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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
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17th Nov 2016, 2:00 pm | #21 |
Dekatron
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
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Re: Pye 838 TV-radiogram.
Actually the first EF50 valves had the rather curious hooked pins, the idea was the valve was inserted and rotated a few degrees into position. A good idea but lateral forces cracked the glass base so very soon after the conventional straight pins were adopted.
Just been checking the EHT supply components. The transformer was rewound some time ago by Majestic Transformers Ltd and is perfect, so is the 0.25microfarad smoothing capacitor. However, the 6.8megohm bleeder resistor is open circuit or has gone to an impossibly high value and will have to be replaced. The EHT capacitor will maintain it's charge for a long period and that's not conclusive to safe servicing. On no account does one want a 5000 volt electric shock! There'll be some power in the charged capacitor: P = 1/2CVsquared. How do you do superscripts? More about power in capacitors here: http://www.engineeringtoolbox.com/ca...er-d_1389.html DFWB. |
17th Nov 2016, 10:20 pm | #22 |
Heptode
Join Date: Jun 2006
Location: Ware, Hertfordshire, UK.
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Re: Pye 838 TV-radiogram.
Hi David,
The PYE 838 at the museum has a Garrard RC4 fitted. Cheers Andy
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18th Nov 2016, 12:03 pm | #23 |
Dekatron
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Re: Pye 838 TV-radiogram.
Hi Andy,
A picture of the RC60 record deck taken at a similar angle. I think this deck should be allowed to remain in the set. IMHO it looks OK. DFWB. |
18th Nov 2016, 1:04 pm | #24 |
Heptode
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Re: Pye 838 TV-radiogram.
Hi David,
I think the RC60 is the best bet for the time being. If you didn't know better, you would think it was original. The RC4 seems a rare beast. Would it have been used in RGD radiograms? Cheers Andy
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18th Nov 2016, 2:12 pm | #25 |
Octode
Join Date: May 2009
Location: Stevenage, Herts. UK.
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Re: Pye 838 TV-radiogram.
I agree, the RC60 looks the part whilst the UA6 is definitely from another era.
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18th Nov 2016, 7:01 pm | #26 |
Octode
Join Date: Mar 2006
Location: Co. Durham, UK.
Posts: 1,117
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Re: Pye 838 TV-radiogram.
That RC60 deck looks very like the one in a Ferguson radiogram I remember from yore. It was said to be able to play the 'new' records, but I never found out how.
There was no 33/45/78 knob, and it took throwaway needles. Just more bull from the 'grown-ups'? |
21st Nov 2016, 5:08 pm | #27 |
Dekatron
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Re: Pye 838 TV-radiogram.
It's unlikely a Garrard RC4 will ever turn up. So for now the RC60 will remain in the set.
The sawtooth waveform present at the control grid of the AC6PEN line output valve. Perfect waveform shape @ 17 volts P - P. Note the flyback, only 5 microseconds, in practise it might actually be too fast. DFWB. |
21st Nov 2016, 6:02 pm | #28 |
Dekatron
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Re: Pye 838 TV-radiogram.
Showing the relationship between the video and line drive waveforms.
Note the position of the line sync pulse and the flyback stroke of the line oscillator waveform. DFWB. |
21st Nov 2016, 6:57 pm | #29 |
Dekatron
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Re: Pye 838 TV-radiogram.
It's becoming all too evident what is causing the foldover at the beginning of the line scan. The duration of the line flyback pulse at the anode of the line output valve is too long, it measures 20 microseconds. This results in the start of the scanning stroke in the scan coils being too late.
DFWB. |
21st Nov 2016, 9:47 pm | #30 |
Dekatron
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Re: Pye 838 TV-radiogram.
A 1 ohm resistor has been inserted into the earthy side of the line scan coils, the oscilloscope is connected across the resistor so that the current in the scan coils can be monitored.
The 'scope remains triggered on the video waveform. Despite the fold over of the left side of the picture the waveform across the test resistor has a perfect sawtooth shape. Peak to Peak voltage is 5V so we now know the current through the line scanning coils is 5 amps P - P. The line linearity control takes the form of the usual variable damping resistor which has a series capacitor with it. No damping diode in this 1938 TV receiver. Refer to the topic: "Evolution of the Energy Recovery Diode". Post #5: Damper diode, reclaim diode and booster diode are all terms used over time. The damper diode was the attempt to improve the efficiency of the line output stage. You have to go back the early 1930s and the beginnings of magnetic deflection in TV receivers. It was known that at the end of the scan the rapid flyback time initiates an oscillation in the scan coils. A damping resistor across the scan coils can reduce the oscillation but this is wasteful and saps up too much energy. A better solution is to make the damping circuit more frequency selective so a capacitor was inserted in series with the resistor, the degree of damping can be adjusted to serve as a linearity control. DFWB. |
22nd Nov 2016, 12:08 am | #31 |
Dekatron
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Re: Pye 838 TV-radiogram.
Fiddling about with various capacitor values in the line linearity circuit has resulted in an improvement in the line scan.
DFWB. |
22nd Nov 2016, 8:36 am | #32 |
Heptode
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Location: Eindhoven, Netherlands.
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Re: Pye 838 TV-radiogram.
Looks like an amazing picture from a 1938 CRT David!
Jac |
22nd Nov 2016, 8:57 am | #33 |
Nonode
Join Date: Feb 2005
Location: Worcestershire, UK.
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Re: Pye 838 TV-radiogram.
78 years old! Television as it was a lifetime ago. Incredible!
Steve
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22nd Nov 2016, 10:39 am | #34 |
Rest in Peace
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Re: Pye 838 TV-radiogram.
You and a few others on this forum must get tremendous pleasure from getting these very early sets going again. Well done.
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22nd Nov 2016, 4:50 pm | #35 |
Dekatron
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Re: Pye 838 TV-radiogram.
And there is more pre-war TV restos to come.
The cathode bypass capacitor of the line output valve is 25microfarads, I've been experimenting with smaller values, 2.2microfarads improves matters considerably. The reasoning for doing this is to reduce the gain of the line output amplifier at the fundamental frequency of 10.125Kc/s. We know that a line output stage must handle frequencies up to 10X the line scan rate. By reducing the value of the cathode bypass capacitor the line linearity capacitor can be returned to it's original value. Way back in 1936 driving a class A timebase amplifier at 10Kc/s and beyond must have been considered as working at the limits of the technology of those times. One of the reasons many manufactures preferred electrostatic scanning. By 1939 almost all TV manufactures had adopted magnetic scanning. DFWB DFWB. |
24th Nov 2016, 7:03 pm | #36 |
Dekatron
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Re: Pye 838 TV-radiogram.
Quick update: the HT supply reservoir capacitor has gone O. C. thus causing the HT to the line output valve to fall to 270volts. It should be about 330V.
DFWB. |
2nd Oct 2017, 11:08 pm | #37 |
Dekatron
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Pye 838TV-radiogram.
Never been entirely happy with the installation of the Garrard RC60 record changer. Nevertheless, it was a more suited unit for this TV-radiogram than the mid fifties BSR deck which it had when I first acquired it in the 1990s. Many thanks to Andy (Beery) for giving me the lead to the 1937 MRG radiogram which was fitted with a Garrard RC4 automatic record deck, the correct gram deck for the Pye 838.
So it's out with the RC60 and in with the RC4. Should be a very simple unit to install in the 838. DFWB. |