EV 7200 power amp

[in a vacuum]

I somehow managed to buy two EV 7200 power amps at an auction for $15 for the pair. Lucky me. Well, they don't work. Not so lucky me. R65 is burnt. It feeds off the power supply to a relay and Q20 and Q21. Both amps have exactly the same issue with a burnt r65. I am somewhat competent around tube amps, and thoroughly incompetent with solid state. I know that r65 burnt for some other reason than old age. Any ideas? I have no idea how to test the relays.

The schematic is figure 6 on page 14 (page 13 of the pdf). R65 is 2/3 down the page.

http://archives.telex.com/archives/E...r%20Manual.pdf

Any help of any sort would be greatly appreciated.

[Enzo]

That resistor is just a currnt limiter for the relay itself. Replace it with a 1 watt and see what happenes. Make sure the diode across the relay is not shorted, D16.

[in a vacuum]

The diode tested good once I removed it from the circuit. Unfortunately, I do not have any one watt 1.5K resistors handy so I will have to order them. I will report back when I have installed them. What does the relay do?
Thanks,

[bob p]

Not having looked at your schem, I'd guess that the relay is part of a protection circuit to protect the amp/speakers from DC on the outs or excessive current. I don't think your two current limiting resistors failed in a vacuum, so before you place the parts order I'd do some basic testing of the output transistors and their drivers using your diode meter. If a driver blew out and took the output stage with it, that might explain the resistor burning up and the relay malfunctioning. Hopefully you can confirm/refute the hypothesis and make one parts order instead of two.

I think $15 was a good buy.

[Enzo]

Bob, the resistor is in series with the relay coil. The whole circuit is V+ supply - resistor - relay coil - transistor - ground. The resistor is not part of the amplifier output itself. That's why I suggested a higher wattage and a diode check. If the transistor croaks, it either leaves the relay always on or always off, either thing the relay is happy with. SO to me anyway, that means either the resistor is just too small - and both units show hte same failure - or the diode shorted adn the whole V+ current is shunting to ground throught he resistor.

On the other hand I could be missing something too,.