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Old 6th April 2011, 10:48 AM   #1
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Default Leak Troughline - high frequency distortion

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
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Old 6th April 2011, 12:20 PM   #2
DF96 is offline DF96  England
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Did the distortion start gradually or suddenly? Could it be alignment?
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Old 6th April 2011, 06:16 PM   #3
kevinkr is offline kevinkr  United States
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Did this issue start when you added the LM4500 decoder? What are you doing to buffer the relatively high source impedance of the troughline discriminator output before feeding it into the LM4500? With the decoder disconnected and the proper 75uS de-emphasis installed how does it sound?
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Old 6th April 2011, 08:00 PM   #4
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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
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Old 7th April 2011, 10:58 AM   #5
DF96 is offline DF96  England
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Strange that it should affect a nearby amp. Could it be a blown mains filter capacitor? A temporary short would affect other equipment. Then the lack of a filter cap might create problems. However, I would expect you to have heard the bang!
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Old 7th April 2011, 06:42 PM   #6
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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
Attached Images
File Type: jpg Leak 01.jpg (329.7 KB, 130 views)
File Type: jpg Leak 02.jpg (433.2 KB, 124 views)
File Type: jpg Leak 03.jpg (469.5 KB, 124 views)
File Type: jpg Leak 04.jpg (478.9 KB, 122 views)
File Type: jpg Leak 05.jpg (461.8 KB, 119 views)
File Type: jpg Leak 06.jpg (443.3 KB, 51 views)
Attached Files
File Type: zip Leak_Test_060411.WAV.zip (534.9 KB, 10 views)
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Old 7th April 2011, 07:08 PM   #7
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I think that yellow jobbie is a power supply capacitor, I don't see that being the cause.

How far up the magic eye does the signal strength go, and are you using AFC?
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Old 8th April 2011, 09:40 AM   #8
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Thanks, the magic eye signal strength is very strong, not much difference AFC on/off.

Thanks again
Nathan
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Old 8th April 2011, 11:45 AM   #9
DF96 is offline DF96  England
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Does the effect change much with tuning (switch AFC off to test)? Is it worse on louder stations (e.g. compare BBC R3 with Classic FM)? If so, can you temporarily insert an audio attenuator to reduce the input to the stereo decoder and see if this fixes it.
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Old 8th April 2011, 09:13 PM   #10
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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
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