I tracked back the negative voltage to R57 which on one end is connected directly to the LM337 -15v supply and infact reads -13.4vDC. On the other end which tracks to pins 8 on all amp chips an to the negative side of C54 reads -2.3vDC. R57 is 10k and after a few seconds of steadily rising readings it reaches 9.91k.

Ok tried without the current limiter and the main 40v rail gives me 37v so that's ok but where I was getting -13v before, now it's stil -2.3 (pin 8 on the amp chips).

the shematic states +/- 40vdc on the main power rail and +/- 15vdc after the VR's. Also the tests I did over a month go gave me these results:

http://www.flickr.com/photos/73835524@N00/11291983513/in/photostream/

whereas now I'm getting these results:

http://www.flickr.com/photos/73835524@N00/12134847064/

there's around 10v difference on all readings

So should I just leave it like this or start tracking down what's eating up +/-ve 10 volts?

Sorry I meant pin 8, measuring as usual to ground, -ve probe of the meter to the heatsink.

I suppose maybe after instaling the heatsink one or more of the solder pins on the amp chips may hve move and with the vibrations may have caused intermittent connections, and also that dead jumper may hve lso been part of the cause.

thanks
J.

Well I was hoping in getting my portable cd player to inject some music into the amp though a jack to RCA cable but when I touch the junction point with the RCA tip I get a loud hum so I'm supposing the power amp is ok. I replaced U4 yesterday and redid all the solder points.

UPDATE: I plugged the guitar in and it works, I get nice loud sound, no noise, the current limiter bulb lights up stronger as I play harder or turn up the volume but there's still -2.3volts off pin 7 on the amp chips... what's going on?

yes the led is green, I found another dead jumper right at the junction R45 & C34, bypassed with a piece of wire but voltages are still the same so I'm just about to do the signal test and let you know the results.

thanks
j.

I also took voltage readings off U3:
Pin 1= -11.7vDC
Pin 2= -12.5vDC
Pin 3= -12.6vDC
Pin 4= - 14vDC
Pin 5= 0
Pin 6= 1.28vDC
Pin 7= -12.6vDC
Pin 8= 13.6vDC

sorry for the confusion, I forgot about the low power rail voltage being odd too and was considering only the high power rail. I'll try injecting a signal at that point today.

thanks
J.

I tried resoldering all the solder joints of the parts that I had changed out, redid the voltge tests but all the same. So I replaced U4 but nothing changed there either. I did notice that U6 is giving me -12.6v on pin 4 again. Wouldn't the lower 10v on the main power rail mean there's a problem in the power supply or somewhere there's something sucking up +ve and -ve 10vDC?

here are the voltage readings I got:

http://www.flickr.com/photos/73835524@N00/12134847064/

the +ve and -ve 29.9 readings follow back to the rectifier bridge, and after its 21 and 22vAC

thanks

well after a very long delay waiting for he new preamp opamps, I swapped ou the BA6110 and one of he BA4560's and sure enough that got rid of the crackling noise. Still usin the current limiter I hooked up a guitar and everyting seemed to be working fine, no noise or problems so I mounted everything back up and playd through it for around 10 mins without the current limiter, eveytn seemed perfect, when all of the sudden it just went dead, vey low sound coming from the horn, it doen't go in protection mode though.. one of the amp chips dead?

thanks

Pulled apart some old burnt up PC power supplies I had lying around and sure enough there were some of those insulators... thanks for the tips!!

Thanks for the reply but I don't need the mica spacer, the original ones are still fine, I need that plastic but that separates the chip amp mounting screw from the base of the chip amp so that there's no connection between the heatsink and the base of the chip amp:

http://www.ebay.co.uk/itm/Insulation-Bushing-for-TO-220D-Transistor-Washer-x-50-/130579772765?pt=LH_DefaultDomain_0&hash=item1e6729355d

these are the only ones I could find

is there anyone in the EU that sells the right sort of insulator bushings for the LM3886? I think the TO-220d type bushing should be correct but searching ebay the only seller is in China.