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Old 9th Jun 2019, 7:32 pm   #1
chrispyduck
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Join Date: Jun 2019
Location: Bath, Somerset, UK.
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Default MPX decoder wanted for a Leak Troughline.

Hi.

I’m currently restoring a Leak Troughline tuner and I’m trying to source a suitable MPX decoder. I know One Thing Audio used to make them but don’t think they do anymore.

Any help greatly appreciated
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Old 10th Jun 2019, 8:18 pm   #2
Biggles
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Default Re: MPX decoder wanted for a Leak Troughline.

Depending on your constructional skills, they can be homebuilt from scratch using one of the commonly available chips. I have successfully used the Motorola MC1310 (now quite an old chip) and this sounds ok, but it depends on whether you want something original, or just something to do the job. There has been quite a bit of discussion previously about decoders on the forum, and there are also plenty of members willing to assist you if you decide to go down the home construction route.
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Old 10th Jun 2019, 9:23 pm   #3
vidjoman
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Default Re: MPX decoder wanted for a Leak Troughline.

Don’t forget that when a decoder was fitted to a Troughline they also made changes to the front end together with a few component changes and also added a switch to select mono/stereo.
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Old 10th Jun 2019, 10:09 pm   #4
Sideband
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Default Re: MPX decoder wanted for a Leak Troughline.

Yes and one important thing is to disconnect the de-emphasis components from the discriminator otherwise no stereo decoder will work......
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Old 10th Jun 2019, 11:25 pm   #5
PJL
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Default Re: MPX decoder wanted for a Leak Troughline.

If its a Troughline 3 it has a multiplex out that is straight off the demodulator without de-emphasis. The Stereo version uses the cathode follower for stereo and mono, and when mono is selected a simple RC de-emphasis is applied to the output.

It will need a buffer if you want to drive an IC decoder. I used a JFET for a Beam Echo tuner powered off a doubler (might have been tripler) from the heater circuit.

It may also need IF realignment to increase the bandwidth.
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Old 11th Jun 2019, 12:30 am   #6
martin.m
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Default Re: MPX decoder wanted for a Leak Troughline.

I had a Troughline 3 in the 1970s fitted with a decoder board that used the MC1310 chip. The input for the decoder was taken straight from the discriminator (de-emphasis capacitor removed) making the triode cathode follower stage and volume control redundant. For mono operation I simply paralleled the L and R channels on the amplifier. The 12v power supply for the decoder was constructed on a piece of Veroboard and mounted on the back of the tuner and the front fascia had been drilled to accommodate a "stereo" indicator LED. Obviously it will be helpful to have a circuit diagram of the Leak before you start.
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Old 13th Jun 2019, 11:58 am   #7
turretslug
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Default Re: MPX decoder wanted for a Leak Troughline.

I use a Mullard LP1401 decoder module (TDA1005 FDM IC) with a Troughline 3- the nice thing about this module is its high input impedance. This module/IC shows its age by its indicator driver, orientated around small filament lamps and including a small "off" state maintaining current, presumably to extend bulb life. I decided that an LED stereo indicator would be thoroughly inappropriate with this venerable tuner, so fitted a lens-end neon in the gap between 88 and 90 on the scale with a BCY70 and 2N3440 for level-shifting and quiescent offsetting.

This LP1401 module had "autumn leaf" status on the surplus market decades ago, but was rapidly latched onto by the Troughline fans when they achieved must-have status, I believe some of the loftily-described aftermarket decoders were little more than one of these modules in a glitzy box with glitzy spiel.
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