On 9/20/2020 10:40 PM, Phil Allison wrote:
David Farber wrote:

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** No schem from me- but it looks like you may only have a bad relay.

The blown device is a PTC thermistor not a MOV - they are fitted to reduce excessive inrush surges at switch on. The relay bridges the PTC out of circuit to stop it overheating and exploding if made to pass the full load current.

You can try using a 5 to 10 ohms, 5 watt or more WW resistor in place as a test and then bring the amp up with a Variac.

If the resistor smokes, you have a blown PSU.




Hi Phil. I think I'll try using one of my audio, 8 ohm, 25 watt dummy
load resistors to test it. I'll also put a 200 watt light bulb in series
with the Variac just in case.

** That's fine.

If there's no overload, then I may as well
pull out the other identical inrush relay

* Are there 2 PSUs involved?


and see if I can figure out
what voltage and current it takes to close the contacts. Any idea what
voltage and current rating those contacts need to be?


** 20A at 120VAC at least.


.... Phil


Before I went to the trouble of dismantling my audio dummy load resistor
setup, I found two, 3.3 ohm, 1 watt resistors, put them in series, and
soldered them in place of the thermistor. (My supply of resistors in the
low ohm, high power range is almost nil.) I know you said to use at
least a 5 watt resistor but I wanted to get some idea of what kind of
current the amp would draw on power up. I did put a 200 watt lamp in
series. By the time the line voltage was up to 50 ACV, the lamp was
glowing at about half strength and the AC current was about 1 amp. The
voltage across the resistors was over 3 volts so they were already
overloading. At that point I aborted the test. If I do repeat this test
with the 8 ohm, audio, 25 watt dummy load resistor, should I leave the
light bulb in series? Would it take more than 2 amps of continuous AC
power even when bringing up the voltage slowly? What if I bypass the
relay contacts and eliminate the resistors? There shouldn't be any
inrush surge while bringing the voltage up slowly, right?

I should also mention that the power amp has a 25 amp line fuse inside.
Does that sound reasonable? Neither the schematic nor the PC board
indicate what the fuse rating should be.

Thanks for your reply.
--
David Farber
Los Osos, CA