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Old 12th Sep 2018, 4:17 pm   #21
Michael Maurice
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Default Re: Hmv 1620

I've only just got in and not had a chance to have a proper look.

The sketch and the circuit diagram are NOT from the same chassis, the circuit is from a 1622 which bears a similarity.

There is a capacitor that is not shown on my sketch going from the anode of the triode section to a wafer switch and then off to various coils.

I will carry out more checks tomorrow.
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Old 12th Sep 2018, 4:48 pm   #22
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Default Re: Hmv 1620

Presume the capacitor is part of the feedback network.
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Old 30th Sep 2018, 11:52 am   #23
ronbryan
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Default Re: Hmv 1620

Michael brought round the HMV 1620 radiogram chassis for me to investigate. It's a LW/MW/SW/Gram band AM set with two HM309 output valves in push-pull.

I spent some time tracing the aerial and oscillator circuit, identifying the coils and trimmers for each band - not a simple job as they all went to the inacessible band switch. I subsequently worked out that the RF circuit was exactly the same as the HMV 1122 (Trader Sheet 1025), with the exception that the 1620 uses the B9A based X79 instead of an X78.

The aerial trimmers are postage stamp compression types mounted on a bracket under the chassis, but the oscillator trimmers are concentric, bolted to the chassis around the tuning capacitor. These trimmers are not the normal Philips type as they are larger and have two concentric cylinders in the rotor and stator. The material could be gold plated, or maybe is a type of brass that does not tarnish.

Boater Sam was correct in post #7, I found that the oscillator trimmers were shorted, the LW one for most of the travel, on the others the short would clear if the trimmer setting was moved. I don't know if this was caused by lack of concentricity or some other effect like metal 'whiskers'. Has anyone had any experience of problems with these trimmers? I wonder if they were originally made for military use.

I checked the max. trimmer capacitance with a TF2700 and it measured 43pF. The 1122 sheet says 30pF, so it is of the same order.

I've ordered some compression trimmers with integral bracket, to see if they can be easily mounted in place of the original ones.

Ron
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Old 3rd Oct 2018, 4:27 pm   #24
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Default Re: Hmv 1620

I had a close look at the shorted LW oscillator trimmer and found a crack in the wall of the inner cylinder in the stator.

I replaced all three trimmers with 50pF compression trimmers bought from Ebay. I tapped the integral angle bracket 4BA, bolted the trimmers to the original trimmer holes and extended the connecting wires a little with some 21 SWG copper wire, covered in Hellerman H12 sleeves.

The trimmers were adjusted on all three bands using the alignment instructions for the HMV 1122. The oscillator frequency was quite stable on SW at 16.8MHz.

I did not have the cabinet, with its associated internal plate aerial, so I strung a temporary 50ft wire down the garden to get round all the electrical noise. A short wire round my workshop is just not good enough these days.

Ron
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Old 3rd Oct 2018, 4:43 pm   #25
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Default Re: Hmv 1620

I feel that there is a tendency to regard mechanical bits as "usually OK" certainly I have fallen into that trap several times.
We get so used to trimmers being reliable and then we meet one that isn't and its a surprise.
This was brought home to me with several VHF Bush sets where the compression trimmer is short circuit yet the slightest movement cures it. I have never really found out why, the mica always looks fine, and they are undisturbed.
I've seen the Philips trimmers of course but yours sound new to me, any pictures?
Glad you got it sussed, another learning curve.
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Old 3rd Oct 2018, 6:51 pm   #26
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Sam

Re. trimmer pictures, please see post #23

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Old 3rd Oct 2018, 9:16 pm   #27
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Default Re: Hmv 1620

Thanks, never seen any like that, I'll be wary in future.
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Old 16th Oct 2018, 5:39 pm   #28
Michael Maurice
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Default Re: Hmv 1620

Thanks to Ron, this is now sorted, the lack of a diagram didn't help either. I'll hopefully get it reassembled into the cabinet soon together with the record deck.
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