Restoring a badly abused '67 Bandmaster Chassis

[mort]

Hey folks, it's been a while since I've been here. Thought I would start a thread showing what I'm up to with this thing. I already have a couple questions.

An online friend sold me this wreck for pretty cheap. Gonna build up a 1x12 combo cab for it once I get the circuit straightened out and mate it up with an EV SRO 12" 8 ohm speaker. First order of business was to break it down to pieces and get the chassis itself cleaned up. Here's what it looked like when I started:





Pretty bad eh?? That tag board was ROUGH. Looks like the thing has definitely seen some water. While I had it apart I checked the capacitance of every cap and all were within spec except for the dual caps which are rated 25/25 and each read 38 or real close. Given that they were consistent I don't suspect a problem with them(or should I?).

Luckily another online friend sent me a stash of tag boards and one of them was an exact match. It cam already loaded with resistors, a photo-resistor, and a pair of 25/25 duals, which also had the same readings as the duals from the original board.

Also, it was missing the two 60uf filter caps and had a 20uf cap mounted outside the doghouse on a terminal rail. I got rid of it and bought a couple of proper caps to go back on there:




Here's how it came out after doing some rust removal and putting it back together. After a few days the rust started coming back in force(visible on the top edge in this pic) so I had to disassemble it again and do another round of rust removal. This time I used phosphoric acid, a baking soda wash, and final rinse.

Here's a pic before the final rust removal:




Anyway, so I got it back together today for the second time and did a quick power up and after about a minute or so (no tubes) it blew the fuse. I tried another fuse and pop, instantly. I unplugged it and didn't have any time to go further into it before heading to work.

Looking for any good resource material on testing the OT and/or PT. Also I will be going through it to make sure I didn't miswire anything, but I feel pretty confident that it is wired correctly.

I have a little experience but am no pro. I've built a couple amps from scratch and had good luck with minimal problems, and I've been able to sort out a couple of repairs on other Fender amps but it has been a while. I need to get my head back into this hobby.

Please comment, share useful resources/advice, or just chime in with whatever. I'll update as I go along. Thanks

[olddawg]

Build yourself a light bulb limiter, float the secondaries on the transformer and turn it on. If you don't blow a fuse or the light doesn't glow bright mote than likely your PT is good and you are drawing current after the secondaries. If you left any old filter caps (no matter what they measured) in it that would be my first suspect. If you have a variac with an amp meter use that instead of the lightbulb of course.

[mort]

By "float" you mean to unattach them and isolate them from any conductors, correct?

And should I do this with all 5 secondary wires? or leave the 6.3v leads attached...

And yeah, I'll build the limiter. Thanks for the reminder.

[olddawg]

By "float" you mean to unattach them and isolate them from any conductors, correct?

And should I do this with all 5 secondary wires? or leave the 6.3v leads attached...

And yeah, I'll build the limiter. Thanks for the reminder.

Yes float them all.

[JHow]

Nice job on the clean-up. I'm sure the gurus will help you get this sorted out. I'd probably go at it like a first-time start up since you had to do so much work. If you don't know this link, have a look:

Paul Ruby Amplifiers

[mort]

Great link JHow! Thanks for that



I took a few photos of how the amp looks now (I will celebrate my victories as I ponder my challenges )



So far I've only put in 2 new filter caps(they were missing before). I would have bought all of them in one shot but I saw just the ones I was missing on ebay for super cheap so I just bought them. Once I get into some more troubleshooting I'll attempt to make only one parts order but I'm sure it will end up being more than just one...


Looks alot better now huh!









I have noticed that the rectifier circuit isn't quite like it is on the schem/layout. Looks like it's missing a pair of diodes. I happen to have some left over from a previous project.




And these are the caps that I was talking about that are wildly out of spec but are very consistent with each other(and others that I have on hand). Each output is supposed to be 25uf but all 4 are ~38uf (the black dual caps with red ends). Should they be replaced?

[olddawg]

Your cap checker most likely does not provide high enough voltage to test your caps for leakage nor will it reliably measure ESR. Most shops don't. I had access to one we used to call the "grenade launcher" because caps had a tendancy to explode under test) You are not building a museum piece restoration. Save yourself a major headache and replace all electrolytic power supply and bias caps. Also buy your caps from a well known (Antique Electronics, Mouser, Ampwares, etc) distributer. You have no idea if the stuff on eBay is a Sino-fake and you certainly do not want or need NOS caps.

[mort]

Just a sidenote about Ampwares, the first parts order I ever made was with them. After a couple weeks I began to worry. Tried contacting them but never could get 'em, and couldn't leave a message either. I also tried emailing them. After several months with no parts and no refund, paypal finally decided to return my funds because they couldn't reach the vendor either. I never ever heard from in any shape or form, other than the automated email that confirmed that they had my cash.

The moral of the story is, don't use paypal for general online purchases and don't buy from Ampwares

[sgelectric]

Mort, Check the polarity on the two 60uf/350volt filter caps you installed.
They should be wired in series, not in parallel as they seem to be in the picture.
If you reverse the one closest to the OT,things should be fine. While you're in there also check the 220k voltage equalizing resistors.

SG

[Mandopicker]

Good Eye, SG!

[mort]

Mort, Check the polarity on the two 60uf/350volt filter caps you installed.
They should be wired in series, not in parallel as they seem to be in the picture.
If you reverse the one closest to the OT,things should be fine. While you're in there also check the 220k voltage equalizing resistors.

SG

You are absolutely correct. Dummy mistake #1 (and counting, I'm sure)

I set the filter cap straight and tried another power up sans tubes. It held out for several seconds but still popped the fuse.

Also, the PT checks good when lifting the secondaries. I removed the power section and checked its comopnents which all check as good but I think I'm gonna build another one anyway.

I have the other filter caps on order so I'm just gonna put this thing on hold until they get here.

Thanks for the help so far guys

[sgelectric]

Mort,
As Olddawg said, be sure to buy your caps from a reputable source.
The visible date code on the 60uf/350volt cap you replaced indicates the cap was made in 1973.

[mort]

You guys warning against buying on eBay have given sound advice. Turns out one of the 60uf Spragues is completely open.

I'm on the way right now to an electronics store to get a few things. I don't think they have exactly what I need for the 60uf caps(some schematics show 70uf). Can I use a slightly higher/lower uf so I can continue troubleshooting the fuse blowing problem until the proper caps can be ordered and delivered?


Edit: nevermind, they don't have anything near the voltage spec anyway.

[52 Bill]

It's possible that the cap was damaged when it was wired into the circuit backwards.

[mort]

It's possible that the cap was damaged when it was wired into the circuit backwards.

It was actually the other 60uf cap that I found open. Perhaps it still could have damaged it.


I'm in the process of rebuilding the entire power supply, filtering sections, and neg voltage bias supply(while studying up on the fundamentals of them). I figure that's probably the best place to start, and then work my way through the rest of the circuits.