JM Fahey,

Thank you for beating me up, I deserve it! 
I'm sorry if I have driven you to 'tear your hair out' with frustration but I did say I was a little green working with Solid State. You have helped me  tremendously for which I am very grateful.
Now you have pointed me in the right direction I can move along. Thank you once again.

Also a big thank you to Enzo and g1 for their help.

Best regard,
Baz.

Hi Phil,

Thank you for your help and those links, that is some very interesting reading and tips.

Regards,
Baz.

Well I'm having a problem obtaining replacement transistors!

I know what you are thinking - you can get transistors most anywhere - I can get Q1 (tip 29c)
and Q6 (tip 47) no problem,  but Q2 and Q7  are both  SM2177  which I am having trouble finding a replacement/alternative. I have found that Semelab PLC appear to be the makers stating its  BJT PNP 200V 30W. This info may be right / maybe wrong but anyway they are discontinued!

I have tried several  'cross reference' sites but I can't seem to find an alternative.  Any help would be appreciated. I don't want fit the wrong thing and cause more damage.

I checked all the said resistors and they are fine.  I have removed the 4 transistors, just need to obtain replacements and fit.
I have a friend bringing a Variac to me later in the week so once the amp is back together maybe we'll get it back up to speed hopefully without anymore problems.
I'll keep you informed on progress.

Thank you both for your help, it really is appreciated.
I will recheck the resistors that I replaced, and test R25 (on both boards!) and R28.

Hi,
You say replace R25/26/27/28  in my earlier post I did say that I have replaced
R26/27.
On the schematic there are two R25's,  one on the Driver PCB and one on the
Pre  Amp  PCB.  I assume you mean the one on the Driver PCB?
I haven't had time to make a light bulb limiter yet.  I'm going to be a bit busy
the next 2/3 day's with other things, so progress might be a bit slow!

Phil,
Thank you for you concern, but since the last repair the Amp has worked fine for the last 30 years, which I think isn't too bad!
Regards,
Baz.

Hi,
Update on amp history, the amp repair from 30 years ago was because I lent it a Social Club when they had an emergency, I don't know what speakers they hooked it to, but that's what
blew the resistors the first time.  It had been running perfect ever since.
I am the only one to use it since and have only used it with the proper HH Pro 100 Speakers,
so no mismatch with impedance.  It has never been driven hard or abused.
I tested the transistors Q1, Q2, Q6 and Q7.....   Q2 and Q6  are faulty.

Hi JM Fahey,
On August 15th 2014 a member on this site by the name of Mike H had a similar  problem and put a schematic on his post by the name of,  HH MA150 Help Please.
I'm sorry but I don't know how to do a link.
The 4 Resistors I replaced were,  R26, R27, R30, R31. Upon double checking R30 and R31 are in spec but were a bit black, this is why I  changed them all.
I removed and checked the 2 internal fuses and they are OK.
I'll build a light bulb limiter.

I have a HH MA150 PA 5 Input Mixer Amp with 2 HH Pro 100 Speakers that I use indoors. It is in near mint condition because it has never been abused.
Just before Christmas I was using it to play some backing tracks, I left it switched on (with nothing playing through it) left the room for 5 minutes when I returned it was making a loud humming noise!  I checked
all the connections etc. and they were all fine. 30 years ago it died and I had it repaired, I was told it was the output resistors. I am more electronic savvy now having done some work on valve amps so I can use a
meter/solder read schematics etc. but not much experience on SS.
I checked the  4 resistors that were replaced last time and 3 were burnt and way out of spec. So I replaced all 4.  The amp switches on, everything lights up, nothing smoking, its cured the humming, but no sound at all?