Thanks Graham,
My old 100w VS combo uses 2N3055 as O/P's. I must compare the specs of both the 3055 and the 3773. The latter might be a better replacement if I ever need to replace the 2n3055's in my 100w.
By the way, I got the MA150 running ok with help from Enzo and others on this site. Replaced all the electrolytic caps that had high esr readings while I had the unit on the bench. It's back gigging again.

Hi Enzo,
Scoped the sine wave on the o/p and it looked fine at the cross over point so I left well enough alone.
I played a guitar through it and it sounds good. I also left it on over night with no signal i/p. Today I played music through it for about an hour and it sounded good. The o/p tranny's feel cool to the touch which I presume is a good sign. The amp belongs to a guy who has been gigging for over 50 years. It hasn't seen a duster cloth since it left the factory but it is now ready to go back into action. Repair workshops are becoming a rarity these days so I was happy to help him out. At least the old stuff doesn't have any surface mounted components.
Thanks for all your help.

Hi Enzo,
To update on my progress. I was using the aux i/p at the rear of the amp while testing and I think I was hitting the amp with to high a level. I replaced the caps that had a high esr value and the resistors that looked like they had suffered from excessive heat in the past although they measured ok. Using the pre-amp i/p with a low level produces a good looking sine wave at the speaker socket with no clipping on the peaks. Should I check the bias adjustment before I put the lid on this one. If so what is the proper procedure for this adjustment.

Thanks for all your help.

Hi Enzo,
Thanks for getting back to me once again. I was a little slow my end as I was locked out of this site due to my IP being mistaken for a spammer.However this is an update on the fault.
I replaced the 741 which restored the center voltage to 0 so things are now looking good. I put some guitar through it and it now does not have the distortion which it previously had after I replaced to o/p transistors. I have not injected a sine wave or scoped the o/p yet but things sound ok, however one thing I am wondering about is R13 (1k, looks like a 2W) feels hot to the touch. Q2, 3 feel cool. This amp has a variable resistor fitted between R14 and R15 which I guess is for setting up the zero or cross over point. The voltage across the resistor is 35v and the current I measured is 40ma. (1.4 watts) ?.
I just noticed that if i leave the amp powered up for about 10 mins or so the resistor cools down. I think I should change the caps at this stage. Some due in tomorrow.
Keep you posted and once more many thanks for all your help.

Update.
I injected a signal and all is not right.  The o/p of the 741 pin 6 looks good at the base of Q1 but the collector is 'spikey' ( well nothing that looks like a sine wave). I will check the bias resistors later as I have to go out on some service calls.

Thanks Enzo,
Your explanation has clarified things a little more for me. I think the problem may be drive to Q1 which is connected to the 741. There is a - voltage there at the base of Q1( holding it off) and the collector is at full + potential and the emitter is close to zero ( - .7V). I have checked Q1,2, and 3 with my component checker all looks good with zero leakage, gain etc. Checked the bias resistors R9, 10, 11,8.
All measured ok, I even replaced one of these that lookes like it got stressed in the past. I am all out of 741's at the moment.

Thanks for your reply,
I am not in the workshop at the moment as it is night time here on this side of the Atlantic. From memory, D1 and the resistors connected to it are different in the amp. D1 does not exist and there is a pot fitted which I think replaces R15 on the schematic. Both 15V +, - supplies are OK and I Isolated the op amp in case it was driving Q1,2 and 3 mad. Am I correct in thinking that Q 2,3 are for biasing / crossover setup.
With nothing connected to the o/p socket the volts swing to -60v. With a wire wound resistor connected it the voltage may swing to +60v.

Thanks guys for getting back to me,
I checked for dry joints, flexing pcb, etc as 30 years on bench repairs would have conditioned me to check for the simple and obvious things first. Power supplies are all OK and smooth. Most of the transistors are dc coupled so measuring voltages under a fault condition can be hard work. The o/p to the speakers swing to -60v ( sometimes +60v ) when the unit is powered up. The dc biasing setup has some caps which have high esr value so I have ordered replacements and will replace them all as cost is insignificant.
I will try and post the schematic here for your comments. At worst it may be useful to some other forum members.

Thanks Enzo for you reply,
I will bite the bullet and make room on the bench where my scope and signal generator reside.
The main caps are mounted on the chassis and are well soldered and wired to the main board. When I saw the esr on the small 6.8uf caps all over the place I thought that it would be no harm to replace them. Your suggestion to use the eco return socket is a good point as it rules out a lot of the pre-amps etc. It's the 'intermittent' element that experience has led me to think capacitor trouble. As I stated earlier turning the unit off and on a few times will bring the unit on with no hum present and zero volts on the o/p to speaker socket. All the bias resistors checked out ok on cold test but as you know components can act funny when working voltage is applied.
I will keep you posted. Thanks for now.

Hi All,
This is my first post as I just discovered this forum today. I have been in the repair of TV, video and broadcast equipment for the last 30 years or so. Recently a friend asked me to repair a HH MA150 amp which was totally dead. After checking it out I found both 3.15ma fuses were o/c. I found one of the output transistors s/c so I ordered a pair and replaced both. This restore life to the amp but it sounded distorted. I checked both + and - HT supplies, the + and - 15V supplies which all checked out OK. I checked out all the other transistors on the driver board as many are dc coupled to the o/p transistor that was s/c. All transistors checked out ok. The following day when I switched on the amp to have another go at the distortion fault I was presented with a loud hum sound. At this point I checked the 0 line which should read 200mV max. This was totally 'off the radar' so I switched off the power to avoid a puff of smoke and a loud bang. I also discovered that if I switch on and off the main power switch 5 or six times the unit will come on with NO him present. I suspect capacitor trouble so I checked the electrolytic s and found that all have a high ESR reading compared with new ones of the same value. Could this be the problem as the amp is over 30 years old. Maybe a block change of the caps is what is needed.
I welcome any comments from other members.
Thanks in advance.