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Think this PT is FRIED?

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11/28/2005 1:18 PM

Ferdinand The Bull

Think this PT is FRIED?

The guy said "the amp you fixed is blowing fuses."

Let's see, 0 ohms from primary to one of the secondaries, 0 ohms from primary to all CTs, 0 ohms from primary to 6v winding. I'm no expert, but that looks bad.

The full story is, I had this amp on the bench a few months ago. It had evidence of flamed rectifier diodes, then a hacked-in RadioShack rectifier. I replaced the section with 1000V diodes, recapped, and biased the Sovtek 6L6's. I left it on for several hours, playing it occasionally. Beautiful. I told him about the evidence of fire, and to keep an eye on the PT--if it seems really hot. Two months later he called and said it's still beautiful.

Now I have it back on the bench with a "slightly irregular" PT, and TWO ODDBALL POWER TUBES! One is an RCA, one is an Ampeg/Sylvania with a suspicious looking frosty top. Who knows if they were matched, and I doubt that this guy bothered to re-bias.

I'm wondering if the PT was already doomed, or if a mismatched/misbiased/faulty power tube could have been the kiss of death.

11/28/2005 5:48 PM

I wouldn't rule out the tranny just yet. Take the load off of the secondaries and then check them to see if you get voltage. Sometimes bad components can cause the transformer to load down and act bad when it's really good. The transformer windings are nothing but a coil of wire so you won't read much resistance across it. Just the resistance of the wire. The frosted tube is bad as it cracked and let the gas out. You still may be ok. Try RG's GEO site as he has some great info on troubleshooting trannys.,

KB

11/29/2005 9:10 AM
Ferdinand The Bull	Thanks for the reply KB. I'm measuring under 50ohms from primaries to chassis. Isn't that fried tranny?

11/29/2005 12:53 PM

Ferdinand The Bull

Aaaarrrrgggg!!

Here's some more info:

I lifted all the secondary leads (HV, heater, bias), I lifted the center taps (HV, heater), and I lifted the strange white ground wire (on schemat it looks like tranny core?). I hardwired the AC (fused) to the primaries. I applied power for a minute and it seemed OK--around 750VAC across the secondaries.

Then I reconnected the center taps and the white ground wire, and it was back to popping fuses.

This amp is strange (Gibson Mercury II). It has a 3A mains fuse in back on the AC neutral, then another 2A mains fuse in front (the one that pops) on the AC hot (following the switch).

My next move was to bypass the power switch and the 2A fuse--hardwiring the hot wire to the primary. Then I plugged in and what do you know, the 3A fuse did NOT pop, but the tranny started buzzing and smoking.

center taps, 2A fuse, white wire, smoke, #*@&!!

11/29/2005 3:50 PM
Jag	Why would you EVER bypass a fuse?!?!??!?

11/29/2005 4:27 PM

Ferdinand The Bull

Hey Jag,

If you read back, you'll see that there are two mains fuses--one for AC hot, one for AC neutral. Have you ever seen this before? I was trying to rule out all possibilities, including the (redundant?) fuse, and the crusty rotary switch. The second fuse is NOT on the schematic. It seemed safe to have only one mains fuse, like most amps.

I don't know if the 2nd fuse was a modification, and I don't know why it would pop a 2A fuse (hot), and not the 3A fuse (cold).

My somewhat crude troubleshooting knowledge has led me to the conclusion that there was a short in the primary. When the CT was lifted, the tranny functioned. When the CT was connected to chassis, it shorted the primary to ground.

Once again I'll ask: Isn't 30ohms from primary to chassis a sure sign of fried tranny?

11/29/2005 5:53 PM
Enzo	With all windings disconnected and lifted, there should be NO continuity from any winding to the frame. That odd wire probably IS a frame connection for shielding purposes.

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