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DX100 Can't control Drive level and Hum in modulator




 
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Author Topic: DX100 Can't control Drive level and Hum in modulator  (Read 635 times)
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K8CA
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« on: December 03, 2018, 05:23:55 PM »

I can't control the drive level in my DX100. I have checked the drive pot and the voltage to 5793 Driver tube varies with adjustment of the pot but, the RF output of the final isn't affected. Final tunes resonate and tune fine!
Also have hum in the modulator.

Any body been through this, and have any ideas??

Steve
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WBear2GCR
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« Reply #1 on: December 03, 2018, 06:13:15 PM »

Check the manual... afaik usually the "drive" control will not alter the power output, as it
usually changes the GRID drive. The meter usually has a position to look at that.

Not looking at a DX-100, so this is a good guess.
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KK4YY
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« Reply #2 on: December 03, 2018, 07:28:53 PM »

I agree with Bear. The drive control is not there to control output power. It's there to drive the finals (6146's) into class C. Set grid drive according to the manual.

Don
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DMOD
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« Reply #3 on: December 03, 2018, 10:44:11 PM »

If you can vary the Final's grid drive the Drive pot is working.

Final operational Grid current should be around 5 mA.

There is some interaction between the grid drive and the 6AQ5 clamp tube. The manual goes into detail about that interaction and adjustment on page 54.

As far as Hum, if the power supply filter caps haven't been replaced, then the obvious course of action is to replace them. Hayseed Hamfest has replacements.


phil - AC0OB
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K8CA
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« Reply #4 on: December 05, 2018, 12:10:53 PM »

Thanks for the input on these problems. I will go back and check the manual for the DX100 regarding the Drive control operation.
I will check the Caps in the Modulator and other HV section as well.
I have one other issue. With the first modulation in "Phone" the final current will go up to about 400 MA and stay there, even without any further voice input. I have to switch back to "CW" or switch off the Plate switch to get it return to mormal.
Any body had any issue with oscillations in the modulator or Final?
Thanks,

Steve
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KK4YY
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« Reply #5 on: December 05, 2018, 05:32:35 PM »

It's not a good idea to use the CW-PLATE switch while the PLATE switch is on. The contacts may arc and fail if done too many times.

Is the audio gain pot at "0" when you turn the PLATE switch on (in PHONE)?

Don
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K8CA
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« Reply #6 on: December 06, 2018, 11:44:59 AM »

I usually don't set the Audio Gain to zero when turning on the plate switch.
If it is not at zero does that cause the lockup?

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WBear2GCR
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« Reply #7 on: December 06, 2018, 01:30:29 PM »

Thanks for the input on these problems. I will go back and check the manual for the DX100 regarding the Drive control operation.
I will check the Caps in the Modulator and other HV section as well.
I have one other issue. With the first modulation in "Phone" the final current will go up to about 400 MA and stay there, even without any further voice input. I have to switch back to "CW" or switch off the Plate switch to get it return to mormal.
Any body had any issue with oscillations in the modulator or Final?
Thanks,

Steve

When you key the rig in "phone" it stays at the proper nominal value. after the rig is tuned and dipped??

Suggest you put a scope probe in proximity to the finals and see what the carrier looks like - oscillations
are always possible.  Did you check the modulator current when this happens? Could be an oscillation in
the modulator too... or a hum...
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K8CA
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« Reply #8 on: December 06, 2018, 03:23:50 PM »

Thanks for the suggestions. I'll give the scope a try.
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KK4YY
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« Reply #9 on: December 06, 2018, 05:16:58 PM »

I usually don't set the Audio Gain to zero when turning on the plate switch.
If it is not at zero does that cause the lockup?
You shouldn't need to put the audio gain at "0". I was suggesting it only for troubleshooting.
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K4RT
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« Reply #10 on: December 06, 2018, 06:09:36 PM »

Steve,

I'm not sure I'm understanding the symptoms described but it sounds like you are able to vary the grid current using the front panel drive pot.

Are you tuning the PA in CW, then switching to fone mode and seeing 400 mA plate current or do you mean 400 ma modulation current?  I think resting mod current should be around 50 mA and maybe peaking up around 125 to 150 mA with modulation. I'm going on memory here, but this is all noted in the manual as you are likely already aware.  If you're referring to plate current are you able to dip the plate current then below 400 mA?

I used Hayseed electrolytic caps to replace the PS filter caps in my Apache. No problems in five years.  Last year, I used new small electrolytics, maybe Nichicon or Panasonic in my DX-100B because I had several on hand. They work fine, too.

I hope you're able to get things squared away soon and get that DX-100 back on air.

Brad K4RT
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Brad K4RT
K8CA
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« Reply #11 on: December 08, 2018, 04:38:27 PM »

Brad,

The DX100 idles fine with the plate switch on until I speak into the mic.
The modulation causes the final current to jump to about 400 ma and stay there!
It's fine in the CW mode. I think the modulator must be going into oscillation and driving
the heck out of the final.
I've got to check further to see if the drive contro lis varying the final grid current.

Thanks for replying to me.
Any ideas are sure appreciated.

Steve
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W4RFM
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« Reply #12 on: December 08, 2018, 05:21:28 PM »

Modulators or finals tube saturated? If it is fine till you modulate it.  I had a broadcast rig that did that once, back in the dark ages.
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BOB / W4RFM
KK4YY
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« Reply #13 on: December 08, 2018, 07:10:23 PM »

Steve,

It does sounds like oscillation. Oscillation needs feedback. But where's the path?

It may be RF feedback into the mic. Connect a good dummy load and see if it still happens. That will help discover/eliminate an RF feedback path as the trouble.

Don
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« Reply #14 on: December 09, 2018, 01:21:52 PM »

Before going further and to save your finals:

1. I know this can be tedious but - check the wiring in this rig against the schematic and note any differences with a red ink pen. I don't recall if you said you had just bought this rig or had brought it out of storage.

2. Make sure the 6146 finals and the 6AQ5 clamp tubes are good and test within specs.

3. Solder a 0.001 uF 1kV ceramic cap to pin 7 and ground at the Clamp tube.

4. With the mic gain at full CCW, adjust the Clamp tube in CW and "0" drive with the 500k pot so the 6146 screen grid voltages are at about 140V. You never said you had adjusted this as yet.

5. This rig has grid-block keying. Check the bias voltages at point "C" (at the junction of the .005 uF and the 2.2k resistor) with a scope and DMM on Keyup and Keydown and compare to the manual.

What is the modulator current when this happens?

This transmitter has a Pi input circuit to the final's grid and is not known for parasitic oscillations. Check the condition of the "Driver Section" Bandswitch and the wiring to and from this switch.

Phil
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K8CA
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« Reply #15 on: Yesterday at 02:01:56 PM »

Phil,

I have not tried those steps yet- but sure will.
The modulator current goes up and own with the voice peaks, but upon the first voice peak the final current goes to about
400 ma and stays thee until I switch to a crystal position or turn off the Plate switch.
I think the final is going into oscillation.
I just ordered  new caps for this transmitter- they have never been replaced!
I'm thinking that may help. An I'll go from there.

Thanks to you and all for the suggestions.

Steve
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