HH MA150 NO SOUND?

[bazmusicman]

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? 

[J M Fahey]

Well, it can be repaired
But now you do a little homework   8|
1) google and post or link the HH MA150 schematic, at least its power amp
2) tell us *which*  "4 resistors" you replaced.
3) read the lamp bulb limiter thread here in this Forum and build one, you'll  need it.

[bazmusicman]

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.

[J M Fahey]

Ok, I found him searching his name and posts.
For the next time, in such a case you simply open the page you ant to show us, copy its address from the top address bar and paste it here, so clicking the"link" sends us straight there.
In this case:
http://www.ssguitar.com/index.php?topic=3550.0

the circuit is:


Now to some diagnostics:
* blown R26-R27 tells me that VERY probably power transistors Q1 and Q2 are blown, probably blown open or you would be blowing mains fuses by now
* toasted R30 tells me the amp oscillated, big time, which is an excellent way to cook the output transistors.
* toasted R31 tells me that the amp was driven into a short or a very low impedance (did they hook half a dozen cabinets to that poor amp?) for an appreciable time, and didn't smell the smoke rising from it.

you'll have to check Q1, Q2 , Q6 and Q7 at least for shorts.

[bazmusicman]

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.

[J M Fahey]

Ok, not the most orthodox way, but to save time I'd replace Q1 - Q2 - Q6 - Q7, even if a couple does not show shorts (as said before, I suspect some might have failed *open* ) , of course desoldering carefully, pampering the PCB, would also replace R25/26/27/28 , plug the amp into a light bulb limiter and turn it on.
No speaker or any load connected, also no input signal and all controls on 0 .

*Maybe*  that's all there is, speaker out rail stays within 200mV of ground and later connecting a speaker and applying some signal lets us hear some Music 

If not, we keep troubleshooting.

I guess power transistors are TO3 metallic, in that case a pair MJ15024 would be fine; also MJE15030/31 would be good as drivers.

Just doublecheck pinout on TO220 drivers, to make certain they match originals.

[phatt]

I'd be concerned that this issue has happened before so something is not right.
Am I right in thinking that the current limit setup may not be working?

Just think'in out loud , Phil.

[J M Fahey]

Agree, also think very possible that those current clamps are shorted, thus always clamping.

But I want to keep this step by step troubleshooting so for now I'd just be happy to restore low DC at the output conditions plus no fuse blowing.

Later, we'll feed some signal and your prediction may very well be found.

[bazmusicman]

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.

[J M Fahey]

Hi,
You say replace R25/26/27/28  in my earlier post I did say that I have replaced
R26/27.

I give you the full recipe.
If you already did some of it, fine.
Not forgetting that if the transistors are shorted emitter ballast resistors might have blown again.

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?

Today I'm worried about a dead power amp, didn't even *look*  at the preamp.

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!

Take your time

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!

Things can work for ages until they die or fail

[bazmusicman]

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.

[bazmusicman]

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.

[bazmusicman]

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.

[phatt]

Unless I'm sadly mistaken just find a 200V 30W rated BJT (or above) and it will work, though check the pin out. No matter what you do, run it with limiting lamp or variarc until you know it's stable.

In my DIY experience, it needs to be able to run at full power into a load before you know it's stable. If something gets insane hot (the instant burnt finger bias test) then it won't last long. Sure if you never turn it up full pelt then it may well last 30years in tender hands but one day it will fail again,,,,, if it was my amplifier *THAT* would bug the hell out of me. :trouble :grr

My recent adventure into design and build of a power amplifier took me quite a while to perfect. My final test, ran for 8 hours into a 4 Ohm load and my temperature probe could not get past 35 Deg C at any point in the circuit.
Only the load box got hotter, which is what you want.

You might find some very helpful info and tips here;
http://sound.westhost.com/index2.html

click on *Articles*

Power amp circuits here;
http://sound.westhost.com/projects-0.htm#pwr

Phil.

[bazmusicman]

Hi Phil,

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

Regards,
Baz.