Help with Reverb/Tremolo wiring Gibson GA15-RVT

[devilsindetails]

You need to download and install ExpressPCB: http://www.expresspcb.com/ExpressPCBHtm/Download.htm

THKS MARTIN...appreciate the pointer

[martin manning]

V2 pin 8 should not be zero; more like -1 to -1-1/2V, but this may be hard to measure as your meter will alter the circuit operation. Try grounding the center lug of the speed pot directly and see if the oscillator starts.

[devilsindetails]

V2 pin 8 should not be zero; more like -1 to -1-1/2V, but this may be hard to measure as your meter will alter the circuit operation. Try grounding the center lug of the speed pot directly and see if the oscillator starts.

OK...I'll try it when I get back to where the amp is tomorrow.

Any other measurements jump out at you as being way off? Thks Leon

[martin manning]

V2 pin 8 should not be zero; more like -1 to -1-1/2V, but this may be hard to measure as your meter will alter the circuit operation. Try grounding the center lug of the speed pot directly and see if the oscillator starts.

OK...I'll try it when I get back to where the amp is tomorrow.

Any other measurements jump out at you as being way off? Thks Leon

No. By grounding the wiper of the speed pot you are bypassing the pot, foot switch jack, the wiring, and the switch. If it works, one of those other parts has an open circuit. Grounding it at the same place where V2 pin 9 is grounded would be best.

[devilsindetails]

Thanks Martin...will post results..

[devilsindetails]

V2 pin 8 should not be zero; more like -1 to -1-1/2V, but this may be hard to measure as your meter will alter the circuit operation. Try grounding the center lug of the speed pot directly and see if the oscillator starts.

Hi Martin and others...V2 Pin 8 reads -.44 vDC to - .48 vDC....Problem?

Also, grounding center lug of speed pot does not start oscillator.

I will note that when I attached an alligator to center lug of speed pot and touched V2 Pin 8 repeatedly I could get some oscillation. Does this indicate anything?

Also AC voltage readings for heaters are:

V1 and V2
Pin 1--0 vAC
Pin 2--6.4 vAC

V3
Pin 4--0 vAC
Pin 9--6.4 vAC

V4 and V5
Pin 1--0 vAC
Pin 2--6.4 vAC

[martin manning]

Was the oscillation at tremolo frequency, like 8-10 Hz? If so that's progress. Is the 4.7 Meg resistor measuring within spec? Is it possible that the socket and pins are not making good contact? Use of contact cleaner and or retensioning the socket contacts might remedy that. This tube works in another amp, right? Do you have another one?

[devilsindetails]

Hi Martin.

Yes I think that's about right but I have to repeatedly step off and on pin 8 V2 with my probe clip to get something that approximates tremolo...contacting pin continuously doesn't.

I'll check the resistor again but it seems that it was within spec according to my resistor check sheet.

Sockets seem tight no ststic or intermittentcies when I wiggle tubes a bit.

Yes the tube works in the other amp and swapping the tubes from one to the other and back does not alter lack of tremolo in the problem GA-15RVT.

[martin manning]

You've replaced C's 19, 20, and 21, yes? See if there is DC voltage on the side of C17 that goes to R31. I'm wondering if C17 has failed.

[devilsindetails]

The resistor checks out...although resistance seems to commence at about 12 megs and then stabilizes at 5 megs...is this telling me something important?

[devilsindetails]

You've replaced C's 19, 20, and 21, yes? See if there is DC voltage on the side of C17 that goes to R31. I'm wondering if C17 has failed.

Yes C19, 20, 21 were replaced....C17 reads about 1.5 volts DC.

[martin manning]

Hmmm... I think it should read zero, but I don't think that would prevent the oscillator from working. Evidently you can kick it into oscillating by grounding the speed wiper, but it won't sustain. Can you check the grounds on R35 and R36? See if the lead from those resistors to ground measures close to zero ohms.

[tubeswell]

The resistor checks out...although resistance seems to commence at about 12 megs and then stabilizes at 5 megs...is this telling me something important?

That you probably used a 4M7 for R36 - which is what's required. Also check that R34 and R35 are the specified values.

Altho' you replaced the caps, you might want to test each one for DC leakage just in case - starting at the cap nearest the plate. (See Fig 4-4 on P57 of the attached article for a testing method). Also test C16 and C17 for DC leakage (I doubt that C16 is strictly necessary - probably put there to shunt HF oscillation).

[devilsindetails]

Hmmm... I think it should read zero, but I don't think that would prevent the oscillator from working. Evidently you can kick it into oscillating by grounding the speed wiper, but it won't sustain. Can you check the grounds on R35 and R36? See if the lead from those resistors to ground measures close to zero ohms.

No, I can only get some oscillation by connecting speed wiper to pin 8 on Valve 2 and tapping it...connecting it directly does nothing...oscillation only occurs if i remove and connect, remove and connect, remove and connect in rapid succession.

[devilsindetails]

That you probably used a 4M7 for R36 - which is what's required. Also check that R34 and R35 are the specified values.

Altho' you replaced the caps, you might want to test each one for DC leakage just in case - starting at the cap nearest the plate. (See Fig 4-4 on P57 of the attached article for a testing method). Also test C16 and C17 for DC leakage (I doubt that C16 is strictly necessary - probably put there to shunt HF oscillation).

Thanks Tubeswell, I'll do my reading.