HH MA150 NO SOUND?

[bazmusicman]

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?

[Enzo]

it sounds more like a connection to me, bad caps don't come and go.

If the preamp works and the power amp works, it only makes sense that straight through works.  So go back and do the isolate thing again.  Play through the input jack, and connect preamp out to some other amp as before, but also leave the amp speakers running.  Now if the preamp sounds OK through another amp, does the speaker still sound OK?  Maybe the bottom panel is touching something, or one of the screws.  Try it with and without the panel. 

The schematic doesn't show a cutout, but just for science, when it sounds crappy, plug a cord from preamp out right back to slave amp jack.  Does that restore the sound?

[bazmusicman]

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!!

[phatt]

My money is on the hidden switch inside the send return or Aux/link sockets (comes under many names but all do the same) It is by far the most common failure point and often intermittent so,, As ENZO mentions establish that Switch is making proper contact by jumping with a cable which bypasses the switch completely. If all runs fine then two options,,,, replace switching socket or if you are like me,, just leave a short cable permanently plugged in,, I'm too lazy to replace the socket. 

Phil.

[bazmusicman]

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!!

[J M Fahey]

I don't follow you.
You said that it worked perfectly on the bench , then mounted it in the case and it lost tons of volume.

What is the situation today?

What if you put it back on the bench as when it worked earlier?

We are not there, don't see what are you are actually doing.

Hard to

Any other suggestions/idea's would be greatly appreciated.

without a clear input.

[Enzo]

If it works on the bench but not when bolted together, only a couple things come to mind.  One is that perhaps tightening down the bolts flexes the chassis in some fashion or more crudely, maybe a bolt comes through the hole and touches something. The other is that maybe a component or a wire sticks up a bit too far, and bolting it together pushes down on that or pushes two things together.

But those arguments only apply if it CONSISTENTLY works open on the bench, but not closed up.

[bazmusicman]

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.

[Enzo]

The essence of troubleshooting is to isolate the problem.

Apply a signal to the input, then use a scope or a signal tracer and look stage by stage along the signal path, and see where it goes haywire.

[bazmusicman]

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.

[Enzo]

Do you have another working amp?  Use it as a signal tracer.  Make up a cable to plug into it, and on the free end put a probe, a straight wire even.  We want to add a series cap of some sort to block any DC the circuit might hold.   Now we probe any point in the circuit and listen to what is there.  Sort of an audio scope, if you will.

[bazmusicman]

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?

[phatt]

Yes it's that simple ))  A cap on the end of a guitar cord. :dbtu:

On the rare occasion you may need a ground clip for a common path return but as most Guit amps are grounded chassis then the power cords Earth wire make the loop.

As you have established that both the preamp and powr amp work then the weak link is in the interconnections which is almost certainly in those 2 sockets and if they are enclosed plastic types then they are suspect to failure.

Although the schematic shows a hard link to both *Slave link and Aux out* it may not be wired as per schematic. the internal contacts may have broken *inside* the socket.  I know the plastic types are a nightmare and at the age of the unit it maybe failing.

Maybe bypass those connections and hard wire the preamp out back to Pwr amp input to be sure?

I have an old Ross 16 Ch mixer with more than half of the 32 plastic sockets broken now the internal contacts no longer switch,, the threaded shoulder has broken on most of them.
Phil.

[Enzo]

Yes, I have suspected all along that the two jacks are not wired as drawn, but have the slave jack as a cutout.

I have to suggest always using a ground wire from the work amp to the tracer amp, they may be grounded together through the mains cord, and that may be good enough for voltage readings, but the potential for ground loops that way is large, and often as not, when we are listening to the tracer, we don't want any added hum we can avoid.

The signal tracer used to be an actual piece of equipment on the bench.  Nowdays we just make one up.  But 50 years ago, no shop lacked one.

Like phatt said, simple as a cap on th end of a shielded wire fron the tracer amp input.  Essentially using your own amp, the signal tracer is little more than a special cord.  Official ones have an actual probe on the end, but many times I spun the cap off a gitar cord and used a clip wire and cap for the purpose.  "Probe simply means something with a live tip you can touch to circuit points.  Use the cap itself as a probe, watch your fingers and don't touch its wires.

The tracer lets you touch the tracer amp to any point in the circuit and listen to what is going on there.  With the DC blocking cap, I can even listen to ripple on the power supplies.  Cap value is not critical, I use 0.047uf/630v because I have tons of them.  Just make sure the voltage rating is at least the highest voltage you might touch.