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Author Topic: DX 100 B behaving badly - Parasitics?  (Read 4606 times)
KD1I
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Posts: 466




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« on: November 07, 2014, 08:09:32 PM »

I hope some of you with experience with the DX 100 B can give me some direction.   I just bought a DX 100 B transmitter from a bone yard.  After a good cleaning, including the switches and tube sockets and pins, I brought it up slowly with a Variac.  OK, so far.  Next, I followed the tune up procedure in the assembly manual. I was able to adjust the driver level and peaked the tuning for 5 mA grid current.   The instructions say to reduce the drive level to zero and switch on the plate voltage to the finals. That's when I noticed that the plate current goes very high.  However, with normal grid drive of 5 mA, the plate current behaves well. It can be adjusted nicely for 250 mA and has a nice dip.   Output is good using a Heathkit wattmeter and a 100 watt lamp, it indicates just over 100 watts out and the lamp glows to full brilliance. I have tried this on both 80 M and 40 M so far.
Plugging the key in has the same effect.  With the key open, plate current is very high, but depressing the key produces normal plate current.  Plate voltage is similarly odd. Key up is 800 volts DC while key down goes to 850 volts if the panel voltmeter is to be believed....just the opposite of what one would expect.
So the bottom line is:  No drive results in high plate current while normal drive of 5 mA results in very normal operation.   There is no RF output indicated during the high plate current condition, BTW. The lamp does not glow and the watt meter does not show power.  I have searched here for parasitics on this transmitter and elsewhere but nothing specific found.   Any thoughts?      73, Jim
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W1BR
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Posts: 4189




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« Reply #1 on: November 07, 2014, 09:23:35 PM »

A Class C amplifier is self biasing, and requires drive to generate bias. A lack of drive puts the 6146 tubes into full conduction, and runaway plate current.  I suspect what you are seeing is normal in that regard.. the RF drive on the control grids is rectified by the finals, and that voltage biases the finals. No drive, no bias, and full plate current!

However, in CW, with the CW key open, there should be no drive, and no plate current.

Pete
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G3RZP
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Posts: 1280




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« Reply #2 on: November 08, 2014, 12:54:17 AM »

As I recall, the DX100 has a clamp tube from the screen grid to ground - possibly a 6AQ5 or similar. It sounds as if the clamp tube circuit is not working. The idea is that when drive is applied and grid current flows, the negative voltage across the grid leak drives the grid of the clamp tube negative and cuts it off. With no drive, the  clamp tube conducts: its plate (and usually screen) are connected to the PA screen grid, and when it conducts, it pulls down the screen grid voltage to the point where the plate current is a safe value. If the clamp tube is defective - or maybe even missing (such things have been known!), you will see the effect you are getting.

For more details of clamp tube circuitry, see an ARRL handbook of the 1950s or 60s.

Hope this helps.

73

Peter G3RZP
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KD1I
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Posts: 466




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« Reply #3 on: November 08, 2014, 03:55:30 AM »

Pete and Peter, Thank you both for your thoughtful replies.    The DX 100 B does have fixed bias for the finals derived from a 6al5 dual diode connected as a full wave rectifier. This provides bias for several of the stages.   I will need to measure this carefully.   There is a clamp tube circuit.  The pot that adjusts the clamp level is a known problem that is posted on the web.   I will need to check this as well.  In normal CW operation, drive is removed from the finals because the first two stages are cathode keyed so it is normal for the driver and finals not to have drive all of the time. So I will check the bias level and then dig into the clamp tube circuit later today.  Right now I'm off to some lectures on radio; what better way to spend an autumn Saturday morning? Thanks again for your input. I think you are correct and my original thought of parasitics is absolutely incorrect.    73, Jim
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KD1I
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Posts: 466




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« Reply #4 on: November 08, 2014, 07:38:36 PM »

Well, the problem is solved. Thanks for the tip on the clamp circuit.   I removed, disassembled and cleaned the drive level and clamp level pots. A retest showed normal operation.  I adjusted the clamp level so there was minimal plate current with plate voltage but no drive (like in CW with the key not depressed).   It works fine.   This certainly shows what collaboration can accomplish. Very Best of 73,   Jim
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G3RZP
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« Reply #5 on: November 09, 2014, 12:03:16 AM »

Jim,

Good - you fixed it! Glad to be of help.

73

Peter G3RZP
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