fisher x 100c problem i can't fix!arhhh

[jstraw]

Ok here's the story.
I bought a scott 299 in semi rough condition off ebay.
I got sent to me a mint fisher x 100 c instead.
Although it's not as pretty as the scott i made a deal with the seller to keep this unit.I like fisher's as i also own a mint 500c.
I proceed to buy a variac and succesfully reform the caps.
It plays for five minuits and then one channel goes south distorting.
I then recap the thing with russian teflons.a pair of russian k 40 pio's and two auricaps.I also replace the under chassis electrolytics.
What i get is simply the best most detailed amp i have ever heard.
It's rated at 24 watts but it felt like 50!Great in every way.
I used it like this for 3 months then i smell something funny.Metalic.
I know something is going wrong most likely the can caps.
I keep useing it any way.I was in denial.
Then i see smoke.Yeah i know i'm a dumb ass
I then go buy 3 can caps and try to parrallel two of them for the power supply.Well you can't parrallel can caps so i get blown fuses.
I try under chassis caps.I get blown fuses.
I find two 250 uf 250 v caps to replace the 200 uf 250 v caps that were stock.
I rewire everything perfect.All new cans all wired right.I have a nice pic.
I get a blown fuse.
I seriously lost it today.
I'm getting afraid to work on it anymore.
What i'm thinking is i might of heated up the high perfrmance fred diodes i was useing.They might be dead as i had to solder and resolder them a few times trying different parts.
Would this blow a fuse or just not turn on?
Here's something else i was wondering when the can caps went south there was a wire that burned but not near the cap.
If i rememeber correctly it runs to the other under chassis electrolytic.I taped it up with electrical tape.
Does or could this indicate another problem a wire burning away from the blown can caps?
What should i do now?
I'm going to get the schematic and trace everything.Just to make sure.
I burned a few wires underneath when i was useing a less powerful soldering iron.I taped all the burns up just to be sure maybe i need to replace those wires?
The first time i fired her up i got power to the tubes at 70v.
Now i just get burned fuses.
Oh just two clarify this amp stock uses 2 200 uf 250v can caps wired in a voltage doubler circut.
The other can cap is a 40 40 20 if i remember correctly.I have a 40,40,20,20 can cap in there now.One 20 uf section is not used.
The 2, 200 uf 250v power supply can were replaced with 2, 250 uf 250v cans.Made for voltage doubler's

 

[Mike Stehr]

Maybe lift the leads of the PS transformer, and check DCR of the windings. Compare them with known specs of the Fisher PS xformer.
(Sams Photofacts or whatever.)

The PS transformer may have issues, unless there is a short to ground from a faulty part or wire.
Don't use electrical tape, use heat shrink or replace the wire.

You can test diodes with a DMM. If you measure resistance both ways, they are probably bad.

 

[audiodon]

jstraw,
i haven't seen an x-100-C. Is that a 7868 output amp or a 7591 output amp?

My first instinct is that that burned wire is leading to a shorted cap. I'd explore that at my first opportunity.

How high can you turn the amp up ACwise with the variac before a fuse blows? If you can run it up to 40VAC or so, great. If not, you've got a short so pull out your meter and compare things to the schematic in the power supply and filtering region.

What I'd do is turn the amp on with the variac just high enough to get it going and then check the voltage rails off the power supplies. So, if your high voltage rail is supposed to be 430 at 117VAC, the next one down is supposed to be 370, then 330, 295, etc, an you get some proportional level, like 200, 185, 140, 125, etc, then you know you're in the ballpark and things are probably Ok. Turn up the voltage again and try the rail voltages proportionally. I don't think you even need the tubes in while you do this part.

I am assuming this is a solid state rectified Fisher and have set up my comments accordingly. If tube rectified, you have to have the AC voltage much higher to get the rectifier tube going before you can try anything.

 

[Tom Bavis]

Check the diodes with an ohmmeter. Zero Ohms is what I predict...

If you can power it up with one side of the transformer HV disconnected, at least you know the TRANSFORMER isn't shorted...

 

[jstraw]

Ok here's what i'm going to do.1replace the burned wire.all of them.
2 check the dioeds.I have replacements on hand.
I did look at the wire that originaly burned and found that two burned right next to each other.They lead from a 40 uf cap to a point thaT CONNECTS TO THE OUTPUT TRANSFORMERS( sry caps).
Originaly the cap that failed was a 200 uf/40 uf cap.
The two 200 uf sections were tied together in a voltage doubler circut. The 40 uf section was joined by a resistor to the two red wires that burned.That went to the output transformers.
I can't replace these wires.
I have a separate 47 uf cap underneath for the 40 uf section now.
I was not sure if i could use one of the tabs of the 200 uf can cap to ground the 47 uf axial cap with.I soldered the negative side to the chassis.
This would cause no problems?
I'm just wondering trying to eliminate anything that was done that was not perfectly stock.

 

[jstraw]

Oh the x 100c is not a tube rectified amp.
Its a 7868 output tube amp.
I can't seem to put any power into it before i pop the fuse.I start at 60 hz.
The amp calls for a 3.2 amp fuse.
I don't have this.I have 3 amp and 4 amp slow blows from radio shack.
The last time it blew with a 3 in there.

 

[dsndblm]

Maybe a few pictures of the repaired and problem areas would help out. Can you also scan and post the schematic?

 

[Fisherdude]

As a general rule, never start a restoration effort without a schematic, unless you've done this particular piece of equipment so many times you have the circuit layout memorized.

Based on your posts, I think there's probably a 90%+ chance you have made a wiring error. There is also a chance you have damaged diodes, and possibly a short between the wires that have melted insulation. Any of these mistakes could cause a fuse to blow.

Trying to replace capacitors that are part of a voltage doubler network can get confusing, especially without a schematic. Replacing can caps with single caps below the chassis also carries a risk of making a wiring mistake, especially without a schematic.

I would suggest not doing anything else until you have time to check your wiring against the schematic.

 

[jstraw]

I have the schematic and a very good pic of a stock 100-c.
That's the thing i have gone over it connection by connection,part by part it's wired correctly.
So i'm thinking diodes.I have read fred dioed are very heat sensitive and i have soldered and unsoldered them 5 times.Even with a heat sink i might of blown one but would that blow a fuse?Or would the unit just not work?
I have some 4007'S i think from another project.Maybe even two fred's.
I'm wondering if i might as well swap out the wire now to but i remember from my car tuneing days make one change at a time or you will get lost.
So i think tommorow i check the diodes with my dvm.As a first step.
I'll post the pic tommorow schematic too.

 

[quicksand]

Ignore the highlighted areas - I have been working on a post but just haven't gotten around to it.

Call this the before photo and schematic.

 

[jstraw]

I found the problem!!! I was searching that 40 uf wire burn and i just kept following it.I have what looks like a 330 ohm 2 watt resistor and the resistor 1k under it touching.All over where they were touching is a white residue which looks like bakeing soda.
So i found the problem.
Thanks all.i might post a pic tommorow just to show you.The bad part is one of the resistors is on pin 6 of my 7868's.
I bet i hurt that tube.