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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Yes, modern ones often don't last very long. Older ones in normal consumer items can last for decades, provided they are kept cool. The decoder caps might be in a worse state than the tuner!indianajo said:
You're quite right sir. Since it's working really well, I'm not going to pull it out of its sleeve, rip the top off and rummage inside, I shall assume I have misremembered. Thanks for steering the thread right.
Cheers Steve
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?
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.
Replacing all components is a good way to introduce new faults, and make tracing the original fault almost impossible.
Thanks DF96, is a multiplex filter easy enough to assemble/construct/implement?
The confusion arises that it worked so well for so long but either way, keen to get it fixed up.
Thanks
The confusion arises that it worked so well for so long but either way, keen to get it fixed up.
Thanks
MPX filter is hard to build, but used to be easy to buy. I'm not sure how available they are nowadays. Have you tried replacing the chip? Also, check all DC voltages and compare with the datasheet.
You should not have changed the valves. This changes the alignment.
It seems to me that your discriminator is misaligned and probably the 19KHz oscillator in your MPX as well.
It seems to me that your discriminator is misaligned and probably the 19KHz oscillator in your MPX as well.
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I have owned and used a Leak troughline tuner for nearly 30 years. It has had several changes of valves in that time, as well as other remedial work to replace faulty caps and resistors. It has never needed to be realigned.
You got lucky then. Doesn't change the general principle. Valves have reactance, which varies among samples and over time, which affects the alignment.
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