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Gates crystal frequency




 
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k7mdo
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« on: December 26, 2017, 03:01:04 PM »

I am just started going through my newly acquired Gates BC1G.  Can't seem to get past the oscillator before I find my first hiccup.

The transmitter was in use on 940 KC and the vacuum packed crystal is clearly marked 940 KC but when I went to check it on my homebrew crystal frequency checker (which works on FT-243's always), it comes up as 4.3838 MC not 940 KC. I tried a little math to see if it is a harmonic but no divisor or multiplier I could think of made sense.

What I was hoping to do is to make an adapter for the FT-243's so I could plug them into the socket on the oscillator.

Then of course I will probably need some change in the oscillator to push it up to 75 meters but that will be the next phase.  For now, I am just trying to get the transmitter working on 940 KC to verify operation.

73, Tom
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N1BCG
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« Reply #1 on: December 26, 2017, 09:02:11 PM »

Hi Tom,

Those crystals need a bit of drive and shunt capacitance. I have two for 1490kc as well as a BC1-G oscillator. Try putting between 30 and 50pF across the xtal and see if that fixes it.

If you use the oscillator in the transmitter, take a sample from pin 6 of the Jones connector. It's D.C. blocked by a cap.

All the low level AF and RF stages are energized when the transmitter filaments are turned on, but the plates not energized. That should give you an idea if things are working up to the 833s.

Congrats on the acquisition!  Awesome transmitter! Do you have a manual for it as well?
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k7mdo
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« Reply #2 on: December 26, 2017, 11:20:10 PM »

Great advice, will try the cap and use your suggestions.  I am still "orienting" myself to the components and diagram so am not ready yet to apply power.  However, it looks clean and I was told it functioned before transport but sat about a year from what I know. It looks like a year of dust anyway.

Yes, I have a copied manual but my circuit diagram is pretty rough.  Otherwise my estimate is it will be quite an education to get it going.

Scouring the web for ideas but will keep the conversion as simple as possible.... Hmmmm, but I want push to talk, maybe remote operation..... It is hard to see the finish line so I just started at the power cord and am progressing slowly.
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N1BCG
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« Reply #3 on: December 26, 2017, 11:30:09 PM »

You can always disconnect the primary of the HV supply, so that no matter what, it won't come on at that level until you're ready.

The BC1-G can be fully remote controlled but it uses high voltage for switching. It would be good to utilize a set of relays to keep that voltage within the transmitter. The manual shows the terminal strip connections. Also, normal operation is to press the Reset button then the Plates On. PTT operation would need to handle both functions.

As with any b'cast rig, they were designed to be turned on and left on. Become familiar with the contactors. They disassemble for maintenance but there are a lot of small parts that will fall out unless you're ready.
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KA3EKH
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« Reply #4 on: December 27, 2017, 05:21:06 PM »

You may want to consider conversion to 160 first being the output tank will go there with very little effort. I had a RCA BTA -1 that I ran on 1.885 for years with no issues running 833 tubes but the original tank and the dual 833 tubes can get squarely on eighty unless your willing to do serious work on the tank. The 833 tubes have buckets of capacitance and that tend to have an effect on design. Also at the end of the day I used just one 833 in the RF amp and two for the modulator along with putting a variac or autotransformer in the primary power for the plate transformer to control the output power. Donít waste time trying to use the original stupid DC resistance stuff to control power unless you donít care about wasting lots of power. 
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k7mdo
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« Reply #5 on: December 27, 2017, 10:41:06 PM »

Hi, problem for me is antenna and local noise floor on 160.  75 meters is almost always open for local contacts so that is the aim.

It looks like the 250 carrier level is achieved by dropping one leg of the 220 to HV transformer....  yes I see some issues to be dealt with but some folks have done the conversion without a lot of grief.  We'll see.

I tried the capacitor parallel with the crystal with no success (the frequency readout changed but mostly went UP) but I admit I am not too concerned as am not yet ready to apply power to the set but the "pin 6" monitor wire will definitely be a great deal of help in seeing what is going on.

My net expense will probably be the microphone amplifier and limiter.... looking at them the cost is pretty high, almost totally in comparison to what I paid for the entire transmitter.  (Oh well, the XYL didn't see the bank account go down with the transmitter purchase! Yet anyway)

I did grub around some of my transformers and found the one in the photo below... might I somehow use it to temporarily get some audio into the line?  No sure of the power required in that line so am just scrambling and hoping to cut a corner somewhere.


* Line Transformer.jpg (118.06 KB, 756x504 - viewed 77 times.)
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N1BCG
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« Reply #6 on: December 28, 2017, 12:08:00 AM »

I'm thinking the unusual frequency reading you're getting is due to the crystal not being properly excited. As for a limiter, you'll need a bit of output, I believe around +6 to +8 dBm for 100% modulation. There are compressor limiters available with balanced XLR outputs that should do the trick. DBX and Symetrix come to mind as two possibilities or you could look around for a used b'cast processor.
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KL7OF
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« Reply #7 on: December 28, 2017, 09:25:48 AM »

You can rewire the gates octal xtal socket to take a standard FT243 xtal.....   they fit right in ... I also use a WRL  VFO


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KA3EKH
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« Reply #8 on: December 30, 2017, 01:38:09 PM »

Nothing to do with the oscillator but a couple other things, first I did the half power thing running 120 volts to the power transformer instead of the 240 volts it was supposed to get and that worked great except one weird thing. On the RCA transmitter it was designed around 240/220 volt input and there was no neutral to return the low side of one leg. Donít tell anyone because I think its way illegal but I retuned one side of the plate power transformer to ground so it was a 120 volt supply and disconnected the other side and left it float.  That worked well until I found a 240 volt autotransformer and installed that. On the RCA the filament transformers, bias and the 600 volt supply along with the cooling fan were all 220/240 volt input. As far as the microphone thing goes if you can find any old commercial microphone mixer that will have a balanced output that will give plenty of audio drive. I used a Shure M-67 being they are readily available and cheap. Thousand years ago when I was still in school and right after use to use M-67 and later M-267 microphone mixers over leased dry pair phone lines for broadcast remotes being they had not only an XLR output but a binding post output too. Also they are immune to RF interference or any issues like that being designed for use in the broadcast industry. Think the M-267 also had a simple limiter in it too.
Dose your transmitter have the dummy load installed? Also what condition is you foil traces in? Think thatís the model with the cheap foil traces on the printer circuit cards unless itís the older version.

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k7mdo
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« Reply #9 on: December 30, 2017, 06:44:47 PM »

I think the supply voltage 120'240 is of no concern for this transmitter.  The foil "issue" does not seem to be an issue and as I will be using it intermittently from here on it won't be hard to see and or repair.  I do see that the circuit boards have gotten a little heat damage in the vicinity of some of the higher wattage resistors but nothing is open (yet).

One hiccup that I missed on my first examination is that there was clearly a capacitor failure at some time in the past in the pi network.  C-41 apparently "blew" from the looks of the sheet metal on the inside top of the transmitter.  However, the diagram has two parallel caps forming that leg of the "pi" and currently there are two there and they appear to be replacement to the original capacitors.  So, I will measure them and verify that the capacitance is now what it should be and go from there.

Thanks for the heads up on the audio processor....  hopefully I can get something that works "for cheap" to start with !!!

I'm getting close to plugging it in....  I have yet to verify that the bunch of 833's I have are all "good" and other than checking the filament I have no other way.

I still haven't figured out how to get some rollers under the thing without raising it much but I am getting some ideas.

73, Tom
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KA2DZT
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« Reply #10 on: December 30, 2017, 06:53:48 PM »

Try some 3/4" galvanized pipe under the cabinet.  This is just to move it around as needed.  Not a final set up.
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N0WEK
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« Reply #11 on: December 31, 2017, 03:43:19 AM »

On my BC-1J I installed steel wheels under the cabinet lower shelf which only raise the cabinet about an inch off the ground. I used fixed, not swivel, casters and mounted then as close to the corners as possible. Normally I only have to pull the transmitter straight out from the wall, but if I need to turn it the steel wheels slide on the concrete.

Greg
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k7mdo
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« Reply #12 on: December 31, 2017, 09:35:46 AM »

The water pipe idea will work but since I have welding capability I'll probably build a very low profile dolly.  It looks like one inch is just about the max it can rise and still clear the doors.  

There are so many things that "need" doing that, as always, it can get a little daunting.  😐
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KA3EKH
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« Reply #13 on: December 31, 2017, 05:36:43 PM »

833 are incredibly durable! I have turned the plates cherry red with no bad effects. In broadcast service itís not uncommon to run them with a little red glowing on the PA tubes. Flogging them like that will cause them to wear out in about a year in normal broadcast service. From what I recall the two in the modulator will hold up well for about a year or two normal service and when we started hearing distortion in the audio thatís when we changed them. They are also common and may still be in production. If you need some can maybe scrounge up a couple for you although other then the RCA that I was running on 160 have not seen one in commercial service for maybe twenty years.
Back when I was working for a little AM station back when I was in school we had a BC-1 as the backup for the main and would run it up regular with no issues except one time when I got a call from one of the other engineers that they had just cleaned the transmitter and now it would not come up. Once you hit the plate on switch it would immediately overload and shut down. He tried several times but the same thing kept happening. After I got to the station and saw it do the same thing started looking at the PA compartment and noticed that one of the two PA tubes was installed backwards so the grid was connected to the plate lead and the plate to the grid lead. Swapped the tube back around and everything worked perfectly. You would think that dumping three thousand volts on the grid would have some effect on the tube but it worked just fine after it was installed in the right direction.
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KD6VXI
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« Reply #14 on: December 31, 2017, 06:11:59 PM »

Chicom still makes 833s.

They purchased the tooling for glass tubes from Eimac.

Which means little.  They can't produce a 500Z that's comparable to Eimac....

But, it is still in production, China and I //think// I read Russia as well.

--Shane
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« Reply #15 on: January 01, 2018, 03:34:27 PM »

That audio transformer looks plenty big for putting audio into the transmitter's line input.
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k7mdo
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« Reply #16 on: January 16, 2018, 06:42:36 PM »

Plate resistance question:

I want to use my "rig expert" to tune the tank circuit and the method I want to use is to remove the finals (833A) and replace them with a resistor representing their total RF plate resistance. Then measure the tank circuit by "looking back" into the antenna line".

From reading the forums I think that the value should be around 2400 ohms.  Arrived at by multiplying the plate current by 2 and dividing it into the plate voltage.

At 250 watts the plate will be 1250 VDC and the current estimated at 270 milliamps. Or, at 1000 watts the plate voltage (per manual) will be 2525 VDC with a current of 525 milliamps.

Are there any arguments to this? Huh

Thanks, Tom
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N1BCG
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« Reply #17 on: January 16, 2018, 06:48:33 PM »

If you "back feed" the output circuit with an antenna analyzer or impedance bridge to check the tuning then the resistor would be Plate Volts / Plate Amps. The tubes need to remain connected since their interelectrode capacitance will affect the tuning.
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k7mdo
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« Reply #18 on: January 16, 2018, 07:00:17 PM »

OK, I thought I read somewhere that the RF resistance was represented at twice the plate current whereas the modulator current was at the DC plate current....  I can try both ways of course but leaving the tubes in was not yet on the agenda.  More to do!  The tubes are not in right now!  Thanks, Tom


* bc1g worries.jpg (175.27 KB, 725x966 - viewed 43 times.)
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N1BCG
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« Reply #19 on: January 16, 2018, 07:29:36 PM »

The resistance to use will depend on the class of amplifier. In the case of Class C I've seen some formulas use a 0.6 multiplier (although others have used 2.0 as you mentioned). My experiments have yielded results closer to what the 0.6 multiplier would produce so I've stuck with that. The formula would then be Plate Volts / Plate Amps * 0.6.

For the Gates BC1-G, that would be 1250 / 0.27 * 0.6 = 2778 Ohms so a common 2.7k resistor would be fine.
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KD6VXI
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« Reply #20 on: January 16, 2018, 08:40:31 PM »

It's called 'The K Factor'.

Ranges from 1.5 to 1.8 or 1.9 depending on whom you ask.

K factor depends on class of operation.

For a better explanation :  http://lists.contesting.com/_amps/1999-05/msg00315.html

--Shane
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KB2WIG
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« Reply #21 on: January 16, 2018, 09:05:41 PM »


'The K Factor'

what?

klc
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N1BCG
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« Reply #22 on: January 16, 2018, 09:07:59 PM »

Interesting...

The "K factor" representing conduction time correction makes a lot of sense, i.e., 1.0 for Class A or D.C. ranging up to 2.0 for Class C. I put it to the test using an impedance bridge on a transmitter with 670 Volts and 0.300 Amps on the plates. A potentiometer was placed between the plate connections and chassis via a 2.5mH choke and adjusted for the closest match to a 50Z reading on the bridge.

Removing RF and measuring the resistance showed that the pot was set for just over 1000 Ohms.

670 Volts / (0.300 * 2) = 1117 Ohms.

Close enough for me, so I'm good with the "2X" K factor but it's still important to have all the tubes installed, and more so as the test frequency increases.
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KD6VXI
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« Reply #23 on: January 16, 2018, 09:27:36 PM »

This would be a great time for John, K5PRO to check in.

He could out it in the best terms, I'm sure.

--Shane
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k7mdo
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« Reply #24 on: January 17, 2018, 12:27:06 AM »

As usual, great information.  I will go with the 2.7 k first and assume the 833's will be in place.

73, Tom
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