Yes Enzo I do have some other working amps.
I'm intrigued by the signal tracer idea, I've never heard of that.  Could you please enlarge on how to make one?

I assume that I use a normal guitar lead with jack plug one end and a piece of wire the other end, but not sure where/how to connect the cap and probe?

Thank you Enzo, yes I agree, to troubleshoot you have to isolate the problem. This is what I have been trying to do.

I have a friend with a scope but he isn't local to me. I'll have to wait until he can come over.

Thank you all again for all your help.

Sorry J M Fahey and Enzo,  I apologize for being a bit vague,  What I'm trying to say is that I put it back on the bench and it didn't matter what I tried, I couldn't get it to work properly again at all. It only worked properly that once.  As I said it does work but its gutless.

I have checked for bad wiring/contacts, wires being squashed, screws touching anything etc. I even re-soldered a couple of joints that may have been dry.   I'll recheck everything again.
Thank you both for your kind help,
Regards,
Baz.

Thank you Enzo and Phil for your help and suggestions, I've tried them and although I get sound with them I also get sound using the amp normally through the inputs. As I said the amp works, but it has a very nasal like sound (read awful) and no volume, turning it up almost full you couldn't blow a candle out, and still be able to talk to someone in a  10' x 12'  room without shouting.

I have gone through the cable joint's/wires anything I can see that might be loose or a poor connection. The main 'on switch' that lights when on, has a very faint flicker, could this be a sign of poor current flow?

As all the channels work/sound the same, I'm thinking its something on the main flow/output that is choking the signal strength?  maybe I'll have to work from the output backwards? 
Any other suggestions/idea's would be greatly appreciated.
   
I've got a feeling that success is NOT just around the corner!!

The first thing I thought of was a bad connection. I tried moving the wires with a plastic chopstick without success.
I did also try it without the bottom panel, and looked to see if it was touching anything.
I'll try your suggestions and keep looking, I've got a feeling that success is just around the corner!!

Well I tried it  'straight through'  and it was back to normal;  full volume, no distortion, brilliant!
I put the bottom panel back on the amp and sat it were it always sits, turned it on and........
that's right, low volume and distortion!!!

back on the bench, remove bottom panel, preamp readings fine but still low volume /distortion.
Perhaps I need to check the capacitors on the preamp and power amp?

I plugged the guitar into  the AUX IN and SLAVE jacks, both sound fine.
I set up another amp as advised to test the preamp, slave jack fine, the echo send is fine and the same on all input channels.  The two 1K3w   R24/R25 resistors both measure fine.

R15 should be 3K3 but is 1K70?  also R17 should be 3K9 but is 1K53?  would this cause the low output/distortion?
A couple of others are a little out of speck.

Measured supplies to preamp;

supply 75  minus  14.9v
supply 76  plus     14.8v
Z1            plus     14.8v
Z2            minus  14.9v

IC3
pin 4        minus  14.75v
pin 7        plus     14.79v
pin 6        plus     00.10v

IC4
pin 4       minus   14.95v
pin 7       plus      14.81v
pin 6                     0.00v

Regards,
Baz.

I fitted the resistor and remeasured:

Pin 4  minus  14.9v
pin 7  plus     14.5v
pin 6  plus     00.6v
ZD1   plus     14.6v
ZD2   minus  15.0v
speaker out   02.2mv

As the speaker out level is now right I tried it. The amp now works but very low on power and distorts.  Is there something I've missed,  Any ideas please?

Regards,
Baz.

No I don't have anything near or I would have used it. the nearest I have is 4.6k. 
That's a good idea with two 6.8k in parallel,  but I have some 3.3k's coming tomorrow.

Thank you for your help Enzo, I really appreciate it.   

Hi Enzo,
Resistor R7 is open, I have a boxful of resistors
but not a 3K3!

I have + 53.2v  power feed to Q6, Q3, and to R7. I will replace the faulty R7 and take some more readings and let you know how I get on.

Regards,
Baz.

Hi Enzo,
Yes I did mean 200mv, not ma.  I did connect the speakers when I tried the amp but the speakers still work and no fuses blown!  Before that I noticed
Resistor R13 was very burnt, but it still measured 1K!  Neadless to say I replaced it anyway.

With your suggestion's this is what I found.

Pin 4   minus  15.03v
Pin 7   minus    0.68v
Pin 6   minus    1.15v
D1      minus   52.2v  anode,    52.8. cathode
ZD1    minus     0.69v
ZD2    minus    15.10v  anode end only?  would this be reversed biased?

These measurements were taken with the light bulb limiter in circuit.

Regards,
Baz.

Latest update.
I made a 'light bulb limiter' fitted the new transistors, used a hundred watt bulb switched on and the bulb flashed and went too dull to see any glow, but the amp was on.  I switched everything off and fitted a 40 watt bulb, switched everything back on, the bulb flashed and went to a very dim red - you can just see it in a very dark room - at least the Amp wasn't cooking - so I disconnected the limiter and tried the amp, but still no sound.
I took a couple of measurements from the Driver PSB that I saw on the other thread that I thought may be relevant.

+C  =  53.4v.} these two I believe are in spec?
- E  =  53.3v.}    "      "          "        "        "
  O  =  minus 51.0v.}  ( I know that this one is way too high) should be about 200ma?

Any help with the next step would be greatly appreciated.

Regards,
Baz.

Thank you Usable Thought,
Yes I did mean the top diagram of the two in the stickied thread.
I was not implying that it was trivial by saying it is just a light bulb circuit, I was referring to the way a ceiling light is wired. As you say mains voltage, lamps, appliances etc are dangerous, this is why I queried a no earth wire.

Thank you Enzo,
I have worked on tube amps and I am still learning but of cause safety is the number one priority,  I liked the idea of the light bulb limiter for that very reason.   
I am at the moment repairing a solid state amp with the help of you all on this brilliant site.
Once again thank you all.
Regards,
Baz.

Thank you Enzo,   
Its dawned on me that this is just a normal 2 wire light bulb setup!!
They say that there is no such thing as a dumb question, but that one must have been pretty close!!!!

Regards,
Baz.

PS  In my defense;  in England now all domestic electrical equipment has to be earthed.

As I am about to embark on making one of these,  on the first diagram on how to make one, it only shows 2 wires. Shouldn't there be an earth wire if working on items that need to be grounded, or is it because with the bulb sucking the load we are only talking low voltage here?

Regards,
Baz.