So I rebuilt the can caps for my latest 800c. I stuffed a 220uf in the black can on top as I showed the other day. The 2 other cans I built on small boards and fit them under the chassis. Today I decided to power it up and had a bad time.:no:

I began with the 7591 tubes not installed. As I brought the variac up to about 80 volts, all looked good and voltages were where they should be, a bit below the specified voltages. Dial lights were glowing dimly, heaters on the small tubes were glowing too. So I began to ramp it slowly up to 100 to check the readings again when poof. Power went off on the variac. There was a small puff of smoke from in the chassis. The fuse in the variac was blown, but not the fuse in the Fisher.
I went over my work again, and all caps have the - tied together, none are backwards. I can't find where the problem was at all.
I tried one more time with the output tubes in this time and it blew both fuses before I got to 80v. Looks like the smoke might have come from the thermal resistor slow start device (CL-80A).
I am going to have another set of eyes check my work, but is there a chance I did damage to the transformer, or does that take time?
Hope I did not toast this thing!:sigh:
John

I have not seen the 800C, but does it have one of those power electrolytics that is not supposed to be grounded from chassis to can? Some tube amplifiers use a floating ground circuit that is not the same as chassis ground. Just a thought. Do you have a schematic for that unit?

Some pictures would be most helpful for us. Like you said, another set of eyes (or more) maybe?

Never attempt to bring up a Fisher without all of tubes in place. The power supply requires the load, it will get damaged without it. Remove the thermistor from the circuit for now. You are stressing it and it may have to be replaced. Here is a good place to point out that it does get hot in normal operatin and needs to be in open air with nothing near it.

I had several similar experiences. Actually as failures go it is a pretty nice one. It probably should not take to much time to locate and fix. Since you were able to bring it up to voltage first we need to look for an insulation breakdown.

I have have OPT's fail like this. When I opened the bell's there was a obvious arc. The solution was "fish paper", a very tough insulation plastic sheet. Far superior to the cardboard originally used. I once pinched the wire supplying the bulbs when I put the bottom on. It came up normally but after a little operating time the pinch rubbed through and blew the fuse.

In your case I would take the ohm meter to your stuffed cans. It is a likely bet that a hot wire has found the side of one of the cans. If that is not the case, examine your work carefully for a mechanical cause. Look for arching and use the ohmmeter to identify the shorted circuit.

If your main fuse is not blown you either have to high a value or have miswired the AC circuit. If the fuse in your Variac is of lower value and blew first then you have too small a Variac or have too small a fuse in it. Put the proper size fuse in the Variac. I often test new refurbs with a 1/2 size fuse in the unit.

Two things immediately come to mind.

First, I don't know exactly how you used the JB Weld on that can you stuffed, but isn't that stuff conductive?? Wouldn't that cause conductivity to the chassis? (if that's how it was used)

Second, I noticed there's a burned area on the white pvc jacket for the wire (with the larger yellow plastic jacket around it) going to the bias cap board you made. Is the wire exposed at the burn area, or melted and touching another wire and/or conducting somewhere in that general area? Although it may not appear to be touching anything, it may flop down onto the chassis after the unit is inverted right side up.

. . Falcon

Thanks guys. The JB weld was used on the can cap that it floated on an insulated base. That one should be OK. The fuse in my variac was a 7A fuse, which has me kind of scared. It is a 7.5A variac.
I will check that wire with the yellow jacket to see if there is a problem there. It has an insulated core with a fine wire covered by the yellow. I think a solid wire there might be ok in it's place.
Should I open the power transformer bell and look for a problem there? When checking the new caps for shorts, I assume I will need to remove the connections to them and check for a short. What should a cap read if not shorted? Man this has me bummed! John

This is where good step by step troubleshooting skills come in handy. First rule: only change one variable at a time. How often this occur? Almost never, most of us panic and shotgun, making things confusing and usually worse.

I would unsolder the power transformer and dangle the leads in the air. If you have EH tubes be sure to take them out or put a piece of wood under the OPT's so that the tubes are not touching when you have the unit upside down (guess how I learned that one). Plug it into the Variac bring it up to about 30 volts and clip your voltmeter (on AC of course) across each of the windings. Each should read something within reason (20% or so of the sections DC output) and the Variac should show no current consumption (assuming yours has a ammeter). If the fuse blows or it fails the above then the fault is in the transformer or wiring to the plug.

Next, unplug it and put an ohmmeter to the downstream connections. The readings will be low, even when right. Put the ohmmeter on its lowest setting and touch the probes to get a "0" reading. It usually is 0.2 to 0.6 ohms. Then check the legs. I'll bet one of them is reading "0".

By the way, I love Shoe Goo or Goop as it is now known. Better than JB weld in this case. Non-conductive, extremely strong but can always be pulled apart.

Case in point. I am sitting here with my daughter's laptop in pieces. It wasn't charging. So I immediately dismantled the whole laptop assuming it was the usual power jack solder joint problem.

It wasn't, so I took a look at the wallwart power supply. The rf coil had a broken leg. Total time to fix, about 5 minutes.

Total time to reassemble the laptop, about 45 minutes. Will I ever learn?

OK drew, I will try. I just did one more test. I lifted the ground leads on the 2 multi section caps. I very slowly applied AC, and noticed the ammeter on the variac was coming up. At about 20v AC it blew the 2a fuse on the fisher. I think it is the transformer. If I check resistance of the main AC leads of the power transformer (2 brown leads) what should this read? I have the fuse out and the power switch in the off position so I think I am just reading the transformer. I am getting 1.4 ohms, which leads me to believe there is a short in there...crap!

I am getting 1.4 ohms, which leads me to believe there is a short in there...crap!

Yes, that is a low reading. Isolate the transformer leads and see if you get the same reading. If so, remove the transformer and take off the bell ends. Often you will see the tell-tale arc mark on inside of the bell. These Fishers had very tight bell ends. Often it is just a matter of removing wire pinches and insulating the bell ends correctly.

By isolate you mean to lift all the leads from the transformer where they connect to the terminal strip? Is there any hope the transformer is salvageable? Then I have to figure out why this happened. John

By isolate you mean to lift all the leads from the transformer where they connect to the terminal strip? Is there any hope the transformer is salvageable? Then I have to figure out why this happened. John

Yes, exactly we are trying to confirm that it is the transformer proper. Yes, they are very often salvageable. Let's isolate the trouble first.

OK. I pulled the transformer out and removed the endbells. There is a copper wrap around everything, I slid that off. Now there is a waxy looking brown paper wrap. It looks as if all the wires are under that layer. Do I slice the paper and look inside? I checked resistance across the 2 brown AC leads, still reading 1.3- 1.4. Across the red to red/yellow is 4.9 ohm. Across the blue and white it is even less, .5 and as I keep the meter on it goes down to .2 after 30 seconds or so. Same across the yellow to green. Down to .2 as well. Not sure what to check now. Thanks, John

OK. I pulled the transformer out and removed the endbells. There is a copper wrap around everything, I slid that off. Now there is a waxy looking brown paper wrap. It looks as if all the wires are under that layer. Do I slice the paper and look inside? I checked resistance across the 2 brown AC leads, still reading 1.3- 1.4. Across the red to red/yellow is 4.9 ohm. Across the blue and white it is even less, .5 and as I keep the meter on it goes down to .2 after 30 seconds or so. Same across the yellow to green. Down to .2 as well. Not sure what to check now. Thanks, John

Ouch! that ain't right. You can get some pretty low reading from windings maybe 15 ohms or so. Also it is normal for them to fluctuate due to the inductance and iron saturating. Is there any evidence of arching on the copper (I assume it is what I call fishpaper, copper colored plastic with a visible weave pattern)? With several windings showing short I am beginning to believe you have a general insulation meltdown.


A little history. Is the first time you have fired up the unit? If the transformer worked before we need to check the circuits each winding was going to on the terminal strips. We need to see what section caused this.

Maybe he could just try connecting the PS transformer unloaded (with secondary and heater windings, negative bias, unhooked) to the variac, and ramp it up and see if the variac blows it's own fuse and at what voltage.

If there is a winding or two shorted, wouldn't it blow the variac fuse whether the PS xformer is under load or not?

I think I found a possible problem. The 2 diodes I replaced. One seems shorted and gives a reading on my DVM in both directions. The other seems normal. I had 2 1n4140's in my parts box and assumed they were right for this amp. Perhaps I fried one when I ramped up the variac. I have not tested the tranny with ac on the variac yet. It is on the bench, so I will try later, if you all think that is a good idea. I unwrapped the paper and see no burns or signs of smoke. I should check my other 800c to see what I get checking the primary on that one. Thanks all, John

Another thought. I was thinking about putting a 1A fuse in the variac for testing. I doubt I would pull that much when just checking to see if I get voltage on the other leads. John

I think the transformer is alive and well. I hooked up a plug to the input leads of the transformer. I put a 1A fuse in the variac and brought it up slowly. Here are my readings at 30VAC.
Red/red-yel wire = 47.7v
Green/Yellow= 1.5v
Blue/White =1.5v
Rectifier leads Brown/Orange =6.8v

So I think this is a good sign.

I looked up the specs on the diodes I had used. They are only rated to 100v so I think this is where the problem was. As soon as I reached their limit, one blew and one shorted. Every test after that was asking for a fuse to blow. I picked up some 1n4007 and other assorted diodes tonight. First I may as well paint the transformer since it is out. Keep your fingers crossed. John

Sloober,
There was advice earlier in this thread about one cap that needs to be isolated from the chassis ground. On my 500-C, that value is 200 microfarad and it has a paper cover. Make sure that your replacement cap is wired properly.

Maybe I missed something in a different thread, or I just keep skipping over it...did this receiver work before you did the work? Because if it did, the chances of you having blown the power transformer by turning it on after you changed the caps is near zero.

Yes the 200uF cap is isolated. I was able to put it inside the old can. I then shrink wrapped it and it is isolated on the top of the chassis.
When I got the receiver it was working, but only on one channel. One of the output tubes was blown and it blew the 10 ohm resistor for measuring bias current. Since I had to use new output tubes (eh7591a) I decided to swap the 330k resistors and .47uF coupling caps. Wanting to be through, I also decided to rebuild the power supply caps. I am confident now that the transformer is ok, but since I had it out, I am painting it. The problem I have found was the diodes I used. They were not rated for a high enough voltage. As I ramped up the variac, one blew and one shorted, blowing the fuse. I can only hope all the caps I just did are still ok. I think they will be. I hope to have it ready to power up again in a few days, after the paint dries. John

Yes the 200uF cap is isolated. I was able to put it inside the old can. I then shrink wrapped it and it is isolated on the top of the chassis.
When I got the receiver it was working, but only on one channel. One of the output tubes was blown and it blew the 10 ohm resistor for measuring bias current. Since I had to use new output tubes (eh7591a) I decided to swap the 330k resistors and .47uF coupling caps. Wanting to be through, I also decided to rebuild the power supply caps. I am confident now that the transformer is ok, but since I had it out, I am painting it. The problem I have found was the diodes I used. They were not rated for a high enough voltage. As I ramped up the variac, one blew and one shorted, blowing the fuse. I can only hope all the caps I just did are still ok. I think they will be. I hope to have it ready to power up again in a few days, after the paint dries. John

I am always amazed at how a simple mistake can drive us nuts. This thread reminds me of my shorting out the new diodes in my 340B by not clipping the lead.

Reminds me of when I installed 10K resistors to the output of my first 800c. Ge I am getting sound, but it is extremely weak.
I just grabbed these diodes because they looked good and beefy. They can handle the amps, but not the volts. Lesson learned.
John

It bothers me that you blew the fuse in your Variac. Do you have an ammeter on it? Is it a 7.5 amp fuse? Is it slow or fast blow? What fuse was in the Fisher? Again, amperage and slow or fast blow. The Fisher should have been a 3 amp fast blow. For fireup I will often use a 1.5 amp fast blow.

Your ohmage readings on the transformer as suspicious. I agree that the transformer is probably ok. I just think it is worth a look at your meter. You need test equipment you can trust.

By the way you can blow a transformer with what you did. The chances are good, not zero. The Variac probably saved it. No output tubes with shorted diodes are a sure way to internally arc a power transformer secondary.

You might want to consider baking out the transformer while you are painting it. I put the freshly painted and disassembled transformer in a very low (200 degrees F) oven for several hours. Stinks like the dickens and will not help with WAF. It will help set the paint and remove any trapped moisture. Don't use high temp paint (barbecue or engine), it will never harden. Also consider getting new plastic washers. I found some nylon washers that worked at my local ACE hardware. These help reduce the buzz that many find annoying at low volume levels.

The fuse on the variac is a fast blow type. I think the Fisher amp fuse was a slo-blow. I hope the meter is ok, it is not great, but it seems to be decent quality. It is a Sperry DM-4100A. What I can do is test the ohm readings on my other 800c. I was pretty sure this tranny was ok as testing did not blow the fuse and I was getting voltage out on all winds. No way I can put it in an oven, wife would surely kick me out. I have it under a lamp and it is getting nice and warm.
You say plastic washers, between the tranny and chassis? There were not any there before, do I need them? I have seen them in the hardware store, I may even have a few. John

I just opened my other 800c and checked the resistance on the tranny. It too settles down to 1.4 after about 30 seconds. It starts around 6 but quickly drops then slows way down. Going from 1.6 to 1.5 took about 10 seconds, to 1.4 took longer. I think someone mentioned this is inductance of the coil etc. I think I am good. Thanks, John

I just opened my other 800c and checked the resistance on the tranny. It too settles down to 1.4 after about 30 seconds. It starts around 6 but quickly drops then slows way down. Going from 1.6 to 1.5 took about 10 seconds, to 1.4 took longer. I think someone mentioned this is inductance of the coil etc. I think I am good. Thanks, John

It's not the primary winding I am concerned about. You took readings of what I assume are dead shorts on the secondaries. I am assuming 0.2 ohms is what your meter reads as a dead short. Or was that 0.2 ohms above dead short (0.6 - 0.4 shorted lead reading = 0.2)? About all you can tell on a transformer with a VOM is dead short and wide open, anything else is non-conclusive.

It is still worth trying. Just be forewarned that when insulation breaks down it can look ok voltage wise until it is under load.

If you have added a soft start thermistor (CL-80) then use a 2 1/2 amp fast blow on the Fisher 500c in normal usage.

I will see tonight when I hook it back up. Keeping my fingers crossed. John

Sloober,
As a thought, what about taking the toaster oven out into the garage and bake the transformer in it there?
Don

Sloober,
As a thought, what about taking the toaster oven out into the garage and bake the transformer in it there?
Don

Naw, just send her out Christmas shopping. Total cost to bake out the transformer = $1000000000000000000000000000

Yah, aside from WAF, the wife acceptance factor, in our home we used to have the TRI factor.
With the TRIfactor, the positioning of any piece of audio gear, and most importantly top heavy speakers on stands, had to pass the test of not falling when a kid's tricycle's back wheel caught on the piece when being ridden at full speed. If the gear fell, it was my fault. Then again, I live in a house full of girls.

But hey, I've got a garage, a workbench and my wife is trained as an engineer! Trifecta!

I don't think I can get it in the slots of the toaster.

Good news, it is playing just fine. It was the diodes for sure. I have it up to 117v and it sounds wonderful. Thanks for the help from everyone. John

Good news, it is playing just fine. It was the diodes for sure. I have it up to 117v and it sounds wonderful. Thanks for the help from everyone. John

Great. :banana: