Problem with 1966 Super reverb (AB763)

[jezzbo]

Hi Everyone,

I'm trying to fix a problem on a friends 1966 superreverb (AB763)

This is the problem: On channel 2 (vibrato channel) when I turn the volume pot between 6 and 8, there is this scratchy noise. (like there is some dust in the pot). Before "6" and after "8" the noise is gone.

This is what I allready did to try to solve this problem but with no succes.

1. Changed the volume pot with a new one (did this two times just to make sure the new pot wasn't also broken)
The new pot makes the same noise at approximatly the same location (between 6 and
2. Changed the 3 tonecap (I thought maybe one was leaking DC)
3. changed the treble bleed cap (120pF)
4. changed V2 tube and V4 with a known good one.

thank you for your help.

[selloutrr]

have you replaced the carbon comp resistor?

[jezzbo]

I did not. But the amp had a major overhaul by someone else.
All original resistors where replaced with carbon film 1W resistors.
All original Molded blue caps where changed to orange drops (715P)
All electrolytics changed with sprague atoms.

Thanks for helping.

[selloutrr]

You said you changed the 120pf cap.
Try replacing it with a silver mica instead of a ceramic disc.
If you still have the issue all that is left that has not been changed in the circuit would be the tube sockets (make sure they are clean and tight) double check all grounds and then the Opto Bug.

[jezzbo]

The 120pF is changed with a silver mica.
I also forgot to mention that I also resoldered all the pins on the tubesockets.

I'm sorry for my ignorance, but how can the opto bug (that is later in the circuit) give problems on the volume pot (earlier in the circuit)?

I also removed tubes that were not needed to test. (imho). Like the reverb driver (12AT7) and the "vibrato- tremolo" 12AX7.

I need to check all grounds. But since it's only at a given point on the pot (between 6 and can this be?

The normal channel doens't have this problem.

Since the vibrato channel has an extra gainstage.
I was thinking it is maybe the coupling cap going from gainstage 2 to gainstage 3. (It's a .02uF cap in series with a 10pF cap. the 10pF cap is parralleled with a 3.3M resistor) But then again why would I hear that on a pot earlier in the circuit?

Thank you for your reply.

[ElectronAvalanche]

Just a long shot:

check with your Voltmeter if there is any voltage on the eyeletboard. Measure from ground to any point close to the anode resistors of the input and Reverb stage.

If you board is conductive, you will measure a voltage that will get smaller the further away you are from the anode resistor eyelets.

I have this problem with my AB763 Vibroverb clone. I have a ticking Tremolo as well.

I plan on swapping all boards to epoxy.

Probably not a solution for your vintage amp.

Would really be interested if you can measure any voltage on the board material.

Thanks and good luck!

Electron

[David Root]

jezzbo, do your friend a REAL favor and get those 715P orange drops out of the amp and put the original blue moldeds back in. Then check for DC leakage on the blue moldeds and replace if necessary (Ebay-not cheap).

If whoever took out the blue moldeds kept them (very likely I think), then get them back, OR, if not possible, get some new Sozo blue moldeds, or Jupiter '60s style caps.

[jelle]

If you have replaced the tone caps, then I would suspect the board. I do not know how clean the work was done but please understand that flux, from soldering, is conductive. This in addition to the possibly conductive fish paper board, so if there is flux from one eyelet to another, clean it with alcohol.

Also, the backing board can be conductive.


Good luck!
Jelle

[Gibsonman63]

Have you checked for DC on the pot when it's in the scratchy region?

[selloutrr]

All good points we are taking for granted the previous work has been done satisfactory.
Depending on where the amp has spent the last 40+ years you may need to clean between the board and backer board as well.

[jezzbo]

Thanks for helping me guys.

@ Electron: I'm gonna check out if the board is conductive and let you know.

@ Jelle: Thanks for the tip about the alcohol. (if needed)

@David Root: I don't have the original blue caps. Someone else took them out. Before my friend bought this amp. Maybe changing them with Jupiter or sozo moded blue caps but that's gonna cost him more money. And budget is really tight.

@ Gibsonman63: No I haven't checked for dc on the pot. I'm gonna do that. Thanks.

[jezzbo]

I did some more measuring:

1. The eyletboard conducts between 3mV DC and 7,5mV DC (depending on where I measure. And it is not consistent.)

Can this be enough to have trouble?

Another thing, the eyletboard seems to be microphonic. When I touch the eyletboard with the probe of my digital meter, I can actually hear a sound comming through the speakers!

2. DC measured at the Volume pot (with the pot dialed in the trouble zone, between 6 and 8:
At the wiper: 3mV
At the right solderlug: 2,7mV

One more thing that I checked and I'm not sure it is related is that when I bias the amp I can only get it to 32,3mA (then the biaspot is at maximum
I know that this is more or less ok. But I think it's odd that the bias pot is full open. and that the amp is still biased rather cold. Tubes used is are JJ 6L6GC (30W), plate voltage = 473V
The biaspot, and resistors all have the correct values according to the schematic. The bias filtercap was changed with a 100uF 100V sprague.

Thanks!

[Reeltarded]

You fount it! Yes. DC causes scratchy crap.

Yeah, you are going to get sound off the board til the DC is gone. SOB is conducting. (business!) It's something like that single needle that pings your wrist when it's the first thing that touches your guitar bridge. With your volume up, the grounding buzz stops, but you hear a punctuated TIC as you make contact, and you feel a micro-zap. That's all it is.

Fix the board. Good deal.

On a hunch, maybe the voltage you detect on the board is bleeding off the bias rig.

Fix the board problem first. Good deal!

[Gibsonman63]

There's been a lot of soldering done to that board. You may try taking out the screws holding them down and try to prop them off of the chassis with something non-conductive.

Take a small mirror and a flashlight and have a peek under the boards. The bottom should completely insulate the top one from the chassis.

It is sometimes hard to make everything fit well in the eyeletts because Fender took the long leads from the cathode bypass capacitors and ran them through the eyelett and grounded them on the bus bar. When you replace the electrolytics, you ended up clipping that part off and having one more wire from the new one in there when you are done.

Have a look at the soldering job with good lighting and lots of magnification. It may be a good idea to suck out the solder and resolder if it is sloppy or at least reflow them. Neatness counts... usually.

I am a little confused on the DC on the volume pot. A few millivolts isn't much. The only way I see for it to get there is from the capacitors on the tone stack leacking DC from the plate voltage from the first stage (which you already changed), but I am at a loss to explain with it only happens in a specific, repeatable range.

You can tweak the range of your bias by changing out the 470 ohm bias resistor. Someone correct me if I am wrong, but I believe you go a little smaller with this one to get a higher voltage.

This amp could really sound great with the right componants. The correct resistors won't set you back much and putting some Sozos in there won't exactly break the bank. If you don't mind spending the time, take some good pics and strip it down to the boards. Then either replace the boards or clean them up really good and dry them out with a heat gun. In the long run it may be easier to really do it correctly than to keep fighting the ghosts.

These guys here are great at working with pictures. Post them if you got em.

[jezzbo]

I allready thought it was the board. Thanks for your confirmation.
Maybe I should build a new board al together? (and keep the original, or what is left of it)