Repair Advice needed...'64 blackface bassman

[KellyBass]

Greetings,

I could use a bit of "service tech" advice. I just build amps, I don't repair them...well, just my stupid mistakes. You service techs have my utmost respect.

Here's the deal...one of my customers dropped off an amp he just bought for $60.00. A 1964 Fender bassman. I kid you not. He bought it from a guy who bought it in about '67 for $60.00. He didn't think it was right to ask any more. Unbelievable, but true. He wants to start playing this thing and it looks like it has very little use. He wants me to look it over. Out of curiosity, I went ahead and accepted the amp. I'd never been inside one of these!

I dropped the chassis to find an amp that looks like it's never been touched. So, I built and installed a beautiful filter cap board using G10, Sprague atoms and 3 watt carbon comps....saving the old cap board intact. I replaced the 25/25's in the preamp and the 25/50 on the bias board with atoms as well.

On close inspection of the PT wiring, I was very disappointed to discover that the filament CT wire was completely fried...and almost took the HT CT with it...they were twisted together. The HT wire is still intact and hangin' in there. I plan to shrink tube it as far down into the bell as I can and make sure there is no further damage to it. The heater CT is gone....as far as I can see.

My question is: what should be my next move? Should I just replace the PT on this completely original classic?. Or, can I pop the bells off the PT and inspect? Can I get away with making a CT?...grounding a couple of 100ohm resistors? Do I need to? Is this safe?

I Really don't want to be "that guy" who ruins a beautiful amp like this by making the wrong move.

All advice is greatly appreciated. Thanks!

kelly

[collinsamps]

My 2 cents.....

Not an uncommon thing at all. I've seen similar cases where the 100 ohm filament resistors are replaces with 2, 3 or 5 watt power resistors that have the same effect on the PT when something shorts. The Schumacher PT's are unusually stout and usually can be repaired with extended leads.

I would try to resurrect the the CT if there is anything left by removing the end bell and Ohming the windings out to ensure that it's not shorted to other windings, adding extensions, and then powering it up on a variac or light bulb current limiter to check voltages out of the circuit, or tubeless at a minimum.

Of course replacing it is easy as well, but just like the fiberboard cap assy that you removed, had you reused it it would retain the amps value more than the G-10.

In addition to the cap job that you've already done, I would also change the plate load resistors which are sure to give you some snap, crackle & pop if they are original CC's. Of course, cleaning pots & retensioning socket pins etc.

[KellyBass]

Thanks very much for the sound advice, Mr. Collins. I will pop the bells and hopefully I will have a little bit of lead to work with. At any rate, I will use my limiter and bring it up nice and slow.

I was really torn between making a new board and keeping the original fiber board intact, or re-using the original fiber board. The fiber board with all those cardboard covered caps seemed too cool to mess with. I probably should have sourced some fiber board and used it instead of the g10. Is this stuff available?

Thanks also for reminding me about the plate resistors...I probably would have missed them.

Kelly

[Firestorm]

The fiber board with all those cardboard covered caps seemed too cool to mess with.

If you're obsessive (like me) you can put the cardboard sleeves over new caps and it will look like it just left the factory. The sleeves fit current manufacture ATOMs; other caps, not necessarily.

[trane34]

if for some reason the pt repair doesn't work out you can send it into mercury magnetics to have it restored. just bringing up the option in case it hadn't already crossed your mind.

[KellyBass]

Using the old sleeves is a great idea! I hadn't thought about that. Atoms are what I used.

I had NO idea that MM offered such a service. I will definitely consider this option if the PT doesn't measure up during my first slow power up. Great suggestions, guys. Thanks!!

[rdjones]

There are a couple other transformer shops that offer a rewinding service as well.
Prices are steep compared to typical replacements other than Mercury.

rd

[KellyBass]

Well, I popped the bells on the tranny and here's the ugliness. The filament CT, as you can see, is barely hangin' in there. So, fellas...what do you think?

There's still some wire there to tie on to, but barely. It got hot in there, without a doubt, but all the other leads look fixable. Why would a CT do this?

Thanks!

[billyz]

You will need to remove the copper shield to get at the heart of it and find good wire and insulation. You'll need a big soldering gun . Maybe even a jewelers torch. Most likely a bad tube tube filament and someone put in a large Amperage fuse or tin foil.

[Structo]

Yeah, that is going to take a lot of heat.

Another option, though not as nice as unsoldering it is to use a Dremel with a cutoff wheel and try to cut through the solder.
I use that trick when taking humbucker covers off of pickups.

Remember that the heater center tap is not that critical to operation since you can use a couple 100R resistors to create a artificial center tap which in most cases is better balanced than the center tap was.

But, you do need to trace that center tap back as far a possible to ensure it isn't shorting something.

[billyz]

Yeah, that is going to take a lot of heat.

Another option, though not as nice as unsoldering it is to use a Dremel with a cutoff wheel and try to cut through the solder.
I use that trick when taking humbucker covers off of pickups.


But, you do need to trace that center tap back as far a possible to ensure it isn't shorting something.

You could use a dremel to cut it off, but you'll need even more heat to put it back on than to take it off , in my experience.

One thing I do like about the 100r resistors is, if not too large they act as a fuse.

[martin manning]

Notice that one of the other heater leads is fried and the other one isn't. That says that the fried side of the heater winding shorted to ground somewhere outside the PT, and that entire half of the winding over-heated. I'm thinking it's toast. PT's arent thought to contain much mojo, so you could replace it without much loss of value. It could be rebuilt, which would pretty much amount to putting the old bells on a new core.

[KellyBass]

Thanks! It sounds like I'm not "dead in the water". I Think I might have access to just the soldering iron and assistance, for this operation. I know an ol' school car body guy who hates bondo. He uses an iron thatmelts that big solder like it's butter.

I can see that the CT wire is good coming out the other side...so the short's gotta be in that first layer. I'll give it my best shot. I've made plenty CT's using the 2 100ohm resistors...but I've always wondered why I've seen 1/4 watt resistors being used for this. Now I know. That WOULD make a good fuse!

I just saw your post before I submitted mine, Martin. Yes, I was worried about that, too. The isulation is bubbled pretty good. I'm HOPING that this is due to the fact that everything was sort of bundled coming out of the bell...and the closest wire to the filament CT was the PT CT. It's insulation is fried big time. The next wire in line was that one side of the filament pair. My hope is that it's a "proximity" melt and nothing more. I'll give it a real close look. Good catch, Martin!

[Jana]

+1 what Martin said. Something has happened in the past to overheat the windings. I wouldn't trust this PT even if you can get it to work. I would replace it and save the old one just so you have all the original parts.

[KellyBass]

Thanks to all who voiced their (well respected) opinions concerning my PT dilemna. My coustomer has decided to go with the replacement from Mercury Magnetics. Cool! I guess he figures he got off pretty well on the purchase price of the amp (uh...yeah), so he could afford a little more $ for the (good) replacement.

BTW, Jana...I really enjoyed your build thread! I've been practicing my "lacing"...and I'm getting pretty good. Very cool technique...

Thanks again