HMV 1200 motor tuned

[boater sam]

Has anyone played with an HMV 1200 with motor tuning? 
I was lucky to find one, looks complicated.  Please let me know your experiences  if you have ever restored one. What a lot of valves! 

[Radio Fixer]

No! I have never worked on one Sam but really its only a 5 valve Supie + rectifier, eye and a double diode for the AFC. The IF second has a complicated tuned circuit which also is the discriminator for the AFC. Should be a good performer, I think a tuned RF stage helps a lot, about 10 dB increase in RF gain for one thing.

Here is a good book if you want to read up on AFC:

www.tubebooks.org/Books/afcs.pdf

www.tuberadioland.com/automatic_tuning_and_AFC.html


The actual motor tuning is a US (I remember it being Delco) mechanism that virtually all the makers used except Philips of course. Well they wouldn't would they !

Have fun Gary

[boater sam]

Thanks Gary.
I will get round to doing something with it when I get back in March.

I like a challenge!

Sam.

[Terry]

Reminds me of the Russian Rigonda radiograms sold here in the mid 60s.

I don't know if the HMV has presets for tuning but the Rigondas didn't: the motor was purely for AFC.

It was very interesting tuning the Medium Wave on this sets because, as you passed through an intermediate carrier en route to the required one, the tuning knob snatched at your fingers as the AFC pulled it first forwards then backwards as it attempted to lock onto the carrier!

[boater sam]

As far as I can tell it is purely electronic AFC. There are lots of push buttons for preset tuning, the motor moves from one to next as you press. There is muting as it moves.
There is a reasonable picture on Radiomuseum site.
Looks like it will be fun especially if the motor selection is broken.

Sam.

[valve-pilot]

I've just finished restoring one of these sets, its a heavy brute and on the face of it a bit complex. However, if you break it down into its stages , then it is no more complicated than other sets. As stated, it has an r.f. amplifier, an a.f.c. stage which operates on pre - tuned stations, and motorised tuning.

The motorised tuning consists of two motors, one to drive the tuning and selector mechanism and the 'cruise' control when on manual tune, The other motor drives the wave-change switch when on auto or manual tuning. Both motors have 'muting' switches which operate only when the motor is energised.
The main problem i had was trying to obtain good service information, the only diagram I could find had little detail of the switches and no under-chassis component layout.
My advice would be to concentrate on getting the set working on manual tuning first, then tackle the automatic tuning. I found it a good idea to insulate the motor 'mute' switches with a slip of folded paper, for if you have the chassis on its side to work on as I did, gravity operates the muting switch and spoils your fault finding logic !
The set works well on manual and auto tuning and I am amazed how good the operation of the a.f.c. is.
Shout if you need more help.

[boater sam]

Thank you for your response and worthwhile advice. I found the only service info which is probably the same as you have. The second page refers to the Ekco motor tuned set! But its the only circuit I have found. Is there  another, maybe Marconi, set similar or does it predate the amalgamations that took place?
 If I get stuck, may I pester you?  Its on my UK round tuit pile so it will be a while, the pile is a mountain, bought far too much whilst we have been away.

Sam.

[valve-pilot]

The service information I used was from the Radio Marketing Service Engineer, dated June 1940. If you are using this diagram, there is an unmarked capacitor across L11/T9, this is C12 (50pF). The equivalent Marconi set is model 880. The radiogram versions are HMV 1600 and Marconi 881. I'm happy to help if you get stuck !

[boater sam]

Thanks, Valve Pilot.

Noted that cap. I looked under the Marconi 880, it was the same incomplete file

Where could I find the Radio Marketing Service Engineer info?  Is it on the net?

[pwdrive]

http://www.radio-workshop.co.uk/service/hmv-1200.pdf

Lawrence.