[TenTec] Orion A9 board operational thory

[Gary Follett]

I have an Orion A9 board that has gone must.

All of the electrolytic are new so not there.

The issue is that, even though I replaced the defective 4.85 volt Zener diode in the base of Q5, the output voltage is stuck at 6.7 volts.

When I look at the bases of Q4 and Q5, there is a strong 25 KHz “square” wave. As near as I can figure, this design applies this signal intentionally to the bases of Q4 and Q5 in hope that the in-phase signals at the bases will cancel out since Q4 is PNP and Q5 is NPN.

That’s the only reason I can see for this design over a simple 7805 regulator for the 5 volt line…

In any case, I replaced both the transistors in the amplifier, Q1 and Q2, and also Q4 and Q5.

Since I had one, I also replaced the -20 volt inverter chip.

I am running out of parts to replace.

Does anyone actually understand this circuit?

If so, some assistance would be appreciated.

Gary

W0DVN