Ampeg Solid State portaflex output transistors

[J M Fahey]

You are looking for "bad parts" and all show healthy part values.
You can replace them 1000 times with fresh new parts and nothing will change,

What I see *wrong biasing* .
Most people care about power transistor/idle current biasing *only* but fact is each and every one transistor must be individually biased.

Normal transistor operation is : Base about 0.6V above emitter and collector above emitter for NPN transistors and samebut opposite polarity for PNP .

All those mentioned above meet this criteria, even
* Q7 , which actually looks fully saturated; 0.7Vbe and collector 0.1 V above emitter. .... check.
* Q8: base 0.5V above emitter , collector 1.1V above emitter .... check.
* Q9: base 0.6V above emitter, collector 75V above emitter .... check.
* Q10 (itīs PNP so normal polarities must be reversed) : base is 0.5V below emitter, collector is 5.5V below emitter ... check.

But I also see that Power Transistors are getting too high base voltage: 0.84V which is **a lot** and makes me think that they are saturating and pulling midpoint rail down, from expected +39V or so to around +6V .

So all parta are behaving as expected, just they are being biased down.

I strongly suspect Q6, measure E/B/C voltages and report.
Q6 might be open, or centering trimmer VR7 was set to 0 ohms, shorting Q6 base to ground, or R36 might be open.
Please measure and post and just in case move VR7 to different settings and see if conditions change.

[pontiacpete]

Thank you very much for that Juan! I went to get voltages on that transistor and knowing that I had already change it out( I know probably for no reason) and saw the low voltages I went for the trim pot. Gave it a turn and sure enough the voltages on the collector of Q7 went up to about 34v. And,,, what you know, sound. Thanks again. Been looking at this thing for many hours, nice to hear it working. I'll do some adjustments tomorrow to balance and see what's up with the other trim pot.

[g1]

I'll do some adjustments tomorrow to balance and see what's up with the other trim pot.

The adjustment for both trim pots is explained in the notes at the bottom of the schematic.

[J M Fahey]

So it was just another case of FATU or FATT (failure attributed to user or to Tech )