pioneer sx1250 power amp problem

[Lawrence James]

I'm working on an old pioneer sx1250 with a weak right amp. I've swapped
the sections all the way up to the output transistors and the problem stays
on the right side. The output transistors connect straight to the speakers
through a set of relay contacts so there is not much left besides them that
could be be the problem. The unit is very modular and it was easy to swap
stuff. The output transistors check out ok with an ohm meter. Each channel
uses push pull 2 2sb600's and 2 2sd555's wired in parallel. I suppose that
and the 140v power supply is how they get 160watts rms per channel out of
the thing I'm guessing one of the output transistors is bad but I'm not
sure how to figure out which one??? Any suggestions. If it was a regular
push pull I'd just buy a pair but I hate to replace all 4 since they're
about $8 apiece. tia

 

[Mark D. Zacharias]

Very doubtful its any of the outputs, lots of possibilities but I'd look at
the small electrolytic capacitors on the driver board.

Mark Z.

--
Please reply only to Group. I regret this is necessary. Viruses and spam
have rendered my regular e-mail address useless.


"Lawrence James" <jamesgangnc@earthlink.nospam.net> wrote in message
news:QA3Lb.32600$IM3.10212@newsread3.news.atl.earthlink.net...

I'm working on an old pioneer sx1250 with a weak right amp. I've swapped
the sections all the way up to the output transistors and the problem
stays
on the right side. The output transistors connect straight to the
speakers
through a set of relay contacts so there is not much left besides them
that
could be be the problem. The unit is very modular and it was easy to swap
stuff. The output transistors check out ok with an ohm meter. Each
channel
uses push pull 2 2sb600's and 2 2sd555's wired in parallel. I suppose
that
and the 140v power supply is how they get 160watts rms per channel out of
the thing I'm guessing one of the output transistors is bad but I'm
not
sure how to figure out which one??? Any suggestions. If it was a regular
push pull I'd just buy a pair but I hate to replace all 4 since they're
about $8 apiece. tia

 

[Sofie]

Lawrence James:
Check the levels of the L and R channels at the tape record output jacks and
at the preamp output jacks (where it jumpers to the main amp on the back
panel)..... if they are not near the same then the fault could be in the
preamp section which is well before any modules you have swapped. Many
times a small bi-polar transistor or coupling electrolytic will be the cause
of the preamp channel imbalance. If you feel that those preamp levels are
OK then try feeding a signal into the main amp inputs on the back
panel...... if the L & R levels of the mains are not balanced then you will
have narrowed it down to precisely where you need to concentrate your
troubleshooting effort...... electrolytics? driver transistors? Most
probably not the power output T0-3 devices..... if shorted or leaky they
probably would trigger the protection circuit or blow a fuse.... but you
can take them out of circuit and easily check them for leakage and open or
shorted junctions with your multimeter if in doubt or even swap them with
the other channel.... they are in sockets so they are not difficult to
remove and replace..
The sx1250 is a terrific unit.... built like a brick..... huge power supply
reserve, tons of conservatively rated clean power with the ability to drive
hard into low impedance speaker loads, extremely low distortion, low hum and
noise..... runs nice and cool..... great FM sensitivity and terrific
selectivity, terrific phono preamp (x2) and will out-perform just about
anything available today at any price...... take great care in repairing
this very fine piece of equipment.
The modern A/V receivers of today are all concerned with DTS, ProLogic,
colorful and flashy displays, etc, etc.... and NOT with the excellent specs
and performance that typified these top of the line, high power receivers
from the mid 70's to late 80's manufactured by Pioneer, Sansui, Yamaha,
Kenwood, Onkyo, Sony and others.
--
Best Regards,
Daniel Sofie
Electronics Supply & Repair
----------------------------


"Lawrence James" <jamesgangnc@earthlink.nospam.net> wrote in message

I'm working on an old pioneer sx1250 with a weak right amp. I've swapped
the sections all the way up to the output transistors and the problem
stays
on the right side. The output transistors connect straight to the
speakers
through a set of relay contacts so there is not much left besides them
that
could be be the problem. The unit is very modular and it was easy to swap
stuff. The output transistors check out ok with an ohm meter. Each
channel
uses push pull 2 2sb600's and 2 2sd555's wired in parallel. I suppose
that
and the 140v power supply is how they get 160watts rms per channel out of
the thing I'm guessing one of the output transistors is bad but I'm
not
sure how to figure out which one??? Any suggestions. If it was a regular
push pull I'd just buy a pair but I hate to replace all 4 since they're
about $8 apiece. tia

 

[David]

The relay contacts are a common problems on those. Insert a mono sine wave
into the left and right channels at the same amplitude. Look at the levels
on a dual channel scope at various places until you see the imbalance.

"Lawrence James" <jamesgangnc@earthlink.nospam.net> wrote in message
news:QA3Lb.32600$IM3.10212@newsread3.news.atl.earthlink.net...

I'm working on an old pioneer sx1250 with a weak right amp. I've swapped
the sections all the way up to the output transistors and the problem
stays
on the right side. The output transistors connect straight to the
speakers
through a set of relay contacts so there is not much left besides them
that
could be be the problem. The unit is very modular and it was easy to swap
stuff. The output transistors check out ok with an ohm meter. Each
channel
uses push pull 2 2sb600's and 2 2sd555's wired in parallel. I suppose
that
and the 140v power supply is how they get 160watts rms per channel out of
the thing I'm guessing one of the output transistors is bad but I'm
not
sure how to figure out which one??? Any suggestions. If it was a regular
push pull I'd just buy a pair but I hate to replace all 4 since they're
about $8 apiece. tia

 

[Lawrence James]

Daniel,
Thanks for the suggestions. I've isolated it to the power section by cross
connecting the channels at the pre-out / amp in. The weak channel stayed on
the right side. So I know I'm down to the power amp. I did not know the
output transistors were in sockets, that will help a lot. I've had it
forever, toasted a lot of speakers with it. It's a great unit. You can
tell it has serious power just from the power cord. Gotta love these new
systems that claim to have some huge output power and then have a wimpy lamp
cord on them, yea right.

David,
You mean the relay contacts on the protection relay where the speaker output
line is? I sort of dimissed it since they had ganged together two contacts
for each channel. I should be able to jumper it easily enough and see if
that is my problem.

I'm gathering from everyones comments that the output transistors would
probably either work or not. A weak channel is probably not them. I'll
keep poking around and see what I find. I've got an old scope but not an
audio generator. I really need to get one of those. Thanks everyone

"Sofie" <sofie@olypen.com> wrote in message
news:vvpl40nic5mk54@corp.supernews.com...

Lawrence James:
Check the levels of the L and R channels at the tape record output jacks
and
at the preamp output jacks (where it jumpers to the main amp on the back
panel)..... if they are not near the same then the fault could be in the
preamp section which is well before any modules you have swapped. Many
times a small bi-polar transistor or coupling electrolytic will be the
cause
of the preamp channel imbalance. If you feel that those preamp levels are
OK then try feeding a signal into the main amp inputs on the back
panel...... if the L & R levels of the mains are not balanced then you
will
have narrowed it down to precisely where you need to concentrate your
troubleshooting effort...... electrolytics? driver transistors? Most
probably not the power output T0-3 devices..... if shorted or leaky they
probably would trigger the protection circuit or blow a fuse.... but you
can take them out of circuit and easily check them for leakage and open or
shorted junctions with your multimeter if in doubt or even swap them with
the other channel.... they are in sockets so they are not difficult to
remove and replace..
The sx1250 is a terrific unit.... built like a brick..... huge power
supply
reserve, tons of conservatively rated clean power with the ability to
drive
hard into low impedance speaker loads, extremely low distortion, low hum
and
noise..... runs nice and cool..... great FM sensitivity and terrific
selectivity, terrific phono preamp (x2) and will out-perform just about
anything available today at any price...... take great care in repairing
this very fine piece of equipment.
The modern A/V receivers of today are all concerned with DTS, ProLogic,
colorful and flashy displays, etc, etc.... and NOT with the excellent
specs
and performance that typified these top of the line, high power receivers
from the mid 70's to late 80's manufactured by Pioneer, Sansui, Yamaha,
Kenwood, Onkyo, Sony and others.
--
Best Regards,
Daniel Sofie
Electronics Supply & Repair
----------------------------


"Lawrence James" <jamesgangnc@earthlink.nospam.net> wrote in message
I'm working on an old pioneer sx1250 with a weak right amp. I've
swapped
the sections all the way up to the output transistors and the problem
stays
on the right side. The output transistors connect straight to the
speakers
through a set of relay contacts so there is not much left besides them
that
could be be the problem. The unit is very modular and it was easy to
swap
stuff. The output transistors check out ok with an ohm meter. Each
channel
uses push pull 2 2sb600's and 2 2sd555's wired in parallel. I suppose
that
and the 140v power supply is how they get 160watts rms per channel out
of
the thing I'm guessing one of the output transistors is bad but I'm
not
sure how to figure out which one??? Any suggestions. If it was a
regular
push pull I'd just buy a pair but I hate to replace all 4 since they're
about $8 apiece. tia