Fisher X-100 from hell

[gadget73]

Depending on the humidity in the garage, that may or may not help. Ideally you want to dry it out, not just make it hot and damp. Maybe sitting outside in the sun, the black transformers will get fairly toasty that way.

 

[Bradbrews]

This time of the year just put it in a car with the windows closed parked in the sun.

 

[andy]

I have disassembled the transformer ready for the next step. Based on the number of turns on the heater windings, I'm looking at around 2000 turns for the B+ winding. I'll use my hand crack winding machine to unwind it for an exact count. From past experience, the biggest challenge will be getting the new windings to fit. I plan to use polyimide tape between layers, which should save some space compared to paper.

 

[Mr Boochie]

I have disassembled the transformer ready for the next step. Based on the number of turns on the heater windings, I'm looking at around 2000 turns for the B+ winding. I'll use my hand crack winding machine to unwind it for an exact count. From past experience, the biggest challenge will be getting the new windings to fit. I plan to use polyimide tape between layers, which should save some space compared to paper.

Good luck with the rewind. That insulation is toast!

You're probably going to need to look at the gut (read: rewind) the OPTs. too.

 

[andy]

I unwound the B+ windings (counted 1969 turns). Much of the wire had the enamel literally falling off, but there were also large sections that looked like new. The insulation between layers was very thin and fragile paper.

One curious design choice was how the center tap was done. I would have expected the center tap to have been in the middle of the 1969 turns, and the two extreme ends would connect to the rectifier, but that's not what they did. The inner and outer ends were connected together as the center tap, and the two 360v outputs came from the middle of the winding. There was a layer of thick paper between the two middle ends, but that still seems like an odd way to do it since you have 720v RMS across that central insulation. Maybe that's normal; I haven't disassembled many power transformers.

 

[mcgjohn]

Will require a bit of effort to clean it up, but cosmetically does not look too bad.

 

[kvflyer]

Almost approaching (unfortunately) a parts unit. I hate to do that but have had to part out more than one Scott amplifier and one 340B. It was just too far gone, but had a lot of good parts for other repairs.

 

[gadget73]

I'm sort of approaching the point where I almost prefer that. Lets me strip them for the transformers to build whatever I want without having to feel so bad.

 

[andy]

It started as not even good enough to be a parts unit, but sometimes the challenge of resurrecting something like this can be fun. It's like restoring a car that's been in a field for decades.

I sprayed some insulating varnish into the output transformers to hopefully fortify the insulation. If rewinding the original power transformer isn't successful, I'll probably set it aside until I find a suitable replacement.

 

[andy]

I've acquired all the materials, and I've started rewinding it. I've got 4 out of 16 layers done (123 turns per layer). So far so good, but it takes some time and care without an automatic winding machine. There isn't an extra mm left over on each layer, even when carefully aligning every turn.

Has anyone tried one of those Chinese $300 electronic winding machines on ebay? They look interesting, but I fear that poor documentation, and firmware bugs will make them useless.

 

[andy]

I've got all 1968 turns of wire on the bobbin, and there's plenty of room for the heater windings! I should know soon if it works. It wasn't that bad winding it, just time consuming (about 20 minutes per layer). Neatness was crucial. I sprayed each layer of windings with insulating varnish to hold things in place, and add some insulation, and then covered it with a layer of Kapton tape.

I can see why power transformer rewinding is expensive. First there's disassembly, and reverse engineering, which wasn't that hard since this one hadn't suffered a severe meltdown. The materials cost is quite high. I spent about $60 on magnet wire, and Kapton tape alone. I already had a spray can of insulating varnish, and one of the magnet wire sizes on hand, so I'm not counting those costs.

 

[gadget73]

Never re-wound a transformer but it doesn't surprise me that its one of those many things that seems expensive to have done until you have actually done it yourself and understand why its so expensive.

 

[andy]

The transformer is finished (I hope). It all went back together without too much difficulty. I dunked the finished bobbin in some marine varnish to try to seal it, then inserted the laminations while it was still wet. It's outputting the correct voltages, and not making any noises. I've installed it on the chassis, but not wired it up yet.

 

[gadget73]

maybe if you want an excuse to buy more stuff, get hold of a vacuum bell for varnishing. Put the whole works under vacuum to suck the air out, then release it so its more fully charged. Useful for re-oiling bronze bushings in small motors too.

 

[andy]

I put some tired old 7189's in, and a random 5R4 for testing, and after repairing a couple of broken wires, and working around a bad tube socket I have audio on both channels. There's a slight channel imbalance in the tone control circuit which is probably caused by the couplates. I also need to replace at least one tube socket since the terminals are so loose they're almost ready to fall out. So far the other sockets seem fine, but I may replace the rest depending on how things go.

 

[andy]

Finally hooked it up to speakers for the first time, and so far so good...

 

[zackthedog]

Call me impressed!

 

[crispycircuit]

Impressive!!

 

[albusark]

Thank you for sharing, very interesting. How is the transformer doing temperaturewise? I think 210-220 F is normal for these right?

 

[andy]

Thank you for sharing, very interesting. How is the transformer doing temperaturewise? I think 210-220 F is normal for these right?

So far it hasn't gotten too hot to touch. I don't expect temperature to be a problem. The materials I used should be significantly more heat resistant that it originally was. The failure I had wasn't caused by overheating, it was definitely water damage.

Today, I replaced one of the crumbling tube sockets along with several out of tolerance resistors. Amazingly, the switches and pots are working fine, and even the power switch is good. There's an obvious high water mark on the chassis. It's definitely not a an amp that should have ever worked again.