Pye/Ekco UHF converter unit

[FERNSEH]

From the 1962/63 Radio and Television servicing book. Details of the introduction of the 405/625 line output transformer fitted to late production V700A models. Information is from service manual RV114B. Later V710A models were fitted with a slightly different 625/405 line output transformer. Co-incident with the introduction of the new transformers was two silicon HT rectifier diodes to replace the old metal rectifier.

DFWB.

Originally Posted by Argus25

That is an interesting converter, but I wonder why they had to duplicate the IF and detectors.

And 625 requires a larger bandwidth, negative modulation and FM sound, it is not an easy "down convert and tweak the line output" job. It is just possible it would have made sense for a new up market 405 TV.

[dazzlevision]

Originally Posted by Heatercathodeshort

I believe the valve Philips UHF tuner, the one with the polythene back and copper foil had an AFC tag that was never employed in UK receivers.

Philips/Mullard certainly made such a tuner, but the version without AFC is the only one I've ever seen in a UK produced TV set/UHF conversion kit. Again, as the UK TV market was very price driven, manufacturers would be unlikely to design in AFC unless really necessary and the 6MHz intercarrier sound technique rendered it unnecessary.

[dazzlevision]

Originally Posted by FERNSEH

The best and most complex 625 conversion kit was the one made by Philips for TV receivers made in 1962.
DFWB.

It was certainly complex, but I'm not sure it was the best, as it only had a two stage vision IF amplifier (EF183 + EF184) and was thus somewhat lacking in gain. You wouldn't see any sign of noise (snow) on the screen with no aerial connected - just like the Thorn UHF converter (same valve line up in the vision IF stages).

The UHF converter unit produced for Murphy Radio's "Astra" series of TVs (V6xx and V7xx models) used three Mazda 6F23 frame grid valves in the vision IF strip and you could certainly see snow on the screen with that. The Murphy unit used a Philips/Mullard UHF tuner of the same type fitted in the Philips UHF converter kits.

[Argus25]

Originally Posted by merlinmaxwell

625 requires a larger bandwidth, negative modulation and FM sound, it is not an easy "down convert and tweak the line output"

I looked at my notes, how I was able to convert the English Philco model BT1215L to 625 lines, with an added VHF tuner and not much else.

I realized for this one that the primary determinant of the oscillation frequency of the self oscillating line output stage was the inductance of the transformer. So I simply increased the gap in the core, bringing it up to 15,625Hz. Worked perfectly. I did have to take the transformer out of its oil filled canister. I can explain how I did that without any damage at all to the metal canister but its a bit long winded. It would have been easier if it wasn't a self oscillating stage.

I added a VHF tuner on the front end connected to the antenna input with the attenuator resistors removed and just a 150R terminating resistor.

I simply re-tuned the video and sound stages for a 5.5 spacing between sound and vision (the system in NZ).The increased bandwidth and loss of gain there in the video RF (which had now become an IF) was more than made up for by the tuner.

I replaced the detector transformer in the final sound stage with a ratio detector type and two OA91 diodes. I coupled the primary of that stage into an OA91 detector to make AGC for tuner. The tuner tubes were a PCC189 and a PCF86 strung into the heater chain. The video detector diode, an EB91, simply got reversed so its anode was the output.

Due to the fact the L/O in the tuner ran above the received VHF frequencies, the sound channel (transmitted higher than the vision frequency) was flipped below the vision carrier to suit the antenna input on the set so it all worked just fine with very good picture quality and sound on rabbit ears too.

So I may just have been lucky with this one, but it is why I asked the question why those converters needed to re-duplicate the IF's, when, if the existing stages are used, they become the new IF's.

[Nuvistor]

I would think, but stand to be corrected, that the two IF strips were due to the set requiring to receive 405 as well as 625 line systems, later sets used a single IF strip with switches to control the bandwidth and different sound circuits.
It would have been easier to add an IF strip to the set rather than convert the original into switched version.

I think there was a Bush TV TV115 that the conversion involved replacing the IF strip with a swiched one, probably the one designed for the TV125 which I think would be an easy but still expensive swop.

There was a great deal of uncertainty in what system would be selected so for those sets sold as convertable it was easier to add a complete IF strip when the standards decision was made.

Just my thoughts

[kan_turk]

I think that conversion from single standard 405 line to single standard 625 line , as in the NZ/Aus case was 'relatively' straightforward but conversion from single standard 405 line to dual standard 405/625 line, as was mostly required in GB prior to all services being available on 625 lines, was a more challenging proposition which was seldom undertaken

[FERNSEH]

The attachments show the UHF tuner in the Philips 625/405 conversion unit. This is the tuner without AFC.
There is no type and date label on the tuner, the only indication to what the tuner type is: A3 285 12

DFWB.

[Argus25]

Originally Posted by kan_turk

I think that conversion from single standard 405 line to single standard 625 line , as in the NZ/Aus case was 'relatively' straightforward but conversion from single standard 405 line to dual standard 405/625 line, as was mostly required in GB prior to all services being available on 625 lines, was a more challenging proposition which was seldom undertaken

Yes, it would have been very awkward to make my set switch between standards, but not so bad just as a fixed change from 405 to 625.

[dazzlevision]

Originally Posted by Nuvistor

I think there was a Bush TV TV115 that the conversion involved replacing the IF strip with a swiched one, probably the one designed for the TV125 which I think would be an easy but still expensive swop.

The conversion of the Bush TV113/TV115/TV118 range was quite straightforward, as the chassis was designed manufactured with a 405/625 line timebase and a front facing "405/625" system selection switch, operated by a Bowden cable.

So, apart from fitting a rotary valve UHF tuner, the plug-in 405 only hand wired "receiver unit" was replaced by a 405/625 unit (although not quite the same as fitted in the TV125/TV128 range).

However, in the previous range of models (TV105/TV108), it was also necessary to fit a replacement dual standard line output transformer and 405-625 system switch assembly (factory assembled and wired) in place of the 405 only LOPT. There were also several other component changes required in the conversion.

If you study the design of UK made TVs in the 1961-63 period, you can see that as the Government finally decided which technical standard the new programme(s) should adopt (after the Pilkington Committee report), greater provision was made to accommodate it. In the earliest incarnations, it was thought that the new programmes would be on UHF with 405 lines. Indeed, in the earliest Thorn/Ferguson convertible sets (which had 405/625 line timebases), it was stated that if UHF/405 was chosen, it would be a simple matter of fitting a UHF channel "biscuit" in the VHF turret tuner (which had a "U" position).

[Nuvistor]

Originally Posted by dazzlevision

Indeed, in the earliest Thorn/Ferguson convertible sets (which had 405/625 line timebases), it was stated that if UHF/405 was chosen, it would be a simple matter of fitting a UHF channel "biscuit" in the VHF turret tuner (which had a "U" position).

Thanks for the info on the Bush range, I fixed many but the only conversions I did was to fit a UHF tuner to the 125 range, just too expensive for the older sets.

Has for the Thorn sets, would this UHF channel biscuit be similar in the design to that used in many USA sets with a crystal diode mixer, no RF stage, LO harmonic injection and the VHF Mixer as an IF amplifier?

[dazzlevision]

Originally Posted by Nuvistor

As for the Thorn sets, would this UHF channel biscuit be similar in the design to that used in many USA sets with a crystal diode mixer, no RF stage, LO harmonic injection and the VHF Mixer as an IF amplifier?

No. The Thorn VHF turret tuner of the period in question was based on an American (Standard Kollsman) design, but used valves (e.g. PC97 and PCF86). The UHF channel coil/biscuit that would be fitted in the vacant "U" position would have to be compatible with the circuitry used for the Band 1 and Band 3 channel coils/biscuits.

On these tuners, there was no provision for connecting the IF output of a UHF tuner for preampfification. It was only later that this facility was provided, when the idea of fitting a "UHF" coil/biscuit in the VHF turret tuner was discarded (i.e. the new UHF transmissions would not be on the existing 405 "System A" standard, but "System I" - 625 lines, negative vision modulation and FM sound).

[Nuvistor]

This is the type of circuit I was referring to, it uses the VHF RF amp and VHF Mixer as IF amplifier, I have no idea how well it worked but from the magazines of the time it was popular. Would have worked with the Thorn UHF 405 sets if they had come into service.
The article starts on page 30, UHF biscuit description on page 32, interesting read.
http://www.americanradiohistory.com/...cs-1956-07.pdf

[dazzlevision]

Hmm, that's very interesting and although the article is from 1956, a lot of technical developments after that could have made it more compact and feasible to incorporate into the type of VHF tuner fitted in the Thorn 800/850 convertible chassis. How well it would have worked in practice, we may never know (unless articles in that US magazine from 1962-3 revisited this topic). Several different UHF "biscuits" would have been required, to cater for different BBC2 transmitter channels (if UHF 405 was adopted).

Thorn had close ties with some US companies (e.g. Sylvania) and made Standard Kollsman VHF tuners under licence.

[Nuvistor]

You may find this interesting from 1964, new valve and transistor UHF tuners. Still using the diode mixer, the European PC88/86 valves were introduced in 1962, it makes me wonder if it was costs or that the diode gave better performance, the PC88/86 combination were not great.
Page 26
http://www.americanradiohistory.com/...cs-1964-04.pdf

[dazzlevision]

I'm not convinced it was due to cost that all UK manufacturers adopted the PC86/PC88 valve UHF tuners. Before UK made valve UHF tuners arrived (e.g. Sydney S Bird), most of them were imported from W Germany (e.g. NSF and HOPT) and that TV market was more driven by quality and performance, than cost.

[Nuvistor]

Sorry, not expressing my self very well, i was meaning the USA manufacturing using the diode mixer with no RF stage using less parts than the PC88/86 tuners being a cost reduction.

I understand why the very early USA UHF tuners were built that way, there was no suitable UHF amplifier valve for those frequencies but by 1962 there was the European type.

I think some early Japanese TV's imported into the UK used a diode mixer but I cannot remember if it was preceded by a transistor RF amp.

All academic now I suppose.

[dazzlevision]

Originally Posted by Nuvistor

I think some early Japanese TV's imported into the UK used a diode mixer but I cannot remember if it was preceded by a transistor RF amp. All academic now I suppose.

I'm sure I've seen Japanese imports with no RF amp in the UHF tuner. "All academic now." Yes, but still of interest to the likes of us!