high pitch tuner noise

drumgodal

New Member
Just picked up another sx 2500 -nice clean unit ! even the auto tune works ! only issue I found is that the fm tuner makes a high pitched soun. it sounds great from the low channels but as soon as you get to the 100 mhz mark on the dial the sound comes in all the way up to the end ! I tried all the antenna input configurations made sure the variable cap( tuner ) was free of dust/contaminants. any ideas ??? Funny thing is that any station under 100 mhz soundsclear and strong -
 
Strange one!! Hard to think it is power supply related, could try some tuning/alignment and see if it makes any difference. Putting a scope to it when it screws up, might reveal something.
Hum!!
 
I was thinking it might be a tuner alignment problem. I'm not a tech (or have all the necessary equipment to do it). It's strange because I did try to use the 300 ohm balanced fm input and the unbalanced and the point where the noise comes in moves slightly to the lower end of the scale. Also if I touch the fm antenna input terminals it gets much louder. I wonder what this would cost to get aligned - I'm in Illinois and repair centers are not all over like years ago.
 
Weird problem, I have not come across that one before. From the clue about touching the antenna terminals I wonder if the front end FET is leaky or has some other fault. If you take the antenna off and tune around is the noise still there? If so, softer or louder? How about when it's in "mono" mode?
 
does it in mono mode too ! I tried the tuner without any antenna and it doesn't do it. The noise stay relative to the volume setting. The amp section is quiet at idle and is ok with cd input (no noise). I tried all the other inputs (including phonos) and all is ok. Strange thing is that the tuner sounds great on the lower side of the scale. I did run it for a long time at a decent volume and at 97.9 (closest station that I can get near the 100 mhz where the noise starts) to see if maybe it might be a drifting or temp related fault. 97.9 played perfectly for many hours. Also the AM section works perfectly
 
does it in mono mode too ! I tried the tuner without any antenna and it doesn't do it.

No noise on other inputs makes me guess the power supply is okay as does the FM working at the lower frequencies. The noise in mono leads me back to the front end and not the multiplex circuitry. No antenna means the AGC is demanding more gain from the front end FET which would be my main suspect. It may just be alignment but I'm thinking not. You might try measuring the resistors in the front end but poking around in there has some risks. It may be time to have a pro look at it, sorry to say. What's your electronics repair skill level?
 
I was thinking it might be a tuner alignment problem. I'm not a tech (or have all the necessary equipment to do it). It's strange because I did try to use the 300 ohm balanced fm input and the unbalanced and the point where the noise comes in moves slightly to the lower end of the scale. Also if I touch the fm antenna input terminals it gets much louder. I wonder what this would cost to get aligned - I'm in Illinois and repair centers are not all over like years ago.

WHERE in Illinois?

Try using keyboard duster on all the tuning fin capacitors. Even the AM one.
 
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I'm on the west edge of chicago. I did blow out the tuner ( that was the first place i looked figured spider web). Mpx switch makes no difference.
 
Some tuners have an undocumented neutralizing cap on the front end. Mis-adjusted, it can cause instability.
 
Ok More strangeness ! I just turned it on today and now the problem is all the way down to the 97mhz . Also as you go up the scale and get to 97mhz the noise is high pitched. on some stations the noise is higher in pitch and some it gets lower. I'm thinking it has to be a cap problem ??
 
I'm thinking it has to be a cap problem ??
Lots of possibilities, have to measure/look at the power supplies, esp. 13.5V tuner supply to see what they are doing or not doing.
 
Indeed rcs, many possibilities. Something's bad I think, not just dirty and it could be that stability cap. There are no electrolytics in the front end and I've never had a bad cap, ceramic or otherwise, in that area but there's always a first time!
 
Yup crystal filters for the IF(W1,2), do they squeal? :) I have no idea of the failure modes for them.
Sometimes it is easier to separate/isolate one section from the other, like the IF from the RF Front end, especially if you can't see with a high quality scope/probes.
Disconnect the RF-out to the IF-in and route the RF-out to another receivers IF. It is a piece of 75 ohm coax.
Ground the IF-in so it does not oscillate and you get maximum agc gain in that feedback loop.
Either that round about way or use a RF signal generator as substitute to test each section.
These things are sorta trial and error, if you do not own a >100MMHz scope and low cap FET probe.

Good luck
Rick
 
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