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murphyv310 2nd Aug 2022 8:46 pm

Band 1 Rod Pentode modulator (Part 2)
It was last December that I lashed up a 405 line Band 1 Channel 1 vision modulator. Where has the time gone?
After thinning down my collection and having decided that I am no longer repairing Vintage Radio & TV for others It's now possible to have some "me" time, like us all we are not getting any younger.
I've now rebuilt the vision stage onto copper laminate board and have tested it.
As before I'm using a 1ZH42B dual grid rod pentode, the HT is 80v and the filament is running 1.12v from an Ni-Mh AA battery via a 1ohm series resistor.
The video input is fed to one control grid via a 470Ω pot (set near max). The interesting thing is the result was a negative going picture. I didn't have this issue in December.......Then id came back to me that the grid needed to be driven positive between 0.7 & 1.0v, the result then flips positive.
The second grid has a crystal connected to it and the other end to the G2, which gets its HT via a 470μh choke, the second grid is bled to ground with a 1mΩ resistor.

There is more info on this video.

murphyv310 2nd Aug 2022 9:08 pm

Re: Band 1 Rod Pentode modulator (Part 2)
1 Attachment(s)
Now revised since the video with a bias pot.
Here is a rough diagram of the video section,

murphyv310 3rd Aug 2022 4:20 pm

Re: Band 1 Rod Pentode modulator (Part 2)
More work done today and some changes to the circuitry.
The 45Mhz crystal is low and doesn't always start and the 41.5Mhz crystal is as dead as a dodo. So I have found a 44.3Mhz crystal for Vision and a 40.815Mhz crystal for sound, both a bit low for channel 1 and of course not exactly a 3.5Mhz spacing but it works extremely well.
It has been pointed out to me on post 1 I have quoted the wrong valves they are 1ZH37B NOT 1ZH42B (there is no such valve as a 42B anyway its a 1ZH42A) Just to clear up any confusion.
I have contacted Vincent Jakomin re 45 & 41.5Mhz crystals.
I have also tidied up the bias arrangements.
I'll upload a revised diagram in due course.
Two further Videos here:

rambo1152 3rd Aug 2022 5:14 pm

Re: Band 1 Rod Pentode modulator (Part 2)
I am a great fan of minimalist design and this project ticks that particular box.

Are you going to build a matching sound modulator, I don't suppose you have found a cheap source of suitable crystals?

The rod pentodes aren't radially available at the minute for obvious reasons.

I've been trying (quite hard!) to get Vidblaster v5 (v5.54) going for our purposes. Why? Because it has a nice built-in sound mixer that can be configured to follow the selected video source.

The problem is, as you know, the video needs s t r e t c h i n g to 16:9 in order to get the correct aspect ratio on 405, and v5 does not have the convenient 1.33:1 / 1.78:1 switching in the GUI's sources modules.

In actual fact, you can force a still-store to 1.78 by altering the aspect ratio parameter of the relevant source in your [profile name].ini file, this works a treat for still-stores but seems to be ignored for "player" modules. Quite infuriating!

Of course I could re-render all my videos, but I'd rather not.

murphyv310 3rd Aug 2022 9:36 pm

Re: Band 1 Rod Pentode modulator (Part 2)
1 Attachment(s)
Thanks Graham, I've now completed the Audio and done some further tweeks. We are not actually using 1ZH37B properly in this circuit according to "those" in the know but it works and quite well.
The logo on Vidblaster isn't really an issue TBH I can live with it.
Here is the circuit so far and I've also added a couple of videos.

murphyv310 4th Aug 2022 5:53 pm

Re: Band 1 Rod Pentode modulator (Part 2)
1 Attachment(s)
Some interesting developments on the modulator this afternoon. For some reason I decided to reverse the grids on the vision modulator and lo and behold I don't need the positive bias to give me a positive going signal! The down side is it needs a lot more drive so its left as was.

Also I have modified the audio stage slightly by adding a 1K resistor between the earth side from the input audio transformer to ground bypassed by a 25μf cap and connecting the HT supply negative to the transformer, this giving the audio section of the 1ZH37B some negative bias (approx 1.95v) this increases the depth of modulation and reduces distortion. I think I'll be doing this mod to my 1ZH37B pantry transmitter.
I've updated the diagram.
The total HT current is a little below 2ma

PS There is a typo on post 1 The valves are NOT 1ZH42B, (which doesn't exist) they should be 1ZH37B, Sorry about the wrong info.

Jac 6th Aug 2022 3:50 pm

Re: Band 1 Rod Pentode modulator (Part 2)

If you are still in need of a 41.5 MHz crystal, I can send you one.
When I made my modulator in the 1980s (David Looser design), I bought two spare 41.5 MHz crystals. (No 45 MHz crystal unfortunately. I only had the one, made to order.)

Just let me know.


murphyv310 6th Aug 2022 8:34 pm

Re: Band 1 Rod Pentode modulator (Part 2)
Hi Jac.
Many thanks for the offer. Vince has come to the rescue and I'm now up and running on the correct frequencies for Channel 1.

Jac 7th Aug 2022 7:54 am

Re: Band 1 Rod Pentode modulator (Part 2)
Excellent Trevor!

Best regards,

murphyv310 8th Aug 2022 8:35 pm

Re: Band 1 Rod Pentode modulator (Part 2)
The project is virtually completed, I've ordered a suitable project box which will house the modulator and power supplies. I've confirmed that I can run this on my 12v solar system with regulated filament supply and video bias, HT is from a small inverter regulated at 80v, max available current is 3ma which is sufficient, there is no noticeable interference from the supplies as they are well filtered. Total current draw from 12v is just below 60ma so quite good. I would say that the modulator wouldn't see a lot of use TBH. The case will have BNC video in, Phono audio in and an "F" connector for RF out. Two controls on the front would be power and Video level, bias is a preset and shouldn't need further adjustment after setting. The RF will be fed to a passive Splitter, which will also be fed from the channel 11 modulator and then to the distribution amp that feeds 9 outputs.

murphyv310 12th Aug 2022 9:17 pm

Re: Band 1 Rod Pentode modulator (Part 2)
Today I started on the completion of the project with fitting it to a die-cast box. After some fettling of the board it's now fitted.
Here is a video of how it looks in the box.
I've done a little more since and hope to have it fully completed tomorrow.

murphyv310 13th Aug 2022 5:18 pm

Re: Band 1 Rod Pentode modulator (Part 2)
It may be quite possible to dispence with using the 1ZH37B and use a 1ZH24B or 1ZH18B. I'd say normal Anode modulation could work with the type of transformer I am using in the existing circuit. The advantage would be lower filament current and possibly lower HT, RF output would also increase.
Perhaps worth a try.
The above two types of valves are also more common and cheaper.

murphyv310 13th Aug 2022 8:41 pm

Re: Band 1 Rod Pentode modulator (Part 2)
Well that was interesting. I decided to try anode modulation using the existing output transformer in reverse basically, it sounded good but needed the balancing resistors for left and right from the PC to be reduced from 180 ohms to 47. It sounds nice and I can achieve over 90% modulation. I decided to try a 1ZH18B which reduced the filament, the only thing is the output went through the roof, the G2 is now fed from a PD but to tame the output I had to massively drop the Anode volts, tapping off the filament supply before its dropped to the filaments via the resistors gives superb results, with an anode voltage of 2.8v.
These valves really amaze me!

kalee20 14th Aug 2022 12:40 am

Re: Band 1 Rod Pentode modulator (Part 2)
Any chance of a scribbled circuit diagram of the sound stage Trevor? I'm a bit lost which electrode is being modulated, where the output is taken from, how the feedback is arranged for oscillation, etc!

Operation with just 2.8V of HT does sound impressive!

murphyv310 14th Aug 2022 7:17 am

Re: Band 1 Rod Pentode modulator (Part 2)
Yes I'll post up a revised diagram soon.
Anode modulation with a modulation transformer is the accepted way to do AM and gives the best fidelity and modulation depth. Basically all you do is modulate the HT supply to the Anode and G2, via the high impedance side of the modulation transformer. In this case I'm not including the G2 as the G2 is used for the feedback loop for the xtal. The lower the anode volts the lower the output of course and the higher the depth of modulation.

murphyv310 14th Aug 2022 8:10 am

Re: Band 1 Rod Pentode modulator (Part 2)
1 Attachment(s)
Here is a quick sketch.

murphyv310 14th Aug 2022 8:46 am

Re: Band 1 Rod Pentode modulator (Part 2)
Here is a quick video of the results with the 1ZH18B

murphyv310 14th Aug 2022 3:32 pm

Re: Band 1 Rod Pentode modulator (Part 2)
I've now completed the modulator, tidied up the work and loose ends.
There is one small issue to investigate which is hum related, I think its because I'm feeding this from my solar 12v system, will investigate later.

Here is the final video.

kalee20 14th Aug 2022 9:50 pm

Re: Band 1 Rod Pentode modulator (Part 2)

Originally Posted by murphyv310 (Post 1491855)
Here is a quick sketch.


I'm curious about the method of oscillation - is the crystal really connected between g3 and g1? And what is g2 connected to?

murphyv310 15th Aug 2022 9:04 am

Re: Band 1 Rod Pentode modulator (Part 2)
Sorry. A mistake here as it was quickly drawn just before leaving for church. The 1ZH18B G3 is internally connected to filament minus the crystal goes between G2 & G1 the rest of the circuit is correct.

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