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Television Standards Converters, Modulators etc Standards converters, modulators anything else for providing signals to vintage televisions.

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Old 29th Oct 2021, 9:06 pm   #1
murphyv310
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Default Rod Pentode Band 1 modulator

Hi.
I do many experiments with Rod Pentodes from small audio preamps, TRF regen radios and pantry transmitters.
I've been experimenting with a 1ZH37B Dual control grid version and recently made a very good Pantry TX for Medium wave. Out of curiosity I wondered if the valve would oscillate with a crystal of 45.033Mhz, only slightly higher than Channel 1, 405 line frequency. It did, the circuit is about as simple as you can get with the crystal connected between one control grid and the G2 in series with a 22pf cap, the screen is connected to HT via a 470uH choke, the grid is returned to filament - via a 1m resistor. I then applied to the opposite control grid base band video from the video out socket on an Aurora. I was rewarded by a negative picture, I then added a series connected pot to vary the level, reducing the level gave me a washed out positive picture, I noticed some RF on the video waveform so I added another choke I had to hand a 470uH and a 100pf to ground from the video input and this removed the RF. At this point I was rewarded with an excellent picture with the exception of hooking at the top of the picture. On the envelope there was some sync clipping. What did make a big difference was to ground the input to the filament positive, this removed the hooking, also the sync clipping stopped.
There is still some tidying up and I have yet to draw out the circuit. A second stage is required for audio as well. I see this as a useful modulator for PC standards conversion, its very simple, only requires 1.2v NiMh AA for filaments and 63v for HT, I'm using batteries but a mains PSU would be very easy to construct.
I've done a couple of videos here:
https://youtu.be/r6j98UlMWFU
and here:
https://youtu.be/_dVMij9vVFU
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Old 30th Oct 2021, 11:14 am   #2
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Default Re: Rod Pentode Band 1 modulator

Great work and impressive results. Just the crystal might be difficult to source. Worth getting a few of those rod pentodes and having a play.
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Old 30th Oct 2021, 12:35 pm   #3
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Default Re: Rod Pentode Band 1 modulator

Fascinating!!

I guess your xtal is a third overtone one.

These were used in their thousands for mobile radio systems back in the 1960s to the 1980s, you usually find someone at a radio rally with a tray of the things, and a while back I donated a bunch of them to a couple of Forum members.
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Old 30th Oct 2021, 8:36 pm   #4
murphyv310
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Default Re: Rod Pentode Band 1 modulator

Hi.
As can be seen this has been assembled on Matrix board. Far from ideal in the relation to RF, moving the board around causes some odd effects. The idea would be a dedicated PCB something I've never previously done, the other idea is manhattan construction on copper clad board, the valve also should be screened.
I'll remake this in a day or two.
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Old 11th Dec 2021, 12:04 pm   #5
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Default Re: Rod Pentode Band 1 modulator

Hi.
Other projects have come in the way of progress on this including the HMV 2807. I now have some double sided copperclad board and the manhattan blocks for mounting the components. I aim to restart the project this coming week.
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Old 11th Dec 2021, 12:26 pm   #6
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Default Re: Rod Pentode Band 1 modulator

Looking forward to that, Trevor!

Most of the little pantry transmitters are MW and audio, yours is one of the few running above SW and modulated with video!
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Old 13th Dec 2021, 7:09 pm   #7
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Default Re: Rod Pentode Band 1 modulator

Hi.
Today I've been working out a suitable layout with dimensions for fitting the blocks and spacings for the valve leads. Next is to decide on the inductor placement and crystals.
All done on paper the same size as the board
I don't want to start on making the board up until I'm confident that the layout is finalised.
I've also another project on the go plus four new TV arrivals, so rather busy.
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Old 24th Dec 2021, 5:00 pm   #8
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Default Re: Rod Pentode Band 1 modulator

Hi.
Progress is halted at the moment due to problem with a Yaesu FT-480R plus other projects and commitments.
Will resume in early January.
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