Amplifier section of a Farfisa Mini Compact

[jpcar]

I'm having problem with a Farfisa Mini Compact that is not producing sound. It lights up!

Doing a few tests I'm finding that the base of the transistor (input signal) has 7.3vdc on it. That's Y363 (First transistor from the left on the attached schematic). And then the base of the next transistor in line (SFT337) has 7.7vdc and the next along (Ac128) has 8.3vdc. The point '3' continues on to the right to a headphone output (with 8.3vdc). And the point '5' goes through a 50K pot and then to a line output.
I have tried disconnected point '2' and the 7.3vdc remains.
I have tested all of the resistors and capacitors on this small board and they all seem fine. I also tested the three transistors and they measure approx 1.50V in the correct direction and nothing in the opposite direction.

Any ideas what I should do next?

[Enzo]

Disconnecting point 2 won't remove that DC bias because it comes from the resistors across the power supply.

It doesn't pass signal?  2 is where the signal goes into this, is there signal present there?  If there is no signal at 2 there won't be any further along.  If you have signal at 2, or if not put some there, then follow it along.  Is there then signal at emitter of Y363?  (A emitter follower).  Is it at the base of SFT337?  The collector?  At the base of AC128?

Got signal at 3?  and at 5?

[jpcar]

Thanks Enzo. Solid advance.

I disconnected '2' and looked for a signal at the organ side. Nothing happening there. Then I put a signal at '2' (500Hz sine through a .2uF cap to block the dc). On the scope I detected signal at Y363 emitter (with AC coupling), and signal at SFT337 base, but no signal at the SFT337 collector. And so nothing after that, 5 and 3.
So faulty SFT337? I disconnected it and I it's reading fine with diode test. All the resistors and capacitors around SFT337 are measuring fine.

What should I do next?

[Jazz P Bass]

Not being able to see the whole schematic I can only venture a guess.

The voltages on A128 are all the same.
That tells me that the device is bad.

[jpcar]

I took AC128 out of circuit and diode tested it and it seems fine.
But the voltages on each pin are the same so that seems strange.
I have attached the full schematic. There is not much more to it. The wiper of that 50K lin volume pot goes to a line out jack, and '3' goes to a headphones jack.   

[Enzo]

So the 337 tests OK as a diode?  That is base to emitter and base to collector?  Did we also test for emitter to collector short?

The diode test is certainly useful, but not complete.  No out at 5, but how about at the other end of that cap, right at the transistor?  Is there signal on collector of the 337?

Since you get no output at 5, I would sure try something in its place.

[jpcar]

The 337 diode tests fine. There is no short between emitter and collector, but there is a diode voltage in one direction. I'm not sure if this is normal but it does seem to happen on a few ge transistors I have.
I swapped the 337 out with AC125 (which has a similar enough spec - same collector current, higher power dissipation)

Exactly the same readings as before. Signal gets as far as the base, but is not present at the collector.

[jpcar]

Ok forgive me. I made an embarrassing mistake. The solution was pretty simple and overlooked!

The wiper of that volume pot went off to the jack - which was shorting the two terminals. So the signal was shorting to ground. Because I had the volume up full the whole signal was disappearing. I noticed this by moving the volume pot while checking for a signal at the collector of 337. With the volume pot at zero it doesn't short any of the signal to ground so I saw full signal at the collector, and then dc removed signal after that cap.
Silly mistake really. I should have eliminated basic problems before getting the scope out. Anyway, I did learn a few things along the way.
Curiously, Y636 and SFT337's bases have a dc offset of around 7V. I didn't think this was normal. And the 25uF cap after Y636's emitter doesn't seem to block the dc. Presumably after that cap the dc is now coming through SFT337. But then the cap 25uF after 337's collector does block the dc and then the signal comes out to the volume pot. Interesting stuff.

Thanks for all the help!