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I'm rejuvenating a 1967 Ampeg GS-12R Reverborocket II. (Sorry but I can't seem to get the schematic to upload). Replaced the electrolytics and coupling caps, she plays clean--but no tremolo. This is the earlier version with the simpler tremolo (compared to the early 1970's version) basically using 1/2 of a 12AX7 and 3 caps as a LFO. I replaced the 3 caps in the oscillator, as well as the cathode resistor bypass cap, and checked all resistors in the tremolo circuit. Everything within spec....but no tremolo.
Checked plate and cathode voltage around that 1/2 of the 12AX7, looked good. Put scope on signal going to neon lamp, got a "trigger" signal at the lamp leads on the TM-1 optoisolator. But nothing on the LDR. Desoldered the LDR from the intensity pot, got continuity (if can you call 12 Mohm continuity?).
Okay, so I've got a trigger signal to the bug, but no response from the LDR. I guess that means the optoisolator is dead. Ouch, $35 from Fliptops, Well, nothing to lose, since it sure looks dead, might as well do an autopsy. (Hey maybe the LDR is still good and I can just put in a new neon?) So I carefully start cutting into the TM-1. Turns out the lower half is brittle black plastic and breaks apart nicely by crushing it slightly with vice-grips. There's the top leads, going through a black plastic piece, and the LDR and neon potted in some clear soft stuff. With a Dremel I cut through enough of the top black plastic to pull the neon, the leads look fine. Ok, lets check the LDR. Lo and behold, it shows low resistance in room lighting, and goes high when I obscure it. The LDR must be OK. So just for giggles, I hopk the neon up to the same trigger signal I had before...and the neon works.
Huh? The LDR seems to work, the neon works. I reassemble the thing (sorta) and slap some black liquid electrical tape on it, and run test leads to everything that it was hooked up to before. And...now I've got tremolo. Nothing else has changed other than partially disassembling it.
I've only got one conjecture at this point. Maybe the clear potting stuff (soft and pliable like clear RTV) had carbonized a little between the two neon leads at the bulb, shunting the trigger signal off the bulb. I'm going to let the liquid tape cure overnight and reassemble it into the amp properly in the morning to see if it continues to function.
So...the point of my long rambling...if someone has a non-functioning TM-1 or its successor, it may be worthwhile trying to disassemble it, maybe it will work if the clear gunk is cleaned off the neon leads like I did. Or if you can salvage the LDR you could fix it for the cost of a neon bulb-- maybe Rat Shack carries them (is it a NE-2?)
edited: Still working this morning when I soldered it back into service. Oh, and it's half of a 12AX7 that drives the LFO not 1/3 of the 6U10 like I wrote originally...too many hours hunched over the soldering iron.
Checked plate and cathode voltage around that 1/2 of the 12AX7, looked good. Put scope on signal going to neon lamp, got a "trigger" signal at the lamp leads on the TM-1 optoisolator. But nothing on the LDR. Desoldered the LDR from the intensity pot, got continuity (if can you call 12 Mohm continuity?).
Okay, so I've got a trigger signal to the bug, but no response from the LDR. I guess that means the optoisolator is dead. Ouch, $35 from Fliptops, Well, nothing to lose, since it sure looks dead, might as well do an autopsy. (Hey maybe the LDR is still good and I can just put in a new neon?) So I carefully start cutting into the TM-1. Turns out the lower half is brittle black plastic and breaks apart nicely by crushing it slightly with vice-grips. There's the top leads, going through a black plastic piece, and the LDR and neon potted in some clear soft stuff. With a Dremel I cut through enough of the top black plastic to pull the neon, the leads look fine. Ok, lets check the LDR. Lo and behold, it shows low resistance in room lighting, and goes high when I obscure it. The LDR must be OK. So just for giggles, I hopk the neon up to the same trigger signal I had before...and the neon works.
Huh? The LDR seems to work, the neon works. I reassemble the thing (sorta) and slap some black liquid electrical tape on it, and run test leads to everything that it was hooked up to before. And...now I've got tremolo. Nothing else has changed other than partially disassembling it.
I've only got one conjecture at this point. Maybe the clear potting stuff (soft and pliable like clear RTV) had carbonized a little between the two neon leads at the bulb, shunting the trigger signal off the bulb. I'm going to let the liquid tape cure overnight and reassemble it into the amp properly in the morning to see if it continues to function.
So...the point of my long rambling...if someone has a non-functioning TM-1 or its successor, it may be worthwhile trying to disassemble it, maybe it will work if the clear gunk is cleaned off the neon leads like I did. Or if you can salvage the LDR you could fix it for the cost of a neon bulb-- maybe Rat Shack carries them (is it a NE-2?)
edited: Still working this morning when I soldered it back into service. Oh, and it's half of a 12AX7 that drives the LFO not 1/3 of the 6U10 like I wrote originally...too many hours hunched over the soldering iron.
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