Fender Amps - Circuit Board Conductance

[GCastor]

Has anyone ever actually seen a fender circuit board collect moisture and become conductive be the problem of hissing and popping noises, or is it "almost" always a different problem. Has anyone not solved a background noise problem by doing everything imaginable, then changed the board out and it was gone?

I have heard of people saying they have heard of this problem but that is about it.

Thanks!

[GCastor]

*If so how common is this

[BTF]

I can only point to one Princeton amp which I believe had a conductive circuit board, but the owner sold it rather than have me service it so I am to this day unsure. It had an unusual low frequency feedback problem much like inter-lead pickup, but the wiring was neat and tidy. Still I only had about twenty minutes with it counting chassis removal.

For what it's worth, ex-Fender engineer/amp designer Paul Rivera mentioned the problem of conductive circuit boards some years ago in an article in Guitar Player (he was a technical reviewer then). I can't recall the nature of the article, but part of it concerned tagboards absorbing moisture and becoming conductive.

Little help, I know. Regards, BTF.

[Structo]

I've heard of it as well but have not worked on one where this is suspect.

From what I read, you can sometimes fix it by replacing the backer board.

If you have a meter, see if you can measure any resistance on the board itself between two points.
The meter should not show anything, but it may not be sensitive enough to measure small amounts of conductivity.

Not sure about the top board, I've heard of guys using wire in some spots to reduce the problem.

I wonder if cleaning the board with denatured alcohol would alleviate any of it?
Of course always try something in a small spot before going whole hog in case it damages the board.

But with any tube amp the first suspects should be the tubes.

If you have other known good tubes, substitute a good tube for one of the same kind in the amp and give it a listen.
Do this to all tubes if you can.

I saved the best for last.
Many times the hissing, crackling and popping is the carbon composition resistors. They absorb moisture too and and it results in exactly what you describe.

Now you could replace them with NOS Bradley's, or go modern and replace them with carbon film and or metal film (no noise but no carbon mojo if you believe the stories).

If this amp has not been used in a long time and has been stored where the humidity is high, it most likely needs a cap job, a three prong cord installed if it doesn't already have one and removal of the death cap.


Hope this helps,
Tom

[GCastor]

In the my previous post the owner described it as hissing and popping noises, if the board is the culprit, would isolating the circuit board from the mounting bolts (standoffs maybe??) be a possible solution?

[GCastor]

This amp will be a potential purchase and i dont have access to it so i am trying to gather as much info to make an educated decision on whether to buy it or not. The seller had modded it to aa864 specs on the normal channel and replaced any faulty components in the process. He rattled off that he put in "sprauge atom caps, carbon comps, and orange drops" though he hasnt told me specifically which ones. He mentioned he put some new carbon comps in so i have not ruled this out. I may just have to buy it and see if i can figure it out with the always great help of this forum. Just trying to see how likely the board conductance thing is since this seems like the most work to fix.

Thanks!

[Gibsonman63]

I have a modest collection of blackface fenders and this has not been a problem for me. I have read about it in some of the other forums, though. From what it understand it is caused by the boards absorbing moisture and the cure is to carefully heat it with a hair dryer to evaporate the moisture.

FWIW, the cracking problem in a couple of my old Fenders was solved by replacing the 470 ohm resistors soldered to the power tube sockets with 2W metal film resistors.

[Firestorm]

I've actually seen one where you could measure DC voltage off the board itself, but it didn't exhibit any noticeable problems. It was necessary to put the probe fairly close to the voltage source (plate resistors, IIRC) to get a significant reading. I suppose it depends on what is conducting to what. I think if you find an offending section of a board, you ought to be able to remove some components and heat the eyelets with your soldering iron to drive off any moisture.

You mention that is was converted to AA864, so I'm guessing it was a Silverface. A lot of these had the boards coated in wax to prevent moisture absorption.

Hissing and popping (that isn't tube related) is usually 1) preamp plate resistors (mostly 1/2 watt types); 2) screen resistors (as gibsonman63 says) since these are located so they "cook"; and 3) dropping resistors in the power supply (these usually hiss, rather than pop).

If the price is reasonable, it's probably worth acquiring as it doesn't sound like a transformer problem. It would be a shame iif it had blue Mallory caps that got replaced with ODs, though.

[BTF]

I must agree with the other posters in that none of the Fender amps I've owned have ever exhibited any evidence of this problem. Still that's not to say a conductive circuit board isn't the problem.

If it helps, I think that I once saw a service bulletin wherein a recommendation was to strip the board and carefully bake it in an oven at low temperature for a few hours. I'd have to try and find the article, but I believe that was the fix. You'd have to be very careful as, just with a cake, you could easily overdo it.

One worrisome problem is that the problem might be something more, such as a power transformer which is shorting or a faulty choke. My '64 AA864 popped a bit when I got it, but new plate resistors made it quiet as a mouse. Perhaps if you're lucky the owner or tech used the original plate resistors and their faulty.

Bood Luck, BTF.

[ElectronAvalanche]

Hi!

I have the same sad problem with a VibroVerb Clone I built in 2005. The chassis and boards came from Natchanis (or so).

The amp had a ticking on the tremolo from the start. I had the amp stored for some time in the basement and fired it up recently. The ticking was much worse, the Reverb pot had DC and low and behold I was able to measure between 0.4Vs from the eyeletboard to ground and close to the anode resistors even some hefty 10Vs to ground.

Grrrrr. I really hate these Fender boards and them absorbing moisture. I reckon that many Fender amps in the day did this, but I can not live with this.

So I will have to rip out the board and start all new with a PCB material board. I know Hoffmann offers replica PCB eyelet boards but not for the B+ board under the dog house. Anyone have a source for these?

Thanks in advance,

Electron

[brentm]

Not sure if this helps, but I did purchase a filter cap board from these guys and was very happy with the product.

http://www.turretboards.com/TURRET_EYEL ... ACITOR.htm

[ElectronAvalanche]

Hi Brent,

thanks for the link!

Just what I need!

Cheers,

Electron

[David Root]

GCastor, I don't understand why you are considering buying this amp without seeing it and checking it out yourself. Risky business......

If it were me, as if that weren't enough reason, putting orange drops in a vintage Fender is not exactly the work of a knowledgeable, professional tech.

Leo didn't use orange drops in any AA864. Caveat Emptor bigtime!

[Richie]

I couldn't count how many fender amps i have worked on over the years,but i have come across this maybe 4 or 5 times.
Not much of a cure for it if you find one. But sometimes if its only happening in one spot,you can lift the resistor or cap from the board,and use a method that isolates that connection.
Silicone or maybe hot glue or maybe a mounted terminal to get the connection off the board. Other than that,you have to replace the board.

And sometimes the symptoms will only be for a few minutes,then the crackling or pops may quit. Or it can do it all the time. But first make sure it not something else causing the problem. CC resistors,dirty tube pins or sockets and many things can make a similar sound or problem.

If it is conductive,you can measure it. Finding the spot or section, you can lightly drag a small screwdriver across or around sections,the louder you hear the static,the closer you are to the problem section.

I picked up a deluxe reverb that looked fairly good from a few feet away. But once i got into it,it is the worst amp i have ever seen. I was hoping it might be repaired,but nothing is left.
Looks like it was left under a leaky roof. The cab is rotten,the speaker rotted,the reverb tank had to be cut out of the bag,and its trash.
The chassis is rusted insde to where the one side has a hole in it! And green mold fungus crap all over the inside of the chassis. So its to the trash can it goes.

[jjman]

My '71 DR had crackling and I could affect the crackling by pressing in the area where the 5 components/leads connect for the grid of V4A. Replacing the solder did not fix it. Either it was moisture or maybe some solder was almost reaching the mounting screw that is very close to that eyelet. I lifted all 5 and used the mounting screw to secure a standoff which now keeps that connection off the board. It looks stupid but no more crackle. This board does not have the wax coating on it whereas my '74 VC does have the wax.