13th Oct 2019, 10:48 am | #161 |
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Re: Bush TV22 with TC184 B3 converter.
Last night a substitute frame blocking transformer was tried out in the TV32.
The transformer chosen for the Bush TV32 is a Radiospares FT7 which was made as an exact replacement for the HMV 1824 series. The frame oscillator is working again, plenty picture height adjustment and the frame hold locks in the centre position of the control. But the set has problems with interlace. The preset interlace control VR9 can be adjusted for good interlace but the setting is critical and not reliable. Extract from #154 Argus25 wrote: "The circuits with the Westector diode in the TV22 are interesting. If you assume the diode has around 100k leakage or less (would be interesting to measure it) the average diode anode voltage would simply climb to the plate circuit voltage of the sync sep stage. Then when the leading edge of a negative going pulse appeared the diode would assume a low resistance and when the plate voltage went high again the diode would stop conducting and go back into leakage mode acting like a high value resistor. So it appears as if the designers wanted a filter with a fast attack and a slow decay. Which to some extent makes sense and another not." Note the HMV frame timebase circuit has an interlace diode with a 2.2Mohm resistor connected in parallel. DFWB. Last edited by FERNSEH; 13th Oct 2019 at 10:54 am. |
14th Oct 2019, 10:09 pm | #162 |
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Re: Bush TV22 with TC184 B3 converter.
The replacement blocking oscillator transformer is now permanently fitted on the chassis. As the Radiospares transformer is slightly larger than the original this necessitated that two holes had to be drilled in the chassis.
DFWB. |
15th Oct 2019, 1:25 am | #163 |
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Re: Bush TV22 with TC184 B3 converter.
Regardless of the quality suitability of the integrator and interlace diode design in this set, I would expect the interlace to be pretty poor. The set looks like its still running original electros and the HT rail will be buzzing with H pulses and probably C67 is high ESR too letting these into the vertical oscillator and fouling up the interlace. Clearly the V oscillator is getting triggered by H pulses and not the leading edge of the V sync pulse, so your half line delay is going down the toilet.
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15th Oct 2019, 3:30 pm | #164 |
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Re: Bush TV22 with TC184 B3 converter.
AC ripple on the HT line is 2 volts P - P.
The TV32 has a 3microfarad ripple cancelling capacitor in series with a 2.9Kohm high wattage resistor. The two components are connected between the top of the heater chain and the HT rail. DFWB. |
15th Oct 2019, 3:41 pm | #165 |
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Re: Bush TV22 with TC184 B3 converter.
Hi David,
My far from perfect TV32 has 600mV p-p ripple on the HT but that is with modern electrolytics. Peter |
15th Oct 2019, 3:43 pm | #166 |
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Re: Bush TV22 with TC184 B3 converter.
Hi Peter, have just checked the ripple cancelling capacitor on a capacitance meter and multi-meter high ohms range. The capacitor is useless!
DFWB. |
15th Oct 2019, 3:53 pm | #167 |
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Re: Bush TV22 with TC184 B3 converter.
Is the 3uF on the top side of the chassis? If so I've not changed mine as I've not taken the chassis out of the cabinet. Ah! found it. Not changed.
Peter Last edited by peter_scott; 15th Oct 2019 at 4:03 pm. |
15th Oct 2019, 4:00 pm | #168 |
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Re: Bush TV22 with TC184 B3 converter.
Hi Peter, yes, the 3mfd capacitor is on the topside of the chassis. The chassis will have to come to gain access to it.
As a temporary measure I've tacked in a 4mfd motor run capacitor and the ripple is now a distorted sine wave and is down to 1volt P - P. Fit a capacitor of the correct value and the ripple should be reduced further. DFWB. |
5th Dec 2019, 1:07 am | #169 | |
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Re: Bush TV22 with TC184 B3 converter.
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5th Dec 2019, 2:48 am | #170 | |
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Re: Bush TV22 with TC184 B3 converter.
Quote:
Still the interlace will possibly be poor if the HT filter caps are in poor condition (the originals always are in poor condition) or the the hum cancelling cap off value. So it is important to rebuild the capacitors for a good result. |
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5th Dec 2019, 10:52 am | #171 | ||
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Re: Bush TV22 with TC184 B3 converter.
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5th Dec 2019, 12:07 pm | #172 |
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Re: Bush TV22 with TC184 B3 converter.
Equalization pulses normally help the probability of a good interlace, regardless of how good or poor the vertical integrator is. Ideally you have a good multi-stage, two or three stage RC integrator and the equalization pulses present in the sync too and the scenario is optimized for perfect interlace. This was standard practice for American TV sync signals immediately post WW2 and their TV sets seldom had any interlace issues as a result. If the vertical integrator is a poor design to the extreme, then, the equalization pulses cannot help.That is why they are not beneficial for the standard TV22
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5th Dec 2019, 12:50 pm | #173 | |
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Re: Bush TV22 with TC184 B3 converter.
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5th Dec 2019, 1:53 pm | #174 | |
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Re: Bush TV22 with TC184 B3 converter.
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There is a very good description of equalizing pulses and their application in Principles of Television Engineering, by Donald Fink, McGraw-Hill, 1940. He points out that the purpose of the equalizing pulses is to make the shape of each vertical pulse the same, after separation. The shape otherwise is not the same as in one case the vertical sync block starts with a 1 line delay from the preceding H pulse and on the next field a 1/2 line delay. The equalizing pulses fix this issue. Still, even without this issue fixed, it is still possible to have a respectable interlace. They also recommend that the filtering is not so heavy that the serrations are completely removed from the V sync, but I have found the interlace is fine if they are filtered to the extent that they are just removed as shown by V sync recordings from RCA, but gross over integration is a bad idea. Also, two stage integrators were being shown at this point in time in the American literature. |
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5th Dec 2019, 8:13 pm | #175 | ||
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Re: Bush TV22 with TC184 B3 converter.
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6th Dec 2019, 7:04 am | #176 |
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Re: Bush TV22 with TC184 B3 converter.
It is an interesting situation in that the American TV engineers were years ahead of the ones in the UK in the 1940's, with designs that included energy efficiency scanning and proper interlace systems.
The paradox is though, that many of the revolutionary ideas about TV technology in fact originated in the UK, from people like Blumlein. However they were only paid attention to in the USA initially, for example RCA labs taking up Blumlein's idea for energy recovery scanning in the early 1940's. So UK TV's were slow to get these improvements, along with slow to get turret tuners as well compared to their American counterparts. The same thing repeated itself in the field of computing where the theory in the UK was well ahead in the 1950's, but more successfully commercialized in the USA. I guess they had the population to support this. |
6th Dec 2019, 9:50 am | #177 | |
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Re: Bush TV22 with TC184 B3 converter.
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6th Dec 2019, 11:30 am | #178 | |
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Re: Bush TV22 with TC184 B3 converter.
Quote:
When FM was invented RCA knew that people would not bring their AM radios back in for modification, so instead they more or less trialed it by bringing it out immediately post WW2, in their first post war TV set, the 621TS and quickly they moved to FM sound for all TV transmissions, while AM sound for TV persisted in other countries for a decade or more. FM sound for radios initially was popularized in car radios before it really took off and now it is being exterminated by digital radio, which has inferior quality. Coming from a small country like NZ, by the time we adopted any new tech in those says it was already mature, and we got monochrome TV, with FM sound around 1958-1960. I was amazed when I first saw a TV set with AM sound in NZ, it was imported from the UK (didn't work in NZ of course) I had to modify the scan stages and convert the detector to FM and because the set was a TRF I had to hang a turret tuner on its front end to get it to work. The self oscillating line output stage was a challenge, I sped it up by parting the two halves of the line output transformer's core to make it run at 15,625 KHz. |
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6th Dec 2019, 3:26 pm | #179 |
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Re: Bush TV22 with TC184 B3 converter.
I have a Stromberg Carlson radiogram dating from 1941 with FM on the 40-45 mc/s band.
I believe it was changed to 88-108mc/s immediately after WW2. The FM unit is a separate chassis with octal valves! John. |
6th Dec 2019, 7:45 pm | #180 |
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Re: Bush TV22 with TC184 B3 converter.
Had to take my TV22 MKII out of the case today to replace a crackly audio o/p transformer (a shorted turn I suspect) and took the opportunity of reducing R52 (R55 in the MKII) to 10k. I really couldn't see any difference, except the picture made a slight twitch as I did it.
Anyway, I think the picture is probably as good as the TV22 gets. I'm lucky with the tube, although it does need a slight refocussing after the first 30 mins or so. Rather a nice set, and MUCH easier to work on the the TV24! Photo of picture with 100k resistor: |