Marconi T21A Background Noise

[Boater Sam]

Great, keep us informed. Its a worthwhile experiment.
You may be able to buy a choke near enough to the inductance required.

[Voxophone]

A quick update. Just in case, I tried warming up the coil with a hairdryer for a while to drive out any moisture. Amazingly this caused the Q to increase to about 20, settling to 17 once cooled.

I tried the coil back in the set and was able to get something of a peak, but nowhere near as sharp as SW or LW and still lacking in amplitude. I think I got about 500mVpp out for 1Vpp in, compared with 1.5Vpp out for SW.

The performance seemed to get worse over time and later measurements suggested the Q had decreased to 11. It seems like I haven’t made a permanent improvement then.

Afterwards I tried a 220uH coil with decent Q salvaged from an old signal generator, connected as Sam suggests and with the existing coil disconnected. Still no peak, and MW performance was about the same as before. On the other hand, the inductance is a bit high and it was difficult to mount the coil in a good position.

I’ll do more testing later today.

Liam

[Voxophone]

Another update.

I tried warming the coil up again, for longer this time. Again I saw an increase in Q which temporarily improved performance somewhat in the set, but again it didn’t last. Interestingly, the Q of the other two coils seemed to rise somewhat when the assembly was warm. This is the opposite effect to what I would’ve expected as wire resistance should surely increase with temperature. Can anyone think of a reason for this?

Afterwards I pretty much wrote off the original coil and focused on fitting the 220uH one from the signal generator in its place. Luckily it’s an exact fit into the middle of the hollow former. I tied the ends of the old coil together with a shorting link in case it might have some effect on the circuit. Not sure if this was really necessary but it seemed like a good idea.

With the replacement coil in position I can get the response to peak. It’s not as good as the other wavebands (0.7 Vpp out at 1 MHz for 1 Vpp in), but it’s still much better and more consistent than the original. Strangely the peak is reached with the trimmer screwed all the way out. Screwing it in reduces the output. I’m thinking this could mean too much capacitance in the circuit? Perhaps a series padder to reduce the trimmer capacitance might help?

Regardless, the set sounds much better now and seems perfectly listenable. I’ll probably do a bit more experimenting to see if I can improve it any further, then fix the new coil in position more permanently.

Thanks to everyone who helped me in figuring this one out. That’s 2/3 sets with weak MW problems sorted!

Cheers

Liam

[ms660]

""I tried warming the coil up again, for longer this time. Again I saw an increase in Q which temporarily improved performance somewhat in the set, but again it didn’t last. Interestingly, the Q of the other two coils seemed to rise somewhat when the assembly was warm. This is the opposite effect to what I would’ve expected as wire resistance should surely increase with temperature. Can anyone think of a reason for this?""

Heating the wire causes expansion so maybe it's the skin effect, eg: more conductor CSA in the skin depth for RF?

Lawrence.

[Skywave]

Just a belated thought - I don't know why I didn't think of this before - but does the offending coil have an adjustable dust-iron core?

Al.

[Voxophone]

No, its just a hollow former with no core. The adjustment is via a trimmer capacitor.

I got everything reassembled on this set last night, only for the drive cord to break or come off the rails inside the cabinet - despite working flawlessly for weeks. I'm never going to get this thing finished!

Liam

[Skywave]

Originally Posted by Voxophone

I'm never going to get this thing finished!

But you will. You need three sturdy tools: Persistence, Perseverance and most of all, Patience. The most satisfying results come after a struggle; if the required fixing is achieved too easily, somehow it's just doesn't feel rewarding.
Well that's my experience, anyway.

Al.

[Voxophone]

Originally Posted by Skywave

if the required fixing is achieved too easily, somehow it's just doesn't feel rewarding.

True. It seems I've had more than my fair share of sets lately with r.f or i.f. problems, but solving these has improved my understanding no end. Much more satisfying than a standard re-cap.

The drive cord will probably be an easy fix, it just picked its time to fail. Afterwards, if the the whole thing doesn't spontaneously combust, I can get back to the Ferranti I'm supposed to be restoring.

Liam