Yamaha CR-2020 / FM distorted - AM wierd

w1jim

I can fix it but good...
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My CR-2020 adventure continues. I've gotten the power amp working and I've done the service bulletins.

Now my current (last?) problem is with the tuner section. The FM reception is distorted, regardless of the signal strength. Also, when I release the FM Mono switch I get a 'pop'.
On AM (which I'm not especially concerned with) I get a clicking sound that increases in frequency the stronger the signal is. When I received this the AM ferrite bar antenna was missing so I installed one from a CR-400; I matched the wire colors.
I've recapped (electrolytics) the tuner/preamp PCB and replaced all the tantalum caps with electrolytics.
I've tested the exposed transistors except for the 2SK68A's (I'm not sure how to test J-FETs). There are no apparent bad solder joints (I've reflowed most). I have not opened up the tuning cap assembly.

When I received this receiver it was packed in a wooden crate that had minimal shock absorbent material so I'm fearful that it was dropped and something has cracked.

Thoughts?

JimB
 
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I tried tweaking VR101 ("Dist Adj") as well as T101 and 1FT but could hear no discernible change. It's kind of difficult to pick out the distortion by ear.

I was thinking that perhaps I inserted a cap in backwards but they all seem to match up with the PCB markings; any errors on the board anyone is familiar with? When I replaced the caps I did them one-by-one to discourage errors.

I do have a 20-mhz scope, an audio frequency generator, various meters and tons of parts to assist in this quest. I also have a Sencore SG165 (which I'm not 100% certain is functional - I guess it's time to read the instructions).
I was hoping that there is some common failure; that would possibly simplify (or mislead) a repair.

JimB
 
It sounds like two problems AM and FM. I would check the B+ voltages to the FM MPX. It sounds like the MPX IC might be toast, based on the Stereo/mono "pop". The ticking sounds maybe the loopstick may not be tuned. Try a non-conductive slotted driver to peak the AM output.
 
I've recapped (electrolytics) the tuner/preamp PCB and replaced all the tantalum caps with electrolytics.

My rule of thumb when restoring a piece is to replace tantalums everywhere BUT THE TUNER SECTIONS, as I don't have the equipment or expertise to do an alignment.
 
My rule of thumb when restoring a piece is to replace tantalums everywhere BUT THE TUNER SECTIONS, as I don't have the equipment or expertise to do an alignment.

Generally speaking, tants and electrolytics aren't used for the RF path or anything tuned. ....but just getting access to desolder them, some inadvertant detuning can occur. Avoidance might be the safest practice unless the plan is to have it aligned after a full recapping.
 
My rule of thumb when restoring a piece is to replace tantalums everywhere BUT THE TUNER SECTIONS, as I don't have the equipment or expertise to do an alignment.
The tantulums I replaced were all in the preamp section so I should be safe there. I don't think I disturbed anything in accessing these but I did have to partially dismount the PCB to get to the underside for those.

The stereo light comes on - but only dimly. I measured the voltages (DC) into IC104/LA3350:
  1. 12.8
  2. 3.7
  3. 4.0
  4. 8.75
  5. 8.75
  6. 4.0
  7. Ground
  8. Ground
  9. 4.0
  10. 3.8
  11. 3.8
  12. 4.0
  13. 3.8
  14. 3.8
  15. 3.4
  16. 3.4

Curious; C142 (0.68/50V) shows as a non-polarized on the schematic but was polar and indicates polarity on the PCB. I doubt that's an issue but I did notice it.
Thanks for the input.
JimB
 
...The ticking sounds maybe the loopstick may not be tuned. Try a non-conductive slotted driver to peak the AM output.
I tried moving the slug in the AM ferrite bar antenna. I changes the clicking just a little bit. Again, the AM problem is secondary. Oh, and I discovered I can't spell "weird"!
Good to see that the MPX chips are still available (if that's indeed the issue). Does that chip control the stereo light?
JimB
 
I ordered a new LA3350 mpx IC and installed a socket. Hopefully that'll do the trick on the FM.
JimB
 
:banana:Yay Team!:banana:
I've got the CR-2020 singing again with the replacement of the LA3350 mpx IC. As mentioned I put a socket in there so as to avoid further abuse of the PCB traces.
And wonders of wonders - the new chip seems to have fixed the AM problem as well.

Thanks everyone and (surprise) merrylander for his behind the scenes support.

Time to play it for a while and see if any other gremlins pop up.
JimB
 

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