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SX-1250 Tuner board.. oops

M

MisterV

New Member
Hi everyone, I revived my very sick SX-1250 using the rebuild kit sold by Peace Love and Music. This is a kit for ALL electrolytic caps, and quite a few transistors and diodes.

Good news... The Amp is fine. Both channels sound equal and clear. The phono board is fine.. both channels sound good. AM Sounds OK...

FM, is not so good. The tuner board has only 1 channel playing. Using a signal tracer, I listened to the audio out pins of the tuner board 40 and 42.

Pin 42 Has adequate volume, and the channel sounds properly through the amp.

Pin 40 has LOW volume, and can be heard through the amp, IF I shift the balance to that side and crank it up... SO, I'm convinced that something on the tuner board. This is where I get perplexed, because I'm just not that good with radio circuits. I'm certain it's something not too complex, but I was hoping that a more experienced radio tech could point me to a few test points.

I usually fix guitar amps.........so this is a bit different of territory. Thank you in advance.
 
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i have not worked on a 1250 but i have the schematic. it looks like it uses a reed relay(circled in red) for fm muting. in my sx1010, the relay was 1/2 stuck and caused one channel to be "dead"

how about using your signal tracer and probing area marked in blue? i'd probe caps c36/38 or maybe r43/41 or r42/40.

good luck. i'm not a tuner guy either but i have been known to pull a few rabbits out my hat.
 

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i'm not sure how far back in the circuit the muting relay will "kill" the signal so you may want to jump the solder pads of the relay's signal path. just be sure you know which pads not to jumper.
 
I have seen the reed relays stick (although usually only one reed). The relay mutes the FM when it is activated. Typically if the relay is stuck closed (muting one or both channels), tapping on it with a screwdriver handle fill free it, but it will stick again on the next cycle. I have tried demagnetizing the relay with a tape head demagnetizer, but that was no help. I have some "Electrol" 24 volt dual reed relays I use for a replacement, but they are obsolete and no longer available. Disconnecting the two pins at the end of the relay will disconnect the reeds in the relay, and prevent muting. Or just remove the relay all together as a test. Disconnecting the outside 2 terminals at the end with 4 (removing power to the coil), will not typically relieve the issue, as the reed is stuck closed.
 
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it slipped my mind earlier...you may be lucky and be able to move the relay by sliding a small magnet next to it. i've done that a couple of times.

a new relay assembly can be made, there is a thread on here somewhere. i've done it on an h/k pre and my sx1010

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I've done a little more probing...... on the tuner board, looks like Pins 1 and 2 go to AWX-099... 1 is strong, 2 is weak. I feel like I'm maybe going the wrong direction though.
 
Two coupling caps C36 and C38? 1/25. Use a film.
Check the signal at the HA1196 pins 6 and 7? There's a 6.3v DC component there too.
 
You need to measure to measure pins 40 to ground and 42 to ground on the tuner board with an ohm meter. If one is showing continuity to ground, one of the relay reeds is frozen closed. The relay shorts the signal to ground when closed, to mute the audio. I have seen the contacts on the muting reed relays stuck closed multiple times. This test will be absolutely conclusive for the condition of the relay.
 
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