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!

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.

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.   

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?

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?

Thanks for the help!

I'll get a TIP42B from mouser and complementing TIP41B and see how that works.

Can someone help me identify this transistor?

It's from a Wurlitzer Orbit 3 organ.

It reads 7179 - which in the schematic is 657179, which I have found on a Wurlitzer page, so I'm thinking that's a Wurlitzer part no. It confirms that it's a PNP power transistor.

Next it reads SP8406A. I can't anything to do with transistors when googling this number.

And next it reads 7346. Is this an age code?

I've attached a pic of the transistor and a schematic of where it's from.

Please help!

I would love to find the specs of this transistor so if I can't find one I can get an equivalent.

Thanks!

Solid advice. Thanks Phil!

I swapped the reverb tank connections and yes indeed they were the wrong way around!
I marked which way around they were when I removed the other tank but they might have been wrong before that.
Now the reverb sounds pretty good! No mod necessary!
Out of curiosity, I tested the resistance of the transducers of both new and old tanks, and they are close, not completely matching.
Old tank was around 32 ohms input and 248 ohms output.
New tank was around 32 ohms input and 204 ohms output.

As for the hiss, you might be right. It's not a offensively bad hiss. But the other Mini Brute I have much quieter so I wonder if there is something that can be done.
I have attached the schematic for the another unit. 

And thanks for the info on Vactrols. Got some learning to do on that topic.

I'm looking to fix up a Polytone Mini Brute II amplifier.

There are a few issues I'm dealing with..

Firstly, the preamp is quite noisy. This is hiss that is present regardless of the volume being up or down and regardless of anything plugged into the input. I'm wondering if this is an inherent problem with this amp or is this something I can do to improve it. I have another Polytone Mini Brute (later model) which is quiet as a mouse.
I tested the preamp from the noisy Brute into the power amp of the other Brute and it's noisy. Then I tested the preamp from the other Brute into the power amp of the noisy Brute and it's quiet. Hence my conclusion is that the preamp is to blame.

Second problem is the reverb is very weak. With the reverb control up full the reverb is just audible but not particularly strong. At half way you can barely hear the reverb. Again this might be an inherent problem with the amp. I recently replaced the old reverb tank (which had a snapped spring) with an Accutronics 8DB2C1C. This was a recommended replacement. Input impedance of 310 ohms and output impedance of 2575 ohms.

There's a trick I've seen with other amps to get more reverb, you can take the input for the reverb driver from before the Gain pot so that it gets maximum gain to the reverb tank all the time, making the reverb louder. In this case that would mean disconnecting R23 from the wiper of R72 (Gain pot) and reconnecting it to the side that's not grounded. (Schematic attached)

Has anyone tried this? Should I give it a go?

Another question is... can anyone tell me about OP1, OP2, OP3 etc.. in the schematic? What are they and what's their purpose. They are covered in black rubber on the circuit board. Are they little current limiters perhaps?

Thanks for any help!

Ok thanks!

Thanks for this info. I'm trying to learn about transistors amplifiers so this is useful information.

So, I found a bad transistor which I think is the problem. I took 40409 out of circuit and it's only working from base to collector, and not base to emitter.

Should I replace 40410 at the same time?

And does anyone know where I can get a good 40409, 40410 transistor or replacements?

There seems to be DC voltage all over the place.

With a signal of 200 Hz at the input. Scope measuring 400mV pp (which is 0.14V RMS)
At C1 - the sine wave is present at 400mV pp, with 280mV DC.
At R1 - sine wave at 48mV pp. with 304mV DC
At 40408 collector - a small portion of bottom of the sine wave is present, with 11.4v DC.
All three legs of the bias pot have measure 11.4VDC.

The signal at C1 and R1 is unclipped. Just when it gets to the 40408 collector that it's clipping and it's really severe and only missing the top of the wave.
I will check out using a lower frequency. Sounds like a good idea.

That's very useful advice. Thank you Enzo!

It's confusing when the transistor type and part number are similar like that but thanks for clearing that up.
Also for advising a sine wave instead of square wave.
Now, when I test for signal with a sine wave I notice that at 40408 collector I get a sine wave with the positive chopped off. In fact more than half the wave cut off. So, mostly flat line with some of the bottom part of the sine wave. I get the same signal at the bases of 40409 and 40410 and R23, 24. At the base of one of the MJ15024s I get the half sine wave. At the other I get -40VDC.
So I am actually getting output of the speaker but very weak and distorted. I didn't get anything before. This signal seems to be intermittent. Most of the time it's there but sometimes it just disappears.

What to do next?

I checked the 2N4248s and they are fine.

Using a square wave 1KHz signal from my scope I found signal at C1. It measured 4 Vpp and then at R1 it was 2 Vpp. At the collector of 40408 I measured a signal of 28 Vpp. Is that very high? Anyway it's there, an undistorted looking square wave.
There is a 480059 replacing the 40408 which I hope is a usable replacement. The amp has worked in the past so I think it is. Some of the other transistors are also replacements.
Where should a trace for a signal next?

Thanks for the help!