FM tuning and stereo

RocketMonkey

Active Member
I've got a 2275 that has reception but no FM tuning meter deflection and no stereo indication. The tuning meter moves just a tiny bit but always comes back to center. It appears that it is playing in mono I'll prove that here shortly. Can someone describe how the FM tuning meter works? Thanks.
 
Ok so no stereo, no lamp. H321 pin 4 and 5 are at about 9.5 volts dc. Pin 6 will not ground, grounding it with a wire will light the lamp. If pin 8 changes state properly but 4 and 5 seem common (I'm assuming they aren't shorted but both output the exact same signal) and 8 never goes low, is my chip screwed up?
 
Im not saying the chips cannot be screwed up, but its rare. Is the chip receiving trash however....peek at the SM and see how the meters are driven and for stereo, that depends on detecting a pilot signal - is the last IF section to the stereo detection ok?
 
Dunno, how do I check it. I'm not really a radio guy. The tuning meter does not deflect so I know there are problems.
 
This applies to the 2275, 2325 and probably the 2250B as well although some of the component designators are different.
Note: An 'ON' NPN transistor has a base voltage about 0.6V more positive than the emitter. This will cause the collector to become very near the emitter voltage.
Wire-anded: the junction of two or more components where either one can pull that point to or near ground potential.

FM Stereo MPX operation:
Intermediate Frequency (IF) signal strength is monitored by H308. If signal strength is above muting level, H308 is ON, H309 is OFF.
If there is excessive noise, then H311 is ON which is wire-anded to H309 keeping H301 (FET) OFF.
If the discriminator is outputting DC due to the tuning being off station, either H314 or H315 is ON which are wire-anded to H309 as well. This also turns H303 ON grounding Pin 8 of the decoder forcing MONO operation.
If the station is properly tuned, not noisy and the signal strength is high enough,
H308 is ON, SS OK
H309 is OFF, SS OK
H311 is OFF, Noise is OK
H314 is OFF Tuning centered
H315 is OFF Tuning centered
H317 is OFF, SS, Noise and center tuning OK then:
H301 is ON allowing audio and MPX signal to pass into the decoder.
H303 is OFF If SS sufficient for Stereo operation and MONO button is not pressed
H302 is OFF VCO runs (AM not selected)

Tom
 
So i ran through all that and H317 is on. Also H314 and H315 never switch on but they are off. Also I just discovered that H208 has .7 volts on the collector.
 
Sound like a discriminator problem.
That Discriminator transformer, L201, LI1401623 is used in a lot of different receivers so it's not too hard to find if that's the problem.

Tom
 
It works correct, I can play static or a station with mute off and mute on wont cut out a radio station. After dinner I'll figure out my h208 issue.
 
Yep,
Just had this exact same problem on a 2285(not b) which ended up being a discriminator problem on the P200 IF board.
The 2285 has identical circuitry to the 2275.

Bob
 
So, lifted R273 and have 21 ohms to ground from that leg of the coil. H208 is out also so somthing is shorted out to the can or something.
 
Ok so still no stereo but i can deflect to the right and center, no left. The little capacitors in the coil looked dark like they have neen wet, i think this thing went for a swim.
 
In my case l201 did not fail but the ferrite core lost its strength with time so i had to screw it in a bit
You need to check voltage on j204 it should be as close as possible to zero (tuning needle is in th middle)
Use plastic hex screwdriver to do so and turn very carefully
And only screw in top core as there are two of them

Luck
Robin
 
is the rest of the ratio detector in good shape? are the diodes equal, the little caps c240/241 the same, the resistors the same? and is Q210 (H210) ok?
 
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