X-100-3 Restoration tips

[Mike52787]

Hey guys, I got an X-100-3 in the mail yesterday, and am wondering how far I should go with a recap/restore. When I got it, the large black 100uf filter was shorted, and the closest thing I have on hand was a 150uf lytic from some old plasma television power supply I had in my parts bin. When I get a chance to hit up my local shop I will get a proper replacement for that. I also replaced the 4 coupling caps on the output tubes with some poly caps I had lying around. I haven't touched any of the rest of the circuit, and have heard its better if I leave it that way. The metal can lytics seem to be fine, after hours of use they aren't getting hot.

On another note, the grid voltage seems to be a bit low on the output tubes. The fisher schematic specifies 18v, and I am measuring about 11.7 to 12. Possibly some resistors drifted due to excessive heat?

 

[Mike52787]

OK... something just happened and my head is spinning with rage. I was using this amp, and all of a sudden... pop. one channel now has very very low gain and some 120hz hum in stereo mode, but is fine in mono. I pulled the output tubes and tested the OPTs, they seem fine. swapped all the tubes... no change. Tomorrow I'll be changing the rest of the original caps... hopefully that has some affect on the current condition. Any tips on what the hell to do to fix this would be incredibly useful.

 

[Tim D]

With any luck the part that popped might be something easily replaceable like a resistor or cap. I'd power it off until you get it sorted to avoid further damage.

You'd need to power it down and open it up and looked for smoked/burnt parts, but that would only be the victim. The culprit could be a lot of things and probably is several together. Do you want to fix it yourself?

 

[Mike52787]

Already opened it and did some poking around, nothing looks burnt. I was terrified that it may be the output transformer, but I checked the primary and secondaries and they are the same as the working channel. so that pretty much rules that out as the issue (thankfully)

 

[Mike52787]

Looking at it one more time, I noticed a burnt resistor connecting to the tone control amplifier 12ax7. That should be my issue (atleast part of it)

 

[Tim D]

Good that the power transformers seem good. You can start by replacing that, but you’ll need either a dim bulb tester or a variac or both to gently power it up next time. If the power supply caps are original and you have the original rectifiers then I’d recommend replacing those too before putting any power on it again.

 

[Tim D]

Lotsa threads about the Fisher integrated amps. You might consider reading up and getting a schematic for your model.

 

[Mike52787]

The main filter has been replaced as it was shorted when I first got the amp, however the other two multicans seem fine. I'll dig into replacing the diodes next. And yes, I have been powering it up on a lightbulb when working on it.

I did find a schematic for my unit, but havent really dug into it yet. Will definitely prove useful when trying to figure out what caused my fault though.

 

[Tim D]

Checking on the voltages is a good idea once you get far enough to bring it up on power. Low voltages should be addressed and might indicate too much current draw. Start with the various voltages at the power supply.

Note that there are some seriously dangerous high voltages in tube equipment. If you aren't confident you can do it safely enlist someone that is. Also, read up on high voltage safety in tube equipment. Be careful.

 

[Mike52787]

I do have some experience working on tube gear, so I'm confident I can tackle this myself. I'm going to stop by my local shop, and hopefully pick up some parts to continue working on this amplifier. I will update once all new coupling caps and filter caps are installed, and the damaged resistor has been replaced.

 

[Mike52787]

Fixed it!
Apparently recapping the power supply was all it took.

 

[Tim D]

Good job! Any thoughts of going further?

I’d go through it and replace any other electrolytics and the rectifiers. Switches and controls might use a good cleaning and checking the tubes for any that are having trouble might be a good idea.

 

[Mike52787]

Doesnt seem like there are any other electrolytics in the unit, I did replace all the original wax coupling caps with polys, and the only others are disc. The diodes seem fine, not really sure what can go wrong with them.
I cleaned the pots and checked all the tubes, just waiting on a matched quad of russian 7189s from ebay. The main preamp tubes are the original mullards, and the phono/tape preamps / tone control tubes are RCA 5751s.