Univox U-400G blatty, distorted, and weak output

[Wes]

I'm pulling my hair out on this one. I have a schematic for it but there are no voltages for anything.

Univox U-400G Service Schematics.pdf

I've tested the transistors and all seem ok and all the electrolytics are new.

On the output rail, before the 1000uF cap, there is 28v. The full ht is 46 or so. Should there be 28V on the output as part of the design and that's why the cap is there on the output, or as usual with many other SS designs, should there be 0v at this point?

[52 Bill]

I've tested the transistors and all seem ok and all the electrolytics are new.

On the output rail, before the 1000uF cap, there is 28v. The full ht is 46 or so. Should there be 28V on the output as part of the design and that's why the cap is there on the output, or as usual with many other SS designs, should there be 0v at this point?

There is only one polarity to the power supply, so the midpoint of the power amp will sit at roughly 1/2 the full power supply voltage. That is the reason for the large output cap going to the speakers.

Have you figured out if the problem is in the power amp or the preamp?

[Wes]

I've localized it to the power amp. Looking there now.

[Wes]

What is the device that is between the bases of the C1567r and A794r? It looks as if it labeled STV 4. It has a dot on one end and behaves as if it is a diode with the end that has a dot being cathode.

[52 Bill]

What is the device that is between the bases of the C1567r and A794r? It looks as if it labeled STV 4. It has a dot on one end and behaves as if it is a diode with the end that has a dot being cathode.

Based on the position in the circuit, it is a bias diode. It may be a dual diode. If you test it like a normal diode you may see a higher than normal reading if there are more than one diode junction in there.

Measure base to emitter voltages on the transistors in the power amp and see if any seem lower than normal.

[Wes]

Yes, that diode/dual diode reads about double what a normal diode would read. Ok there.

I'll check those voltages.

[Wes]

All the transistors in the power amp have about .55V to .65V B to E.

[Enzo]

Scope the output, run a sine wave through, what does it look like?

Trace signal through the amp, does it change shape along the way?

[Wes]

Ok. I'm a dummy when it comes to using the O scope, but here is what I have.

I have the preamp and the power amp separated after the volume control at the 2.2uF coupling cap. Pre-out and power-in both have a .01 coupling cap in place for sending/receiving.

The preamp ran out to another amp's power amp in sounds just fine, no issues there. Another amp's pre out into this power amp reproduces the issue and sounds blatty and bad.

1Khz in at the power amp gave me these on the output:









At the A608F base:



At the A608F emitter:

[g1]

What are your no signal DC voltages on the A608F ?

[Wes]

Emitter to base -.54

Base to collector -18.45

[g1]

How about E,B,& C with respect to ground?

[Wes]

E 20.1

B 19.08

C .613

Thanks for your help!

[Jazz P Bass]

That 2SA608 circuit is a Class A circuit.

I would expect the base voltage to be much lower.

Start by checking the voltage divider chain on the base connection.
If all is well, remove the transistor and remeasure that base voltage.

If it does go lower, then the transistor is bad.

Here are the spec's:

2SA608
Description: TRANSISTOR,BJT,PNP,30V V(BR)CEO,100MA I(C),TO-92

2SA608.pdf

[nickb]

I note the B-E readings in posts #11 and #13 are not consistent - It may be your meter is loading the A608 bias network and causing the reading in #13 to be lower than reality.