Help Me Fix Fender Vibrato in AB763 Deluxe Reverb

[s2]

So I have this Deluxe Reverb on my bench. The owner says it was his late father's amp and has always been in their family. He says the vibrato has never worked.

I've checked all the obvious stuff. I've replaced the tube. The wiring seems to be correct. All the component values measure right. I've replaced the LDR to no avail.

What I do see is the plate voltages are too high on both triodes and neither cathode has any voltage on it.

I've been staring at this thing for 4 hours now and am out of ideas.

What am I not thinking of?

[jem]

Are you sure there is a ground connection for the cathodes resistors?

[Johnhenry]

You do know it takes the pedal to turn the Vibrato on, Unshielded wire to the Vibrato, Shielded wire to the rca jack marked reverb, if you installed the LDR properly your lightbulb should be hooked to the 100K and 10 meg resisitors, if you have voltage and the circuit's complete you should get vibrato, I have found bad .01 cap's or 1 of the 3 .01 cap's broke loose from the board, and you will see higher voltage reading's in this circuit than what you see on Fender's layout's,
Check all connections and the value of the intensity pot, it's not a big circuit ! you'll find it if you keep looking and testing.
Johnhenry

[s2]

Thanks for the ideas, guys!

I've checked the pots. Those are correct and functioning normally. The cathode resistors are indeed grounded. I do have a footswitch plugged in. I know that one

That's just it! This should be a no-brainer, but it has me stumped!

All the caps and resistors measure correctly. I've checked every wire 10x over and they are all OK too.

The LDR does not light.

HELP!

[Johnhenry]

Nix on the easy stuff, Just keep at it, You'll find it S2.
Johnhenry

[Firestorm]

Hmmm ... make sure the footswitch really is grounding out the negative voltage being fed through the 2M2 resistor (you've got to have the reverb lead plugged in too, because that's the ground). The negative voltage shuts off the oscillator. Replace the cathode bypass caps on both sides of the 12AX7, especially on the oscillator. If there's not enough gain, the oscillator won't oscillate. And just replace all the the .01 and .02 caps. Even ones that test good may be leaking so much current that it screws up the phase lag loop and again the oscillator won't oscillate. Are you sure the LDR is good? Some of the available replacements are kind of suspect. Amazing how such a simple little circuit causes so much frustration!

[s2]

OK. I finally figured it out. I think there was a conspiracy in the Fender factory in the 60s to drive me nuts.



Anyway, there was a whisker on the tube socket that was causing me pain and shorting the LDR. Once I found that (it was hiding behind the filament wires), the circuit started functioning according to my scope, but not according to my test speaker.



On a whim, I plugged the signal generator in the other channel, and whaddya know? One of the lovely ladies reversed the leads to the mix resistors at the PI, so the vibrato was wired to the normal channel.



I fixed it. All is well with the world again. Except that I spent 5 hours on this amp and can only charge him for 1



Anyway, thanks for the help, guys!

[Structo]

Wow! Good find!
That's crazy that it had been broken all these years.

[Phil_S]

OK. I finally figured it out.
I fixed it. All is well with the world again. Except that I spent 5 hours on this amp and can only charge him for 1

This amp stuff is a hobby for me, but in my day job, I'm self employed and charge by the hour and I'm not exactly inexpensive. My rule of thumb is that I only charge the client for productive hours. When I have to learn something to complete the task, that's on my dime. In the end, work has a finite value value and there isn't much you can do about it.

IMO, you are to be applauded for 1) charging the appropriate fee -- people "know" when they've been ripped off -- that's bad for business and they won't hesitate to tell others, 2) having the commitment to stick to it and see the job through, even thought it "went bad" because you couldn't "see" the problem.

One think I know, is that when nothing makes sense, there's a good chance that the problem is hiding in plain site. This is a classic case of that.

[s2]

This guy said he's taken this amp to half a dozen techs over the years and no one could fix it. So I'm glad to be able to help him out.

As it turns out, he insisted I charge him 2 hours for it and wants me to do a cap job on it now, so it's all good

[Phil_S]

I'm a big believer in treating your customers right. It brings it's own rewards. Sometimes, it is not as immediate as this. Building trust creates long term relationships and brings referrals. Congrats!