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The Trapezoidal Thing




 
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Author Topic: The Trapezoidal Thing  (Read 2975 times)
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WV Hoopie
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« on: September 26, 2009, 02:25:36 PM »

First of all there is no problem with the math as to the value of C or values of R1 & R2 for displaying a trapezoidal pattern on an O scope. I built this circuit in a project box years ago for a DX100B and it worked just fine. For the last few years it has been doing great on a Junkston 500.

But now I'm thinking about another project box with parts n' pieces for the Johnson Desk, which leads me to the question:

I've never giving much thought as to the insulation needed on the wiring from C to the voltage divider circuit. The old DX100B might of had 700 or so volts of B+ DC, the Junkston 500 has 2000VDC plus what is added by the mudulator. The Desk, with todays line voltage in the shack will do 2600VDC on high tap.

What yun's guy's been using?Huh? The Junkston 500 to date just has stranded #14 with an insulation value of about 600 volts. My luck might change and let some magic smoke excape if I don't change my ways?

Tnx & 73's

wd8kdg
Craig
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WD5JKO
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WD5JKO


« Reply #1 on: September 28, 2009, 07:44:22 AM »



Craig,

    The modulated B+ (for scope horizontal axis) voltage divider and coupling capacitor could be mounted inside the rig. Then output at junction of the voltage divider with a chassis mount BNC connector. Go a step further and do the same thing for the RF output (scope vertical axis) and add another BNC connector. This way you can simply hook up two BNC-BNC cables to a scope in X:Y mode to view the Trapezoid, or turn on the internal scope sweep to see the modulated envelope.

    The suggestion made overcomes the insulation issue you pose. Also, if the voltage divider ratio for the horizontal axis is large, then eliminate the coupling capacitor, and instead just AC couple the scope input.

Regards,
Jim
WD5JKO
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WV Hoopie
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« Reply #2 on: September 28, 2009, 11:10:04 AM »

So far: On the Junkston 500 the coupling cap is inside the PS/Mod supply. I've used one of the spare pins of an octal socket to run a wire to the voltage divider circuit which is in a project box sitting on top of the scope, (Eico 460). Been getting away with this JS for 5 years, but....................... Huh

I didn't want to drill n' blast the chasis of the PS/Mod supply. Edgar might roll in his grave.

The Johnson Desk would or might pose another problem as it can be run at two different power levels, about 1300VDC or 2600VDC B+ to the plates of the RF amp & mod toobs. Which could be real fun rolling the guts of the desk out of the cabinet to adjust the voltage divider circuit. The desk has one octal socket that has a few spare unused pins, its that arc n' spark thing which has me thinking, I might not get away with this type of JS set up on the desk. Pin spacing might not be great enough, etc.

Craig,
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WV Hoopie
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« Reply #3 on: September 28, 2009, 03:23:12 PM »

Jim,

Tnx for the input, I'm now thinking about drilling n' blasting. The "500" would be the easiest, voltage divider is a set and forget. Or could drill n mount the pot where it could be adjusted from the terminal side of the PS/Mod supply with another hole for the BNC connector.

Would have to do almost the same thing with the desk. On this station the oscope is a Tek 2336 YA and I think the horz gain is adjustable. So I could adjust the voltage divider for low power and use the horz gain adj on the scope to tame things down on high tap.

Craig,
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WV Hoopie
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« Reply #4 on: October 02, 2009, 11:03:58 AM »

Update:

Junkston 500; looked thru the catalogs for suitable wire,etc. Test probe lead wire is rated for 5000V/punture at 20,000. Mouser listed it @ $27 and change for 25 feet, pricey. Found the same wire 1/8 dia #18 stranded local for 5 bucks and change. Hope this local electronics store stays in business. So the 500 is a done deal......

The Desk; The rainy season has started in the Pacific Northwest. So, sometime soon I'll get the guts out & on the dolly. All the HV transformers should be mounted on insulators by the end of fall/winter. As noted in another thread, might need to use a few blocks of wood to keep the roller bearing fixtures from spreading. "The" plan for a trapezoidal pattern is to use one of the pins on the octal socket and use test probe lead wire to a project box. Coupling cap will be inside the Desk, voltage divider circuit inside of this project box with a switch to change the resistance for low & high power. As to date this station's oscope hasn't been used for a trapz pattern. I think to display one, the X-Y buttons on the vertical section are to be used. Which means more stuff in the project box. Currently CH1 the X position is being used for an envelope pattern, CH2 the Y position sees the IF output of a R-390A. Another switch in the project box will be needed to switch X from the audio sample of the Desk to the IF from the receiver.

Gotta find suitable switchs,,,

Craig

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k4kyv
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Don
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« Reply #5 on: October 02, 2009, 12:43:37 PM »

The modulated B+ (for scope horizontal axis) voltage divider and coupling capacitor could be mounted inside the rig. Then output at junction of the voltage divider with a chassis mount BNC connector. Go a step further and do the same thing for the RF output (scope vertical axis) and add another BNC connector. This way you can simply hook up two BNC-BNC cables to a scope in X:Y mode to view the Trapezoid, or turn on the internal scope sweep to see the modulated envelope.

    The suggestion made overcomes the insulation issue you pose. Also, if the voltage divider ratio for the horizontal axis is large, then eliminate the coupling capacitor, and instead just AC couple the scope input.

You still may have a phase shift problem with the pattern that requires some capacitance across the voltage divider.  In mine, I use the same resistor network that serves as the multiplier resistor for the PA plate voltage meter, consisting of several wirewound 50K resistors in series with the 10 ma meter movement.  I tap audio off the next to bottom multiplier resistor, using a mica coupling cap.  But to solve the phase shift problem I had to put  some capacitance across each one of the individual series resistors.  Apparently, the capacitance in the audio line from the resistor tap point to the scope, which is located about 10 ft away, is enough to cause phase shift and mess up the  pattern unless additional capacitance is also distributed along the series the multiplying resistor.  I used trial and error until I got the scope pattern cleaned up.
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Don, K4KYV                                       AMI#5
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« Reply #6 on: October 02, 2009, 08:37:39 PM »

I'd like to put the CRT in the HB transmitter's RF deck for easy access to everything and less issues with phase shift because the thing could have its own small divider. The OM however built the RF deck's underside super close to 5" DOH!! I'll have to pull it apart to check the fit. A pity because otherwise I could just bore a hole right through as it is spacious in depth and width.

What do you do Don for the adjustment of the pattern size? The audio would seem simple, but the RF - -a pot can't be right, can it?
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k4kyv
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Don
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« Reply #7 on: October 02, 2009, 09:07:40 PM »

I use a tuned circuit at the deflection plates of the scope tube, which is link coupled to a toroidal pick-up coil at the feedline.  I adjust the vertical size of the pattern by varying the tuning capacitor.  I don't use the vertical amplifier in the scope.  The rf feeds the deflection plates directly.
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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This message was typed using the DVORAK keyboard layout.
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WV Hoopie
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« Reply #8 on: October 02, 2009, 10:03:46 PM »

Don,

Just about the same set up here on the "500". A RF sampler is in the TX output line, nothing fancy, a couple of SO-239s with #12 solid copper between them in a small metal box. Another connector with insulated hook up wire wrapped a few times around the #12. Maybe a little overkill, but I use several small toroidal cores and a 100pF receiver type air variable cap. A switch selects the toroidal core for the band in use. Adjust the air variable to vary the pattern heigth. This output goes straight to the verticle deflection plates via 300 ohm TV ribbon.

Works fine, last long time.

wd8kdg
Craig
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« Reply #9 on: October 03, 2009, 02:17:48 AM »

This is the coolest modulation scope I've ever had. CE Multiphase MM-2. Someone else did the scans, I think. But it has a few nifty features that make it good for TX and RX both.

There is supposed to be an "RM" adapter option, which I would like to find. It lets the unit cut automatically from TX patterns to an RX I.F. scope in the absence of TX RF.
RM-50 (50KC IF)
RM-80 (80KC IF) ?? what is that for?
RM-455 (450-500KC IF)

MM-2 stock features are:

built in 1KC osc. 15mV/1.5V
10:1 sweep locks to incoming audio component.
5W to 5KW, 1 to 55MC no-tuning capacitive attenuator calibrated in 3dB steps
AM teapezoid - shows the topic under discussion here.
AF trapezoid - show modulator symptoms (overdrive, overmod, linearity, etc)
RF trapezoid - shows RF symptoms (nonlinearity (amplifiers), flat topping, parasitics)
sine or speech envelope
AF and RF hi-level & lo-level inputs acomplish these tests, can be left connected to rig.

I thought this would be worth mentioning, because the schematic could help those planning acording to the topic. Not all the features might be desired, but the different arrangements for trapezoids are interesting.

I'm sure I am not explaining it all correctly, and I do not have the proper manual, only this brochure and schematic.

I do not have the RM-455 adapter (or other) schematic. I sure would like it, and the proper MM2 manual, if someone has it.

If I EVER find another one of these scopes I will pick it up!

The point is that, in addition to AF vs RF trapezoid, the scope of this topic could also be made to incorporate inputs for looking at both sides of a modulator and RF amplifier. It does not look that complicated. No I have not tried to put 5KW to this thing.. scared to do it without the book.


* central_labs_mm2_brochure_pg1.jpg (520.98 KB, 1200x865 - viewed 298 times.)

* central_labs_mm2_brochure_pg2.jpg (316.07 KB, 799x865 - viewed 282 times.)

* multiphase-sm.gif (1046.42 KB, 3043x2161 - viewed 306 times.)
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