1960 Vibrasonic Tweed Disease?

[Voxbox]

Hi,

I was about to replace the transformers in a 1960 Vibrasonic after waiting over a month for them to arrive in the UK from Mercury. (nothing to do with Mercury, just the christmas post).
The amp has been sitting in my outside workshop all this time whilst the weather has been incredibly wet all the time. I wouldn't say the workshop is bone dry but maybe a 1 on the dampness scale of 1-10 at the moment, just due to moisture in the air.

The amp was working on the whole before, with some intermittent noise and oscillations which I was going to fix once I'd installed the transformers (the ones in the amp weren't original or correct for the amp).
However, now the amp is acting up, and without going into a lot of detail about the symptoms, (mostly dc voltages are incorrect around V3 and V4) the main thing I've found which really has to be fixed is the presence of low DC voltages all over the fibre board. These voltages get higher the closer the probe gets to the B+ on the right hand side where it connects to the board. The voltages are mostly around 1 to 2 volts dc.
A really weird one is the tag on the board where one of the grids for the PI connects. At this point, there are only three things connected to the tag: a 0.1uF cap, a 1M resistor and the wire to the grid. There is nothing connected under the board at all - I looked. If the three wires are removed from the tag and reconnected mid-air with no connection to the tag, there is 12 volts on the tag, which is just dc as looked at it with the scope. There is no connection I can determine visually or with a ohmmeter to anything else. The board is also microphonic.

I can only deduce that the fibre board has become conductive due to being in the damp atmosphere in my workshop. Doh.....
At the moment the chassis is in the drying cupboard (dunno if there are such things in the US! its where the hot water tank is), so it will be getting gently heated and hopefully dry out.

My big question is: can it be dried out, and can it be prevented from happening again?

I saw a video on youtube where a guy just replaces the fibreboard under the tag board to reduce tweed disease, but I doubt that will help in this instance.

I really don't want to rebuild the circuit boards as it will cost my customer dearly, probably about 4-8 hours work? He's already paid out for this amp and the only thing original is the chassis and the cabinet. And the valve bases....and three pots. And the knobs.

Any help would be great!!

Many thanks y'all..

Voxbox

[PRR]

Don't store amps outside. Treat them like your children.

It may take days or months to dry the boards. (About as long as it took to get damp.)

> I wouldn't say the workshop is bone dry but maybe a 1 on the dampness scale of 1-10

Cold humidity is tricky. I wouldn't say it is real damp here today, but the hygrometer in the garage says 86% RH. And the garage is a few degrees above the outside temp (freezing), so a shed off the ground would be colder and thus the RH of the same air would be higher, easily 90%.

And the day/night warm/cold cycles work the damp into the boards.

[sluckey]

I really don't want to rebuild the circuit boards as it will cost my customer dearly, probably about 4-8 hours work? He's already paid out for this amp and the only thing original is the chassis and the cabinet.

It got messed up while in your possession. You know the right thing to do.

[Willabe]

You might try a hair dryer but don't go to hot to roast the parts especially any filter caps. Take the screws out that hold the 2 boards down to the chassis and put a spacer in between both boards and the bottom board and the chassis so air can flow on all sides of the 2 boards.

I've read where guys have baked the boards at a very low temperature (125deg.?) oven for an hour or so. But you still have to take everything off of them. I never did see any report back if the boards went bad again but that doesn't mean that they didn't go bad again.

Once that type of board goes bad it's probable a flip of a coin if you can get them dried out how long it will/would take for them to absorb enough moisture and go bad again.

I had a BF SR for years and towards the end of my ownership of it I had started working on amps. So I was doing something on the amp (I don't remember what) and I had the boards unscrewed from the chassis And I did a test.

                    WARNING-DANGER-DON"T DO THIS!!! WARNING-DANGER-DON"T DO THIS!!!WARNING-DANGER-DON"T DO THIS!!!

   Anyway, I had my guitar plugged in laying on the bench to the side of the amp and I took an insulated screw driver and I hit the strings, let them ring with the guitars volume full up and the amp was on about 4. As the strings rang out I started to screw in the middle board screw and it was very clear that tone and even some volume thinned out. Yet the amp worked fine and sounded the same to me as it always did. It was not easy to get a set of boards back then if at all and I sold it not long after that. And yes I told the buyer and he didn't care as he thought it sounded fine and it did.

I had played through a good number of BF SR in the blues clubs and if anything mine was a bit fatter sounding than the others I played through. I always thought it was because I had alnico 10" Jensen's in mine instead of ceramic speakers but who knows.

It got messed up while in your possession. You know the right thing to do.

I think he's right.


                          Brad          

[Voxbox]

Thanks folks. If something goes wrong on the bench then I always fix it for no charge, no worries there. It was duff when it came in, and got worse whilst it was here, so you're right, I'll foot the bill for that element of the job, the board rebuild, that is. The cost of the rebuild is nothing compared to the value of the relationship with my customer and my conscience!

However, I'm trying to determine the best course of action to repair it.

What I don't know is if the change in the board is reversible or not. The amp was showing signs of tweed disease before now with the sporadic oscillations, board micro phonics and noise, though that could be a number of things.

It's become one of those "get the head down, get it going and get it out the door" jobs. Even if it does dry out, the South of England is a very damp place to be at the moment and has been for quite some time, I've never seen anything like it. So the fault symptoms would probably recur.

Template for a 1960 Vibralux circuit board anyone? I've got one of Dougs drilling jigs.

Love Willabe's test though! sounds like you can tune amp "mojo" 

[sluckey]

turretboards.com has 5G13 and 6G13A boards.

[Voxbox]

Slight update. After gently cooking in the airing cupboard for over 24 hours, all voltages are good and there are next to no stray voltage readings. The amp sounds loads better, very quiet indeed, no odd oscillations either, and the vibrato is now working again. Its back again for another 24 hours gentle cooking.

I think its a candidate for fibreglass boards though, so I'll take that it up with the owner.

Thanks for all your help!

Cheers, VB

BTW, Thanks for the link Sluckey, however the link to Watts Tube Audio has now disappeared?

[sluckey]

the link to Watts Tube Audio has now disappeared?

Doug doesn't allow linking to that site.

[Stankfut]

I don't know PRR, I sometimes I would like to store my kids outside.....

[Voxbox]

  Never did my kids any harm....

Yep, its going to be a board-replace with fibreglass boards and turrets.

Let you know how it goes, it's been a bit of a saga this one.

[PRR]

OK, in this case... treat them like the children of someone who is paying you to treat them well.

My dad denies making me lay under a Ford in the snow for a weekend (replacing an oil-pump). Actually I think it did me good. (At least it taught me not to lay under a Ford in the snow for a weekend.)

[Willabe]

Actually I think it did me good. (At least it taught me not to lay under a Ford in the snow for a weekend.)

     

[Voxbox]

Well I ordered some blank boards from Doug as a back-up to getting them here in the UK and guess what? Only took 5 days to arrive - that's from North Carolina to Hampshire in England. Bloomin' heck! That's fast!!
Good service Sir Doug. 

That's my weekend sorted. Now just got to tell the missus....

That must have been some oil-pump, PRR. How many fingers do you have left? And the amp is getting plenty of TLC and warm fuzzies.