70’s Twin with conductive board

[Mark]

Hi, thanks for looking at my post.


I have a 70’s Twin on the bench with a conductive board.


To put it into perspective, the previous tech took the tone control caps out of the eyelets attached the wire to them and covered the joint with heat shrink.


I measured this empty eyelet and noticed that there is between 20VDC and 30VDC on the six unused eyelets.


I have had a little experience with conductive boards before and I tried removing components from my 6G7a Bandmaster, I tried cleaning and even soaking the board in alcohol to remove foreign matter. I then dried the board in an oven used for fault finding for eight hours and the board was still conductive. This board wasn’t as bad as the board in the Twin.


I’m wondering if there are any pearls of wisdom in regards to dealing with this particular issue?


Regards


Mark

[sluckey]

I would replace the board.

[Mark]

The first shot is the Twin came out of the factory with conductive board issues.


The next two shots are the voltage on empty eyelets. I haven’t seen voltage on eyelets to this extent.


Regards



Mark

[Mark]

I would replace the board.

I know we are on the Hoffman site, but is there any particular board it should be replaced with?


I don’t want to use fibreboard again as the last boards I bought were conductive from new. The boards need to be tested with an insulation tester after manufacturing.


Garolite is my friend generally.


Regards


Mark

[tdvt]

That is crazy high leakage.

I am pretty much in a similar boat to you. I have 2 '79 UL Pro Reverbs, one waxier than the other, both had/have noise that was somewhat beyond tolerable.

I rebuilt the first, keeping/cleaning the stock board but gutted it in the end, as even after the rebuild, there was intermittent noise that I couldn't fix.


I made a new eyelet board out of some .062 Bakelite/Garolite but have not yet finished wiring it yet.  The thinner material is plenty sturdy but is not the fiberglass composition so I would probably not use it for a main board again.

McMaster Carr sells the G4 in several thicknesses & Doug has eyelets for .125 stock &/or turrets (+ his G4 blanks), so you have some options.

I know Lyle the Psionic guy has stripped them, soaked them in isopropyl & repopulated, but after dealing with a few of these, I feel like that typically warped fiberboard is a lost cause & better to put the effort into something that will hold up.   


I am holding off starting the second one until the first one is finished.

[sluckey]

IMO, replacing that conductive board is the only option for having an amp that you have confidence in.
The first questions I have are... Do you own this amp? How much effort/time are you willing to put into this amp? Do you want a Twin Reverb? Do you have skills and tools to make your own board? Would you consider a Dumble conversion?

AA Electronics (TAG member) makes a nice PCB for this amp. Hoffman sells an AB763 board that uses a bias vary trem circuit. Or, he can make an AB763 TR board using the original Fender layout.

What would I do??? I'd remove the conductive board and completely strip it. Then I'd tape it to a blank 1/8" fiberglass board that Doug sells. The old board simply acts as a drill template. Drill it and load eyelets. Repeat for bias and filter cap boards. This is very time consuming so you would have to love the process. The end product is worth it IMO.

[Mark]

That is crazy high leakage.

I am pretty much in a similar boat to you. I have 2 '79 UL Pro Reverbs, one waxier than the other, both had/have noise that was somewhat beyond tolerable.

I rebuilt the first, keeping/cleaning the stock board but gutted it in the end, as even after the rebuild, there was intermittent noise that I couldn't fix.


I made a new eyelet board out of some .062 Bakelite/Garolite but have not yet finished wiring it yet.  The thinner material is plenty sturdy but is not the fiberglass composition so I would probably not use it for a main board again.

McMaster Carr sells the G4 in several thicknesses & Doug has eyelets for .125 stock &/or turrets (+ his G4 blanks), so you have some options.

I know Lyle the Psionic guy has stripped them, soaked them in isopropyl & repopulated, but after dealing with a few of these, I feel like that typically warped fiberboard is a lost cause & better to put the effort into something that will hold up.   


I am holding off starting the second one until the first one is finished.

I went done the soaking in alcohol path with my Bandmaster 6G7a, the oven would have sorted out the alcohol and any water in the board. In the case of the 6G7a board it’s not bad enough for me to get too worried about yet.


I did order new fiberboards for it thinking they would be okay till they got humidity in them. I put the rail voltage of amp on one eyelet and measured around the other eyelets and they had voltage on them. I notified the seller and asked him if he tested the boards with megger. He said no, and I was left with a useless set of boards.


Regards


Mark

[shooter]

tested the boards with megger

 
that's a tool not many a folk have.

[tdvt]

I was left with a useless set of boards.

That is indeed a bummer.

If you are into keeping it "stock", I think Sluckey's suggestion of using the old board as a drill template for some G4 & moving everyone over, is the most cost & time-effective.

I, for better or worse, was sucked down the rabbit hole & heavily modified the layout, switched over to radial caps, ditched the overdrive circuit, got rid of all the spaghetti, added dwell, etc.

[mresistor]

I'd just buy a Hoffman AB763 TR eyelet board and convert it.

[Mark]

I was left with a useless set of boards.

That is indeed a bummer.

If you are into keeping it "stock", I think Sluckey's suggestion of using the old board as a drill template for some G4 & moving everyone over, is the most cost & time-effective.

I, for better or worse, was sucked down the rabbit hole & heavily modified the layout, switched over to radial caps, ditched the overdrive circuit, got rid of all the spaghetti, added dwell, etc.

The main factor will be what the customer can pay. Replacing the board is now out of the question. I will have to improvise in order to get the amp functioning. I may have to use tag strip to lift high voltage off the board.


Regards


Mark

[Mark]

I'd just buy a Hoffman AB763 TR eyelet board and convert it.

That is a good approach, but I will still have to have a master volume and working out a function for the switch on the master volume.


Regards


Mark

[tdvt]

That is a good approach, but I will still have to have a master volume and working out a function for the switch on the master volume.

I would suggest, changing the "Master" to a LarMar master, which uses a different pot (ganged) & the switch would go away (along with the distortion circuit)

I hadn't done one before, but I recently put a LarMar in a Bassman & it is actually useful.

[stratomaster]

I'd at least attempt a deep clean before rebuilding.  Remove all the pots from the front panel and unsolder the ground connections to the brass plate and the chassis.  Also loosen the bias pot.

This will allow you to flip the board up towards the tubes and access the underside.  Use a heat gun to melt the largest chunks of wax. Since the board is now vertical, the wax will just run right off.  Catch it with paper towels.

Follow with naphtha to remove the smaller amounts more strategically.  Only after these steps use the alcohol for drying only.  I use 99% as it absorbs more water before evaporation than 70% and 91%.

You will need lots of paper towels, cotton swabs, and a few detail paint brushes.

You don't need to strip the whole board.  I would just do the eyelets where DC power node connections are attached - and the eyelets the plate resistors span. 

[NewYorker]

I was going to make some comments and ask some questions, but the ground (subject matter, that is) I was about to cover has been covered before:

https://el34world.com/Forum/index.php?topic=10120.0

This was so long ago, Radio Shack apparently was still in business.   

Ed

[Mark]

I was going to make some comments and ask some questions, but the ground (subject matter, that is) I was about to cover has been covered before:

https://el34world.com/Forum/index.php?topic=10120.0

This was so long ago, Radio Shack apparently was still in business.   

Ed

Thanks for all the replies, I appreciate you taking time to help me.


Regards


Mark