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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
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6th Mar 2023, 11:42 pm | #21 |
Pentode
Join Date: Dec 2007
Location: County Durham, UK.
Posts: 243
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Re: It's another Sobell! ST197 D/S
I too have a soft spot for these sets , as a kid I grew up with the family’s set a GEC 2001 a19”set that uses the same chassis as the 1000/2000 sets
I have in my collection 4 sets using this chassis and all 4 are in good working order up to now |
7th Mar 2023, 1:02 pm | #22 |
Octode
Join Date: Apr 2016
Location: Ventnor, Isle of Wight, & Great Dunmow, Essex, UK.
Posts: 1,377
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Re: It's another Sobell! ST197 D/S
Thanks for posting those pictures! Apart from the GEC 2000 in the last picture, I've not seen any of the other three before.
Does the first one say Electra on the badge? I'm guessing the second one is a Masteradio, judging by the 'M' on the badge? The third I think must be a Sobell as the speaker cloth is the same as my ST197. The front panel layout is different though. All interesting & unusual sets! Cheers Nick |
11th Mar 2023, 10:34 am | #23 |
Pentode
Join Date: Dec 2007
Location: County Durham, UK.
Posts: 243
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Re: It's another Sobell! ST197 D/S
Hi the sets I posted all use the 2000 series chassis
The first one is the GEC Electra model 2001 the second in a McMichael model 3001 the third is a sobel ST something as the model number is missing Chris |
31st Mar 2023, 4:10 pm | #24 |
Octode
Join Date: Apr 2016
Location: Ventnor, Isle of Wight, & Great Dunmow, Essex, UK.
Posts: 1,377
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Re: It's another Sobell! ST197 D/S
Time for a progress report...
I had reached a point where I was very happy with the picture on 405 - picture geometry and linearity were good and the picture nice & sharp and contrasty. However, the sound was 'off'. Rotating the fine tuner after optimum picture had been reached, dramatically increased the volume. On several channels, it was not possible to turn the fine tuner far enough to get any sound at all. So time for some alignment of the sound IF coils. The AM sound on 405 is on a carrier at 38.15 MHz. This is 'picked off' from the whole IF signal by the tuned circuit (T2) in the anode of the common IF amplifier, V3. (EF183) It's then passed to the sound IF amp, V6 (EF80). This also has a tuned circuit (T4) in its anode set to 38.15MHz. The signal is then demodulated by a simple diode before going to the audio amplifier. My test equipment is rather basic, consisting of an ancient Taylor signal generator and a frequency counter to see what it's actually generating. To be fair to it, the dial calibration is pretty good and it doesn't drift appreciably after its been on for 5 minutes. It took me a while to figure out why the frequency counter was showing garbage, until I realised that with the output modulated by the 1KHz tone, the counter was getting confused! The manual says to inject a modulated 38.15MHz signal into the grid of the sound IF, V6 and tune T4 in its anode for maximum output. Rotating the bottom core of T4 produced a huge increase in volume. In fact it needed 3 whole turns! I guess it had been fiddled with, as before I started, the core was flush with the underside of the PCB. Moving the injected signal to the grid of V3, the common IF amp, and tuning its anode circuit, T2, produced a further though less dramatic increase in volume. Now, rather satisfyingly, the test card from the aurora on CH1 tuned in perfectly with the sound, as did most of the other channels. There are still a few that need the coil biscuits in the tuner adjusting, but I can't see an easy way to do this. There is a hole above the channel change knob, but it doesn't seem to go anywhere near the tuner! I'll need to investigate further. So that's about it for the 405 side of things, now I'll turn my attention to 625 where there is a very loud vision buzz on the sound - more alignment needed I think! Cheers Nick |
31st Mar 2023, 8:48 pm | #25 |
Pentode
Join Date: Apr 2021
Location: Paris, France.
Posts: 242
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Re: It's another Sobell! ST197 D/S
Regarding the sound buzz in 625, it is generally due to non linearities in the common demodulation for vison and intercarrier sound.
It's a grid testpattern which produces the most important buzz, it's much less audible on a live picture. In order to obtain totally free of buzz sound quality a "quasi-parallel" sound processing approach has been used in the 70s/80s (separate sound IF amplification and synchronous demodulation for intercarrier recovery). |
31st Mar 2023, 9:45 pm | #26 |
Octode
Join Date: Apr 2005
Location: Peterborough, Cambridgeshire, UK.
Posts: 1,898
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Re: It's another Sobell! ST197 D/S
It sounds like you are getting there with the set. I wonder if the IF alignment drifts with time? The reason I say that is because that is two sets now that I have heard of that needed a tweak. My Mate Mick has a 23" Electra that needed realigning we like you thought at the time that it had been twiddled with but it makes me wonder if they are a bit prone to drift? Lovely sets though! Nice to see a few have survived especially in such good condition!
Rich. |
1st Apr 2023, 11:04 am | #27 | |
Octode
Join Date: Apr 2016
Location: Ventnor, Isle of Wight, & Great Dunmow, Essex, UK.
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Re: It's another Sobell! ST197 D/S
Quote:
Dual standard sets must have been a nightmare for designers & manufacturers as almost everything is different between the two systems. Only the frame timebase remains unchanged! I guess that's why pictures on 625 often seem poorer than 405 on DS sets. Trying to design an IF strip that caters for both standards is going to be a compromise and I imagine it was more important to ensure good operation on 405 as that was the system most people were still going to be using. I suspect many DS sets never got used on 625 at all, unless the owner was particularly keen on BBC2 and a UHF aerial. I often find sets need 'tweaks' in the IF stages, especially the sound. I suspect there are several factors involved. They have been fiddled with in the past, the cores have moved due to vibration, the characteristics of the tuned circuit have altered over time due to damp, dirt & drift in component values. The cores I adjusted on my set were very loose and one had unscrewed to the point where it was flush with the PCB, so it is possible it had vibrated downwards. The slot in the core was undamaged, so if it had been twiddled, it had been done carefully! I'm looking forward to actually using this set to watch vintage programs on a regular basis, especially as it has an excellent tube and the later type LOPT which runs almost cold, so should be reliable. At least there is no danger of this one being turned into a plant garden or cat bed!! Cheers Nick |
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1st Apr 2023, 6:20 pm | #28 |
Pentode
Join Date: Dec 2012
Location: Herne Bay, Kent, UK.
Posts: 233
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Re: It's another Sobell! ST197 D/S
Hello Nick. Nice work. Could I just deny any "twiddling" on my part. I learnt very early on( the hard way) not to twiddle.
Regards Steve. Last edited by Cobaltblue; 1st Apr 2023 at 6:49 pm. Reason: Automotive removed |
1st Apr 2023, 7:31 pm | #29 |
Octode
Join Date: Apr 2005
Location: Peterborough, Cambridgeshire, UK.
Posts: 1,898
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Re: It's another Sobell! ST197 D/S
The only D/S sets I can remember having to twiddle back in the day was the Thorn 1400. Carry out all the mods to cure 625 sound buzz and then resort to tweaking when all else failed ! In those days there were no (or very few) captions, gawd knows how we would have cured caption buzz on a 1400!
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1st Apr 2023, 7:37 pm | #30 |
Banned
Join Date: Dec 2022
Location: Peterborough, Cambridgeshire, UK.
Posts: 418
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Re: It's another Sobell! ST197 D/S
One of the drivers for eliminating "caption buzz" was the use of anti-aliased captions: when a conventional caption was hard-keyed via a clipper into a vision signal it generated out-of-band products that intruded into the sound signal. By using anti-aliased captions and linear keying the signal was kept within the vision bandwidth.
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