Band 1 Channel 1 Rod Pentode modulator

[Murphyv310]

Hi.
Here is what I have done so far.
I did try again feeding the top end of the 470μh choke form a PD and decoupled, at 65v screen volts the 44.3Mhz vision stage shut down the Audio though soldiered on to 40v on the screen.
  Rpentode ch1 mod.pdf (Size: 75.93 KB / Downloads: 21)

[radioman]

Well done Trevor - it's great to read about somebody who is experimenting with these valves and making something useful !

Cheers,
Andy

[Mike Watterson]

Great experimenting!

Don't chuck out the apparently dead crystal! Try it in a few transistor and IC crystal circuits. If it's an overtone crystal it may oscillate at fundamental on an unbuffered IC inverter.
I'm sure Trevor knows, but overtones are higher frequency mechanical modes, so are not integer multipliers of the harmonics. A fundamental mode osc needs no tuned parts, but an overtone mode needs additional L & C in the feedback to select the required overtone (a 3rd Overtone crystal can sometimes be persuaded to oscillate at 5th. Some 40 MHz approx crystals are fundamental and others are 3rd overtone, so won't always work in same circuit. Harmonics are different and always an even or odd multiple of the oscillation, so need a separate oscillator followed by an amp tuned to desired harmonic. Frequency doubling is simple, but 4th and 6th are best done by two doubler and doubler and tripler. The 3rd and 5th are easy directly, 7th needs more care.

Above 40 to 50 MHz the crystals (such as 101 MHz) are almost always overtone mode and in a simple IC osc + counter crystal checker will either run at the fundamental (no integer relationship) or not at all.

Some crystal circuits will work with some resonators or even an IF Filter (455 KHz, 6.0 , 6.5, 10.7, etc). Higher frequency monolithic filters ( 20 MHz to 200 MHz) may be SAW types and need more gain. A narrow filter is more stable than a wider one as an oscillator.

My last modulator/mixer - oscillator experiments were with the double balanced mixer-osc SA612A IC with on board regulator for osc. Oscillators benefit from a noise free stable supply, as the supply noise and ripple can AM, PM and FM the oscillator. This matters for a LO on TX or RX on very poor signal links, not a local RF source.
Anyway, by default the SA612 has a DSBSC output, nothing on the output from the LO or input (BB or LO). It can do PM by adding in the LO from pin 7 to the output via a buffer at the correct phase. Then multiply for NBFM, or more for WBFM.
The trick to get a carrier for AM is to deliberately unbalance the input pins at DC, by pulling one down more with a resistor to 0V and pulling the other up a little to noise free regulated voltage via a resistor, though just one pull down will work. The input will ALSO appear at the output, but if it's baseband that's not an issue. In normal use the output is Fosc-Fin, Fin- Fosc and Fosc + Fin, filtering giving the desired signal. With 38 kHz and L to one input and R to other input you'll get something like the stero subcarrier, then divide the 38 KHz by 2 and add it (19kHz) in to the L+R and 38 kHz DSBSC. Except it's pointless as an entire IC single chip 76 MHz to 108 MHz FM Stereo modulator ready made is about €8.

Anyway, with adding one or two resistors at pin 1 & 2 (input one to 0V and one maybe to LED on lamp) it works as AM for 60KHz to 500 MHz and audio or video baseband. Positive or Negative modulation.

Vcc 4.5V to 8.0V
Current about 2.6 mA
Input DC to 500 MHz
LO some 10s of kHz to 200 MHz or more.

Can be LC, crystal or ceramic resonator. A varicap can be added for a PLL (add external dividers & comparators) but not worthwhile with excellent LMX series now. or direct FM.

It's actually easy to get LC to work, but it's tricky to get crystals and resonators to work. Overtone mode crystals need a separate oscillator driving pin 6, which is disappointing, or else I'm doing it wrong!

[Murphyv310]

Hi.
Thanks Michael for the info. A closer look at the 41.5Mhz crystal revealed some rust at the leads at the body, perhaps its O/C internally.

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. Total HT current is 1.95ma.

  rpentodech1upd.pdf (Size: 380.31 KB / Downloads: 10)

[Mike Watterson]

Put 10K in series with crystal to gnd. Scope across crystal. Drive the 10K with a signal gen at 10 MHz to 45 MHz. If the crystal is O/C you'll see  no effect. If there is something between about 10MHz and 15 MHz, then it's a 3rd overtone crystal. A 5th Overtone is unlikely, but you can try 5MHz to 10MHz too. It's impossible to predict what the fundamental frequency is for an overtone mode as it's the physical shape of the crystal.

See https://en.wikipedia.org/wiki/Tuning_fork to understand why an overtone is not a integer number of fundamental as harmonic multipliers always are.

Sub-miniature crystals actually are tuning forks, which allows a low frequency in a small package. Higher frequencies often use a disk or rectangular piece of quartz.

Overtone circuits
https://www.robkalmeijer.nl/techniek/ele...index.html

Most overtone crystals, being of the plated variety, are incapable of dissipating much heat. This means that the crystal oscillator must be operated at low power level, and with no more feed-back than is necessary to maintain good starting characteristics under load. The crystal oscillator should never be thought of as a power-generating device, and this is particularly true of overtone oscillators. The oscillator should generate a stable signal; stepping up the power should be left to succeeding stages.

When fundamental crystals are used on overtones, the frequency of oscillation may not be an exact multiple of the marked frequency. And the frequency will be different for series or parallel resonance. Moral: When working anywhere near band edges, have some accurate means of checking frequency; a crystal marking is no guarantee that you will be inside the band.

The common test for self-oscillation, pulling out the crystal to see if oscillation stops, is not applicable to most overtone circuits. The capacitance of the crystal and its holder is a part of the feed-back circuit. If there is self-oscillation present, it will almost invariably stop when the crystal is removed.

Many Crystals marked at over 30Mhz are actually overtone, so without a crystal osc circuit for the required parallel or series overtone mode it may run at the fundamental or not at all.

Overtone crystals are not well suited to a combined modulator-oscillator, but the oscillator is best driving a separate mixer.

If you want a combo single device modulator/mixer and oscillator you need a fundamental crystal, but even then there is risk of oscillation ceasing at lower carrier levels (greater modulation depth) and FM effects at higher levels.

[Mike Watterson]

See also https://www.electronicproducts.com/advan...-crystals/
If the crystal is an overtone type the feedback gain should be low to zero below the desired overtone mode and also it's probably a good sign if the circuit oscillates without the crystal, but is at desired overtone frequency when crystal inserted.

[Murphyv310]

Hi.
Obviously I don't know the history of the crystals I have nor if they are fundamental or overtone. There is around 10v p-p on the crystal on sound and 8v on vision,
The ones that are coming are 3rd OT and rated at 1mw, well above what I'm doing. So I'll need to make some further changes I would say so the circuit oscillates slightly above the frequency with the crystal removed. I have managed to get the 1ZH37B to oscillate at 120Mhz but obviously with a different approach to this, as this is as minimalistic as possible.
I have had the modulator running for around 4 hours this evening and have been monitoring any drift. So far there has been a drop of 3hz over that period which I think is pretty dam good, it might even be the frequency counter drifting, will also check tomorrow with different test equipment.

[Mike Watterson]

You can tell with the test setup I suggested. You'll see a peak & dip at fundamental, 3rd and 5th overtones. Which ever freq approximates to labelled frequency tells you if overtone or fundamental.

120MHz is good

Some hope of adding airband if I get back to my 1j24b based all band superhet. The dual 1j24b class B push pull with small 110-0-110 mains transformer with 4" speaker nice and loud it has 2 x 0.6V filament VFDs for scale pointer lamp and one is used as phase inverter. The other will be an AGC amp to drive g2 for AGC as the valves are sharp cut off.
1N60s for FM and AM detectors. All the battery valve FM sets used semiconductor diodes.

[Murphyv310]

Hi.
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]

Hi.
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. https://youtu.be/c7CrMLkN4sw
I've done a little more since and hope to have it fully completed tomorrow.