A Stereo decoder for Leak Troughline 3 - Page 3 - UK Vintage Radio Repair and Restoration Discussion Forum

sonata149 · 11th May 2011, 10:01 am

OK I'm getting signals from both channel now. Problem was with the rca plug -somehow the two wires were shorting - defective plug. The remaining problem now is that the decoder is not working. (I'm using a battery to operate it.) I can't tune to a fixed broadcast. I'm getting a lot of noise from various stations wherever I move the tuning dial. Is there a way to test the decoder or perhaps the IC? With the decoder disconnected the tuner was OK yesterday.

G8HQP Dave · 11th May 2011, 11:07 am

You will get noise between stations, as this is a feature of FM - the tuner does not have any squelch to silence this. The easiest way to test the decoder is to feed it a stereo signal and check that it decodes it! Tune to a known stereo station (if necessary, via the tuner output rather than the decoder). Adjust the decoder preset until the stereo LED lights. You should then be getting L and R outputs. You may need to check that the input to the decoder is not too small or too large. Too small will mean that it stays in mono mode, but it should otherwise be OK. Too large will cause distortion. Check the DC current draw of the decoder against the data sheet, bearing in mind that the LED may be a significant fraction of this.

sonata149 · 11th May 2011, 7:38 pm

Hi friends. Problem solved.

I can't say why or how. After a lot of trial and error changes I decided to take voltage readings again. First thing I noticed was that R13 was blackened. I changed it and started taking the readings. Voltages around V4 (EF80) didn't seem right, so I changed it. I eliminated the decoder to make sure I had signals coming in right, then connected the decoder and this time I was getting a clear sound and could adjust the LED. From then on things got a lot easier and it's playing right now.

So, thanks a lot to both of you because you gave me the courage to keep on trying until the end. Besides, it was a very worthwhile learning experience as I had never worked on tuners before.

Warm regards and best wishes,
Joe A

G8HQP Dave · 11th May 2011, 8:53 pm

Glad it's sorted. Moral of the story: old equipment often has more than one fault (but they don't all appear at once), and fixing faults can accidentally introduce new ones.

MrElectronicman · 12th May 2011, 4:02 pm

That's great I knew you could you it, sometimes just takes a little time and patience

sonata149 · 12th May 2011, 7:42 pm

Thank YOU guys; you've been a great help. Your continuous comments and suggestions kept me struggling to succeed. I even found a way to operate the decoder from the 6.3V taps on the tuner's transformer. The problem seems to have been caused by the fact that the original voltage doubler was full-wave. I changed this to half-wave with a common earth. This is good enough to accept a 12V voltage regulator.

I've being listening to it for a few hours and to my ear the Trough line is sounding better by the hour. This wasn't my impression at first, thinking that the mono version sounded better. Maybe my ears are slowly getting accustomed to the new sound.

Best regards,
Joe A

MrElectronicman · 12th May 2011, 9:49 pm

The DC power was a potential problem, which is why I asked about it. You cannot use a full wave supply from the 6.3 VAC heater source since it is already grounded at one side.

I used a very small independent 6.3 VAC transformer for my set with FW rectification which gave me 12 VDC.

The original design from Leak used the HT via a dropping resistor with a 250 uF capacitor, however, this idea was was not good since if you disconnected the decoder, the 250uF 25VDC capacitor would end up with full HT on it. BANG!

The stereo sound does "sound" different, and as mentioned earlier can be more noisy if the signal is a bit weak, so you need a better aerial.

layumhs · 24th Jun 2012, 8:25 pm

Hi, I'm new here and just found this thread on a decoder for trough line stereo. I just got this tuner in original form and I'm after upgrading the decoder. my electronics knowledge is basic, but I'm willing to learn. whats the best way of getting a new decoder for this tuner? many thanks. M

11th May 2011, 10:01 am	#41
sonata149 Retired Dormant Member Join Date: Oct 2006 Location: Malta Posts: 62	Re: A Stereo decoder for Leak Troughline 3 OK I'm getting signals from both channel now. Problem was with the rca plug -somehow the two wires were shorting - defective plug. The remaining problem now is that the decoder is not working. (I'm using a battery to operate it.) I can't tune to a fixed broadcast. I'm getting a lot of noise from various stations wherever I move the tuning dial. Is there a way to test the decoder or perhaps the IC? With the decoder disconnected the tuner was OK yesterday.

11th May 2011, 11:07 am	#42
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: A Stereo decoder for Leak Troughline 3 You will get noise between stations, as this is a feature of FM - the tuner does not have any squelch to silence this. The easiest way to test the decoder is to feed it a stereo signal and check that it decodes it! Tune to a known stereo station (if necessary, via the tuner output rather than the decoder). Adjust the decoder preset until the stereo LED lights. You should then be getting L and R outputs. You may need to check that the input to the decoder is not too small or too large. Too small will mean that it stays in mono mode, but it should otherwise be OK. Too large will cause distortion. Check the DC current draw of the decoder against the data sheet, bearing in mind that the LED may be a significant fraction of this.

11th May 2011, 7:38 pm	#43
sonata149 Retired Dormant Member Join Date: Oct 2006 Location: Malta Posts: 62	Re: A Stereo decoder for Leak Troughline 3 Hi friends. Problem solved. I can't say why or how. After a lot of trial and error changes I decided to take voltage readings again. First thing I noticed was that R13 was blackened. I changed it and started taking the readings. Voltages around V4 (EF80) didn't seem right, so I changed it. I eliminated the decoder to make sure I had signals coming in right, then connected the decoder and this time I was getting a clear sound and could adjust the LED. From then on things got a lot easier and it's playing right now. So, thanks a lot to both of you because you gave me the courage to keep on trying until the end. Besides, it was a very worthwhile learning experience as I had never worked on tuners before. Warm regards and best wishes, Joe A

11th May 2011, 8:53 pm	#44
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: A Stereo decoder for Leak Troughline 3 Glad it's sorted. Moral of the story: old equipment often has more than one fault (but they don't all appear at once), and fixing faults can accidentally introduce new ones.

12th May 2011, 4:02 pm	#45
MrElectronicman Heptode Join Date: Feb 2011 Location: Washington DC, USA Posts: 619	Re: A Stereo decoder for Leak Troughline 3 That's great I knew you could you it, sometimes just takes a little time and patience

12th May 2011, 7:42 pm	#46
sonata149 Retired Dormant Member Join Date: Oct 2006 Location: Malta Posts: 62	Re: A Stereo decoder for Leak Troughline 3 Thank YOU guys; you've been a great help. Your continuous comments and suggestions kept me struggling to succeed. I even found a way to operate the decoder from the 6.3V taps on the tuner's transformer. The problem seems to have been caused by the fact that the original voltage doubler was full-wave. I changed this to half-wave with a common earth. This is good enough to accept a 12V voltage regulator. I've being listening to it for a few hours and to my ear the Trough line is sounding better by the hour. This wasn't my impression at first, thinking that the mono version sounded better. Maybe my ears are slowly getting accustomed to the new sound. Best regards, Joe A

12th May 2011, 9:49 pm	#47
MrElectronicman Heptode Join Date: Feb 2011 Location: Washington DC, USA Posts: 619	Re: A Stereo decoder for Leak Troughline 3 The DC power was a potential problem, which is why I asked about it. You cannot use a full wave supply from the 6.3 VAC heater source since it is already grounded at one side. I used a very small independent 6.3 VAC transformer for my set with FW rectification which gave me 12 VDC. The original design from Leak used the HT via a dropping resistor with a 250 uF capacitor, however, this idea was was not good since if you disconnected the decoder, the 250uF 25VDC capacitor would end up with full HT on it. BANG! The stereo sound does "sound" different, and as mentioned earlier can be more noisy if the signal is a bit weak, so you need a better aerial.

24th Jun 2012, 8:25 pm	#48
layumhs Retired Dormant Member Join Date: Jun 2012 Location: Lund, Swden Posts: 1	Re: A Stereo decoder for Leak Troughline 3 Hi, I'm new here and just found this thread on a decoder for trough line stereo. I just got this tuner in original form and I'm after upgrading the decoder. my electronics knowledge is basic, but I'm willing to learn. whats the best way of getting a new decoder for this tuner? many thanks. M