A Stereo decoder for Leak Troughline 3

[sonata149]

I managed to restore a Leak Troughline 3 and it sounds amazingly good, even in mono. I am populating a stereo decoder pcb based on the Motorola MC1310P bought from Maplin many years ago. It will be housed in a separate box but I need to know how to connect it to the Leak for stereo reception.

Besides power supply connections, there is an input which I suppose connects to the Multiplex output from the tuner. Now comes the confusing bit:

1) Where are the L-R signals connected to? Are they connected directly to the pre-amp (from the decoder)?

2) If so, is the audio output from V5b (ECF80) left floating? Again, if this is so, is this valve rendered superfluous/ useless?

3) Out of curiosity, is it possible to take 2 (two) audio mono outputs from the present audio output position from V5b cathode? Will this scheme work or are there other repercussions which I don't foresee?

Thanks for your help.
Joe A

[sonata149]

Just to make things a bit clearer I attach some images in connection with the above. WRT the Leak circuit diagram, MPX is taken from the bottom end of R33 in parallel with OA79 on V5a (ECF80 pentode section). This can be seen a circuit diagram of the Troughline 2.

Then there are 2 images of the decoder I'm using.

Thanks for your input.
Joe A

[MrElectronicman]

I have a leak trough line stereo that had the original decoder, later on I changed it for an IC decoder.

On my tuner the output from the ECF80 triode cathode (V5B) is developed across a 47K via a 1uF capacitor and then goes to a wafer switch which combines the on-off switch and the stereo/mono switch.

For stereo operation this output goes directly to the decoder and then comes out of the decoder as two outputs, left and right.

If you want Mono, the switch takes the output from V5B as above, and connect them together at the left and right output cables after disconnecting the decode outputs.

[MrElectronicman]

I just looked at your diagram and it might be better if you looked at the stereo version.

[G8HQP Dave]

The Troughline 2 takes the MPX signal straight from the discriminator, but the stereo version uses V5b as a cathode follower for MPX. It does this by moving the de-emphasis from before this stage to after this stage. In the 2/3 de-emphasis is done by R32 and C50. In the Stereo version mono de-emphasis is done by R42/R43/C50, and stereo de-emphasis by R48/C59 and R49/C60.

I think your best option is to modify your 3 to make it more like the Stereo version. The chip decoder will expect a reasonably low impedance input, so the cathode follower will provide this. MPX is destroyed by de-emphasis so you need to remove these components: replace R32 by a wire link, and simply remove C50.

[sonata149]

Thank you guys for your replies.

G8HQP Dave has confirmed my suspicion that I need to do some surgery to my Trough Line 3 in order to bring it in line with the Stereo version.

MrElectronicman has led me to the full Stereo circuit where the power-ON and volume control switch is replaced by an OFF-Mono-Stereo switch. This is a nice to have all-in-one switching facility. Not being very technical I cannot see how I can implement it in practice.

Can anyone of you illustrate in more detail the type of rotary switch to use and how to connect the combination of power switching and mono-stereo mode. I understand that these two functions have to be independent of each other, but how do I combine them together without passing ac mains voltage to the output from the CF and input into the decoder.

Thanks a lot for your help.
Regards.

[G8HQP Dave]

My advice is to keep mains well away from audio. Mono/stereo switching can probably be done in the stereo decoder - most chips have an input for that.

[MrElectronicman]

I agree with Dave, if you are not very technical I would leave the mains switching as it is, and just add the decoder.

The decoder IC will usually switch between Stereo and Mono on its own, the only time you may need to go to Mono is when you get a weak or multipath signal, which causes excessive noise from the stereo outputs, and then going back to mono alleviates the problem somewhat

[sonata149]

Thanks again for your interest and help, but before I read your replies I had finished stuffing the decoder pcb as well as the 12V dc supply to be taken from the LT transformer via a voltage doubler.

On the tuner side I have SHORTED R32 and removed one side of C50 as suggested by G8HQP. These two components should have been removed but it’s very difficult to unsolder components on this tuner. Is the way I got around it correct?

I also disconnected the audio output from P1 (50K) - on-off switch broken -and connected a 47K between C53 (1uF) and earth as per the stereo version.

I've been thinking hard how to implement the switching scheme adopted by the Stereo version. My ON-OFF switch does not function and with the potentiometer being bypassed it makes sense to adopt this scheme.

I intend to use a 4-pole 3-way switch; two poles will be used for mains switch and the other two poles for the Mono-Stereo positions from the decoder. I attach a diagram showing how I think it should be wired and I would greatly appreciate it if members check my intended wiring. Does it make sense to you?? For this it will be useful to refer to the circuit diagram of the tuner as well as the decoder posted earlier above.

Thank you very much once again.

[MrElectronicman]

DO NOT put the mains on the same switch as the audio.

The way the Leak Stereo is arranged is there is a separate switch, like the one on your volume control, that is used for the mains. The audio switching is then done by means of a wafer switch. These two switches are on the same shaft, but are physically separated. I have attached a picture but I think it's not too clear, so I have also attached a sketch.

I do not know if you could find such a switch today, but you could put a three way, three pole switch for the Audio, and find another way to switch the mains safely, possibly by an external switch

[sonata149]

Thanks MrElectronicman, you saved me a lot of trouble.

[G8HQP Dave]

Yes, keep the mains separate.

Disconnecting one end of C50 is sufficient, so I think you have done this mod correctly.

[sonata149]

OK, I kept the mains separate from the audio switching. I connected the whole thing to my system and there's problems! With the sensitivity switch on Local and the AFC in either On-Off position I only get hum from one channel.

With the sens. switch on Distant (again in or out of AFC) I get some noise on the same one channel. It's a station but there is a lot of other signals in the background. The curious thing is that wherever I tune across the tuning range there is no change in the signal I get. The LED is flickering all the time irrespective of the position of the preset.

I checked my wiring but I can't find anything wrong so far. Do you have any ideas or an explanation for this behaviour?

[G8HQP Dave]

Temporarily put the decoder on one side. First you have to get audio out of the tuner again. The output, from the cathode circuit of V5b, will lack deemphasis (so will sound shrill) but should otherwise be OK. Get this working first. You should not have changed much in the tuner, and you should be able to temporarily undo the changes. Ensure you have identified the right components/wiring to change.

The decoder LED is probably just flickering due to noise. Sometimes the noise will happen to look a bit like a 19kHz pilot tone for a brief time.

[MrElectronicman]

I agree with Dave, make sure you have sound across the new 47K (which replaced the 50K volume control) first. If you do then, check the connections to the switch.

Looking at your diagram I see that you have C48 which is not in the stereo version, so you could try taking one lead off that, since it might be attenuating the 19KHz pilot tone.

David

[MrElectronicman]

In the stereo version the output from the discriminator is sent directly to the cathode follower via C51, so in the mono version both C48 and C50 should have been removed, and R32 shorted out.

David

[sonata149]

Hi Dave, Thanks for helping me out. In between posts I checked the voltage at the input to the decoder and to my surprise there is only 4.5V when the voltage (without load or decoder) is 17V. So there must be something wrong with my decoder. The question is where?

About the LED flicker this goes out when the mono/stereo switch is shorted - the LED remains on all the time. Meanwhile I have disconnected the decoder from the supply and took my output from the cathode of V5b.

Again there is one station clearly audible but then a lot of other stations coming in (more like noise). As before, tuning to different frequencies does not change the signals coming in. So now it seems the problem is originating from the tuner.

[sonata149]

Hello Electronicman, your post came through while I was sending my previous reply. I have now disconnected C48 (50p) and listened again. There has been no change from my earlier post in the reception front.

!!!!

[MrElectronicman]

Since there maybe a problem with the decoder, I suggest that that you disconnect it completely for the moment and try to get the tuner back to normal.

Without the decoder you should have something like the attached sketch at the audio out from the tuner. If you then connect this to your amp it should work, but be careful because the output will be a full volume since there is no volume control.

I also noticed something else in the mono version and that is another capacitor C55 which is connected to the cathode of V5B. That too should be disconnected as it may be affecting the 19 KHz pilot as before.

Let us know what happens.

[G8HQP Dave]

You do have a DC isolating capacitor somewhere between V5b cathode and the decoder chip input pin? Connecting the decoder should not change any DC levels.

It sounds like the VHF front end has stopped working. Have you disturbed anything there? The station you can hear: is it an FM station or short-wave AM pickup?