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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 7th Jul 2021, 1:41 pm   #61
Techman
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Default Re: HMV 501 Radiogram issues

Ah, just remembered that it's shorted out in one mode by a switch, so would work in one mode but not the other.
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Old 7th Jul 2021, 1:54 pm   #62
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Default Re: HMV 501 Radiogram issues

Quote:
Originally Posted by Techman View Post
The first valve is biased to cut off to mute the radio when in gram mode.
Yes, I'd forgotten that I'd noted this part of the circuit when I first commented last month after looking at the circuit. S5 opens in 'gram', but without R8 in circuit nothing happens...I should have kept up with the thread as it progressed. Great that it's finally sorted out in the end.
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Old 7th Jul 2021, 3:38 pm   #63
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Default Re: HMV 501 Radiogram issues

I am still restoring one of these, Infact I've just picked up this project again to finish it.

The capacitors in the pitch block will need replacing, but its not a difficult job. The trader sheet for the Marconiphone 42 / HMV 501 was quite useful in addition to the 501 service manual as it shows the positions of the block capacitors in the layout.

What I did was to de-solder all connections, remove the block, stick it in a plastic bag and put it in the freezer over night.

The bottom of the block could then be prized out, and with some taps on the can, out came all of the caps and the pitch in convenient frozen chunks.

I then cleaned it up, installed new caps on the tags, and sprayed the can white which was near enough to how it looked when new.

I've added some pictures of this process so you can see how it comes apart.

Overall, my chassis runs quite well. I've had to replace a smoothing capacitor, the pitch block caps, plus one or 2 others. But actually EVERY single resistor proved to be completely within spec. They seem more reliable than later ones.

Im unable to align the set perfectly for reception at the top end of medium wave however, because it seems the coils have absorbed moisture and this has reduced their inductance somehow. I can get to about 1530 khz with all trimmers at their minimum value. This set did spend 30 years in someones damp garage, so no surprises with cardboard coil formers. I might revisit this issue and see if I can gently bake them...

The pickup was open circuit, but I sent it to Ed Dinning who rewound it for me.

The electric motor and governor assembly was completely stripped, degreased and re-assembled, and some wires replaced. That now runs fine.

I'm currently smartening up the cabinet for when it all goes back in.

The chassis is an absolute pig to work on with the way speaker is attached to it. I've had to remove the speaker and support the chassis on blocks of wood

Adam
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Old 10th Jul 2021, 7:11 pm   #64
mancunian69
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Default Re: HMV 501 Radiogram issues

I agree that the chassis is a pain to work on. It is difficult to access some parts, and because the chassis takes its power from the grmophone in normal opertion, i had to devise a way to power the chassis on its own This meant using a highly dodgy mains wire with a plug at one end and two croc lips at the other connected to two self tappers gently secrewed in to the socket board at the back.

I did make sure everything was locked down before powering up though. I have not replaced any of the caps in the box, as I couldn't find a way to get access to them, other that drilling or chiselling the rivets off the tag board. Looking at your photos would suggest the top can be opened up somehow.

I will be posting an article about the resto on this set soon. Maybe it will give some members who have such a set some food for thought, you never know.

Rgds
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