Triplett 1295 Modulation Monitor

[N1BCG]

Has anyone seen one of these before?



It sold on ebay recently and caught my attention for its simplicity.





I almost bid on it but decided to try my hand at making one. How hard could it be? The result is attached below.

Aside from not having peak indication, it does show average modulation fairly accurately when calibrated. I used a 0-100uA meter for the carrier level, and when the modulation (or VU) meter was set against a scope, the 10k pot was adjusted for a midscale (50uA) reading. This meter could even be 50uA fs since there was plenty of headroom. The circuit needed just under 5V PtoP for proper modulation indication.

The 400pF cap value was just enough to prevent RF from affecting the modulation meter, preventing it from zeroing, yet did not affect high frequency readings and was tested out to 20kHz. This was tested in the BC band so a smaller value could be used for 160m and up.

The carrier level meter only reacts to carrier shift and is quite steady with symmetrical modulation. I was surprised how a simple circuit could separately read RF level and modulation compared to the multi-op amp circuit in my homemade mod monitor!

I thought I'd post it here in case someone had some meters and parts needing a purpose.

[Opcom]

It looks pretty nice on ebay. I like this kind of instrument design.

[ka1tdq]

Thank you for posting the schematic. I bought some magic eye tubes from Ukraine and was thinking about what to do with them. I thought of an SWR meter showing both forward and reflected power with two tubes. Not practical other than cosmetics and as a 'duh' check for SWR and power. However, this is more practical, and cosmetic. It would be easy to use a seeing-eye tube in place of the meter.

Jon

[N1BCG]

The two eye tubes would certainly work, and since the carrier set indicator is mostly for calibrated scale meters, you could use the second tube as a peak flasher and borrow the design from the EICO 730 modulator:



This handy indicator flashes the display area of an eye tube when peaks reach -100% (or wherever you design it for). It can also be reworked for solid state transmitters.

Now you'd have modulation and a negative peak indicator.

[ka1tdq]

That’s a cool idea for the flashing negative peak indicator. I have an LED for that purpose in my current modulator. It’d be sort of like a cataract for the seeing-eye.

Jon

[W2JBL]

I have seen ads for that mod monitor in late 1930's QST's and ARRL handbooks. Nice find!

[Opcom]

Why stop with one or two eye tubes?

A row of EM80s behind a 2U panel, each opening in succession like the Krell power plant on the forbidden planet.
- 3 or 10dB per eye
- modulation or RF power
- RF: FWD or FWD/REF with a split scale.

The calibrate or set control could be a dc-controlled remote pot at the operating position since, if this was ever realized, it could easily be seen from across the room, therefore should be built into a full size transmitter or amplifier.

[ka1tdq]

I must admit, that would be cool. So much energy to produce though (not being an energy weenie). It would be difficult to reproduce that imagery digitally though, so it could be a worthwhile project. I watched a YouTube video of a guy trying to mimic the imagery of a magic eye using a spinning LED on a wheel. It's pretty pathetic looking, but I definitely give him an 'A' for effort. After all, there are zero magic eyes still in production.

You could try an elongated digital screen with complex programming, but it still wouldn't be analog authentic. Look at the Kenwood TS-890 for example. Really cool and awesome, but, digital.

Anyway, one of the countless ideas to build but won't be.

Jon

[Opcom]

The eye tubes with their 3D target construction all have a depth and a life to them that can't be matched by flat displays. We're on our own there.

Let the drawing stand on its own as a fantasy.