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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
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12th Apr 2019, 11:48 am | #21 |
Dekatron
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 9,427
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Re: Pye T25Y New Zealand B&W Hybrid TV Set
Not sure about UK televisions but there is and advert on this web page for a Geloso tv using a nuvistor, presume for Italy.
http://www.r-type.org/articles/art-150.htm
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Frank |
13th Apr 2019, 2:24 am | #22 | |||
Nonode
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
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Re: Pye T25Y New Zealand B&W Hybrid TV Set
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DFW had 5 VHF TV transmitters in 1985, channels A4, A5, A8, A11 and A13. Another, channel A2, was added at the end of the 1980s (or it might have been in the early 1990s). Even with just two VHF transmitters, cross-modulation could be a problem with bipolar tuners. I saw a bad case in Auckland in the 1970s, with a Philips K9 receiver fed by a typical suburban outdoor aerial, 3-element Band I and 5 or 6-element Band III. An earlier monochrome receiver with a typical front end (ECC84 and ECF80) handled the incoming signal with ease, but the K9 required 18 or 20 dB of attenuation. Then there was a repeat in Wellington in the 1980s. I had arranged for a professional installation of a combined Band I/II/III VHF aerial (3 elements Band I, I forget the rest) at a friend’s house, and although that mostly fixed the ghosting problem, it did cause cross-modulation with a Philips receiver, a later model than the K9 although I did not recall which. 18 dB of attenuation was required to clear the cross-modulation. I imagine that Pye was aware of the potential problem with cross-modulation in NZ conditions when it opted to use a fet tuner for its Vidmatic. The use of fets in TV tuners was I think covered to some extent in this older thread: https://www.vintage-radio.net/forum/...ad.php?t=98398. In brief, what might be thought of as a standard configuration for American solid-state VHF TV tuners became a dual-gate mosfet RF amplifier, with agc bias on gate 2, followed by a dual-gate mosfet mixer in full cascode mode, both signal and oscillator on gate 1. This was in the established valve-era 4-gang type circuit, with a single-tuned input and a bandpass tuned interstage. Jfets were investigated, and whilst the jfet cascode worked well enough as an RF amplifier, it did not have enough agc range. Jfet mixers were good but lacked gain; their use necessitated an additional gain stage to provide the requisite output level to feed standard IF strips. Early on UHF tuner outputs were fed into the VHF mosfet RF stages as per established practice. But that appeared to change with the advent of varactor tuning, where tuning the RF stage to the IF “channel” might have been awkward. Then the UHF tuner output went direct to the VHF mixer mosfet acting as an amplifier, and typically the UHF tuner included a post-mixer gain stage to restore overall gain. (Although in some cases active UHF mixers with gain were used in place of the customary diodes, obviating the need for a following gain stage.) RF amplifiers were also sometimes added to UHF tuners to offset varactor losses. Initially these were bipolar grounded base; RCA I think was the first to use a UHF-capable dual-gate mosfet RF amplifier in its KRK226 tuner of c.1975. As cable TV expanded during the 1970s, there was an even greater need for tuners having good resistance to cross-modulation, given that the cable systems often used consecutive channels. (Although my experience was that cross-modulation and other spurs were supplied by the cable operators as extras free-of-charge.) Cheers, |
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13th Apr 2019, 10:29 am | #23 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
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Re: Pye T25Y New Zealand B&W Hybrid TV Set
The only television receiver released in the UK to use compactron valves was the TELETON colour hybrid portables [1971?] They performed extremely well. John.
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