LM3886 Power Transistor help needed

[jimmy74]

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?

[Roly]

Pin 4 on U6 is the control input, and from my reading of the circuit will limit/disable the signal to the power amps when it goes +ve, and I would expect the red LED to light; -ve should be enable, green.

Wouldn't the lower 10v on the main power rail

Sorry, what?  In the voltages you give on flickr the lower power rail is -28V.  After the regulator U10 it should be about -15V.

Have you tried to...

inject some sort of a signal to the join of R45 and C34 you should get some sort of output confirming the main amp is okay.

?

[jimmy74]

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.

[jimmy74]

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

[Roly]

From your voltages I assume that the LED is green, and therefore U6 should be enabled.

[jimmy74]

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.

[jimmy74]

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?

[Roly]

Pin 7 is the ground pin.

What are you measuring -2.3V with respect to, i.e. where are you connecting the -ve probe of your meter?

So what do you think you did that brought it back to life?

[jimmy74]

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.

[Roly]

K.   :dbtu:

[jimmy74]

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

[Roly]

Very bottom right or cct is R57 and C54.  These are the switch on anti-thump, and operate on the Mute like of the power amps.  This will normally be pulled high at switch on because the cap is uncharged.  As the cap charges through R the voltage tracks from +ve to -ve rials, but somewhere it gets to the Mute threshold voltage, and the rest is of no interest.  So what is the Mute/Un-Mute Voltage.

Pin 8 Open or at 0V, Mute: On
Current out of Pin 8 > 0.5 mA,
Mute: Off

The voltage is not stated since this is a current-operated switch; if you draw more than half a mil out of the Mute output, the chips un-mute.  The 10k pulldown to to -15V, R57, 15 - 2.3 =12.7V, a bit over a mA which should be enough to un-Mute all three chips.



Sorry, but I'm back to "um what?"  Specifically what missing 10V are we talking about?

[jimmy74]

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

[J M Fahey]

Your mains voltage dropped or you are using a different tap in your transformer or your meter battery is flat (no kidding).
Or you are using a lamp bulb limiter.
Please confirm which one is it.

[Roly]

lamp bulb limiter

Oh, that 10 volts!   :lmao: