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Radio Leak Trough Line stereo

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I've noticed an odd problem with my Leak tuner. It's only started happening this last couple of weeks. Everything starts fine. After about 3/4 hour the volume starts to drop...just as if it's drifting off tune which is what I thought it was doing. However, the tuning indicator is still showing maximum signal. With the AFC off if you retune slightly the tuning indicator shows less signal but the volume then comes up to normal. So it seems that maximum tuning signal doesn't correspond to maximum output. My first thoughts were some sort of imbalance of the FM detector but that would affect AFC, which it isn't. It doesn't appear to be AGC since the tuning indicator grid is fed from the AGC rail and it's still showing maximum signal when the fault is on. Basically when the fault is on, maximum signal does not relate to maximum output. Off-tune slightly and the output comes up. I'm wondering if it's some sort of instability? I'll have to get it on the bench at some point and have a dig around. I suppose if an HT decoupler is failing, that might cause the problem but it's not affecting the tuning indicator......


I'd like to hear from anyone with ideas....

Posted : 02/12/2023 10:05 pm
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@sideband You have said it yourself, I would be checking decoupling. I would be interested in the resolution.


Posted : 03/12/2023 9:34 am
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I'd second what Frank says. Time to get a scope on it. The only time I've seen a tuning meter say it's off tune when it clearly isn't was when someone had been mucking about with the cores in an Armstrong tuner-amplifier, which I *think* isn't getting stereo because someone has peaked the coils and reduced the bandwidth of the IF amplifier.

Posted : 03/12/2023 1:07 pm
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As a sidenote, the Troughline 2 and 3 have something of a reputation for marginal IF stability that can be nudged over the edge by deteriorating contact between the decouplers' riveted solder tags and chassis, not helped by the way that the various tubular ceramic decouplers are connected to chassis "where it seemed convenient", rather than the RF good practice of a single input-and-output point per stage to keep feedback paths between stages at bay. Unsurprising, really, that three cascaded high-gain pentode stages can misbehave if they get the opportunity.

Posted : 04/12/2023 9:24 pm
sideband and Cathovisor reacted
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You've just reminded me ......when I got the tuner, there were three of faults. One was intermittent low gain, another was no movement of the EM84 and the third....intermittent full deflection of the EM 84 (once I'd cured the no deflection). Low gain was caused by a bent pin on one of the ECF80's, the no deflection of the EM84 was a leaky cap on the grid (the only paper cap in the tuner....no surprise there), the intermittent full deflection.....an intermittent earth rivet on an IF decoupler.....I think it's time to check those earth rivets again....! I seem to recall I cured it by soldering the tag to the chassis....

Posted : 05/12/2023 6:06 pm
Marconi_MPT4 reacted