Re: Pioneer M-790 amp adjustment

From: sofie (sofie_at_olypen.com)
Date: 06/01/04 

Date: Mon, 31 May 2004 19:41:45 -0700

Mark, James, & Asimov:
Quite frankly when I get these models in my shop with blown outputs I
usually recommend that the unit is not economically repairable.
Proper component level repairs are not only very time consuming and are
prone to "blowing" again for a variety of malfunctions .... also replacement
power output modules are becoming quite scarce and again can be prone to
blowing because of the very poor design..
In my opinion this is obviously not a shining example of Pioneer's
engineering and manufacturing prowess...... gone are the days of the very
well designed grand Pioneer "super" receivers of the 70's and 80's like the
SX 1250 and other fine pieces. 

-- 
Best Regards,
Daniel Sofie
Electronics Supply & Repair
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
"Mark D. Zacharias" <mzacharias@yis.us> wrote in message
news:2i23meFhvv3aU1@uni-berlin.de...
> 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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