[John Cook]

Originally Posted by Tony Pike

Does anyone have a schematic for a Melos TM-90 ST power amplifier? I
believe I have a blown output transformer, amongst other probs.....

Hello Tony,

I have a schematic that I drew for the melos tm 90st, however I modified the screen power supply to a series pass regulator instead of the shunt regulator that George Melos designed. It was prone to failure.

If you would like a copy will be happy to send your way. I have two of these amps with the mods I made, they are very reliable and sound great.

The 6LU8 is a Plate supply regulator (cathode follower). If the KT88 is getting hot you have lost bias control on that tube. Sometimes the window comparator (LM339) gets fried and the bias LEDS don't work right. adjust the bias pot so that the cathode resistor has about 0.43 volts across it .

Regulated Plate voltage should be 575 Vdc
Screen voltage should be 325Vdc
Negative grid bias should be -26 to -31Vdc