AM issues with SX-1250

nrenter

nrenter

Active Member
My SX-1250 seems to have issues with AM reception. Even with an external powered antenna I can *barely* tune in any AM stations. I'm in between Dallas and Ft. Worth (and I'm an AM radio junkie) so I know signal power is not an issue (as my car can receive DFW AM during the drive from Austin to Houston - 4 hours away). FM reception is fine.

Any thoughts or help would be appreciated.

On a side note, why are the AM radio markings not equidistant from each other (like the FM radio markings)?
 
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Is the ferrite bar antenna on the back of the unit OK and its wires intact? The bar antenna has a tunable slider or core which might have been moved. From the factory that bar antenna alignment would have been locked in place with threadlock. Otherwise might need an alignment at the least, or have a component failure.
 
The antenna on the back of the unit appears to be in perfect physical shape and all wires appear intact. Looking down the barrel, the threadlock does not appear to have been disturbed. I was really hoping this wasn't a component issue.
 
Could be an alignment problem, maybe one of the trim caps TC1-3 have drifted off, or less likely T3, T4 coils, or maybe the HA1138 AM IC itself or associated circuitry. :scratch2:
 
I have noticed that if the bar antenna is kept flat to the back of the case, reception is severely compromised. Get it a ways out, an perpendicular to the case.


OR the power to the chip could be compromised (by a shorted or leaky C50 220uf, 16v), so check pin 12 of the HA1138 chip for +12v .

Also check the voltage at pin 19 of the tuner board itself (outbound to function switch) and pin 22 of the tuner board (inbound from the function switch to certain parts of the tuner chip) as a bad contact here could compromise the local oscillator. Both should be at 12v with AM selected.


Otherwise some tweaking may be in order.

RE: AM markings - that's the "curve" of capacitance versus rotational position on the tuning capacitor. I'd have to look at the equations a lot harder to see the derivation of frequency in the circuits, BUT one thought to contemplate is that from the bottom of the band's 520kHz to 1610kHz at the top of the band is a 3:1 (300%) frequency change while FM at 88 to 108 MHz is a much smaller ~ 20% proportionate change.
 
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Actually, reception is best with the bar flat against the back of the receiver. When I move the arm, reception goes from very, very poor to nonexistant. I also have a powered Terk antenna attached to the terminals (and the external antenna does help FM reception).

Also, the volume must be about 25 to 30 dB higher for a comparable FM volume.

Should the signal and tuning meters move with respect to AM reception? neither of mind budge. The function with FM reception.

Thanks for the AM markings insight! I thoght it was interesting that the distance between 140 and 150 was significantly larger than the distance between 150 and 160, and the distance between 80 and 90 was significantly smaller than between 60 and 70.
 
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nrenter said:
Actually, reception is best with the bar flat against the back of the receiver. When I move the arm, reception goes from very, very poor to nonexistant. I also have a powered Terk antenna attached to the terminals (and the external antenna does help FM reception).

Also, the volume must be about 25 to 30 dB higher for a comparable FM volume.

Should the signal and tuning meters move with respect to AM reception? neither of mind budge. The function with FM reception.

Thanks for the AM markings insight! I thoght it was interesting that the distance between 140 and 150 was significantly larger than the distance between 150 and 160, and the distance between 80 and 90 was significantly smaller than between 60 and 70.
Click to expand...

Get those voltage readings.... negative (black) probe to bare metal chassis ground, probe with red. Insulate all but the very TIP of the probe so when you slip (we ALL do), you don't short anything out.

Yes, signal meter should deflect. No voltage on pin 22 means the AM function switch needs cleaning.
 
OK...I pulled out my trust DMM:

Pin 19 of the tuner board itself - 11.23 V
Pin 22 of the tuner board itself - 11.23 V

Pin 12 of the HA1138 chip - 12.33 V
 
It's been a while since I've worked with ICs. IIRC, the pinout is as follows...

|(16) (15) (14) (13) (12) (11) (10) (09)|
|----------------PIONEER-------------(0)|
|(1)------------HA1138 6K2--------------|
|(01) (02) (03) (04) (05) (06) (07) (08)|
 
nrenter said:
Pin 7 of the HA1138 chip - 1.913 V
Click to expand...


So much for that theory.

While you're at it might as well check all the voltages.
Don't know if you've downloaded the service manual or not.

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Thanks for the diagram! I grabbed the SM from another site, but don't see this detailed of a drawing.
 
I have poor AM radio reception inside my house with my 1250 too. Have you tried taking it outside to see if it works any better? What I ended up doing was getting a Twin Coil Ferrite external Antenna from CC Crane http://www.ccrane.com/antennas/am-antennas/twin-coil-ferrite-am-antenna.aspx
The stations come in like gang busters now. But if you really want to bring in those AM stations from all over the USA, get their CCRadio http://www.ccrane.com/radios/am-fm-radios/ccradio-2.aspx and put that Twin Coil Ferrite antenna on it. That is what I ended up doing.
 
I have not taken it outside. I do have it connected to an external, powered Terk antenna (with no reception difference with AM). However, it does help with FM.
 
Well I would take it outside to see if it works any better. If it doesn't, then there is something wrong with the receiver. If it works better, then something in your house is blocking the signal.
 
I'm back from the holiday travels...Here are all the readings from the HA 1138:

1. 11.22 V
2. 2.675 V
3. 11.23 V
4. 11.23 V
5. 4.70 V
6. 9.23 V
7. 1.92 V
8. 0.867 V
9. 0.0006 V
10. 3.560 V
11. 4.15 V
12. 12.33 V
13. 2.670 V
14. 2.666 V
15. 2.640 V
16. 0.0001 V

Nothing really seems out of order with regards to the measurements.
 
Could be an alignment issue. You could try tweaking trim caps TC2 and TC3 for max signal. TC1 sets the local oscillator along with with coil T3 so if the dial calibration is close no need to mess with those.
 
AM 820 (WBAP) is "received" right at 95.2 on the FM scale (about where it should). I'm guessing AM 1540 (KZMP) is "received" right at 107.2 on the FM scale (pretty close to where it should).

I say "received" because I have to have the volume at -28 dB (with Pioneer CS-99a speakers) to hear it, and it get about as much noise as I do signal. The pegged at 0 and the Tuning meter does not budge.

I'm willing to try anything, but go slow and assume I'm not that bright. :)
 
I had trouble with my SX-980 recently with the scale not being accurate for AM. There are adjustments inside for this. I would get a service manual and follow the procedure for AM alignment. My scale is now accurate and everything seems right. It receives local stations well without an external antenna, but seems like it should work better than it does. Something may still not be perfect, but the best advice that I know of is to just follow what they say in the "service manual".:thmbsp:
 
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