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Author Topic: Thyratron RFI: what to do?  (Read 10381 times)
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WB6VHE
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« on: August 09, 2010, 01:16:58 PM »

Greetings, all!

I have a vintage General Radio 1570-A AC line voltage regulator that I use
to run a rack full of receivers.  This is an electro-mechanical device
that uses buck-boost via a motor-driven variac to do the regulating.
The motor is driven by a pair of 5727 thyratrons.  It's a nifty machine, but
there is one problem: the thyratrons generate a nasty load of rf hash that
is particularly bad on 75 meters.  For 7 MHz and above it's not really a problem,
but really messes up things on the lower frequencies.  Any ideas as to what to do?
I tried a brute-force line filter with negative results; the hash appears to be radiated,
since disconnecting the antenna from the receivers gets rid of it (i.e., it's not coming into
the radios through the ac line).  Any suggestions will be welcome!

73,
Ken
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Pete, WA2CWA
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« Reply #1 on: August 09, 2010, 01:55:40 PM »

Sounds like it could also be the brushes in the autotransformer.
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« Reply #2 on: August 09, 2010, 02:03:52 PM »

I am no expert on Thyratron operation but I wonder if it could use the same RF choke treatment in the anode (and perhaps cathode) leads as used in MV rectifiers to reduce RF noise?   I don't know if the use of the choke will negatively impact the normal operation of the circuit.
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« Reply #3 on: August 09, 2010, 02:28:24 PM »

Ken

I have found that the following may happen when a piece of equipment in a building is generating RF at the lower end of the HF band of frequencies (i.e. at a relatively long wavelength):

1. The interfering RF couples out of the equipment and into the AC power line through any path it can find. It will flow through relatively high impedance paths... so things like 100uH RF chokes, ferrite cores, etc., that you would normally use to block RF, might not work (or might not work as well as you think they should, based on past experience). E.g., at 7MHz, the impedance of a typical 100uH RF choke (with an ~ 1MHz self-resonant frequency) is going to be controlled by its equivalent parallel capacitance... and will likely be around 100 Ohms or less.

2. The buidling's AC wiring will become an antenna, that will radiate this RF power.

Thus, the interfering RF will come into the receiver's antenna, even if the receiver is well- isolated from the AC line (e.g., even in the case of a battery operated receiver).

In some cases, you may hear 60Hz hum (or harmonics of 60Hz) modulating this interfering RF...due to nonlinear components in the paths (direct and indirect) between the interfering rf source and the various wires that make up the building's wiring system.

Again, because the interfering RF will push through relatively high impedances (because it has no place else to go that presents a lower impedance path) it can be tricky to get rid of. Trying to block the associated RF currents with inductors may require much larger values of inductance (with much lower parallel capacitance) than you would normally use.

You may have to provide something equivalent to a dummy load for this RF to go into (to dissipate it) ...i.e. an appropriately rated .01uF capacitor in series with a 10 ohm resistor between each of the 60Hz AC conductors (going into or out of the variac unit), and a common point... with the common point connected to your building's r.f. ground. That in combination with some ferrite cores on each of the incoming and outgoing AC wires may be sufficient to keep the building's AC wiring from radiating too much of this interfering RF.

Stu
 
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« Reply #4 on: August 09, 2010, 02:34:22 PM »

I used to have two of those that came from military communications shelters. Tube type microwave relay stations. I don't remember the military part number but they worked the same way.

I used them to watchdog the output of an older gas powered generator set for field day. Fortunately I didn't have the problems your faced with. My problems came from spark plug static and other hash from the gas motor.

Is that set in a sealed steel box? I ask because I believe my boxes didn't have even ventilation slots if I recall correctly. I do remember wondering how the two tubes stayed cool enough in a sealed box. Could be more shielding might be necessary? If the enclosure is wide open maybe some screen and a DC ground.

Mike
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« Reply #5 on: August 09, 2010, 03:15:59 PM »

In looking at the manual earlier, the unit is enclosed in a steel box and all the tubes have shields over them. I believe (from memory) the schematic shows an AC line filter but can't remember if it's on the input or output side.
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« Reply #6 on: August 09, 2010, 11:16:49 PM »

thyratrons are not usually noisy by themselves in the same way a MV is but I am no expert on them either! If the motors have brushes i.e. universal type, I'd look there. Is there sparking on the commutator? Can you temporarily short across the thyratron's place to see if the motor is making the noise? then try just a diode in place of the thyratron? I understand you have only a few seconds for the test.. might be fun.
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WB6VHE
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« Reply #7 on: August 25, 2010, 11:18:42 AM »

Thanks to all who replied!
I know that the noise is coming from the thyratrons; the motor is stationary most of the time,
so brush noise is eliminated since the noise is constant.

Hello to you, Mike.  How is that Cushman working?  Still going strong, I hope!

I have an idea for a fix, and will post it here after I implement it and it's successful.

Regards to all,

Ken
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« Reply #8 on: August 25, 2010, 11:35:08 AM »

OH Yeah! The CE4-B service monitor and the CE-15 spectrum scope are doing great. I'm using them to get that Motorola Syntor X-9000 (mentioned in another thread) up to speed. You did a great job. Thanks!

Mike
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« Reply #9 on: August 26, 2010, 12:24:19 PM »

I made some 866 style parasitic suppressors maybe 15 years ago for a machinist friends thyraton motor controls as he couldnt listen to AM BCB radio. Didnt completely kill it across the band but it couldnt be heard on his favorite stations. The input suppressor did the best. He still uses the same equipment and radio.

Carl



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WB6VHE
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« Reply #10 on: August 26, 2010, 01:35:45 PM »

Mike, glad to hear that!  Thanks for the compliment!

Carl:  did you put the suppressors in the grid or plate circuit, or both?

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k4kyv
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Don
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« Reply #11 on: August 26, 2010, 05:41:35 PM »

I have a vintage General Radio 1570-A AC line voltage regulator that I use
to run a rack full of receivers.  This is an electro-mechanical device
that uses buck-boost via a motor-driven variac to do the regulating.
The motor is driven by a pair of 5727 thyratrons. 

Sounds a lot like something I have, called a Stabiline and made by Superior Electric.

Mine is missing a proprietary plug-in sensing unit that is shown only as a rectangular box on the schematic.  I tried contacting the manufacturer, but they responded that the unit was made in 1956 and that they no longer have any documentation on it.

Wonder if you could make me a copy of any documentation you have on yours, since maybe they are similar enough that I could figure out how to fabricate something to  replace the sensor unit in mine, which is shown in a drawing to look like a metal octal tube.

Mine uses a midtapped 220v variac set up to buck/boost the 110v line voltage, and the output from  the variac is stepped down through a transformer with the high-current low voltage secondary in series with the main power line.  It is supposed to boost/buck the nominal 115 v.a.c. by  something like +/- 15 volts to keep the average output voltage constant. It would come in handy if I could get it working, because my line voltage here randomly varies anywhere from 109 to 125 volts, sometimes in a matter of a few minutes. I would use it to run all my tube filament transformers and low power audio stuff, as well as the receiver and VFO.
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Don, K4KYV                                       AMI#5
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WB6VHE
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« Reply #12 on: August 26, 2010, 10:24:19 PM »

Hello Don:

I'll scan the schematic and email it to you.  Send me an email to: shaw@swcp.com so I can
get your address.  I might not get to this tonight, but will certainly do it by tomorrow.

73!
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« Reply #13 on: August 27, 2010, 12:36:36 PM »

Ken, I went with both, I didnt want to aggravate the guy who gives me great deals on machine work. Grin

Carl
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WB6VHE
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« Reply #14 on: August 27, 2010, 05:03:52 PM »

Thanks, Carl!  I put them in the plate circuit for now; the grids have 1 M Ohm resistors in series,
so the plate seemed the place to put 'em (not much current in the grids).

The noise seems much less, listening on a small hand-held shortwave receiver sitting right
on top of the regulator cabinet.  The real test will be when I bring the box back into
the shack; haven't gotten to that yet, but perhaps tonight.  I'll post the results here.

Don, I've sent the schematics and theory pages from the manual.  Hope they are
of some help to you.

73,
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« Reply #15 on: August 27, 2010, 09:16:16 PM »

Replace the Thyratrons with SCRs or Triacs??

I have heard of this being done on the Monarch 10EE lathe.
Some of them used large Thyratrons for motor control, and they were hard to find large ones
so people have replaced them with SCRs iirc...

A snubber would be in order in that application I expect.

I have no personal experience on this.

                 _-_-bear
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WB6VHE
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« Reply #16 on: August 27, 2010, 10:04:28 PM »

Replacing thyratrons with solid-state devices is against my religion! Wink Grin
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KM1H
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« Reply #17 on: August 28, 2010, 05:34:33 PM »

If you saw the price some pay for thyratrons you would change religions! That same machinist was quoted $250 a pop from Richardson, I got them NIB at a hamfest for $1 a pop so now he has spares as the suppressors fixed the problem. Also be sure the filament isnt overvoltage, they are fussy.
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WB6VHE
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« Reply #18 on: September 04, 2010, 12:35:10 AM »

A 1 mH choke in the plate leads of the thyratrons got rid of the noise!
Just some info for all who read/replied to the topic.
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« Reply #19 on: September 04, 2010, 07:22:19 AM »

Ken,

glad you cured it! 
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Rodger WQ9E
WB6VHE
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« Reply #20 on: September 04, 2010, 11:40:51 AM »

Thanks Rodger!  And thanks to all who replied to the topic!  By the way, I used a very
low-resistance choke (about 1 Ohm, available from Newark) in order to have
minimum impact on the normal operation of the circuit.  The regulator still works
great, but noise is gone.
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« Reply #21 on: September 04, 2010, 03:17:47 PM »

Sounds good to me Ken, glad it turned out easy.

Carl
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