Another Mk I TR82C.

[Joe]

I'm currently working on one of these which is working on MW but not so on LW.

I can't get Radio Four but on 1400m it's picking up Five Live!

The Wave switch appears to be working correctly according to my continuity tester and prodding and poking makes no odds.

The IF alignment is fine as is the RF on MW; the trimmer caps are still sealed so The Phantom hasn't been at it.

I've also changed C10 - Bush number - the other electrolytics test OK, C3, C5, C7 and C13 which all tested leaky. I dare say that the remaining four Hunt's are also past their prime but can't see them causing the trouble.

As long standing members will know, I am bear of very little brain and transistors bother me and I haven't a clue where to look next.

Cheers

Joe

[Refugee]

Measure the aerial coils.
Two of them are in parallel with the switch in the MW position so set it to LW.
They all go to chassis so you also have a common point.
Avoid measuring them on the rod as the turns are very fragile and it could get a whole lot worse if you do.

[Robert Darwent]

I can't get Radio Four but on 1400m it's picking up Five Live!

Measure the aerial coils.
Two of them are in parallel with the switch in the MW position so set it to LW.

I think its the other way around, the inductance is increased by switching the two coils in parallel on long wave, with the larger coil used just by itself for medium wave.

Anyway, the problem is probably either with the switch contacts or a break/poor joint on the coil wires themselves. Continuity/resistances checks should show up which is the cause.

Regards

[Refugee]

I would go for parallel in MW.
The capacitance is higher with the switch in the other position to that shown.
Also look where the base goes to. L3 for LW and L5 for MW.

[Robert Darwent]

I would go for parallel in MW.

Yes, you're correct of course - apologies.

the inductance is increased by switching the two coils in parallel on long wave, with the larger coil used just by itself for medium wave.

In fact, I had just logged back on to the forum to correct my earlier post after thinking I should have typed "series" instead of "parallel" in the sentence above - wrong on both counts!

Oh well, I know what I mean - its just that no one else does!

Hopefully, my final comment about the switching and/or the coils themselves being the source of the problem will still be valid?

Regards

[Joe]

Thanks, chaps. I'll see what's what tomorrow.

Cheers

Joe

[Robert Darwent]

Joe,

Take a look at this thread; VTR103 receiving MW on LW. In that instance the LW coil was open circuit. On the VTR103, the MW/LW front-end circuitry is basically the same as that used on the TR82.

I haven't had this specific fault happen to any of the Bush TR82 series that I own, but it jogged my memory of reading the above thread a while ago.

Regards

[pwdrive]

I think the switch is shown in the MW position and in that position the MW and the LW tuning coils are in parallel, the total inductance value of such would be less than the value of the MW coil on it's own.

A possibility is that they have done that to reduce the self resonant frequency effect of the LW coil due to the LW coils relatively high self capacitance. On a lot of receivers the coils not selected were shorted out by the wave change switch to stop self resonance, especially on some shortwave receivers.

If left undamped then any transmission picked up by a self resonating LW coil could well interfere with the MW band as to a certain extent the LW and MW coils are mutually coupled due to the ferrite rod, that's if the self resonant frequency of the LW coil falls within the MW band.

There may be another reason?

Lawrence.

[Joe]

I found that L2 had a break in it which I managed to bridge and the DC resistance was thereafter more-or-less on spec - 13.5Ω against the specified 12Ω. L3 is also slightly high so the discrepancy is probably the test leads.

LW still doesn't work though and is still picking up MW signals.

However MW itself is now much improved. It was working before but not very well. As this is the first Mark I I've dealt with I'd nothing to compare it to and assumed it was par for the course.

I still can't find anything amiss with the wave switch and have also tested the low value capacitors which are switched in on LW and they all appear to be in good health.

I'm now pondering and hoping that if I leave it for a bit I'll spot something I've hitherto missed.

- Joe

[pwdrive]

It sounds like the osc could be at the wrong frequency when switched to LW, osc frequency for 198Khz (R4) = 668Khz, a quick f/counter or scope check would confirm what's what if there's nothing else obvious.

Lawrence. [EDIT: If scope/counter aren't available you could beat it with another radio tuned to MW (frequency wise that is)]