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SX-1250 discriminator adjustment

bzb882

bzb882

AK Subscriber
Subscriber
I’ve finished recapping, etc. my SX-1250 (didn’t do the tuner, I counted around 166 parts replaced). It sounds terrific, but the tuning meter needle is off to the right of the black area when fm is properly tuned in. From the service manual (page 36) it looks like I need to adjust the discriminator (T1) to center the needle with the dial adjusted to noise (needle is still over to the right when noise is present and I’m not picking up a station). This will be my first slug adjustment, does anyone know which specific tool I need and can someone give me some tips on how those slugs adjust (do I just insert the tool and turn, or do I first loosen the slug, adjust and then tighten)?
 
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you need a non metallic tool .plastic of some sort .
my uncle used to use my aunties plastic crochet needles and file the end to suit ..
i will never forget her complaining about it
 
I had the same issues with my 1250. I had finished the repair and recap of all boards except the tuner. What the heck! So I recaped the tuner knowing I have a local Tech (Bobby) to fix or align if I needed it. FM/AM reception and FM separation seemed to be back in specs as well as the tuning functions (as well as the meter indications)! From the sounds of it I got lucky but this is the third tuner recap and only very miner adjustments were needed by Bobby to put it back into specs. The tuner board is very accessible and parts may add up to $15.00 at the most. I used MTF's list posted on Matts Thread (page 7).

Continue at your own risk!

http://www.audiokarma.org/forums/showthread.php?t=420950&page=7

sx-1250,tuner,ok,,,,,,,
sx-1250,awe-068,C4,220,16,cea,,220,16,647-UPW1C221MPD
sx-1250,awe-068,C10,0.47,50,cea,,0.47,50,667-ECQ-V1H474JL
sx-1250,awe-068,C23,10,16,csza,,10,50,647-UKL1H100KDDANA
sx-1250,awe-068,C24,47,16,ceanl,,47,25,647-UKL1V470MPDANA
sx-1250,awe-068,C25,470,16,cea,,470,25,647-UPW1E471MPD
sx-1250,awe-068,C26,4.7,25,ceanl,,4.7,25,647-UKL1E4R7KDDANA
sx-1250,awe-068,C28,10,16,csza,,10,50,647-UKL1H100KDDANA
sx-1250,awe-068,C29,10,16,csza,,10,50,647-UKL1H100KDDANA
sx-1250,awe-068,C32,100,16,cea,,100,16,647-UPW1C101MED
sx-1250,awe-068,C33,1,50,cea,,1,50,647-UPW1H010MDD
sx-1250,awe-068,C35,0.1,35,csza,,0.1,50,598-DSF050J
sx-1250,awe-068,C36,1,25,ceanl,,1,50,647-UKL1H010KDDANA
sx-1250,awe-068,C38,1,25,ceanl,,1,50,647-UKL1H010KDDANA
sx-1250,awe-068,C41,1,10,csza,,1,50,647-UKL1H010KDDANA
sx-1250,awe-068,C42,3.3,10,csza,,3.3,50,647-UKL1H3R3KDDANA
sx-1250,awe-068,C43,0.22,25,csza,,0.22,50,667-ECQ-V1H224JL
sx-1250,awe-068,C48,4.7,25,ceanl,,4.7,25,647-UKL1E4R7KDDANA
sx-1250,awe-068,C56,10,16,csza,,10,50,647-UKL1H100KDDANA
sx-1250,awe-068,C59,0.1,35,csza,,0.1,50,598-DSF050J
sx-1250,awe-068,C60,220,16,cea,,220,16,647-UPW1C221MPD
sx-1250,awe-068,C61,47,16,ceanl,,47,25,647-UKL1V470MPDANA
 
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Not recommended. That said:

there are two slugs in the transformer and they INTERACT.

just getting the center of channel set may hurt the distortion performance IF the upper coil has not moved since it was last aligned.
You MIGHT get lucky that the zero is off and bringing it back in gets it to the original setting.

The tool is getting harder and harder to find.

It is a plastic hex that has a longer shaft and is undercut so that it can access the lower coil without disturbing the upper coil.

try "alignment KIT" on ebay, then sort them out from the car stuff. usually multi-colored.

warning - you HAVE to find a dead spot on the dial, where there are NO stations, ANY distant station will pull it out... shorting the antenna inputs (all of them) to the antenna ground screw right there may help. I have the handicap of a full spectrum here in the Chicago area. But I manage...
 
T1 is the center meter adjust only. T2 is the ratio detector discriminator. Check DCV as in step 20, if it is within tolarance +/-10mV, then you do not have to adjust the top slug and leave it be. In order to adjust the bottom slug, you must use a distortion analyzer to properly set the ratio detector discriminator for minimum distortion with a 75KHz deviation, modulated signal (400/1K Hz) at 98MHz.
I'd monitor as in step 20, as you adjust T1 for center, there maybe a point where both coincide, so that you do not need to adjust T2, but of course it is always good to go through the whole procedure and complete the adjustment procedure.
See SM section 9.2, steps 15-23
Boy am I glad I bought those old TV tuning kits years ago.
By rights, changing ecaps in the tuner should not require a tune up, (they are only used for coupling and filtering) but removing the varcap shaft may screw it up, if you don't do it right.
 
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rcs16 said:
T1 is the center meter adjust only. T2 is the ratio detector discriminator. Check DCV as in step 20, if it is within tolarance +/-10mV, then you do not have to adjust the top slug and leave it be. In order to adjust the bottom slug, you must use a distortion analyzer to properly set the ratio detector discriminator for minimum distortion with a 75KHz deviation, modulated signal (400/1K Hz) at 98MHz.
I'd monitor as in step 20, as you adjust T1 for center, there maybe a point where both coincide, so that you do not need to adjust T2, but of course it is always good to go through the whole procedure and complete the adjustment procedure.
See SM section 9.2, steps 15-23
Boy am I glad I bought those old TV tuning kits years ago.
By rights, changing ecaps in the tuner should not require a tune up, (they are only used for coupling and filtering) but removing the varcap shaft may screw it up, if you don't do it right.
Click to expand...

There's one or two lower value filtering caps in some detector IC circuits I am leery of, but the real effect is the stereo decoder section where the separation and pilot rejection gets knocked way off, that's where I say totl performance drops to motl or botl levels...
 
I see that C28,29 might affect separation and 19KHz pilot rejection, so of coarse a complete tuning is the best route to follow.
What do you use for a stereo modulator? or is it built into your RF SG? and what type of RF SG do you use?
 
The 1250 has two discriminators, one is the meter section HA1137 IC, but the actual sound discriminator is the TA7061ap and the two have to agree.

The meter sets the one, but the other is the voltmeter reading.

Luckily the st1000a has a CW setting where I can set exact center frequency using a frequency counter.
 
Well, I finally decided to recap the tuner board. I first checked with my local repair shop to make sure they could do an alignment if I needed one and they could. I also numbered all the caps on the board and took pictures of them numbered (in case I really ended up with a mess and wanted to put the old caps back). Things turned out pretty good. FM sounds a little better, AM is pulling in more stations than before the recap. The tuning meter needle is now dead center when a station is at max strength. The FM dial is a little off, but that was the case before the recap.
 
I had been using Terk Technology's AM/FM Radio Antennas on all my connected (operational) sets. I live on the west side of the Las Vegas valley and could get the stations in reasonably well. At a yard sale this weekend, I found a RCA Rabbit Ears with a AM loop antenna attached to a small heavy base for 50 cents, so I bought it. Thinking that the modern Terk would out perform it, I connected to the 1050 and man what a surprising difference in reception. All the stations come in near 4 or better on the strength meter and separation is dialed in to the degree (by ear).

Knowledgeable AK members all mention making sure that a good antenna is connected and a good known station is tuned in for the best reliable checking of the tuner. I thought I had a good antenna until I found this 50 cent one.

I just wanted to mention this because of my surprise in old technology. :D
 
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markthefixer said:
Not recommended. That said:

there are two slugs in the transformer and they INTERACT.

just getting the center of channel set may hurt the distortion performance IF the upper coil has not moved since it was last aligned.
You MIGHT get lucky that the zero is off and bringing it back in gets it to the original setting.

The tool is getting harder and harder to find.

It is a plastic hex that has a longer shaft and is undercut so that it can access the lower coil without disturbing the upper coil.

try "alignment KIT" on ebay, then sort them out from the car stuff. usually multi-colored.

warning - you HAVE to find a dead spot on the dial, where there are NO stations, ANY distant station will pull it out... shorting the antenna inputs (all of them) to the antenna ground screw right there may help. I have the handicap of a full spectrum here in the Chicago area. But I manage...
Click to expand...

Finding and using those dead spots is critical and I find a set of headphones really helps.
 
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