Hi all, thanks
I've a Leak Troughline Stereo tuner going through a LM4500 decoder.
When listening to spoken word everything seems fine, but as soon as music is transmitted there's some distortion (predominantly in the high frequencies).
Has anybody else experienced this, can you shed any light?
I'll have a look tonight, my hunch is a cap need replacing or the like but diagnosis could be a pain.
Thanks
Nathan
I've a Leak Troughline Stereo tuner going through a LM4500 decoder.
When listening to spoken word everything seems fine, but as soon as music is transmitted there's some distortion (predominantly in the high frequencies).
Has anybody else experienced this, can you shed any light?
I'll have a look tonight, my hunch is a cap need replacing or the like but diagnosis could be a pain.
Thanks
Nathan
Thanks for the replies.
It was sudden, I've just moved things around and when I turned the power back on (tuner) even my amp cut out for a split second then came back (it was on another channel).
It has worked wonderfully with the decoder for years, so I think it must be the tuner itself at this rate.
For the faff I may be better off in the long run trying to pick up another if I can get it cheap enough, but I'd sooner at least diagnose this one first.
Thanks again
Nath
It was sudden, I've just moved things around and when I turned the power back on (tuner) even my amp cut out for a split second then came back (it was on another channel).
It has worked wonderfully with the decoder for years, so I think it must be the tuner itself at this rate.
For the faff I may be better off in the long run trying to pick up another if I can get it cheap enough, but I'd sooner at least diagnose this one first.
Thanks again
Nath
Indeed.
Thanks all. I've added a recording. Oddly enough it only seems to do it on certain stations but it's more of them than less (Radio 1 didn't seem to mind but I don't listen to it).
There's feint interference I think on the presenters voice but it really kicks in when the music is heard.
I wonder if it is adjustment. It does seem to be very close to finding another station, as if the boundaries "between" the stations have changed.
I changed the valves and it's still there as expected really, though looking underneath I think the yellow (bubbling) 350V cap needs replacing.
Would this be it?
Thanks
Nathan
Thanks all. I've added a recording. Oddly enough it only seems to do it on certain stations but it's more of them than less (Radio 1 didn't seem to mind but I don't listen to it).
There's feint interference I think on the presenters voice but it really kicks in when the music is heard.
I wonder if it is adjustment. It does seem to be very close to finding another station, as if the boundaries "between" the stations have changed.
I changed the valves and it's still there as expected really, though looking underneath I think the yellow (bubbling) 350V cap needs replacing.
Would this be it?
Thanks
Nathan
Attachments
Thanks for sticking with this.
I'm trying to attach a WAV sample going through a few stations but I'm struggling to do it (bear with me). The high pitched interference is fairly constant I'd say though, but in terms of distortion it seems to be more apparent on Classic FM as heard nearer the end of the (albeit short) recording.
Magic Eye wasn't on when I did this (I think).
Off on hol soon so might have to wait, even so it's a bugger.
Thanks again
Nathan
I'm trying to attach a WAV sample going through a few stations but I'm struggling to do it (bear with me). The high pitched interference is fairly constant I'd say though, but in terms of distortion it seems to be more apparent on Classic FM as heard nearer the end of the (albeit short) recording.
Magic Eye wasn't on when I did this (I think).
Off on hol soon so might have to wait, even so it's a bugger.
Thanks again
Nathan
Right, hopefully some of you will bear with this. I've made a little diagnostic progress and 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?
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?
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?
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?
Attachments
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I think your yellow bubbling cap is just the tip of the iceberg. This unit is over twenty years old, right? All my old radios ,tuners are fuzzy sounding, don't do stereo very well, and have no sensitivity. I quit working, have some time for some cheap entertainment. I started with a Readers Digest 12 transistor portable radio, that I inherited from my father. Not worth much. Without touching it with a meter or scope, or having a schematic, I replaced every electrolytic cap with one of equivalent tolerance or better. Voila- sounds great and pulls in the 100 W high school FM station from the next county. Listening to it right now in fact, use it 16 hours some days if they don't play too much baroque **** whist playing music in a row.
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I have a Troughline 3. I would suggest the power supply cap. I think that yellow cap may be a replacement for the original which is 3 capacitors in a big can, yeah? I had this replaced by three caps, but the lack of real estate in the unit caused heat issues, and it melted. Replaced it and it sings like a bird again. I love it. Found an East German (RFT) ECC83 inside - I wonder if it was original equipment?
Good luck - these are the dogs if you have a big signal.
Cheers Steve
Good luck - these are the dogs if you have a big signal.
Cheers Steve
There should not be any ECC83's in the Leak Trough-Line 3 the valve line up is as follows.
ECC84 (ECC88 in later models), ECC85, 2 x ECF80,EF80 and a EM84 Magic Eye.
Thanks for the replies gents.
It's actually the stereo decoder that's at fault though, rather than the unit or the caps (although they should be replaced granted). I say this with some certainty too, as there's no distortion if I remove the stereo decoder and listen direct to the troughline, yet there is destortion when I use the decoder with the troughline.
The above pics are from the signal going through the standalone stereo decoder unit, the Troughline itself wasn't even in the building.
In essence, I'm trying to repair the Stereo Decoder rather than the troughline.
It certainly sounded much better with the standalone decoder rather than the poor one inside the unit. Until now. Hence my attempts to correct it.
Info about the decoder is here:
FM MPX Adapter
Many thanks again
Nath
It's actually the stereo decoder that's at fault though, rather than the unit or the caps (although they should be replaced granted). I say this with some certainty too, as there's no distortion if I remove the stereo decoder and listen direct to the troughline, yet there is destortion when I use the decoder with the troughline.
The above pics are from the signal going through the standalone stereo decoder unit, the Troughline itself wasn't even in the building.
In essence, I'm trying to repair the Stereo Decoder rather than the troughline.
It certainly sounded much better with the standalone decoder rather than the poor one inside the unit. Until now. Hence my attempts to correct it.
Info about the decoder is here:
FM MPX Adapter
Many thanks again
Nath
The FM mpx adapter link you provided shows a PCB with some pretty obviously Electrolytic capacitors on it. The first question to answer, was the manufacture date over 15 years ago? Save your trash caps, listen to trash signals. Some grades of e-cap go bad in 4 or 5 years, like the ones on my PC mainboard I replaced in March, or the ones in my HDDTV converter remote that are draining a new battery in weeks.
If you indeed have a bad solder joint or the rare expired semiconductor, use your analytical skills after the weeds are pulled. While you worry about which $.09 component is bad, I'm listening to Respighi The Birds on my 197? FM radio re-e-capped in 2011,with a 1970 assembled power amp repaired in 2010. The speakers are 1998, the rubber suspensions are just as limited life as the rubber sealant in consumer grade e-caps.
If you indeed have a bad solder joint or the rare expired semiconductor, use your analytical skills after the weeds are pulled. While you worry about which $.09 component is bad, I'm listening to Respighi The Birds on my 197? FM radio re-e-capped in 2011,with a 1970 assembled power amp repaired in 2010. The speakers are 1998, the rubber suspensions are just as limited life as the rubber sealant in consumer grade e-caps.
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