Paramour Voltage problem

[Aural Robert]

One of my Paramour monoblocks has been making a crackling sound on startup which stops one the circuit is fully powered (say ~ 30 seconds). A few days ago, this very same one started playing a LOT quieter.

So... I've looked under the hood and ran some diagnostics and discovered that my voltages on 2 terminals are off by 100VDC - T5 and T15, which is where the plate loading choke also connects. I read 100V less than the manual - 295VDC on T5, and 280VDC on T15. Also I've discovered it's ok when I first turn it on, but within 2 minutes the voltages have dropped.

I'm really quite illiterate when it comes to the theory of how this stuff all works. Therefore I'm hoping someone will help me out here - where should I look next? I've checked all my connections and the resistance checkout per the manual is all good. ???

Thanks. Aural Robert.
... confusion will be my epitaph ...

 

[Paul Joppa]

First thing to check is the power transformer high voltage secondary winding. Measure the resistance from the brown wire to the orange one, and again from violet to orange. (I am away from home for a few days, so I hope I have the right colors!) Both should be a bit over 200 ohms. Don't run it any more if they are more than 10% different!

If the PT is OK, then we'll work our way down the power supply path.

When the voltage drops, did you notice if the power transformer buzzed or vibrated?

 

[Aural Robert]

OK, measurements as follows: brown to orange is 211.0 and violet to orange is 224.3.

Yes I heard something buzz and thought I'd see smoke or smell something, but just the buzzing for a brief period. Interesting, when I'm probing the circuit I can hear static in my computer speakers at the exact same moment. I don't know if that helps but it's sure strange imo.

THANKS Paul.

Aural Robert.

 

[Paul Joppa]

OK, the power transformer is good.

I don't have much for a theory right now; perhaps one of the power supply diodes has failed or has a solder joint that has gone bad. I'm out of town and don't have my manuals at hand, but you can re-flow the solder connections in the power supply - those same three wires, the 450v capacitors, all the diodes, the 270-ohm resistor, ...

Check the resistance across the capacitors, too - it's possible at this age that one of them has dried out and died.

If that gets you nothing, cliplead the meter across the 270 ohm 5 watt resistor in the power supply, check its resistance to be sure, and then set for high voltage. Then monitor it as the amp is turned on. It should drop 15 volts; if it's off then we have a good idea which direction to look further.

Wild goose possibility - did you make the grid stopper correction, way back when? Of course if you're like me you don't remember ... there should be a single 220 ohm resistor connected to the 2A3 pin 3, with a 270K and a 0.1uF cap conected to the other end of the resistor. The manual had an error on this and there was a lot of discussion on the old Forum ...

I'll be back in town in a few days so if those things don't pan out we'll look elsewhere.

 

[Aural Robert]

OK, my bad. While I was touching up the joints I noticed one of the cap leads was not actually soldered to the terminal. It was just floating there for . . . 9 or 10 years???? Since I built it. HOLY CRAP. At any rate, a wee bit of solder and we are back to 2 channel stereo. THANK YOU Paul. I appreciate it.

Aural Robert
... there goes a super-nova ...