Help: Pioneer M4000 Home Stereo Amp

I got this item and the 2SA1264 IC's on one side of each of the boards that
make up the amplifier block were FRIED!! I replaced them as well as a few of
the other IC's that were on the board. Well of course they instantly cooked
again right as I plugged the unit in and hit the power button. Does anyone
have any idea what needs to be replaced (other than 2 more 2SA1264 IC's) to
keep it from cooking these? I am assuming it is frying either the right or
the left side or maybe the front or the rear; not sure how this thing is set
up being I don't have a service manual. Thanks in advance for any help
anyone can offer.

 

This is the typical scenario with a DC coupled amp of this sort of
complexity. You will need at least a full set of schematics, a decent
multimeter, possibly a 'scope, preferably a variac, and an awful lot of
experience to get to the bottom of this item's problems, and effect a
reliable long-term repair. As well as the output TR's, you are like ly to
have failed drivers and possibly pre-drivers. Also coupling resitors, and
possibly the bias sense transistor, and any diodes in that circuit.

I don't want to put you off if you are determined to have a go, but
honestly, if this is a first repair project, or you don't at least have
*some* experience with this type of equipment, it is unlikely that you will
succeed in doing anything more than teaching your kids some new swear-words,
and emptying your bank balance ! Just an honest opinion ...

Arfa

 

LOL good advice.

 

You usually have to also replace the 2SC3181, as well as the 2SA1264
outputs, & look for surface mount 220 ohm resistors
right by the outputs. They usually go also, make sure the mica
insulators are in the right location. These Pioneer units use
a 'self destruct' trigger that activates if the bias drifts which is
designed to intetionaly destroy the ouput transistors & blow the fuse.
Use fresh heat sink compound on the outputs to keep them cool. Finally
make sure the speakers that were on
the amp before it blew have good woofers, as this will blow it all
again. Hope this helps, Dani.

 

And don't forget to resolder the bias transistors.

Mark Z.

 

Actually it's a fuse blow circuit that uses the audio outputs like a
crowbar to intentionally blow the fuse in the event of overvoltage on the
line. Same output design as the RX series all-in-one units, and VERY hard to
work on. I'd suggest finding the "Blow" line and check it to see if it's
going active, and certainly check all the 220 Ohm surface mount transistors.
With all the finals out of circuit the foil on the pcb should read 220 Ohm
between the base and emitter connections, if not your resistor is bad.

I've made a "test jig" using a NC latching relay and a FET, removing the
Fuse Blow line connection on the amp module ( be sure to ground the amp side
otherwise it'll float up and activate the fuseblow circuit) and using it to
turn on the FET and relay, thus removing power from the main transformer
instead of applying forward bias to the finals. This approach allows
troubleshooting without killing the finals every time, it functions kind of
like the powerdown circuit in the Pioneer PJTV's, removing Line voltage from
the main power xformer. Just reset and go again

Good luck

Jammy

 

Well I appreciate the advice. At least I now know a little more about the
"path" of what leads to the amp output block. I know there is some other
parts that are obviously bad but I did not have a clue where to begin and I
did not want to waste the time looking at everything being there are a lot
of components. Now I have at least a few more items to check out before
putting in my last two IC's. I actually did replace a few of the 2SC3181
because they did test bad as well.

 

Its definitely not my first repair project but I am mainly into car audio
stuff not home audio. I have a ton of car audio service manuals and they
help a lot but I don't have anything else so I didn't even have a clue where
to start other than to replace the obvious and obviously that did nothing
other than cost me money. I knew I was not going to get lucky; it has never
happened when I have tried repairing anything that plugs into a wall. I was
just hoping my luck would eventually change.

 

Don't give up on AC powered stuff just because of a bad experience on this
Pioneer model. Lots of people had bad experiences on these. It was a stupid,
bad design to start with. If you need me to fix it for you, I can - I have
an M-4000 "test bed" to hook it to.

Mark Z.