Pioneer SX-1250 -what have I done?

randballen

randballen

Active Member
Today I was working on a recently acquired SX-1250. This unit was in really good condition but the left channel ran very hot. So, today I decided to work on the DC offset and the bias. After getting both channels darn near zero on the offset and the bias at 100v I thought I had everything in good shape. However upon turning back on, the left channel ( the previous hot one) was dead. Worked on mono, dead on stereo. What the heck did I do that may have caused this?
 

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I did not touch the stabilizer ..the directions confused me regarding the. +/- connections (page 38) Can you confirm the connections?
 
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think you mean 100mv ?
100v would be smoking well before you got there .
not sure on this but is it not recommended to set these lower than 100mv ?
 
100 MILLIVOLTS

what avionic was gently prodding about was using the correct units.

100 volts at those test points is impossible without "severe tire damage".


ok, now the phrase:
Worked on mono, dead on stereo
Click to expand...

exonerates your work on the power amp. MONO working means that the switches somewhere are not passing along the signal for that channel, so when the "switch of last resort" MONO is turned on, the signals from both channels are connected together. Check the manual's block diagram for the "control section".
S20 is mono, while one or more of the: function selector switches (AM, FM, PHONO 1, PHONO 2 & AUX ), TAPE 1, TAPE 2 & ADAPTER switches may be interrupting the signal.

the cure: deoxit and exercise.

And yes, 100mV is high, 50mV is also high, while 35mV isn't LOW.
The setting determines where the point is that the amp switches over from class A operation to class B operation, and realistically - somewhere lower than that is the distortion "knee" where further increases do NOT decrease distortion, only increase dissipated heat.


edit: the guys beat me, because I stopped to get and open up the service manual.... :deal:
 
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UPDATE: switched to a tape source. Perfect! Both channels sound great and left channel now staying cool... Niw, I noticed when the channel problem appeared on FM, the pressing of the multi path button made the sound very distorted in stereo but when the button turned off, no left channel.. What is the multipath option and could this be the dirty scoundrel causing the FM problem?
 
6 o'clock setting from what i see .. but what i see that meter is far from worthy of the job . you need to invest in something more worthy . i went and bought something similar for setting up a 7kva generator output current .it was less than useless .. the 1250 deserves more accuracy ....
 
petehall347 said:
6 o'clock setting from what i see .. but what i see that meter is far from worthy of the job . you need to invest in something more worthy . i went and bought something similar for setting up a 7kva generator output current .it was less than useless .. the 1250 deserves more accuracy ....
Click to expand...
Agree. Next on the purchase list...
 
exercise your FM multipath switch (with deoxit if possible)

the stereo output of the FM tuner board circuitry goes into the multi-path switch, as well as the multi-path signal - which is MONAURAL.
When the multipath switch is off (OUT) the stereo signals come into and go out of the multip[ath switch and continue over to the FM function selector switch.
When the multipath switch is ON (IN) the stereo signals are INTERRUPTED and the
MONAURAL multipath signal is sent out on both the left and right channels over to the FM function selector switch.

Thus having L and R working when multipath is selected exonerates the FM function selector switch.
That only leaves the left channel contact feeding the left channel part of the multipath switch, the output (center) contact and the multipath contact are ok.
]

Now as to what the multipath channel signal is: it's the same signal as being fed to the signal strength meter, without the filtering. It's used to point the antenna to AIM at the best received signal at that location, for stereo. There is interference that you would HEAR even though the meter isn't capable of indicating it - it interferes with stereo.
https://web.archive.org/web/20070708132643/http://www.wgbh.org/cainan/article?item_id=1360980
 
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How to deoxit switches.

Remove face plate.
Get hose berween spring and aim down shaft.
Squirt and work switch.
Repeat if needed.
 
randballen said:
I did not touch the stabilizer ..the directions confused me regarding the. +/- connections (page 38) Can you confirm the connections?
Click to expand...

Connect to a pin with one lead.
Connect to chassis with the other lead.
Ignore the sign on the meter and adjust to 65V.

The pins are labeled on the board.
Read the instructions again.
The ground pin is not there. Use the chassis.
I think there are only 2 pins so the instructions above are the simplified version.
There are only 2 pins on the one I fixed last night.

The old service manuals were written for analog meters that only swing one way.
Digital work either way. They read negative if you connect backward.
 
Well, I was able to spend a little time with the 1250 today. Took on the faceplate and sent stone contact cleaner in to the multipath switch and worjpked it on and off for a few minutes. Turned it on to check the fm and I had stereo! Unfortunately it lasted for only a few minutes. Shot it with a little more cleaner, worked the button and let it sit for a while. Back to right channel only in stereo. When I depress the multipath butto, both channels work but it is very messy scratchy sounding. Is it possible that the switch itself is bad? So close, darn it.
 
blhagstrom said:
I have had to open switches and polish the bar.
One looked like assembly glue seeped in.
Click to expand...
Yes I concur, I have done recently a SX750, 850 and 950, ALL of them needed the switches to be completely dismantled and the contacts polished to remove the oxidation.
They literally turn black with oxidation and no amount of contact cleaner can remove it whilst the switch is together....
It's a very delicate job, time consuming and requires a lot of patience...
 
I had this same exact problem recently with a 1050, it was a faulty reed relay, try tapping on it and see if it comes back, mine had rusted inside the relay from being in a shed for 25 years. The 1250 and 1050 use the same reed relay, you could also remove the relay and see if you get your both channels working.Tuner board before.JPG
 
tsd71 said:
I had this same exact problem recently with a 1050, it was a faulty reed relay, try tapping on it and see if it comes back, mine had rusted inside the relay from being in a shed for 25 years. The 1250 and 1050 use the same reed relay, you could also remove the relay and see if you get your both channels working.View attachment 940641
Click to expand...
Great! I'll give it a look this evening. Thanks!
 
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