Re: Pioneer M-790 amp adjustment

From: Mark D. Zacharias (mzacharias_at_yis.us)
Date: 06/01/04 

Date: Mon, 31 May 2004 20:16:06 -0500

Comments inserted.

"James Sweet" <jamessweet@hotmail.com> wrote in message
news:R_Nuc.33672$n_6.12557@attbi_s53...
>
> "Mark D. Zacharias" <mzacharias@yis.us> wrote in message
> news:2i1i5aFi2ljuU1@uni-berlin.de...
> > The bias is set by soldering together or unsoldering specific adjacent
> foil
> > pads near the bias transistors. No practical way of telling you which to
> use
> > via these postings. For each bias transistor there is one pad pair which
> is
> > soldered to increase bias, and one to decrease bias. If both were
> soldered,
> > there would effectively be no change in bias.
> >
> > There are surface-mount transistors which can be shorted emitter-base to
> > defeat the "blow" function. Assuming there is no other problems, the amp
> > would fire up OK after replacing the outputs and 220 ohm resistors.
> > The Blow line must measure less than 100mV under all conditions before
it
> is
> > safe to unsolder the surface-mount transistors mentioned above.
> >
> > ALWAYS bleed off the power supplies - all 4 of them - before any
soldering
> > etc. These supplies do not go down or bleed off when the power is
removed,
> > not even when unplugged.
> >
> > Post back if your still into doing this one, and I can get more info -
> I've
> > done lots of these.
> >
> > Mark Z.
> >
>
>
> Oh joy, if I'd known how much hassle this would be I wouldn't have ordered
> parts, oh well. Now that I'm into it I may as well see if I can finish. So
> let me get this straight, the "protection" circuit *intentionally* blows
the
> output transistors?! What the hell were they thinking??

Yeah - their engineers should have gone to jail for this one. The
"protection" circuit monitors the output of a voltage divider, the output of
which is zero under normal circumstances. If the trigger line goes over
about 100 mV, a destructive bias voltage is applied. Just to save the cost
of a relay. In their all-in-one units, like the RX-570 and RX 590, etc, even
a bad cassette motor can blow the outputs.

>
> Where are the 220 ohm resistors? I was looking casually for the usual
> largish resistors but I didn't see them.

They are surface-mount types near each output transistor. You can measure
across the E-B connection after the shorted transistors are removed. One or
two will probably be good, a couple probably bad. If the resistor is bad
when the amp's power is applied, the amp will blow again.

The transistors involved in defeating the blow line are Q7401 and Q7406.
They are surface-mount types, and they are pretty hard to find, but once you
do, you can solder across the E-B junction of each one to defeat the BLOW
function. After the amp is repaired you must remove the short and restore
normal operation of this circuit. Otherwise someone could come along later
and sue your ass if the thing burns their house down, which is quite
conceivable on one of these if they fail and there's no "protection".

Pioneer says NOT to use a variac, use a 100W light bulb across the main fuse
instead. If the lamp glows bright for more than a split-second, cut power
immediately.
The way it should work if the amp is OK - the lamp glows for an instant as
the main power supply caps charge, then goes out, then may glow dimly
beginning a few seconds after that. I believe the blow line is monitored at
pin 6 of the connector at the end of the longer board. Should read less than
100 mV with respect to ground. In a borderline situation, say 90 mV, a 15K
resistor may be added from BLOW to ground to reduce the level a bit without
defeating the circuit.

What's the general procedure for
> setting the bias?

This usually isn't necessary if the amp is OK, but there is NO emitter
resistors to measure across, so you theoretically would have to break open a
B+ line to monitor current. I haven't done this, just looked at the schem
and eventually decided which pads to solder together to reduce bias and make
it run cooler. As I said, this isn't usually necessary if the other stuff is
OK.

Mark Z.

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