Modulation Monitor?

[WA1GFZ]

Precision active rectifier eliminates worries about junction voltage. Pretty easy to get over 60 dB of dynamic range. The best circuit I have seen is Rob K2CU's circuit.
1N34 is a leaky hos very unstable over tempreature.
Stewve I wonder if you looked into LCD panel displays to make it stand alone.
Now that would be cool and run at a lower power than having a computer running just to write a display. Might take another pic but would br cooler.
Another option would be to drive a flat screen monitor.

[KM1H]

Analog 30MHz scopes (to get you through 10 meters) are a few hundred bucks these days, and well worth it for air signal and bench work.

The 50MHz Tek 453A and several nice Leader models are under $50. For $200-250 and a little looking you can find a working HP 141T spectrum analyzer. I use the 453A to look at patterns and the 141T to see what is really happening beyond the sidebands as well as their actual transmitted width.

Having a PC display would be nice but if its pricey I can live without it.

Carl

[K5UJ]

Anything that requires running a computer in the ham shack isn't gg to get off the launch pad here.  I guess I'm comfortable using a scope to monitor my negative peaks--I let the positives land where they may, but I run compression and peak limiting.

[Opcom]

Re: Questions from: ssbothwell KJ6RSG on Yesterday at 14:47:16"

1. A scope or CRT circuit is used always while transmitting. I like it because it is always easy to see if 100% negative modulation has occurred. Used CRT-type scopes are pretty cheap for now.

2. I think you still need a dummy load there. Usually a monitor and a load are separate items when any power is involved.

3. It is crude and may leak a small amount of RF but I use a SO-239 "TEE" where the PL259 port is screwed into the amp output and the coax screws into one SO239, and the other SO239's 'socket' has one pin of a dual banana pin connector plugged into it. The small capacitance between the two banana pins couples enough signal to the scope to display well. A X10 probe is hooked to the other banana pin and the probe GND goes to the amp GND. It does not seem to disturb the amplifier output and I don't have to worry about cooking the scope.

http://www.cleanrf.com/ makes a ready to use sampler or demod., could be used with anything.

There are enough choices for doing this monitoring function to pick what you like!

[n1ps]

Well I did build a variation of the analog monitor that Steve published.  But I did not use meters...I drove some LEDs....I think 20 per side.  I don't recall the exact drivers but it is all downstairs somewhere.  I built it for my bench as I was tired of unhooking the REA unit and bringing it over.  I did not use the carrier osc circuit and used a voltage divider pick-up to pick up the RF.  Oh and it was a PITA to get it to work correctly.  Having said that, it is nothing like the REA as far as accuracy, but does the job for me when checking out a TX.   For a station monitor....I highly recommend Steve's monitors and suggest either find one of the REA units or wait for his newest. 

The fellow who posted about not wanting to use an REA (if he could find one) for his vintage radios....I completely disagree.  The REA is dead on accurate and will help you to look at your modulated signal.  A scope is always useful, but the REA is the best modulation monitor I have yet to see whether looking at a modern rig or something 60 years old. 

Peter

[w3jn]

A mod monitor is fine, but don't even consider getting on the air without a scope!    And a HO-10 or SP-610 is fit only for swinging it around by the line cord and smashing it against the wall  

A HP8901A mod monitor is the bees' knees if you can't afford or can't get a REA.  Built in freq counter/receiver too.  I've gotten several for about a hundred bucks.  It certainly doesn't fit with a vintage station though.

A simple mod monitor is a fun and interesting HB project if you don't want something more complex.  Score a scope first though - not only is a decent scope necessary if you're running AM but it's extraordinarily useful for troubleshooting.   Next to a DVOM or VTVM, a good scope is the most useful accessory you can own.

Here's another thread discussing the same issue http://amfone.net/Amforum/index.php?topic=15743.0

[Jeff W9GY]

I built Steve's analog monitor (before the LED one hit the market),  Its in use every day...Bought a bunch of cheap dual op-amps, and tacked it together on a hunk of perf-board. I calibrated the unit somewhat differently than the procedure Steve outlined.  Basically adjusted things using voice waveforms and a scope looking at the modulated output of the transmitter. Also, changed some time constants to use a big buzzardly VU meter.  Audio output on the unit is indeed high fidelity and, with a little more amplification, drives headsets for constantly monitoring your signal quality.

[W1AEX]

Here's another thread discussing the same issue http://amfone.net/Amforum/index.php?topic=15743.0

From the information in that thread, I built several variations of the ECS monitor mentioned by Jeff W2NBC. Stu AB2EZ suggested a few changes to allow use with higher power and I ended up with this circuit:

http://www.w1aex.com/756AM/RFsamp.jpg

The audio output will drive a pair of headphones fine, but at the Flex operating bench I run the audio output into one of the 2 inputs of a set of Bose Companion II Series speakers with the other input of the powered speakers connected to the Flex Radio. That gives me an automatic T/R type setup so with headphones plugged into the jack on the Bose speakers the receive audio comes through and then switches over to the transmit audio as the Flex receiver is muted with PTT. The scope pickup works fine running into my $40 Tektronix 485 flea market scope. At the Flex operating position the sampler is inline between the Flex and the amplifier as I didn't think it would handle the +250 watts of carrier and the +1400 watts of positive peaks for very long when running AM. When I want to sample the output amplifer's RF rather than the Flex Radio's output with the scope I simply switch the scope over to a 2 meter rubber ducky and it sniffs plenty of RF from 160 meters to 10 meters to display a clean envelope pattern.

At the Viking III operating bench I use the same circuit with a relay to handle the PTT switching of audio from the receiver during RX and from the RF sampler during TX. The scope output is connected to a Textronix 453 scope. The sampler has no trouble handling the +150 watts of carrier power on AM and whatever positive peaks that the Viking produces.

I ended up using an NTE equivalent of the 1N34 in both RF samplers, but I have to agree with the other comments that there's probably something that would work much better out there. I did stick a 1N914 in there at one point and would have to say that 1N914 switching diodes definitely suck in this application!

Rob W1AEX

[John K5PRO]

Or if you want to homebrew a mod monitor completely analog, that was ready for FCC approval in 1979 and never produced, here is a circuit I designed, being just out of school. It required an RF detector stage in front, all I had was the Dc/audio circuitry to get very accurate metering and peak flasher. I had even prototyped most of it at the time, including the meter peak hold circuit. The op amps chosen are ancient JFET types, no longer desirable. This was the way many commercial mod monitors worked from that vintage, like Belar, TFT, Harris, etc.

[The Slab Bacon]

I have a SB-610 and a B&K shop scope,  the SB-610 has focus issues still remaining after replacing the HV rectifier and filter caps to get the trace back so it's only marginally useful, don't really want to drag the "real" scope to the operating position.   Even if it was working correctly, I would prefer meters.
Thanks to everyone for the input.
73 Jack KZ5A

Jack,
       The little greifkit scopes are POSs for the money people want for them. But for a dedicated do nothing else but a mod level monitor they aren't all that bad. The biggest problem with them is the prices that people want for them, for what they bring you can easily buy a descent older lab grade scope.

They all seem to have an asthignatism / focus problem. The chinese replacement CRTs, I'm told are even worse. There is a pretty simple mod out there to add an asthignatism adjustment pot to them that really helps out with this problem. I did it to a couple of them years back and it worked pretty well. It's been so long that I cant find the paperwork but it was pretty simple to do.

As far as an RF pickup for a lab grade scope goes, almost anything will work. I usually just use a couple foot piece of wire hanging off of the vertical inpoot jack. Or if it makes you feel better you can just jack in the coax from whatever antenna you have and are not using at that particular time.

just my $.02 worth................

[KZ5A]

Slab,

Fortunately, I didn't pay too much for my SB-610 because it had "no trace".  I gambled on it just being the HV filter caps/diodes and lucked out.  Replaced those and now it has a strong trace that doesn't focus.   So there's a good chance that a the other HV coupling caps are also bad and one of them is known to cause the "out of focus toward the right" problem.   So I'll replace those before giving up on it.  It's cosmetics are about 9.5 so I'm hoping I can use it.  I'll look around for the astigmatism mod.

There is also a CE MM-2 in the restoration queue but I'm thinking more along the line of using it as a RX IF monitor for the 75A4.

However the whole modulation monitoring issue has taken a turn for me since I just picked up a RCA BW66F.   Needs major TLC, but is complete and looks quite restorable.  Hey and just the thing too compliment the "new" RCA BTA1R1.

73 Jack KZ5A

[The Slab Bacon]

Jack,
        I'll look around (no promises that I'll find it) I may have the asthignatism mod written down somewhere. IIRC it was rather simple, just a couple of resistors and a pot. I remember that it only took a few minutes to do. If I can find it, I'll post it to this thread.

If you have no focus at all, check ALL of the resistors in the focus circuot and connecting to the CRT, they are notorious for the high Voltage opening them up or causing them to go way up in value. I had to replace a bunch of them in my 610.