'65 Twin Reverb R-Issue B+ noise problem

[Ampsmith]

I have a '65 Twin Reverb Re-issue in the shop that makes a lot of noise.

I traced the source down to the B+ rail having low frequency noise on it. If I move the negative side of the first electrolytic filter cap (actually it is the two 220 uf stacked caps with the 220K equilizing resistors) so that it is grounded to the metal chassis and not to the point on the circuit board then it behaves.

I've checked the integrity of the grounds and it just seems like it is a weird ground loop/bad ground scheme noise. I've also replaced the rectifiers in the bridge and the 220uf filter caps. It's bizzaro to be sure.

Any help from experienced techs welcome. I'm not a newby but this one has me stumped.

Thanks,

Mike

[MWJB]

Are the PI & preamp filters on the same ground buss? Does the noise diminish when all the vol & tone pots on both channels are turned up?

[Ampsmith]

Are the PI & preamp filters on the same ground buss? Does the noise diminish when all the vol & tone pots on both channels are turned up?

Thanks for the reply! I can actually pull all the tubes and still have the fluctuating B+ noise present. The filters appear to be working and you can see the progressive decrease in the supply ripple with each stage of filtering but the noise in question does not also decrease which pointed me towards some weird modulation of the ground buss with noise. 30 + years as a tech, this is a new one!

Thanks,
Mike

[MWJB]

You're absolutely sure the noise is coming out of the speaker? Not from the PT or any other components rattling?

[Ampsmith]

I'm sorry I should explain that I initially heard the noise through the speaker which was the customer's complaint. From that point on I've been tracing it with the scope. At this point the amp is on the bench with the power tubes pulled and at one point I had all the tubes pulled so it is definitely a power supply/grounding buss issue. I was hoping someone might have come across this before. I called another amp tech in the area, a friend of mine even though we are "competitors", and he has the same amp on the bench which similiar problems...and is baffled as well! So we might even be looking at a factory defect of some sort.

Mike

[MWJB]

Can you hear the noise now (with tubes pulled dummy load)?

What condition was the amp in when you last heard the noise?

[Ampsmith]

It is not a mechanical noise. The last time I heard it was when the speaker was connected ;-)

The noise is electrical in nature as per my first post.

Thanks,
Mike

[MWJB]

You actually claimed to hear the noise with all the tubes pulled, see post of 3:41.

So...Are the PI & preamp filters on the same ground buss? Does the noise diminish when all the vol & tone pots on both channels are turned up?

The reason I asked these is because you often get noise if a common ground is used for all the filter caps. Sometimes grounding the main B+ (usually the screen node cap too) to a seperate ground cures the problem.

[Ampsmith]

You actually claimed to hear the noise with all the tubes pulled, see post of 3:41.

So...Are the PI & preamp filters on the same ground buss? Does the noise diminish when all the vol & tone pots on both channels are turned up?

The reason I asked these is because you often get noise if a common ground is used for all the filter caps. Sometimes grounding the main B+ (usually the screen node cap too) to a seperate ground cures the problem.

I think you may have misread my post:

"I can actually pull all the tubes and still have the fluctuating B+ noise present. The filters appear to be working and you can see the progressive decrease in the supply ripple with each stage of filtering but the noise in question does not also decrease which pointed me towards some weird modulation of the ground buss with noise." Present visually on the scope since I also made visual reference to supply ripple as well.

I do realize the issues with improper grounding. If that is the case then we are talking about a Fender design issue. The other tech I mentioned in an earlier post is going to call Fender later to see if they can shed any light. I'll let you know what, if anything, that turns up.

Thanks again,

Mike

[Mark Black]

Howdy -

First off I haven't had to attack that particular problem on that particular model, but a look at the schematic shows an awful lot of what I would assume from other late-model Fender experience to be fast-on connectors in the grounding circuit of the power supply filter section. CP17 through CP20 specifically, with CP15 & CP16 also being involved in the standby switch part of things.

Those connectors could be suffering from slight oxidation at the wire crimp (seen that before) or solder joint cracking at the PCB-mounted mating terminal (seen that plenty too).

Just an idea...

[Ampsmith]

Howdy -

First off I haven't had to attack that particular problem on that particular model, but a look at the schematic shows an awful lot of what I would assume from other late-model Fender experience to be fast-on connectors in the grounding circuit of the power supply filter section. CP17 through CP20 specifically, with CP15 & CP16 also being involved in the standby switch part of things.

Those connectors could be suffering from slight oxidation at the wire crimp (seen that before) or solder joint cracking at the PCB-mounted mating terminal (seen that plenty too).

Just an idea...

Thanks Mark, I have already resoldered the fast-ons and the connectors look good and have measured with the meter to make sure I had good continuity on all the wire runs etc. The search continues.

Mike

[jrfrond]

I just had one of these on my bench, so it's fresh in my head.

OK, it seems to me as if you have already solved the problem by establishing a chassis ground on the B+ filter caps, which is SUPPOSED to be at chassis ground potential anyway. You replaced the caps, diodes, soldered the Faston's, yadda yadda yadda.

Somewhere, you have resistance in that ground that is causing motorboating. The series resistance is making it look like a partially open cap.

When you can't lower the drawbridge, raising the moat can be just as effective. Just run a separate chassis ground to that point. There are no electrical, quality or safety issues with doing this, and the repair will be done and gone. Yes, I know it's not factory. But, do you want to be right, or do you want to finish the repair? It's that simple.