Strange FM reception problem 30 year old JVC receiver

bluebelly

bluebelly

Member
IMG_3629.JPG I have a JVC R-X 500 receiver that is 30 years old and I have a problem with the FM receiver not pulling in enough signal to play in stereo. A strong station where I live is 97.1 so I tune to 97.1 and I have a weak signal and it will not play in stereo but it will in mono. Now tune to 97.2 and the signal is stronger and will play in stereo but it is fairly distorted. Kind of like a "spitting" distortion. If I tune to other strong stations I have similar results. Curiously, if I remove one lead from the FM antenna then the signal seems to increase some according to the status bar on the display of the receiver but I still have the same problem as before.

What do you guys think this is?

Thanks,
BB
 
ELzekio said:
What is the strongest db you can pull in for reception?

How close is that station located?
Click to expand...
I can get it up to about 25db. On this receiver it should be reading about 75db. That station is so close my other receiver does not even need an antenna hooked up in order to get good reception.
 
Sounds like something has drifted. Maybe replacing the electrolytic caps on the tuner board would fix it. Check PCB for bad solder joints. You might have to take it to someone that can re-align it.
 
There's a specific procedure for adjusting the FM section in the service manual. Have you already carried it out?
 
Although your receiver may have other issues, this is a common symptom of an issue with the FM discriminator circuit.

It can be a simple adjustment using a DMM and with the receiver tuned to a blank spot on the FM dial and with no antenna connected. This is explained in the service manual.

Although the service manual indicates using a signal generator tuned to 98 MHz to do this adjustment, you can get very close using the procedure above.

Be sure to use the correct non metallic alignment tools.

CaptureA10.JPG

This adjustment may or may not help with your issue, but it is a quick and simple adjustment and a good place to start.
 
Thanks for the lead on the free download for the service manual. I read the FM section and unfortunately I do not have those tools at my disposal at this time. Looks like I may have to take it in for this repair. Would it hurt to make slight turns of the adjustments that are listed in the manual just to see what happens?
 
The only thing that you need to check is the adjustment that I mentioned and it does not require any type of test equipment. Just set the receiver to blank spot on the dial and adjust T132 for a zero reading from the test points.

If you do not have a DMM (it is not a bad idea to have one) you can tune the receiver to a weak station and adjust T132 for the clearest audio and strongest signal strength and this may help (this is a second choice work around for not having a DMM).

Do not just start turning things. This is a receiver that one can do a fairly good alignment (likely better than 95 percent) without any test equipment other than a DMM, but you need to understand what you are doing and the expected results with each adjustment. We can help you with this.

The first place to start (and it is possible that this is not your issue) is the with the single adjustment that I have described. Your symptom is related to the FM discriminator part of the receiver and that is what this adjust is for. The other adjustments are for different things.

First things first and one thing at a time.
 
I do have a digital multi meter and will proceed tomorrow. Hopefully that will fix it. In the service manual a VTVM is mentioned. What is that? And does a DMM replace it? Since this manual is 30 years old I suppose no DMM existed at that time.
Thanks again for your help.
 
I LIKE MUSIC said:
The only thing that you need to check is the adjustment that I mentioned and it does not require any type of test equipment. Just set the receiver to blank spot on the dial and adjust T132 for a zero reading from the test points.

If you do not have a DMM (it is not a bad idea to have one) you can tune the receiver to a weak station and adjust T132 for the clearest audio and strongest signal strength and this may help (this is a second choice work around for not having a DMM).

Do not just start turning things. This is a receiver that one can do a fairly good alignment (likely better than 95 percent) without any test equipment other than a DMM, but you need to understand what you are doing and the expected results with each adjustment. We can help you with this.

The first place to start (and it is possible that this is not your issue) is the with the single adjustment that I have described. Your symptom is related to the FM discriminator part of the receiver and that is what this adjust is for. The other adjustments are for different things.

First things first and one thing at a time.
Click to expand...



+1

Also, as warned above, you need to perform the adjustment using plastic tools, otherwise the adjustment will shift when you take away the tool from the transformer.
 
bluebelly said:
I do have a digital multi meter and will proceed tomorrow. Hopefully that will fix it. In the service manual a VTVM is mentioned. What is that? And does a DMM replace it? Since this manual is 30 years old I suppose no DMM existed at that time.
Thanks again for your help.
Click to expand...

VTVM vacuum tube volt meter. That's one obsolete tool nowadays lol
 
Before adjusting the discriminator, let the unit warm up with the cover on for at least 30 minutes.
My experience, even tho limited, is that the discriminator, LA1235, can drift a bit before reaching working temperature.
Between 50 to 200mV.

Dunno if it's a aging problem with the component or if it's the normal behavior.

But with the behavior your unit show, I'll guess the discriminator is off by more than 500mV.
 
And by the way, that adjustment is very touchy so it can take a few minutes before manage to get a reading below +/-20mV.

Getting it to 0mV as the instruction says, sounds a bit optimistic.
 
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