I'll try replacing the 741's and the electrolytics too just to be sure.
Wish me luck......

Tried it last night. Output from Echo Send to a separate power amp didn't cause distortion. Took output from Aux In but got no sound, master vol control made no difference. I guess the problem lies around pre-amp output IC3 and IC4.

Right I'll try that but can't do it till a week Tuesday. That's when we rehearse next and I can use a bit of volume. Can't try this in the house - neighbours might complain.
Mike

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

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.

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.

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

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