AB763 Vibrato issues (64 deluxe)

MrCoolGuy

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The Vibrato on this blackface deluxe isnt working. The roach is flashing and reacts to the speed control. It was working yesterday. Nothing had changed.
Why would th he plate voltage be low on v3 (trem tube)
V3a 185vdc, V3b 192vdc.... I have tried swapping tubes.
 

Wally

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The photocell in the ‘roach’ may have failed?
voltages?? Are the other voltages at and fed by the ‘A’ node correct? What are the cathode voltages there on V3? Excessive current draw will pull those plate voltages down.
 

Phrygian77

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Is this a build or the real deal?

The photocell in the ‘roach’ may have failed?

Had that happen on my '67 SR. Lamp was flashing, all resistances and voltages were okay. I cut it open after replacing it and the LDR was brown and burnt looking.
 

MrCoolGuy

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The numbers are different every time I check. The plates of v3 are now
263v and 350v. Cathodes are 2v and 7v.
I'll look at the other A nodes.
 

tubedude

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Check the LDR path. Sounds like new roach time. The shifting plate voltage suggests oscillation is good, the neon pulses, so the LDR side is likely. What do you read across the LDR?
 

MrCoolGuy

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I didnt mean the type of "shifting" plate voltage you ever to... that would indicate oscillation. I mean its stable but different during different readings.

Anyway, the "lamp off" resistance is only 12.5r... shouldnt it be much higher?

Wait, thats across the top rwo legs... it is 11m across the bottom legs...???
 

schmee

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Posted over in your other thread:
I've had Fender Trems not work but visibly flashing. Often they will occasionally start working out of nowhere as you are playing, or they may stop suddenly. Most often I just replace it and it works. I think there's a resistor in there with the opti, maybe the resistor goes bad? It seems to rarely be any of those 3 capacitors. I'd put the stock ones back in into that amp.
 

MrCoolGuy

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Yeah, I'm replacing it. Was just curious what kind of readings I should get on the roach. For some reason, it's not an easy topic to google.
 

Silverface

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The numbers are different every time I check. The plates of v3 are now
263v and 350v.
If the voltages are inconsistent check the solder joints leading to and after the dropping resistor; the resistor itself; and it DOES have fairly fresh filter caps, right? If not that's priority #1
 

MrCoolGuy

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If the voltages are inconsistent check the solder joints leading to and after the dropping resistor; the resistor itself; and it DOES have fairly fresh filter caps, right? If not that's priority #1
Yes, all the electrolytic caps are new. All the solder joints look fine, that dropping res. Is fine too. Just waiting on a new roach.
 

MrCoolGuy

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The photocell in the ‘roach’ may have failed?
voltages?? Are the other voltages at and fed by the ‘A’ node correct? What are the cathode voltages there on V3? Excessive current draw will pull those plate voltages down.
So, the plates of V1 are also "A" node... correct? Their voltages are good. (165 and 170)
 




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