Re: Pioneer M-790 amp adjustment

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

Date: Tue, 1 Jun 2004 05:11:35 -0500

I still make good money on these - I just educate the customers WITHOUT FAIL
regarding speaker wires etc. Re-do's haven't been a problem for this reason.

The "prop" I use is a AA battery and a small speaker with wire. I
demonstrate how the condition of the wire should be, then "POP" the speaker
with the battery. Have the customer check theirs likewise, and if they hear
no pop, don't hook up that speaker / wire. You can almost see the light come
on above their heads. They really seem to get it for a change.

Mark Z. 

-- 
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have rendered my regular e-mail address useless.
"sofie" <sofie@olypen.com> wrote in message
news:10bnr9eomdpmke0@corp.supernews.com...
> 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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