HH MA150 Help Please

[Mike H]

Hi, started rehearsing after a 13 year break and set up the PA using my HH MA150 mixer amp. For those who are not familiar with this amp, it's a 150 watt 5 channel mixer amp with reverb. Anyway, when we started to use it there was distortion, like a compressed digital distortion, or like a speaker failing, when the vocalist sung loud, and it got worse when the master volume was turned up. When the singing was quiet or normal it was ok. We temporarily overcame the problem by turning all the channel volumes to max and kept the master volume down. But we can't go on like this as we might get a gig in a large venue. I tried the amp thru my 250watt 15" bass cab and the distortion was still there, so it's definitely the amp and not the speakers. I've had some advice from tech minded friends........change the coupling caps around the master volume pot..........change all the electrolytic caps on the power amp board..........but I think I recognize the type of distortion. I was modifying a treble boost to include different caps to get different boost tones and in doing so it changed the biasing of the transistor and the result was this type of distortion. I could be well off course here tho......Any advice would be welcome. I have a schematic for this amp and a difference I have noticed between the amp and schematic is the coupling cap from the master volume pot to the power amp is 470n 100v in the amp but 220n 100v on the schematic.
Mike

[J M Fahey]

Don´t replace parts at random because that way you will NEVER solve anything.

Try to check whether you have a real problem and what it is.

BEST is an oscilloscope to see actual waveforme, nothing beats that.

Next best is to inject a clean  tone, say 400 or 440Hz, start very low so you recognize the sweet undistorted sound and then start slowly rising it until it starts becoming shrill/buzzy which indicates distortion.
Then raise and lower volume slightly until you find the point where it changes.

After that you can make some measurements.

Note 1:

We temporarily overcame the problem by turning all the channel volumes to max and kept the master volume down.

Weird.
Usually for clean sound best is rising master to 10 , then starting with each channel volume very low and raise them slowly until sound dirties.

Note 2: please scan and post a clean readable schematic.

[Enzo]

Perhaps the master circuit has a DC offset and the waveform runs into it, getting squashed against a rail.

[Roly]

Hi Mike, welcome.

I've had some advice from tech minded friends........change the coupling caps around the master volume pot..........change all the electrolytic caps on the power amp board..........

This is called "scattergun" or "blunderbuss" servicing.  Sorry, but it's the favored method of the clueless, and it never works out.

First we diagnose, then we replace the faulty part(s).  It may be a cap (or a resistor, or a transistor, or a dry joint, or a ...)

Are we to take it that this amp has been sitting in storage unused for 13 years?

Post circuit.

the coupling cap from the master volume pot to the power amp is 470n 100v in the amp but 220n 100v on the schematic.

This will only give you a little less bottom end.  Not the cause of your problem - ignore.

[Mike H]

Hi again. Here's the schematic fir the HH MA 150.

[Roly]

709's!  741's!  Ye gods this is a blast from the past.

The good news is that I have reworked kit of similar vintage with more modern op-amps and the difference was well worth the trouble and minimal cost ... however;


Okay, first all the boring stuff.

Starting bottom right, unplugged from the mains.

There are two fuses, F2 and F3.
Take them both out and test for continuity with your meter.

Power up, no speakers connected, all controls down.

Measure the power supply voltages.
These are found at connections "+, C" and "-, E" on the main power amp "Driver PCB" board, and should be around 50V+ and 50V- to ground/chassis.

The preamp supplies are +15V and -15V on "76" and "75" connections on the "Preamp PCB".

Last and by no means least, the voltage on the power amp "half rail", "O" on the driver PCB, near where the inductor is (or at the inductor if you like).  This voltage is quite important and should be no more than about 200mV (0.2V) max above or below ground.

If you can take these measurements and report the results we can take the next step.

[g1]

  A couple more things you could try even before you open it, just to make sure:
you mentioned trying other speakers, you need to also try other speaker cables.
Also, try going straight into the aux input with something that puts out a fairly hot signal (some kind of preamp out) to see if the problem still occurs.

[Roly]

Yup, check everything.   :dbtu:

[Mike H]

I'm rehearsing tomorrow evening so I'll try different speaker leads and I'll plug a mixer into the Aux input and check what happens.
Thanks.

[Roly]

If you're in a band you have to maintain your gear.  I've roadied/sound engineered/theater teched quite a few bands and shows, and every so often you have to divert a practice gathering into a fixit session.  I was running a PA with a huge chest of leads, and a couple of times a year I'd go over the lot, checking, cleaning, fixing, but particularly checking by taking off covers and looking at the actual lead connections, how they were traveling.  It doesn't just need to look right, it has to actually be right.

Serious protip: If you want every show to go off without a hitch then you have to stay ahead of the curve with your gear.   :dbtu:

{your band can't gain ground struggling with a duff PA, so fixing it is a high priority.  Looking forward to those voltages}

[Mike H]

This is strange......Last night we plugged a Behringer 12 channel into the aux input, ie., directly into the power amp, and using the master volume on the HH I increased the volume and there was no distortion. So logic dictates to me that the power amp section is working fine. Yet, when using the HH mixer amp on its own I have to turn the channel volumes on nearly full and the master volume right the way down to prevent the distortion. When the channel volumes are turned down and the master volume turned up, the amp distorts......indicating that mixer preamp section of the HH is working fine because surely, distortion would normally occur if the channel volumes were turned up?? Don't quite understand this.....
We all check our equipment regularly, and I checked this HH PA amp in my house prior to our first rehearsal...it turned on, the sound thru a mic was fine, but it wasn't until we started using it at venue volume that the distortion was evident.

[Roly]

Measure and report voltages as requested, in particular the +/-15V preamp supplies.

[DrGonz78]

...I checked this HH PA amp in my house prior to our first rehearsal...it turned on, the sound thru a mic was fine, but it wasn't until we started using it at venue volume that the distortion was evident.

This happens time to time again where you're only barely pushing an amp to test it. Then you push it harder and realize there is a problem that was not present before. Roly's advice to check the +/-15v preamp supplies is a must. Do you feel confident measuring voltages inside the amp? Best regards and good luck  :dbtu:

[Mike H]

Hi, just took the continuity and voltage measurements using a Fluke 73 multimeter.

1. Fuses F1 & F2 are fine.
2. Power amp supply voltage is +59v and -59v.
3. Pre-amp supply voltage is +15.08v and -15.30v.
4. Power amp half-rail to ground is 0v.

Mike

[phatt]

OK if all is well with supply voltages then take a signal from ECHO SEND socket into another amplifier. Is it clean or distorted?

That bypasses IC3 and 4 (the main mix amp) 

To test IC3&4
The main mix amp output is also present at the Aux input so try taking a signal out from Aux input to another clean power amp, see if that distorts.
Phil.