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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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26th Jan 2021, 1:36 pm | #21 | |||||
Octode
Join Date: Sep 2020
Location: Thetford, Norfolk, UK.
Posts: 1,741
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Re: HMV 1252 FM Tuner - front end problem
Thank you all so far for all the suggestions.
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I remain totally stumped. I do wonder if part of the problem is the aerial and whether it needs a coil of some sort between the aerial feed and ground. I will attempt to rig up some form of dipole and run more tests. Seems a pretty standard VHF design of the time, but just zero performance! |
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26th Jan 2021, 2:59 pm | #22 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: HMV 1252 FM Tuner - front end problem
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26th Jan 2021, 5:46 pm | #23 |
Octode
Join Date: Dec 2019
Location: Owston Ferry, North Lincolnshire, UK.
Posts: 1,704
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Re: HMV 1252 FM Tuner - front end problem
I would still try these valves in a known working RF stage in another VHF tuner that uses the same valve. The ECC85 is pri-marily an audio valve, so a high spec one will be needed for RF use. I had to go through 4 valves a few years back before I found one that worked at RF, yet they all worked perfectly OK at audio frequencies.
Dave |
26th Jan 2021, 5:59 pm | #24 | |
Octode
Join Date: Sep 2020
Location: Thetford, Norfolk, UK.
Posts: 1,741
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Re: HMV 1252 FM Tuner - front end problem
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I have seen other conversations about ECC85 VHF stages and I am starting to think this set might just be poor quality. |
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1st Feb 2021, 5:07 pm | #25 |
Octode
Join Date: Sep 2020
Location: Thetford, Norfolk, UK.
Posts: 1,741
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Re: HMV 1252 FM Tuner - front end problem
Still struggling with this set, and would welcome any more input.
So, latest tests. Still don't have another VHF set to test valves in, but have now tried 3 different ECC85's. I think the problem is the oscillator, despite all voltages being roughly correct. With the probe of my scope on x10 to avoid too much loading, I cannot detect an oscillator output anywhere, so I don't think it is running. All components test passively OK. I think the 'locally generated' test FM signal was a red herring as probably only bleed on the harmonics through to the IF stage. Can anyone help with how this thing is supposed to oscillate? I think the first RF amp stage is probably working. |
1st Feb 2021, 5:48 pm | #26 | |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: HMV 1252 FM Tuner - front end problem
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Lawrence. |
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1st Feb 2021, 6:41 pm | #27 |
Octode
Join Date: Sep 2020
Location: Thetford, Norfolk, UK.
Posts: 1,741
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Re: HMV 1252 FM Tuner - front end problem
Thank you Lawrence for such an excellent description. Very helpful.
On that basis I think L5/L6 is the primary problem. The original former was broken and the ferrite slug was wrecked. As it was only 3 turns each, I re-wound them onto a new former with new slugs, but clearly the spacing of the windings is not correct so the oscillator is defunct. I will have to play around with the coil and see if I can get it to oscillate. |
1st Feb 2021, 6:55 pm | #28 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: HMV 1252 FM Tuner - front end problem
Make sure the new slugs are VHF friendly, some aren't.
Lawrence. |
1st Feb 2021, 6:58 pm | #29 |
Octode
Join Date: Sep 2020
Location: Thetford, Norfolk, UK.
Posts: 1,741
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Re: HMV 1252 FM Tuner - front end problem
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1st Feb 2021, 7:05 pm | #30 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: HMV 1252 FM Tuner - front end problem
It's to do with the composition so far as I know, in the old days ferrite wasn't much cop above 2MHz.
Lawrence. |
1st Feb 2021, 7:10 pm | #31 |
Octode
Join Date: Sep 2020
Location: Thetford, Norfolk, UK.
Posts: 1,741
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Re: HMV 1252 FM Tuner - front end problem
I didn't know that! Oh - I don't think there will be any way to tell! All I know is that I got them from some old coils in an AM radio, so maybe not. Yet another fun problem to add to the mix!
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1st Feb 2021, 7:24 pm | #32 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: HMV 1252 FM Tuner - front end problem
Also the connections of the windings matters too, eg: if L5 connections were swopped round or L6 connections were swopped round then the feedback would be -ve and not +ve, that also means it wouldn't oscillate.
Lawrence. |
4th Feb 2021, 4:44 pm | #33 |
Octode
Join Date: Sep 2020
Location: Thetford, Norfolk, UK.
Posts: 1,741
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Re: HMV 1252 FM Tuner - front end problem
What a dope! While working on this set yesterday, I realized that I cannot check if the set is oscillating with my scope anyway, as it is only 20MHz.
Still think the fault is around the coils. Back to the drawing board! |
4th Feb 2021, 4:56 pm | #34 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: HMV 1252 FM Tuner - front end problem
If the oscillators working you should be able to measure some grid current or 'scope a signal at the input to V2.
Lawrence. |
4th Feb 2021, 4:58 pm | #35 |
Octode
Join Date: Sep 2020
Location: Thetford, Norfolk, UK.
Posts: 1,741
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Re: HMV 1252 FM Tuner - front end problem
There is something, but not sure if it is correct at the moment. My gut feeling is the oscillator is probably working but way off frequency due to L5/L6 being previously damaged.
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