon fails to produce light, the signal level goes way down because it can only pass through the 5.6M. Later Ampegs replaced the 5.6M with a 4M Intensity pot. With this Intensity pot if the opto fails you could at least turn the pot to zero and keep the guitar signal happy.
Sunn used the same method and when the opto died the amp produced almost no guitar signal. I bought a Sunn Sceptre head really cheap because it had very low output. The opto was the culprit.
-
To check it :
1- disconnect the resistor at X Y
2- put voltage on the (neon ) bulb ; + and - ( ground)
3- Read resistor at X Y
4- voltage on = low value on resistor
5- Voltage off = high value
Better use a series current limiter resistor or the neon bulb will be destroyed.
-
To check it :
1- disconnect the resistor at X Y
2- put voltage on the (neon ) bulb ; + and - ( ground)
3- Read resistor at X Y
4- voltage on = low value on resistor
5- Voltage off = high value
Better use a series current limiter resistor or the neon bulb will be destroyed.
You are right .
It was a short answer.
Many details have been deliberately omitted.
-
I just rehabbed a '67 GS-12-R. Fortunately replacing the e-caps and the oscillator caps fixed the tremolo.
Just for reference, reproduction TM1 modules are available from Fliptops: https://www.fliptops.net/catalog/p-101220/tm1-tremolo-module-for-vintage-ampegs-fliptops-version (https://www.fliptops.net/catalog/p-101220/tm1-tremolo-module-for-vintage-ampegs-fliptops-version)
Dan
-
Unfortunately, I probably have the cheapest meter money can buy. I am getting 25V AC at Pin 1, so that may be the culprit. The DC voltage IS oscillating, though, within range of the spec on the schematic, so I am not quite 100% ready to ditch the photo mod. I should also add that the sound output is way less than what I would expect from an 18 watt amp, which is a concern and jives with what you are saying about the photo mod being bad.
Any other thoughts on what to check here before I decouple the photo mod for testing?
-
Pin 1 on tremolo tube ar DC not AC . I see 125 to 195 volts
-
I am getting 25V AC at Pin 1, so that may be the culprit. The DC voltage IS oscillating, though, within range of the spec on the schematic, so I am not quite 100% ready to ditch the photo mod. I should also add that the sound output is way less than what I would expect from an 18 watt amp, which is a concern and jives with what you are saying about the photo mod being bad.
Any other thoughts on what to check here before I decouple the photo mod for testing?
Your oscillator is working just fine. Oscillating DC voltage is AC voltage. :icon_biggrin:
The low sound output is a big clue pointing to a bad optocoupler. Very simple to test without disconnecting anything. Simply put a jumper (gator clip test lead works fine) across points X and Y. If the guitar signal jumps way up to normal levels then replace the optocoupler.
-
Oscillating DC voltage is AC voltage. :icon_biggrin:
I see a big difference between.
-
Pin 1 on tremolo tube ar DC not AC . I see 125 to 195 volts
Pin 1 also has a healthy AC voltage on it. On my Ampeg J12-B there is 66VAC @ 5Hz present on the plate as well as the expected DC voltage. Easy to see with a scope. Easy to measure with a Fluke 87V meter.