'74 Fender Silverface Twin Reverb Project (w/MV)

[Basslice]

Hello there. I am new to this forum. I recently acquired a '74 Silverface Twin Reverb [**EDIT I know the title says '73. Some pots have '73 codes, but it may well have left the factory in '74]**. The guy I bought it from had apparently plugged it in and the power lamp lit and took pics for his CL sale. That was not a great idea, but he probably didn't know that powering up an old amp (and one with no speakers) was a bad idea.Upon delivery, it was in pretty poor shape and has seen some time enjoying nature. The cab is solid - tolex good and no wood rot. Speakers had been removed. Ditto no reverb tank. Some mice had moved in but were recently evicted. A good amount of surface rust on exposed metal parts. A cat or some other clawed creature used the grill cloth for practice. I shaved off the loose loops. Here it is before I opened it up. [**Edit The amp was missing the power tubes, but all the other tubes are there and many if not all look to be original RCAs **]

PXL_20220224_134918082.jpg

I removed the chassis and it was rather disappointing inside. The PT looked like a problem and was rusty. The MV was frozen. All other pots were ok. There are surface rust spots on the chassis here and there. Was this thing under water? (the cab itself has zero water damage). Anyway, the real problem were the boards. It looked like someone had coated everything with candle wax (still can't figure that out). More rust. One of the screen resistors was in two pieces. However, no obvious signs of major failures. Pot codes say this is a '73 build.

1974 Silverface Twin Picture with damage 2.jpg

PXL_20220225_170428696.jpg

I pulled the PT and put in on my Variac. Surprisingly, it was all working to spec. First happy moment since opening this thing up. Also checked the OT and that was fine too. I decided to remove the board and replace (same with the rectifier board and the cap board). I am going to strip the old board and test the individual components and try to reuse as much as I can.

Currently waiting for parts. I am cleaning up the rusty things and dirt. May repaint some surfaces. I am new to this forum, but I have rebuilt several old Ampeg bass amps, but this is my first old Fender.

Questions:
Any advice for speaker replacements?
Any advice on the frozen MV?

[Ten Over]

It might be difficult to find a MV with a tap and with a switch like that. I would try to dissect the thing to free it up.

Fender Twin 100 1974 Schematic.png

Fender Twin 100 1974 Layout.png

[martin manning]

Any advice on the frozen MV?

Put a drop of penetrating oil in the shaft bushing and let it soak overnight. Dioxit might work. Gently apply torque. If it won't free up, repeat.

[Basslice]

Any advice on the frozen MV?

Put a drop of penetrating oil in the shaft bushing and let it soak overnight. Dioxit might work. Gently apply torque. If it won't free up, repeat.

Ya. Yesterday I dumped some deoxit on the shaft, followed by some contact cleaner oil. I took it out of the chassis and with two sets of pliers was able to get it to turn full swing. I twisted it back and forth a bunch, but still need pliers to turn it. It doesn't seem to push in or out if that is what it is supposed to do. Not familiar with this type of pot.

[Basslice]

It might be difficult to find a MV with a tap and with a switch like that. I would try to dissect the thing to free it up.

Fender Twin 100 1974 Schematic.png
Fender Twin 100 1974 Layout.png

OMG Ten Over! I have been looking for a '74 layout and schematic for days! Lots of great info out there, but what you just shared is perfect!

As described above, I was able to get the MV to turn, but it is hella stiff. I may just set it on full and leave it alone. We will see. I have a lot of other things to work on in this beast before I cross that bridge!

[Ten Over]

Some people remove the MV switch circuit or perpetually leave it pushed in. It doesn't seem to be well liked. If you remove the circuit, then you can use a regular 1M audio taper pot for the Master Volume.

[Basslice]

I removed all the caps from the contaminated board. I was wondering if I could get some opinions on which ones to keep and reuse when the new boards arrive. Here are the representative examples after wax removal.

PXL_20220301_231547139~2.jpg

The camera is dodgy. The blue caps (B) are deep navy blue, not black.

Thanks.

Ken

[ampdan]

During that time period the boards came dipped in wax from the factory. Every one I've ever worked on was like that.
It makes for some interesting smells when removing components...

[Basslice]

During that time period the boards came dipped in wax from the factory. Every one I've ever worked on was like that.
It makes for some interesting smells when removing components...

Ya. but for me, the gross contamination of the waxed parts and board have created a problem. I would love to trust things as is, but it looks so dodgie that I feel compelled to correct. The whole board was warped and rusty.

[Basslice]

During that time period the boards came dipped in wax from the factory. Every one I've ever worked on was like that.
It makes for some interesting smells when removing components...

I am allergic to flux fumes, but even when masked I can smell the wax!

[johnnyreece]

Honestly? Were it mine, I'd replace all those caps. I don't believe any of those are supposed to have any special mojo, anyway. Given the condition it was in, I'd feel a lot more comfortable with fresh parts.

[Ten Over]

I removed all the caps from the contaminated board. I was wondering if I could get some opinions on which ones to keep and reuse when the new boards arrive. Here are the representative examples after wax removal.
PXL_20220301_231547139~2.jpg

The camera is dodgy. The blue caps (B) are deep navy blue, not black.

Thanks.

Ken

E and F must be replaced along with ALL of the other electrolytic capacitors. A through D are up to you.

[Basslice]

UPDATE:
After removing the boards and any components I might reuse (few), I am all in. I was able to free up the MV pot and I am gonna keep it.

MW pot.jpg

I looked at all the rust and just couldn't see the point in putting my time into this if the result was a rust-bucket.

Rust Never Slppes.jpg

I broke out the old Dremel and went at it with a flatwheel (80). It did a job on the rust, especially on the power supply cover (not shown). I then cleaned up the rest of the chassis as best I could. I will hit it with some grit brushes tonight and finish by hand.

Rust-B-Gone 2.jpg

Rust-B-Gone 1.jpg

I will repaint the PT in engine block black and the chassis with Flat Antique Nickel while I am waiting for new components. What did I order?

• Replacing all the resistors with metal films (carbon film? Why would I?)

• Upping the wattage of the screen and grids

• all electrolytics

• New ceramics for the tremolo circuit

• About the only thing I am keeping are some signal caps that tested out ok

So far things are ok money-wise, but once I throw down for new 100W 12" speakers I will have spent enough that I will be maybe at break even if I bother to sell it (not counting my time ). But heck, $$ was not the point to begin with. I am having fun and would rather have a 40 year old amp saved from the dump. As I get older, I really appreciate older things. I was 10 when this thing was built. I will say, whoever assembled this thing probably did not keep his job for long. WHAT A MESS!
I have been collecting documentation of '74 Silverface w/MV. There is a hole out there for these things. I also have been compiling a new BOM spreadsheet I will share.

Oh, I still need a reverb tank as well.... Updates to continue.

[bepone]

I removed all the caps from the contaminated board. I was wondering if I could get some opinions on which ones to keep and reuse when the new boards arrive. Here are the representative examples after wax removal.
PXL_20220301_231547139~2.jpg

to preserve the tone as much as possible, keep the tone stack original if the caps are good, specially ceramic treble cap. how you are progressing to the end, better are new capacitors (specially to the output tubes, if the old one fail, can bourn also the output section too - old output transformer primary wire per example)

[bepone]

I broke out the old Dremel and went at it with a flatwheel (80). It did a job on the rust, especially on the power supply cover (not shown). I then cleaned up the rest of the chassis as best I could. I will hit it with some grit brushes tonight and finish by hand.

as you are here you can go to the end.. remove all from the chassis, clean it , put the new sockets and do the total refresh, new wires and everything. restoration. you will be satisfied with the results at the end, now you are just on half