SX-205 KaPutt Revisited
labX...@erols.com
Dear friends -
A couple of weeks ago I posted here about my friends Pioneer SX-205
receiver which started flashing "POWEROFF" one day. I volunteered to
repair it, and sought your input as to just what was going on. A couple
of you very knowledgable folks responded quickly, and I ordered the
output transistors that I needed.
Here's where I stand now:
I replaced the outputs (it turned out that they were both cooked), but
before powering-up the unit I did an in-circuit diode check on 'em.
Here's the results -
(I'll call the transistors "A" thru "D", with "A" and "B" being one side
[channel] and "C" and "D" being the other "bad" side)
"A" 2SA1943 PNP base/collector .550 base/emitter .154
"B" 2SC5200 NPN base/collector .525* base/emitter .155
"C" 2SC5200 NPN base/collector .540* base/emitter .154
"D" 2SA1943 PNP base/collector .650 base/emitter .654
Now as you can see, it's a good thing I didn't plug this thing in yet,
as there's something else going on here. Whatever caused the transistors
to "pop" to begin with is still here. I was hoping that they just kinda
went on to Vahalla on their own, and a simple Yin and Yank replacement
of the outputs would do it. But alas, tis not to be. Crap. Next time
I'll keep my volunteering mouth shut...
It should be noted here that when I removed the bad transistors, I
noticed a great potential (arcing) when the collector/emitter was
shorted on transister "D" and "D" only, as if from a straight-shot from
the filter caps(?).
There's no other sign, none, of anything wrong. Nothing fried anywhere
(at least not to the naked eye), and the dual-emitter style resistors
are all OK. All I have is a DVM and I'm afraid my next step is to remove
the board and start desoldering componets one by one to check them
out-of-circuit.
I'd rather jump off the gable end of my house stark naked...
Any *more* help/info would go far in saving a friendship.
TIA,
Lynn
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Arny Krüger
labX...@erols.com wrote in message <357498...@erols.com>...
(This should be on rec.audio.tech)
>
>A couple of weeks ago I posted here about my friends Pioneer SX-205
>receiver which started flashing "POWEROFF" one day. I volunteered to
>repair it, and sought your input as to just what was going on. A couple
>of you very knowledgable folks responded quickly, and I ordered the
>output transistors that I needed.
>
>Here's where I stand now:
>I replaced the outputs (it turned out that they were both cooked), but
>before powering-up the unit I did an in-circuit diode check on 'em.
I've played with power amps for decades, both tube and SS. If you seriously
play with SS amp designs, you get to see more than a few that are broken. At
least I did.
My usual procedure for dealing with a situation like this is to do the
following:
(1) Do an out-of-circut check on every power transistor and diode on the
card.
(2) Replace everything that seems broke and bring the unit up with a variac,
with a signal applied and monitoring circuit operation with a scope, a
meter connected to the output terminal. If the waveform is asymetrically
clipped or if the output terminal is stuck on one of the power supply rails
after applied AC power gets past about 25 or 30 volts, more carefully
compare operation of the newly repaired channel to the good channel.
jj, curmudgeon and tiring philalethist
In article <6l36gg$777$1...@excalibur.flash.net>,
>>I replaced the outputs (it turned out that they were both cooked), but
>>before powering-up the unit I did an in-circuit diode check on 'em.
>I've played with power amps for decades, both tube and SS. If you seriously
>play with SS amp designs, you get to see more than a few that are broken. At
>least I did.
Using variac, as Arny suggests,is a good approach.
It looks, from the measurements you show, that at least one of the
drivers is dead.
In general, I've found it wise to replace all the driver transistors
any time that the outputs blow. It's often a bad driver that led
to the problem in the first place, and if it wasn't bad before it
took all that abuse from the shorted finals, it might be now.
A very common failure mode is high-voltage second-breakdown in a
driver, btw. Often that brings the rails down so fast that the
driver isn't completely cooked, and only the finals are toasted,
but oh yes, you DO want to replace that driver, and you need a
(*&*& curve-tracer to even find out if it's cooked out of circuit.
Oh, and check the values of any base and emitter resistors. Carefully.
Along with the behavior of the bias circuitry. An open bias diode
is a MEAN thing.
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Arny Krüger
jj, curmudgeon and tiring philalethist wrote in message ...
>
>It looks, from the measurements you show, that at least one of the
>drivers is dead.
>
>In general, I've found it wise to replace all the driver transistors
>any time that the outputs blow.
This is particularly true if the drivers in question are in a darlington
configuration with a regular output transistor - their emitter is connected
to the base of the output (for complementary amps).
I don't know offhand if the SX205 has a regular discrete output stage or
whether the outputs are really darlingtons in one package, themselves.
However, my approach of simply checking all the active devices would have
brought the bad driver to light before the first parts order is placed.
>It's often a bad driver that led
>to the problem in the first place, and if it wasn't bad before it
>took all that abuse from the shorted finals, it might be now.
True.
>A very common failure mode is high-voltage second-breakdown in a
>driver, btw. Often that brings the rails down so fast that the
>driver isn't completely cooked, and only the finals are toasted,
>but oh yes, you DO want to replace that driver, and you need a
>(*&*& curve-tracer to even find out if it's cooked out of circuit.
Yes. There is a failure mode of abused transistors where they just loose
their gain. Because of feedback, if the abused transistor is an output, then
its driver ends up working harder, and perhaps becomes abused itself. Then,
it gets hard to predict which device actually shorts out first, but both are
defective.
>Oh, and check the values of any base and emitter resistors. Carefully.
Those wirewound emitter resistors tend to die with the output device
attached to them.
>Along with the behavior of the bias circuitry. An open bias diode
>is a MEAN thing.
Sort of like putting a crowbar across the power supply. Trouble is, the
crowbar is made up of potentially expensive silicon parts that might just
give up the ghost in the process.
Leonard Weldon
Ref: SX-205 problems...
Keep us posted on this situation. I have been using one of these
receivers as a "switch" on my PC to route/drive the audio from the
ICOM PCR1000 scanner/ham radio/SW receiver running
under Win95. I fully intended this neat, cheap little Pioneer
to last forever...it just sits there and runs...low level audio
running the RS LX-5 speakers all emanating from a neat
Turtle Beach soundcard. Pretty good audio too!
Just what seems to have gone wrong. I was aware that the
PS is a bit puny, to say the least...but does it have some other
malady?
Leonard...
____________________________________________
On Tue, 02 Jun 1998 17:26:18 -0700, labX...@erols.com wrote:
:Dear friends -
:
:A couple of weeks ago I posted here about my friends Pioneer SX-205
:
labX...@erols.com
Thanks to all for the good advice. You given me new hunting grounds...
The emitter resistors and driver transistors checked OK before the first
parts order. But I still should have gone further in my troubleshooting.
Now to look this evening at the diodes, as I think (judging by your
description) this could very well be the problem. Give me a couple of
days to get back to you, as I'll be more methodical this time around as
my time/schedule allows.
Thanks again,
Lynn
Leonard, I didn't forget you - I'll get back to you, too.
Sander deWaal
On Tue, 02 Jun 1998 17:26:18 -0700, labX...@erols.com wrote the
following:
>"A" 2SA1943 PNP base/collector .550 base/emitter .154
>"B" 2SC5200 NPN base/collector .525* base/emitter .155
>"C" 2SC5200 NPN base/collector .540* base/emitter .154
>"D" 2SA1943 PNP base/collector .650 base/emitter .654
It's likely, that there's some parallel resistance btw. B/E of the
2SC5200s (which are NOT darlingtons, as suggested in another post).
Try measuring them out of circuit , likely they're still OK.
Now check the drivers.
They're most likely NPN for the NPN outputs, and PNP for the PNP
outputs.
BTW, you knew that 2SA/SB *always* are PNP, and 2SC/SD
*always* are NPN?
The drivers should read about .550 from B to E, and from B to C.
All other directions should give no resistance (in the diode test
range).
In commercial repairs, we almost always replace drivers, too,
since searching for the culprit costs much more than just replacing a
defective component.
>Now as you can see, it's a good thing I didn't plug this thing in yet,
>as there's something else going on here. Whatever caused the transistors
>to "pop" to begin with is still here. I was hoping that they just kinda
>went on to Vahalla on their own, and a simple Yin and Yank replacement
>of the outputs would do it. But alas, tis not to be. Crap. Next time
>I'll keep my volunteering mouth shut...
:-) Well, you still have RAO (should be RATech, but at least,
this thread is *on-topic*) .
>It should be noted here that when I removed the bad transistors, I
>noticed a great potential (arcing) when the collector/emitter was
>shorted on transister "D" and "D" only, as if from a straight-shot from
>the filter caps(?).
Probably a charged supply capacitor. No harm done, since they already
were to be replaced, no?
>There's no other sign, none, of anything wrong. Nothing fried anywhere
>(at least not to the naked eye), and the dual-emitter style resistors
>are all OK. All I have is a DVM and I'm afraid my next step is to remove
>the board and start desoldering componets one by one to check them
>out-of-circuit.
Don't need to.
Check the drivers, and surrounding components.
Another poster suggested checking the bias diodes, but I'm afraid
this poster has not repaired a solid state amp since the '60s :-)
There is a transistor near or under the heatsink, thermically
coupled to it, that regulates bias.
IMO, no need to replace it, since this transistor almost never dies.
>I'd rather jump off the gable end of my house stark naked...
>
>Any *more* help/info would go far in saving a friendship.
In short: drivers, and surrounding resistors.
Check the output transistors again, but likely, they're OK.
Fire it up with a variac, if possible.
Start at hald the mains voltage (60 V for 120 Vac), since some
amps don't like it to run on low votages (due to protection
circuitry).
Don't have a variac? Nevermind, use a 100 W bulb in series
with the mains.
Warning: mains voltages are lethal, so be careful!
Good luck!
_
Sander deWaal
postm...@pegasus.demon.nl
www.pegasus.demon.nl
_______________________________________________
labX...@erols.com
Sander deWaal wrote:
>
> On Tue, 02 Jun 1998 17:26:18 -0700, labX...@erols.com wrote the
> following:
>
> >"A" 2SA1943 PNP base/collector .550 base/emitter .154
> >"B" 2SC5200 NPN base/collector .525* base/emitter .155
> >"C" 2SC5200 NPN base/collector .540* base/emitter .154
> >"D" 2SA1943 PNP base/collector .650 base/emitter .654
The * was for the missing note: "best reading before cap rise".
Unimportant.
> BTW, you knew that 2SA/SB *always* are PNP, and 2SC/SD
> *always* are NPN?
I forgot. I was a tech back in the '70's before life's many twists and
turns took me elsewhere. My knowledge of applied electronics has been
shed on the way (along with all my tools and electronics during the lean
times- one must eat). I feel as though I'm back to 101 at times...
Now that I can afford a nice HP or TEK digital scope etc., my mind just
won't let me justify it. Or a Corvette. Or a mistress. Common sense
intrudes upon the heart (and as to mistress, lawyers would cut it out).
> In commercial repairs, we almost always replace drivers, too,
> since searching for the culprit costs much more than just replacing a
> defective component.
Without a well-equipped bench and stock, this would be a luxury for me.
> >Crap. Next time I'll keep my volunteering mouth shut...
Who said that? Now there's some good advice for me!
> > :-) Well, you still have RAO
I don't know, Sander. The folks around here do tend to skewer each other in
the public square (subjectionivists/objectionivists et al). I respect the
diversity of opinion, but not the personal attacks that have been appearing
as of late. One can have an *honest* and determined viewpoint yet not
trespass into such violence of word. It underminds participation and hides
"truth", or at the very least circumvents mutual grounds for understanding.
I think I'd rather duck into the local pub and watch the excitement thru
the safety of burglar-barred windows wilst quaffing down gin and tonics ;-)
As Rodney Dangerfield has said many times, "It's a tough crowd!".
> >(should be RATech,
This is true and duly noted... You people were nice enough to help me
anyway, continue the thread, and not flame me overboard.
> >but at least, this thread is *on-topic*).
Let's not count our chickens before they're batter dipped.
I'm not thru yet...
> >It should be noted here that when I removed the bad transistors, I
> >noticed a great potential (arcing) when the collector/emitter was
> >shorted on transister "D" and "D" only, as if from a straight-shot from
> >the filter caps(?).
>
> Probably a charged supply capacitor. No harm done, since they already
> were to be replaced, no?
Well... no. What that charge was doing on the C/E of D is beyond me, see?
(I had to say it!) We're talking macho mfd's, as could only be served-up
by the main PS filters. It shouldn't have been there. At any rate, the
spark proved that at least *they're* still good. The decoupling failed.
> >... I'm afraid my next step is to remove the board and start
> >desoldering componets one by one to check them out-of-circuit.
>
> Don't need to.
I'm glad you said that. I don't know why, but I *hate* desoldering-
could it have something to do with all those legs bent-over flat
against the etch before being soldered? I'd rather have my teeth cleaned.
> Another poster suggested checking the bias diodes, but I'm afraid
> this poster has not repaired a solid state amp since the '60s :-)
Ahm... well... he's one step ahead of me. I think my electric toothbrush
has tubes and selenium rectifiers. Why do I suddenly feel so old?
> There is a transistor near or under the heatsink, thermically
> coupled to it, that regulates bias.
> IMO, no need to replace it, since this transistor almost never dies.
One for each side. Pulled/tested/compared anyway. They're OK.
> >I'd rather jump off the gable end of my house stark naked...
Cancelled. No refunds on tickets already sold.
> >Any *more* help/info would go far in saving a friendship.
I'm not going to tax you kind folks any longer. I'm in over my head
and I (we) know it. I no longer have schematics, X-refs, support
electronics of any kind except a DVM. This problem requires better. I
called an acquaintance who owns a shop, and he said that he'd put it on
his "tracker 2K" (whatever the heck that is) and find the problem for me.
All it's going to cost is half a bushel of Chesapeake Bay blue crabs.
It's a deal, and relatively cheap to save a friendship. Whew... (I'm still
advising my friend that next time he wants to buy a low-end receiver,
don't buy new, buy a good used Marantz/Sony/etc. from the late '70's thru
mid '80's. And if it breaks, don't call me. I moved out-of-state).
This should pretty much end this thread on the "SX-205 KaPutt".
I feel a bit let-down, though. I'm sure technicians know the thrill
they get when a light goes on in their heads, and they find the
solution to the problem at hand. Nobody to see or understand the stroke
of genius, no applause. Just a quite sense of self-satisfaction, of
accomplishment. I do miss that. But not the Boss who wants you to
write-up a used part as new, or to tack-on an additional $40 labor
(no splits) because you're too darn good/fast/or just a hard worker...
Thanks to *all of you* for your kind and sincere efforts on my behalf.
"I've shoveled this problem out the door, now I'm going to Disneyworld!"
I wish.
Hot dogs and lemonade on the patio.
Lynn
DAMN! I JUST FOUND OUT HOW MUCH CRAB$ CO$T!!!
Well... maybe I shoulda' pulled a Poppins off the roof...
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