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Old 12th Aug 2020, 4:42 pm   #1
PsychMan
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Default HMV 501 TRF Antenna and Alignment Questions

Hello Folks,

I've spent some time on my old HMV 501. Capacitors in the pitch block replaced, new smoothing cap and HT resistor.

All other caps are bang on spec, as are the resistors. Pretty amazing.

Anyway, I couldn't pick up my pantry transmitter that was operating at the top of the MW band, so thought I'd poke about with alignment for a change. Given its a TRF this is quite a simple process, and I didn't need it perfect by any means, just good enough to receive my own little station.

I found that I got the best reception possible when the trimmers attached to the tuning capacitors were fully open. There's no slug tuning of the coils on these, so I wondered why this might be. Has the inductance in the coils changed somehow with age affecting its ability at the top end of the band?

In general even with those trimmers open the reception with a long wire is a bit pants, given how strong my signal was in the vicinity to it.

If I stick a ferrite rod antenna and a tuning cap between the grid of V1 and the chassis, I can get as much signal as you like. I might consider adding a screened socket for this as a workaround.

Just thought Id ask for views as I haven't worked on something this old before

Cheers
Adam
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Old 12th Aug 2020, 6:08 pm   #2
cathoderay57
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Default Re: HMV 501 TRF Antenna and Alignment Questions

Hi Adam, most likely cause I would think is a change to the inductance and Q of the coils through moisture ingress into the coil formers. Even though no doubt now dried out they never seem to recover entirely from ageing in this way. Since you have too much capacitance (trimmers fully unwound) obviously it is no use adding extra fixed caps in parallel with the trimmers to bring them within range. Dirty main tuning capacitor vanes and insulators could be adding a tiny bit of extra capacitance but unless they are really grotty I wouldn't think significant. Make sure you have all stages properly aligned though because if one or more stages is out of kilter the others won't peak properly. Cheers, Jerry
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Old 12th Aug 2020, 7:23 pm   #3
PsychMan
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Default Re: HMV 501 TRF Antenna and Alignment Questions

Thanks Jerry, nice to hear the suspicion confirmed. The tuning capacitors are fairly clean, so I doubt that’s the problem. With the trimmers open I can only get my station with the main tuning caps unmeshed completely, and admittedly it comes through ok, but I’d be happier if I could tune beyond it a little bit to be sure I was tuning it for best sound.

Unfortunately there’s no inductance values in the service data to test the coils, only ohms readings which of course will be unchanged as it’s a problem in the former material.

I might experiment adding a bit more inductance in series and see what happens. Or consider removing one and seeing how easy it might be to open up. They’re in large metal cans
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Old 12th Aug 2020, 7:34 pm   #4
crackle
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Default Re: HMV 501 TRF Antenna and Alignment Questions

If you want to extend the tuning to a higher frequency I think you would need to remove turns. Could you not put a capacitor in series with the tuning cap, it may need to be fairly large, maybe .1uf. I am sure someone here can advise better.

Mike
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Old 12th Aug 2020, 7:55 pm   #5
G6Tanuki
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Default Re: HMV 501 TRF Antenna and Alignment Questions

I'd take the coils out and bake them at a low temperature [50C?] for a week or so to drive out any moisture.

This is not an uncommon problem: the Trio 9R59-series of radios from the 60s/70s have similar problems with aged wax-coating and coil-formers causing excess losses at RF.
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Old 14th Aug 2020, 1:26 pm   #6
PsychMan
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Default Re: HMV 501 TRF Antenna and Alignment Questions

Well after a bit of mucking about I’ve worked out the covers unscrew.

Looking at them I’m not sure I’d like to do any baking, they look a bit too fragile. They look actually in very good order visually, but not sure that means a lot.

I might just add a socket that I can connect a ferrite rod to for better performance at the top end of the band
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