Improving old TV sets.

[Geordie McBoyne]

On the subject of cars, the earlier Aston-Martin models such as the DB4, 5 and 6 were fitted with a clunky old David Brown gearbox. A recognised modification is to fit the six speed Tremec 'box. Doing this mod transforms the drivability of the cars. Also, out with the old contact breaker points distributor and fit a more advanced electronic ignition system, now available are systems that employ crankshaft sensors, even better.
As for the HMV 1804 I have to say I was initially put off by the modifications that had been done to it, but it soon became clear that the work had been carried out by a very competent engineer, whoever had done the mods knew what he was doing.
The Sobell T143: that set was only bought for the hard to find MW36-24 CRT, try and a 14" rectangular CRT these days! Even the round 12" tubes are easier to find. The CRT from the Sobell was fitted into a Bush TV32. Anyway, consider all the modification sheets issued by the manufactures during the course of the lifetime of TV receivers, all these modifications were introduced to improve the performance and reliability of the receivers. Again, if you think about Sobell, in due course the original frame timebase circuit in the T143 was replaced with a much better one in the firms' models on sale in the following years. And, if you think about it, do you honestly believe that in say twenty years time anyone will be all that bothered about some circuit modifications had been done to any TV set in the long distant past?

Geordie McBoyne.

[peter scott]

I must admit to tampering with values in the HMV901 frame oscillator.

I checked all the resistor and capacitor values and the oscillator transformer resistances. They were all correct except for the anode load resistor of the frame oscillator. The schematic shows this as 3M but mine had 3.8M so I changed it to 3M but the outcome was worse 240 line linearity than I got with 3.8M. I also tried 4M but 3.8M seems to be the sweet spot. I did wonder about messing with the line oscillator to improve its linearity but haven't done it.

Peter

[Murphyv310]

I read on another forum a few months ago about a young lad with a rather rare 40s TV that he got and was looking for advice. Amongst the rather belittling replies was one offering that he would take it away as it was dangerous for the lad to work on. I mean to say what a way to get hold of an old set for your collection. Some need to have a deep look at themselves, it is only a hobby for goodness sake.

I know the above is OT but I do believe it is relevant to adding modifications, it comes down to it has to be the owners choice as to do what they feel fit to do with it. Its not really up to others to tell them "its dangerous" so give it to me or whether you should improve a circuit or smash it up for parts. Any responsible engineer or collector will look after the set or decide its a basket case but it could fix two or three others.
Like I say I've been on the receiving end of abuse over a jellypot mod and an NTSC 405 Decoder before, not nice I'm afraid.

[Geordie McBoyne]

Took this off-screen picture today from the HMV 1804.  Slightly defocused picture, this might improve when the sound output valve is refitted, the extra 60mA HT current will increase the focus coil field strength.

Geordie McBoyne.

[Geordie McBoyne]

"I did wonder about messing with the line oscillator to improve its linearity but haven't done it.," Hi Peter, I intend to fit a timebase switch to my Marconi 702.   As well as changing the repetition rate of the blocking oscillator the switch introduces an additional ramp forming capacitor C3.  On 25c/s scanning C4 would be fully charged before the end of the frame scan, so C3 in introduced so that the sawtooth waveform will be as linear as possible.   These may be ancient circuits, it's likely design work started on these sets in 1935, but the circuit diagram shows that a great deal of attention was taken to produce a linear scan.
Note that the bottom end of the ramp forming capacitors are returned to the output valve cathode and scan coils.

Till Eulenspiegel.

[peter scott]

Hi Till,

I'm using one of these switches that are far too big for the correct position but I removed my push about coil because it affected edge focus so I've used the push about control bracket for the switch.

Peter



   

[Geordie McBoyne]

The Sobell T143.   The chassis has been reinstalled into the cabinet. The original metal rectifier still delivers 205 volts to the HT line. Note the position of the mains dropper resistor. It's simply screwed on to the rear CRT support bracket.  This set was certainly built on the cheap.
The service notes in the 1953/54 R & T servicing book inform us that the calculated boost HT voltage should be 400volts. In fact it is 395volts in this set. When the voltage readings were taken in 1953 an AVO model 7 multimeter was used. EHT is 12KV.
The AVO 7 is not really suitable for TV servicing. The model 8 is a much better multimeter for TV work because of it's much better 20,000ohms/volt sensitivity.

Geordie McBoyne.

[Murphyv310]

Cracking picture David.
Those rectangular 14 inch tubes in my mind were just about the best ever, beautifully flat sides with just little spherical corners.

[colly0410]

I suppose most 405 tellies didn't need a lot of gain & high performance sync gubbins as they'd be used in strong-ish signal areas. I understand they made fringe models with extra gain & flywheel line & frame sync. I suppose in fringe areas viewers would notice when performance dropped off as valves aged, but in strong signal areas old bangers on their last legs would still work to a fashion. Mr & Mrs public seemed happy if they got a picture..

[Murphyv310]

Hi.
Well some of the old bangers actually worked well. If you couldn't afford or for some other reasons didn't have a fringe area set then the aerial could be improved. Even in the 70's a lot of basic 405 line sets were still around giving sterling service. We were in the fringes in south Ayrshire and UHF was dreadful so many 405 line sets were common before the fill in transmitters started up. Basic sets were often fitted with 30L15's in the tuner and even an EF184 in place of an EF80 would help so long as there was no instability, dodges of course but it made the customer happy.