Leak Troughline - high frequency distortion

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15 year old caps are probably fine. 40 year old caps will be worn out. 5 year old caps may have been bad from new.
Yeah, but the 6 year old PC mainboard worked fine until January. then the DVD writer wouldn't write. In February, the CD writer stopped writing, followed by the disk drive. Finally in March, the DSL modem stopped talking. I had pulled the PS conn. and I/F to the drives as they went, no improvement, which locates it to the mainboard or the power supply. Power supply is 2 years old. Inspection of the PC mainboard showed a 3300 uf 10V cap bulging. I replaced both of that size, and ordered all the others for next winter. Everything works fine, again. Point is, this consumer grade **** e-caps can start failing at 4-5 years. My DTV converter is just about 5 years old, too, The first set of batteries in the remote lasted 4 1/2 years, the next ones 2 months, next set 1 month. Total signature of leaky caps. Yeah, 15 year caps might be fine, but with freight at $10 and caps at $.09 to $1, what is the point of not changing them all. He has a symptom after all, he wouldn't be fooling with it if everything was perfect. You yourself said "decoupling components". E-caps are one kind of that, ceramic caps the other. Ceramic caps tend to go early, or not, IMHO. Sakas does all electrolytics, he says. 1/5 of what you monkey with damages the PCB land,if you take a cap out to check it, may as well replace it with a long life (>5000 hours) quality cap. I date them as I put them in so I don't retrace my tracks if I don't get 100% the first time into the box.
 
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I've just found these components after I've moved house, sadly I'm no further along in terms of fixing it.

To recap (no pun intended) the troughline is fine, there is a fault with the decoder (LM4500).

Back in the day I made a little diagnostic progress and still need a little further guidance/advice please.

Pic 01 - Parasitic oscillation at output, approx 50kHz frequency.

Pic 02 - Input clean 1kHz sine wave.

Tried parallel caps across the decoupling caps to see an improvement but no difference.

With access to the test pin it doesn't appear to do it in mono mode only stereo.

Does anybody know how to solve this? Shall I just replace all of the passive components starting with the caps and narrow it down from there?

Many thanks indeed.
Nath

PS the LM4500 datasheet (http://fmmpx.com/LM4500.pdf p2-137) mentions 3rd harmonics of pilot tone 57kHz (Auto Radio Information service), maybe this is involved?
 

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Could that "50kHz" actually be 38kHz? or 57kHz? If so, you just need a multiplex filter.

Does anybody know how to solve this? Shall I just replace all of the passive components starting with the caps and narrow it down from there?
Replacing all components is a good way to introduce new faults, and make tracing the original fault almost impossible.
 
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