Acoustic 370 repair

[Thomas Robert]

Thanks. I plan on removing all the TO-3 tabs tomorrow.

[Dave H]

Adjusted base voltage to 11 volts gets 11.6 on the emitter and I was reading about 4 volts on the collector. This is with Q303 out.

I think there's 4V on the collector because it's trying to turn on the non existing Q303 to bring the output voltage down. Where is the output sitting with Q303 out? It should be high (~90V) if it's not the bootstrap capacitor (C307) could be leaky. Disconnect one end of it to check (I'd replace it anyway).

[vintagekiki]

My schematic shows 100 ohm resistors at R339 and R340 yet my 370 does not have them. It also does not have double diode CR310.

http://mikesgig.com/wp-content/uploads/2013/11/Acoustic-370-manual-410056.pdf
Acoustic 370 instruction manual

Question
Instead of resistors R339 and R340 whether there are jumpers or not. From the layout of the printed board, there must be traces of the intervention of the soldering iron.

If 370 does not have a resistor R339 and R340 (100 ohm) it indicates that someone for some reason removed them, and separated the driver and power transistors from the driver part of the power amp.

Without the resistor R339 and R340 power amp can not work, it just raises the question of whether the malfunction is in the driver part of the output stage, or power part (transistors Q305 - Q314)

In order to repair the amplifier, you must first bring it into an operational state, that is, you must return it to the factory schematics and by measuring to determine which component is defective.
Return R339, R340, CR310 and all the missing components of the power amp.

To start repair first disconnect preamplifier (resistor R145) from power supply (+ 87V) and embed limiter protection (resistor 220 - 470 ohm / 50W and more) as described on schematics.

https://music-electronics-forum.com/attachment.php?attachmentid=54046&d=1560978663

Limiter protection (resistor 220 - 470 ohm / 50W) prevents the power amp from "pulling" more than 0.5A current, and also enables it to comfortably perform all the necessary tests.
Sensor resistor (1 ohm / 1W) makes it possible measure current through power amp with DVM.

Check all driver and power transistors correctness with ohm meter (range x1000). Replace faulty transistors with the same type.
When replacing the transistor, it is desirable to be matched hfe in the limits +/- 10% and tested on the breakthrough voltage (min 120 V).

Visually and measurement check R320 - R327 whether due to a malfunction did not change the value.
Reliable operation of the power amp depends on their correctness.
Be sure to check C308, 311, 312 to not leak.
Preventively change (due to age of the amplifier) C306, C307
All suspicious and defective components replace with new same values.

Turn on power amp. Limiter protection is in repair mode.
Measure voltage at + UC404. Whether and how much change by changing the resistor R304.
Using the R304 set the voltage at UC404 to +45V (half voltage UC403)

Measure voltage at sensor resistor. By measuring the voltage on the sensor resistor, indirectly measure the current of power amp.
Edit: 1mV = 1mA
Question How much is your measured values?

On the schematics 370 are given some voltage measuring points.
Question How much is your measured values?