Bassman won't stay fixed!

[GregS]

I had to troubleshoot a static noise problem with a 66 Bandmaster a while ago. It turned out to be a cracked resistor - the tail resistor of the phase inverter IIRC. Amazingly it was either the first or second component I probed with a chopstick. The crack couldn't be seen by eye. I think the old carbon composition resistors can get brittle and maybe are prone to this.

You might want to probe components with a chopstick while the amp is still cool and working ok. You may find the culprit that way.

[AmpRX]

Bruce - I worked on one yesterday. A Silverface Vibrolux had 1.5v DC on channel 2's Volume pot and the pot was noisy. Lifting one leg of the 250pf cap off the board eliminated the DC and the noise. However, that does not mean that a conductive board is the solution to this problem..............

[tboy]

A Silverface Vibrolux had 1.5v DC on channel 2's Volume pot and the pot was noisy. Lifting one leg of the 250pf cap off the board eliminated the DC and the noise.

I can't help wondering where that DC was coming from, and if it's not still present on the now empty eyelet, where did it go?

[Chuck H]

Probably a leaky cap. Unless the lead that goes to the treble pot was lifted along with the cap. Did a new cap fix the problem???

[tboy]

I was assuming the possibility of a leaky cap had already been eliminated, because of the "conductive board" context, but I guess he didn't actually say that, did he? I envisioned lifting both the cap and treble pot lead and reconnecting them in the air, in order to eliminate the board as a possible source of DC. Maybe my imagination ran away with me.

[Pedro Vecino]

Does anyone see something strange in this photo?

[AmpRX]

The three caps in the tone stack were changed first. Plus I tried the cathode bypass cap just for kicks. None changed anything. There was still 1.5v DC on the Volume pot. Lifting one end of the 250pf cap (the end facing the front panel) and soldering the wire from the Volume pot directly to the cap, above the board, eliminated the DC effects on the pot. I didn't measure to see if there was any voltage on the eyelet or on the board. I just know the problem was eliminated. And I can't now that the amp is back with its owner. The only reason I tried lifting the cap was after changing all the caps, I didn't know what to do, and I remembered a friend who had worked on thousands of Fenders said that he had seen conductive boards. It was a last resort!

[J M Fahey]

Our colleague danlampton posted for the last time on July the 4th, saying he would start to measure voltages .
Any news?

[Chuck H]

Lifting one end of the 250pf cap (the end facing the front panel) and soldering the wire from the Volume pot directly to the cap, above the board, eliminated the DC effects on the pot.

Exactly as tboy envisioned... I only see one answer for this, and that's a conductive board. Now imagine what an extra volt does to the bias of a preamp tube when it happens to land there!?!

I've heard sooo many old Fender amps that sound like a$$ with issues and the owners think they sound great. Fine, good for them, but, just because the conductive circuit board issue doesn't come up much doesn't mean it isn't happening. As I said, I've seen it on three of five black fiberboard amps I've worked on. Some forums and techs report it as a moisture issue... Nonsense. The F'ing carbon somehow aligns under high heat or high voltage and forms a semi conductor. Sometimes triggered by the usual carbon build up that happens in old amps and sometimes (as in my case) from excessive resoldering and repair. After such stresses those damn things absolutely DO develope high value resistance paths between eyelets. I WILL NEVE BUILD ON ONE. That isn't to say tha there aren many amps operating just fine with this type of board. I'm sure there are. But compared to other board materials there is a disproportionate number that aren't. I'm not prone to blowing smoke up my own a$$ and my shoulders slump just like any other tech when I find out I need to replace an entire board. In fact wiring and soldering are my least favorite things about this avocation. Maybe I've just had an unusual experience with this material.

[MWJB]

Bruce wrote: "I've been doing this for many years.... I can't tell you how many hundreds and hundreds of vintage Fender amps I've worked on, where the eyelet board was defective, conductive or shorting out somewhere as others have described... because the answer is: So few that I can't remember any of them, and that's if there were any at all."

I guess the weather in Colorado, compared to drizzly, damp NW Europe might have a bearing on that :-). I've seen a handful, oddly they were all SF amps, often with the wax coated boards. Every now & then someone will send an amp my way with additional insulator boards/material sandwiched between the originals, or spacers/grommets packed between the 2 boards. Chopsticking the board, I was able to induce symptoms at will. Conductivity between the 2 boards & presumably the blank board and the chassis seemed to be the issue, rather than from eyelet to eyelet on the top board.

[AmpRX]

Yes. No solder on the lugs. Factory?

[mac1amps]

For a number of years, it's been a practice of mine to treat the component boards on the "classic" Fenders (or other amps that used fibreboard componant boards) with a [believe it or not] petroleum product (with a high "dielectric" rating) that's marketed as a "heavy-duty rust inhibitor". The solution restores pliability to the boards (preventing [further] cracking), instills moisture resistance, and restores "non-conductivity" to the boards. Yes, I know that this procedure sounds too "radical" to most of you, but having done this procedure on MANY units (Fenders and various other amps) over the years (having initially experimented with it on my own amps), with nothing but positive (and long lasting) results, it will always be part of my "repertoire"....call me eccentric if you like, but it works......BTW, this procedure also works well in preserving the paper core of old transformers.

[Danlampton]

Thanks for all the suggestions. Everything is clean under and between the boards and the boards are not conductive. I replaced every resistor that was even a little out of spec. Cleaned everything I could and re-flowed a lot of solder. I've spent way too much time playing this amp way too loud, but I think it's ready to go (I hope).

Thanks again for the thoughts and suggestions - and the stories!

[trem]

I am struggling with an AB165 right now that has what sounds like blocking distortion. This is an amp that has all new parts, but I re-used the fiber board. I have re-flowed all the joints twice, including all the ground connections. I actually installed a ground buss for the the preamp section. I still get a distortion that starts as the note peaks, and continues as the note fades.
I addressed the possible blocking distortion by raising the value of power tube grid resistors, and lowering the value of the PI input cap , and lowering the value of the power tube coupling caps. Nothing has helped get rid of this distortion. I still have a few avenues to explore. But, it makes me wonder.......How common of a problem was Tweed Disease in these black fiber boards that Fender used. How often does the problem occur.?
Thanks

Exactly as tboy envisioned... I only see one answer for this, and that's a conductive board. Now imagine what an extra volt does to the bias of a preamp tube when it happens to land there!?!

I've heard sooo many old Fender amps that sound like a$$ with issues and the owners think they sound great. Fine, good for them, but, just because the conductive circuit board issue doesn't come up much doesn't mean it isn't happening. As I said, I've seen it on three of five black fiberboard amps I've worked on. Some forums and techs report it as a moisture issue... Nonsense. The F'ing carbon somehow aligns under high heat or high voltage and forms a semi conductor. Sometimes triggered by the usual carbon build up that happens in old amps and sometimes (as in my case) from excessive resoldering and repair. After such stresses those damn things absolutely DO develope high value resistance paths between eyelets. I WILL NEVE BUILD ON ONE. That isn't to say tha there aren many amps operating just fine with this type of board. I'm sure there are. But compared to other board materials there is a disproportionate number that aren't. I'm not prone to blowing smoke up my own a$$ and my shoulders slump just like any other tech when I find out I need to replace an entire board. In fact wiring and soldering are my least favorite things about this avocation. Maybe I've just had an unusual experience with this material.

[Chuck H]

Trem, Have you tried measuring voltage on the board surface? I think it's also possible that board conduction problems may sometimes be dependant on a particular operation in the amp. As in you need higher current than the standing idle current for he problem to occur. Try floating your cathode circuits off the board. If that doesn't help try floating the downstream end of the coupling caps. If it's board conduction causing an odd bias condition on the preamp one of these should help.

FWIW I have heard distotion that sounds like you describe. I was too green to diagnose it at the time, it was twenty years ago. But it was a PCB Mesa product which. of course, doesn't have a black fiber circuit board.