University Sound 1790 Testing

[ewhiz]

Don't let the username throw you off... I hope to become a whiz at electronics "repair" with some help from you guys.

I am working on a University Sound Model 1790-6A PA Amplifier. This amp has a 8 ohms out, 25v out, & 70v output. The amplifier was damaged by lighting/power surge. There are no fuses. Evidently, the transformer is thermally protected because it will power up and stay on until one side of the transformer gets to hot to touch. The schematic shows the output of the power supply section to be 30 volts, but while powered on, I'm only getting 27.5 volts. I'm not sure if this voltage drop matters or is within tolerance.? I have checked the two diodes on the power supply and they read to have continuity in both directions(damaged). Also, there are two transistors in the amp section at the opposite end from the power supply. Can the diodes or transistors be checked in the circuit or do they have to be removed? Could I check voltages to and from transistors (E,C,B) as stated in the schematic? Finally, could transistors that are damaged in the output section of the amplifier cause problems with the power supply? ie. overheating transformer. Thanks, in advance, for your response!

University Sound 1790 Amplifier & Schematic:

http://archives.telex.com/archives/U...0-6A%20EDS.pdf

[nigelwright7557]

If the output transistors have shorted E-C then they will short out the power supply.

You need to replace the two diodes then check with a multimeter the output transistors.

[ewhiz]

If the output transistors are found to be damaged, do you usually find other components next to the transistors in that circuit to be damaged, as well?
Moreover, given the transformer, EIA Code 606 (Woodward - Schumacher), has overheated several times tripping the thermal protection, should I be concerned about it's future performance? It is clear that one side of the transformer has been extremely hot because the rosin has deformed some on that side.
Once repaired, this amplifier will be put to continuous use. I just want to prevent any further problems even if it requires replacement of multiple components.

[jaycee]

If the transformer is showing signs of damage from overheating, then it would be best to scrap it. However whether this is feasible is another matter - if the transformer has custom secondary windings, it is unlikely you will be able to use an off the shelf replacement.