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Leak Throughline Stereo T4 Discriminator 0A79 Germanium diodes

Big John

Big John

pfm Member
My leak troughline Stereo tuner with Tim de Paravicini stereo decoder has very low output on the left channel; When this happened a few years ago it turned out to be the 0A79 germanium diodes in theT4 discriminator had decided to go out of spec. they were replaced.

I have sourced some NOS OA79 and will try to get a matched pair, however they will be old.

Does anyone know if I can replace them with the much newer & reliable 1N34A germanium diodes & would the 100k resistors value need to be changed.

I want everything ready for when my isolation as a 70+ is lifted & I can get to a friend's for the work he has the required oscilloscope, caps, resistors I have a collection of suitable valves.
 
Arkless yes you are correct T4 is a Foster Seeley discriminator as shown below. With desoldered 0A79's which I know must be a matched pair.


LEAK-TL-T4-diodes.png
 
My first thoughts on this are that I've never known an OA79 in a discriminator to either fail or go out of spec and that it's not possible for this to effect just one channel of a stereo decoder.... however as I don't have a circuit diagram for the decoder and "there are more things to heaven and earth...."
I strongly suggest that the problem is with the decoder though....

Yes you can use 1N34's and I've used modern RF Schottky diodes here in the past also. The 100K resistors do drift and are likely to be out of spec.

Any work carried out on this will put the discriminator out of alignment to one extent or another and there's nothing much can be done about this as a load of test gear and the knowledge to use it are required.
 
Arkless thank you very much the original 0A79's took until 4 years ago to go out of spec. The 0A79's I bought were smaller they may or may not have been crap. It could be a cap or resistor I dont think its the TD Paravicini unit as the EM84 magic eye is unbalanced whether the TDP decoder is connected or not. As I have the time I want to be ready for all eventualities. again thank you.
 
I have had an OA79 fail in a troughline tuner, so it does happen but rarely.
Agree with Jez that it's very unlikely to have that effect on the decoder, but never say never.
Better to use 1N60 rather than 1N34, as the 60's are specifically for demodulator use (lower capacitance and all that).
Failing those then AA138 should be Ok.
 
I can't see the capacitance mattering much as it would become part of the tuned circuits anyway... 1N34 was used up into UHF band. All are fine here and AA119 is another common one plus OA81 if available.

Distortion due to misalignment of the discriminator is my main worry.
 
Gentleman I really appreciate your valuable comments. as I postulated it might well be a capacitor or resistor I just want to be ready with the more difficult to obtain parts so that it will be a relatively easy job for my friend, I have a set of NOS valves + the originals though I re-valved a couple of years ago, I have tried 2 different EM 84's the top half moves down but the bottom only rises a little.
 
Only if no shared cathode or anode connection. I used separate HP schottkys in mine. Choice of diodes is academic in comparison to the issue of alignment of the discriminator IMHO!

The EM84 takes it's inputs from before the discriminator and there is no separate circuitry to drive either end of the bar... drifting resistor values around it quite likely the culprit.
 
I am open to the resistor suggestion. at the moment I am using a Meridian 104 tuner linked to the John Linsley Hood "Quad" aerial I made in the eaves which provides sufficient signal for the Trough line the 104 has no problems with signal and sounds very natural, good though it is, it is not as open and natural as the Leak. The Tim de Paravicini FM stereo multiplex decoder
will have to be checked of course though it was new in the early 2000's after another friend persuaded him to make it for me.
 
Big John said:
I am open to the resistor suggestion. at the moment I am using a Meridian 104 tuner linked to the John Linsley Hood "Quad" aerial I made in the eaves which provides sufficient signal for the Trough line the 104 has no problems with signal and sounds very natural, good though it is, it is not as open and natural as the Leak. The Tim de Paravicini FM stereo multiplex decoder
will have to be checked of course though it was new in the early 2000's after another friend persuaded him to make it for me.
Click to expand...

Ah yes I built one of those aerials as well years ago. It worked pretty good but was bulky.

'Fraid I'm no fan of the Troughline... Even though I've fixed loads of them, converted several to III spec, built several decoders for them and own about a dozen of them... I'll take the Meridian 104 all day long;)

Check the resistors with the diodes disconnected. Those Erie resistors are prone to drifting but the conditions they see in this usage means that they may be OK... matching between them is more important than absolute value. They will have drifted high but if still only say 110K (ish) and within about 3-5K of each other then just leave them.
 
Arkless comments understood. The aerial though bulky 1mm mild steel plate fits in a space that was left after our loft conversion it has 1/2" clearance all round or so my son told me as my 135 kg was a tight fit in the eaves. I have had contact with a nice guy in Hungary who has computer matched 2 pairs of 0A79 nos diodes for me at the listed price.
 
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