EMI Push About Coils

[peter_scott]

Many years ago I removed the push about coil from my HMV901 because it appeared to distort the edge focus and I didn't really need it to get my picture centred.

More recently I decided to try it again and once again decided to remove it but removing it didn't get me back to the edge focus I had previously and my scan now had some trapezoidal distortion. Checking the push about coil I found that its iron core had some retained magnetism and I wondered whether the CRT cage and the iron core of the scan coils also had retained magnetism. Just today I tried using a degaussing coil around the CRT area and have been able to largely remove the scan distortion and also improved the edge focus.

If you have one of these early EMI sets fitted with a push about coil I would highly recommend removing it and degaussing the CRT cage and scan coils. You my need to make other arrangements to hold the scan coils up close to the cone of the CRT.

Peter

[FERNSEH]

Hi Peter,
It's worth noting that later production Marconi 702 sets employed a CRT housing assembly made of aluminium. It's most likely later HMV 901 sets were the same.
The push-about coil is disconnected in my Marconi 702. Picture centering is perfect.


DFWB.

[peter_scott]

Hi David,

I think my 901 is about the youngest of the known TV only sets but it definitely has a steel CRT cage. It's certainly interesting that they tried an aluminium one though.

Thanks,

Peter

[FERNSEH]

Hi Peter,
because my Marconi 702 has no serial number plate we cannot determine the precise date it was made but it's safe to suggest it was in 1938. Perhaps the reason why the set has an aluminium CRT housing was because even in 1938 they were having problems with magnetised steel CRT mounting assemblies. Or it could have been simply nothing more than economics.

DFWB.

[peter_scott]

Hi David,

Could you give me the chassis serial number on the TRF of your 702?
It's just above the output terminals. (The photo is actually from a 900 but the 702 has it in the same place.)

Peter

[FERNSEH]

Hi Peter,
The Marconi 702 TRF chassis serial number is: 0884.

As for the HMV 901 I can't find any numbers stamped on the metalwork.


DFWB.

[peter_scott]

Hi David,

My reason for asking was because I think EMI did swap assemblies on sets they were refurbing (just post war??) principally the timebase so they could remove the Baird switch.

You were suggesting that your 702 might be of a late production run on account of the alloy CRT cage but looking at the dating table I think there are quite a few reasons to suggest it is older than you thought.

I don't think the TRF chassis were often swapped and with its 0884 serial number I think it fits well with your timebase that has an exposed V8 socket and your sound chassis. Also the CRT gun assembly in your set is similar to that in other sets in the middle of the table. Later sets had more substantial locating discs in metal rather than the mica discs of the earlier sets. The power supply chassis also had serial numbers although sometimes hard to see.

I suspect that your set probably dates from early 1937.

Peter

[FERNSEH]

Hi Peter,
My reasoning for believing my Marconi 702 is a late production set is because the pre-set controls escutcheon does not have the hole for the 240/405 timebase switch. See the attachment. Also a picture of the TRF chassis serial number.

DFWB.

[1100 man]

Alright, I've got to ask, what on earth is a 'push about coil'?

Cheers
Nick

[peter_scott]

Hi Nick,

It's an electro-magnet for picture centering but its magnetism can have undesirable side effects such as causing trapezoidal distortion and poor edge focus.

Hi David,

But do the holes for the standards switch show any sign that they previously had self tappers screwed into them?

Peter

[FERNSEH]

Hi Peter,
I'll check out the timebase chassis metalwork tomorrow.

DFWB.

[Rejectostat]

Hi David,

I think that maybe the escutcheon is not original to your receiver as built in the 1930s. I have examined at least three of the early serial number receivers and all have an escutcheon the same as the one you picture. The instructions issued by EMI for the immediate post-war preparation for service include the removal of the existing escutcheon and replacement with a version without 240/405 switching provision. EMI may well have recalled the receivers starting with the earliest originally sold, then as per procedures fitted the new replacement escutcheon. Then, perhaps someone realised it would be much cheaper and easier to just place a small Bakelite cover over the 240/405 toggle switch hole itself, rather than providing a complete new escutcheon. So later recalls were done by this method. Recalling receivers as per original sale would seem a fair way of selecting receivers for updating, as those are likely to have had the most service life so far.

Unfortunately I don’t have any data or pictures to hand, though I am pretty sure the early 702/900’s I examined were H1029, H1067 and H6080, these three representing some of the earliest known receivers by back plate serial number, and timebase chassis 253, 656, 441 respectively. The only other one I have seen is H1007, though that still had a system toggle switch in position as I recollect.

John.

[Jac]

Originally Posted by Rejectostat

The only other one I have seen is H1007, though that still had a system toggle switch in position as I recollect.
John.

John,

I suspect that the system switch of H1007 is not original, since it is mounted in the Bakelite escutcheon, while it should be mounted on a metal bracket on the time-base chassis.

Jac