Fender 68 Custom Vibrolux Reverb Tremolo/bias Problem

[jasonguitar]

This is a brand new amp and the owner brought it back to the shop dead after a few days. Upon inspection, I discovered that the power tubes were cooked and the power transformer was open. I installed a new trasnformer and power tubes and discovered what caused the failure. Bias modulating tremolo is not working properly. There is oscillation at tremolo tube v5 but it's not modulating the bias so when the intensity pot is turned up with trem switched on or off the bias current just shoots up and doesn't modulate - this is what caused power tubes to blow and kill the PT. Having trouble locating the problem. Test point voltages are good so far. Somehow the oscillation at v5 is not making its way to the bias circuit. Pots are good, tubes are good, the footswitch functions properly, the issue is unaffected by bias setting. Any help is greatly appreciated.

[clintronics]

First thing I do is replace C49, C22, C21, c20 with high quality caps. Then re-start your trouble shoot.

[Jon Snell]

Apart from an intermittent bias voltage, which will be obvious;
If V5 stops working that will not affect the bias voltage because the voltage is AC coupled via C24 & C50.
The only cause must therefore be from a leaky C24 or C50.
A simple 100mA HT fuse would have saved all your work and spares. False economy on behalf of the manufacturers in my opinion.

[jasonguitar]

First thing I do is replace C49, C22, C21, c20 with high quality caps. Then re-start your trouble shoot.

Arbitrary replacement of parts is never the right solution. In John’s reply he pinpoints two capacitors that would be the likely culprits and neither of those parts are mentioned in the list you do you provided. Thanks anyway!

[jasonguitar]

Apart from an intermittent bias voltage, which will be obvious;
If V5 stops working that will not affect the bias voltage because the voltage is AC coupled via C24 & C50.
The only cause must therefore be from a leaky C24 or C50.
A simple 100mA HT fuse would have saved all your work and spares. False economy on behalf of the manufacturers in my opinion.

Thank you John! This makes sense and I will check those components out first thing tomorrow. Thank you for your time and expertise!

[Pedro Vecino]

I discovered that the power tubes were cooked and the power transformer was open. .

In the schematic there is a symbol that indicates the existence of an internal fuse in the transformer.

.

[clintronics]

Arbitrary replacement of parts is never the right solution. In John’s reply he pinpoints two capacitors that would be the likely culprits and neither of those parts are mentioned in the list you do you provided. Thanks anyway!

Time is money. every repair Ive ever done has been one of these inexpensive caps going bad in this circuit. I like to start out knowing they are upgraded.

[jasonguitar]

In the schematic there is a symbol that indicates the existence of an internal fuse in the transformer.

.

Yes, most newer Fender transformers have this internal fuse but have you ever tried to repair one of them? The time it takes is more expensive than the new factory part.

[nevetslab]

Yes, most newer Fender transformers have this internal fuse but have you ever tried to repair one of them? The time it takes is more expensive than the new factory part.

I have pulled some apart. Tedious operation, and not always successful. I had posted this thread some time back. It was actually a transformer from the Hot Rod Deluxe, and NOT the Fender Deluxe Reverb, as I had mistakenly stated.

https://music-electronics-forum.com/...r-thermal-fuse

I did pick up the replacement Thermal Fuses like that I had removed from the cardboard insulated tube placed at the base of the xfmr windings, next to the core. I had another Hot Rod Deluxe power xfmr's thermal fuse open, but it wasn't just that thermal fuse. There was another open wire I had discovered during the tear-down.

And yes, it is a somewhat laborsome task....takes over an hour, with no guarantee the surgery will be successful. The one used in the photo tear-down from the thread I posted, I put that back together just recently, added some aluminum extrusion to it for vertical mounting in my Hiwatt DR201 Bass Amp clone to use for the HT Screen Supply.

Hmmm.....I see you're relatively close by to my shop, up in Reseda, CA. I'm at CenterStaging, slingshot distance from the Runway of the Burbank Airport in Burbank.

[jasonguitar]

Time is money. every repair Ive ever done has been one of these inexpensive caps going bad in this circuit. I like to start out knowing they are upgraded.

I get your point but this is a brand new amp and those caps are not the cause of the trouble. The Illinois filter caps do tend to fail prematurely these days though. I wish Fender would switch to a higher grade cap.

[Jon Snell]

I get your point but this is a brand new amp and those caps are not the cause of the trouble. The Illinois filter caps do tend to fail prematurely these days though. I wish Fender would switch to a higher grade cap.

If brand new and failed so soon, I would return it to the supplier for replacement or at least order free of charge parts from Fender as a warranty repair which will mean I would get paid for repairing their new faulty amplifier.
If I were the purchaser, I would expect a replacement and not accept a repair as is my consumer right.

[Chuck H]

First thing I do is replace C49, C22, C21, c20 with high quality caps. Then re-start your trouble shoot.

Time is money. every repair Ive ever done has been one of these inexpensive caps going bad in this circuit. I like to start out knowing they are upgraded.

FWIW there's no reason to argue this point. The caps you indicate are in the oscillator circuit which Jason indicated was functioning. Though it's common practice to shotgun the trem circuit in older amps when the oscillator isn't functioning that doesn't seem to be the case here.

[pdf64]

I think that a bad trem intensity pot seems a more likely culprit than leaky / shorted coupling caps from the oscillator’s buffer. As the 330k resistor should take up most of the VDC differential.
It would be best to remove the output valves until a proper reliable bias has been established.

[Helmholtz]

I think that a bad trem intensity pot seems a more likely culprit than leaky / shorted coupling caps from the oscillator’s buffer. As the 330k resistor should take up most of the VDC differential.

With a shorted coupling cap the grid voltage would be increased by more than +16V (I think).

But you're right, the trem pot losing contact to bias supply or wiper (as well as a bad P30A-1/P30B-1 connector) would cause a loss of bias.

[jasonguitar]

I think that a bad trem intensity pot seems a more likely culprit than leaky / shorted coupling caps from the oscillator’s buffer. As the 330k resistor should take up most of the VDC differential.
It would be best to remove the output valves until a proper reliable bias has been established.

But the bias voltage is controlled smoothly by adjustment of the intensity pot. The pot is working “properly.”