1946 HMV 1804.

[Geordie McBoyne]

I recently acquired this HMV 1804 which is a post-war development of the 1939 model 1800.
The two sets are very similar except for different valves in the RF and IF amplifiers.
Unfortunately my set has well and truly been "got at". The ebay seller told me that the set came from a farm house in Cambridgeshire, well away from the Alexandra Palace transmitter. Well, it seems a long time ago an engineer had attempted to improve the fringe area performance of the set. In the 1804 the RF and IF amplifiers valves were Marconi-Osram Z66 pentodes, a valve that resembles the Mazda SP61.
These have been replaced by the EF50.  However, again I suppose to squeeze a bit more gain from the receiver the MO X81 triode-hexode frequency changer valve was replaced by an EF50 as the mixer and a separate local oscillator which uses the EC52 all glass triode. Perhaps this was an attempt to improve the noise performance of the frequency changer. The X81 triode-heptode must have been at it's limits at 45Mhz?
 http://www.radiomuseum.org/tubes/tube_x81m.html     The X81 resembles the American 7S7.
So as well as the funny signal circuits the rest of the set isn't all that good either. Other strange devices have been introduced into the set, one appears to be the type of interlace filter circuit used in many Pye TVs.  Anyway, if I decide to return the set back to the original circuits I'll gain a few B9G EF50 valveholders.
Or, shall I keep the strange circuits? After all someone took the trouble to modify the set to operate in difficult reception conditions.

Geordie McBoyne.

[Murphyv310]

A mess indeed.
At least there was some research done to use circuitry that just might have improved matters rather than tweeking it up and reducing the vision bandwidth.
Would be interesting to see if the mods actually worked.

[Geordie McBoyne]

Hi Trevor,
apart from the strange add-on circuits the work on the RF and vision IF circuits has been done in a competent manner.

Geordie McBoyne.

[Murphyv310]

Hi.
We had Joseph Urban up here in Anstruther and before Kirk o' Shotts opened he was receiving usable signals form I believe Sutton Coldfield (correct me if I am wrong).
He had two band one Yagis phased up and on the roof of his workshop, I remember seeing his 2 valve mast head amp that he wouldn't part with, he also modified a few sets too.
On the downside Joe's work was rough, a neater approach would no doubt have made his creations work better at RF than they did. I believe he did at one time suggest a relay system for Anstruther to distribute the signal around the village. Once BBC on Ch3 stared from Kirk o' Shotts the idea was scrapped

[ppppenguin]

Sounds like that set was modified quite professionally. Might be worth keeping the mods.

Receiving Sutton Coldfield from Scotland sounds like a very big ask. Perhaps Holme Moss? Or Pontop Pike though I think this was opened after Kirk'o'Shotts.

[Murphyv310]

You could well be right Jeffrey, it's 13 years since he told me and i really don't remember the transmitter he was receiving.

[Geordie McBoyne]

More strange modifications found this morning. The sound IF amplifier V14 should be a MO KTW61 which is a vari-mu beam RF tetrode, the more common 6K7G could be considered a replacement. However, and no doubt to improve the performance of the set V14 has been replaced by a high slope RF pentode, a Mazda SP61. Actually it's the CV equivalent the type CV118. The CV118/SP61 has the Mazda Octal base. OK, I can live with the EF50 and EC52 valves but a Mazda valve in an HMV receiver, no way!

Geordie McBoyne.

[Geordie McBoyne]

Another one of the original valves has been substituted with another type. The vision demodulator diode in the 1804 should be a Marconi-Osram type D43. This is a largish signal diode on the B5 base, it has a 4volt heater. Top cap anode connection. It has been replaced by a D63/6H6 octal base double diode, 6.3volt heater.
http://www.radiomuseum.org/tubes/tube_d43.html
http://www.radiomuseum.org/tubes/tube_d63.html

Geordie McBoyne.

[boater sam]

It would appear that someone either fancied him/her self as a set designer or wanted to keep this set going indefinitely.

Are these the sets that almost bankrupted HMV due to their unreliability?

If the modifications really do enhance the performance it may be better to keep them but you will need another one to compare it with.  Another collection being started?

Sam.

[Geordie McBoyne]

Hi Sam,
The range of HMV TV sets that it is said almost bankrupted EMI was the 1807 series which was introduced late 1949. Although alleged to be the most unreliable TVs of all time I'm beginning to question if these sets were really that bad after all?
As for the post war models including the 1804 these were built to the same standards as the pre-war models, everything was built on a truly massive scale. The 1804 weighs in at a hefty 84 pounds!
I can compare this HMV with a similar set, the Marconi VT50. Same electrically, different cabinet presentation.
In an early post I mentioned that the 1804 we are discussing has a Pye FV1 type of double diode interlace filter wired in. In the original EMI design a single diode interlace filter is employed, one section of a D63/6H6 double diode. I will remove the alien interlace circuits and reinstate the original EMI interlace filter. Same goes for the sound IF amplifier valve, the SP61 and it's Mazda Octal valveholder will be taken out and the correct International Octal valveholder fitted along with a KTW61 or equivalent vari-mu pentode.

Geordie McBoyne.