trough line III multiplex output

[waaza]

I have acquired a Leak trough line III (mono) tuner in good condition (it works very well!), and would like to convert it to stereo. However, it does not have the multiplex output (like the trough line II), only a 'mono' lead. I want to use the decoder in my Hitachi FT5500 tuner, in which I have located the multiplex input (confirmed with a scope). The nice thing about the FT5500 is that it has a digital readout signal strength meter (in dB) which I have found useful in the past.

My question is: can I just overide (cut out) the CRC filter (50pF 470Kohm 50pF) preceding the final output valve, which cuts off the signal below 16kHz or so, I would guess? The 'trough line stereo' model seems to work like this, before feeding its rather rudimentary inbuilt decoder.

cheers
Waaza

BTW, my trough line already sounds better than my Quad FM4 (the Quad cost me 30 times the price of the Leak)! bah

[Phil Y]

Hello Wazza,
That seems very strange, my Troughline III has a phone socket on the back marked "multiplex" (which i use ) as well as the captive mono lead. I have never heard of a mono only III

Regards, Phil.

[pre65]

Hi-i have 3 Troughlines,a 11 and two 111's and they all have the multiplex phono on the back.


Philip

[John T]

Hi Wazza,

Mono versions of Troughline III's can have an "audio output" which is controlled by a 50k pot on the front panel (usually combined with the on/off switch) To convert to stereo, connect a suitable decoder to the position you refer to.

Leak must have had a habit of using old rear panels - possibly at the end of a production run - I have a Troughline II (with the maroon facia) which is marked Troughline III! This model didn't have a multiplex output either - just the audio output, which was (until I removed it) on a flying lead!

Hope this helps!

John.

[waaza]

thanks everyone.

John T
my trough line III has the volume control (which I shall use to set the input to the decoder), but it also has a pi filter (CRC) which reduces output above about 16kHz or so before the final amplification stage. Therefore, it will not have the 19 kHz pilot tone nor the L-R signal out around 38kHz, both necessary to derive the stereo signal, only the L+R one.

I suppose I'm going to have to bite the proverbial, I'll let you know how I get on, if only to inform others of the outcome!

cheers
Waaza
I've posted a couple of pix which may help.

[soulminer]

Hi waaza.

My TL III was a stereo version, but I have removed the old decoder and replaced it with a "Studio 12" decoder. I think at the time I tried using the original decoder input (straight from C53) and it was overloading the "Studio 12" decoder, so I "broke in" where the multiplex is shown on a mono TL III. This is before the last ECF80 valve stage. I soldered a single cable at the junction of C51 (5uF) (which is a green PIO IIRC) and R33 (100k). There is no CRC on the stereo version and on a mono it is situated between the multiplex and the C51 cap. Personally, I would ignore the CRC and pick off the multiplex signal before it, between R32 and R33, as shown on the circuit diagram. There must be a reason that filter was left out completely in the stereo version.

I have posted both of the circuit diagrams on another thread here in case you don't have one.

If you get really stuck, let me know and I will endeavour to photograph the pick-off point of the multiplex in my TL. Might be worth a try for you.

HTH.

Cheers,

[Shane]

I wonder if you've ended up with my old Troughline? It was only when I put it on Ebay that I found out it was a Troughline two to which someone had attached the front panel of a Troughline three. The chassis are pretty much the same.

[soulminer]

I am wondering if this is the case with JohnT and his TL is actually a mark III with a mark II facia plate. As you say Shane, the chassis' are all alike and the facia is only a couple of screws, knobs and wires.

However, I think judging by the photos here supplied by waaza, his is indeed a mark III through and through. I am sure my friends has no multiplex thinking back, as his has (had) a hard wired interconnect from within for his TdP valve decoder. There was never a multiplex on the back of his either. Maybe it was a date thing or even an optional extra that was later added as a standard feature.

Cheers,

[waaza]

Originally Posted by soulminer

Hi waaza.

My TL III was a stereo version, but I have removed the old decoder and replaced it with a "Studio 12" decoder. I think at the time I tried using the original decoder input (straight from C53) and it was overloading the "Studio 12" decoder, so I "broke in" where the multiplex is shown on a mono TL III. This is before the last ECF80 valve stage. I soldered a single cable at the junction of C51 (5uF) (which is a green PIO IIRC) and R33 (100k). There is no CRC on the stereo version and on a mono it is situated between the multiplex and the C51 cap. Personally, I would ignore the CRC and pick off the multiplex signal before it, between R32 and R33, as shown on the circuit diagram. There must be a reason that filter was left out completely in the stereo version.

I have posted both of the circuit diagrams on another thread here in case you don't have one.

If you get really stuck, let me know and I will endeavour to photograph the pick-off point of the multiplex in my TL. Might be worth a try for you.

HTH.

Cheers,

thanks everyone,
I did buy it in Devon this summer, though it is clearly a Mk III if the writing on the back is anything to go by.

Soulminer,
I have checked both your diagrams careully, and notice my Mk III is slightly different in many respects. Had I just wanted a fixed output then I think your suggestion has merit, though the capacitor you mention (C51) I read as 5000pF, not 5000 nF. There is only one small electrolytic capacitor, and that is smoothing the supply. Also, I don't seem to have an R33 (associated with a multiplex output).

I still like the idea of a variable output voltage (in the short term at least) as I do not know the requirements for a low noise input to the decoder.

The reason the filter is not there in the stereo version is surely because it would filter out the pilot tone and L-R signals, obviously vital for stereo decoding. Also, your take off point is between the two legs of another filter (C51 R34), the multiplex output of the mono version takes it from before the C51 capacitor, this may make no difference, of course.

It is good to know someone has done it already.

One further question, my (mono) version is very quiet with only a half decent aerial signal (about 46dB), will it become noisier in stereo? I should be able to get up to 70dB+ where I am, with a suitable j aerial.

cheers
Waaza

[soulminer]

Well here are some photos of mine anyway. The white wire is the pick off for the signal to the decoder.