HMV and Marconiphone sets had tried to market themselves as up-market sets, and indeed had been most successful. However, with television begining a rapid expansion plus the success of low cost sets such as the Pye B18T and LV20 sets, they decided to attempt compete with a low cost model.
Oh dear :-( In their attempts to cut costs, they produced what was probably one of the least reliable sets of of 1949/50. One thing that really wouldn't help is the compactness of the chassis - the photo opposite shows the entire chassis, and you can use the size of the valves and arse end of the C.R.T. to judge the scale ; 16 valves crammed into such a tiny space must have caused problems with heat, especially when it was stuffed in a teenie little table-top cabinet. Maybe it fared better in a larger console cabinet - er, probably not !
So what was wrong ? Well, the first clue comes from a HMV "Supplementary Information" sheet ; no fewer than 3 stock faults were attributed purely to an "unsuitable" valve - not a duff valve, just one that was within tolerance but not "just right" for the badly designed circuitry to which it was fitted.
We now see Hunts capacitors as scum, but compared to EMI's own efforts they would be seen as highly desireable ! Intermittent open circuits were common, particularly as the set warmed up, often leading to RF instability ; HMV's recommended fault finding technique was to cover the chassis with a duster to help it get _real_ warm, then remove the duster and blow on each cap in turn to find when the fault went away. Presumably this assumed you hadn't burnt your workshop down in the first place !
As for the CRT's, well they were EMI and hence afflicted by the same problems as the models they'd actually bothered to deign properly. Problems with secondary emission only seem to have afflicted EMI's tubes and resulted in a background of an unmodulated raster, effectively degrading the pictures contrast.
Not that it really mattered. The focus arrangements were so stupid that you'd struggle to get a decent focussed picture even the the CRT had been perfect. This is attributable to a ""clever"" piece of circuit design whereby the focus was adjusted by varying the EHT. Bad enough to start with, but the way it was implemented also affected the line drive (i.e. picture width) - and the picture width control would also affect the focus - what a balancing act ! You choose, dim and focussed or bright and blurred ! I mean, look at any service manual and they'll quote an expected EHT voltage; examine this particular chassis' service data and they quote the range "2-to-9KV" - not only an extraordinarily wide range, but also bear in mind that even the feeble brightness of a pre-war set's picture would have involved an EHT of at least 4KV.
Other oddities include the use of the then recently introduced Z77 valve. Internal shorts often cased the main fuses to blow, but the biggest puzzle is this ; it is an RF pentode. That is, it is a pentode for amplifying small low power high frequency signals. Fine. So what kind of burk uses one as an audio ouput (power) valve !? Not exactly the way to maintain HMV/Marconi's reputation for high quality sound, as it wouldn't exactly fill the room with sound. Or maybe it didn't need to - the receiver itself was a tad deaf anyway.
When the set was lauched, the Birmingham transmitter was about to open, being quickly followed by tranmitters in other parts of the country. What a great idea of HMV to introduce an un-tunable TRF design ! They must have realised they'd cocked up as non-London versions were superhets. Except these didn't work well either ; whilst the Osram X78 oscillator/mixer would do stirling service in your average table-top radio at upto a couple of MHz, it seriously struggled at the 45-60MHz needed for television and this did nothing to help the stability of the receiver.
Finally the heat generated in such a confined space lead to problems with drift. In the RF strip, this would result in an irritating buzz-on-sound which the telly engineer could come in once a week to tweak. Even EMI's supplement highlights that you'd expect to fiddle with controls such as frame hold regularly within the first 20 minutes. These controls were at the back of the set - as would be the case with any other contemporary set - because they would normally be intended for very occasional use. But with this set, adjustment was needed so frequently that they'd wear out comparatively quickly.
I do wonder if HMV had poached a design engineer from Murphy ; supporting evidence the bonkers aspects of some of the ciruit design. Evidence against this is the fact that it wouldn't have been so darn unreliable !
|SERVICE DATA||I've got a copy of the Trader service sheet #974.|
The cabinet looks nice. Yes its had knocks but its been brightened up and looks very nice. The chassis ? Fine layer of cack, no rust and looks untouched. The back ? excellent. The C.R.T. getter is perfect - these old E.M.I.'s are apparently prone to a bit of gas.In fact no probs except for a mild outbreak of uncharacteristically thoughtful worms, who have confined almost all of their damage to the inside of the cabinet where their work is out of site.
|WHERE I GOT IT||October 2005 Radiophile auction at Shifnal for 120 telly tokens. Funny thing was, the bidding started at £50 - by me - and I sat back and waited for the flood of competing bids to come flying in. It was like a cartoon, with total silence other than the crickets and the odd cough. So it didn't sell, but the deal was struck later with the owners (via Chas, who ran the auction).|
|The compact RF strip ...|
|.. and the even more compact power stage.
Note the metal rectifier ... I doubt
it's fins ever saw a cooling draft !
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THE TELLIES GALLERY
2nd October 2005