Dx-100 modulating downward--why?

[wb3eii]

I picked up a  rig from Craigslist, "dusty, rusty, all complete, lites up, real cheap, will deliver" reads the ad. I'm hooked.
A bit of cleaning, & such later I fire it up. "Makes quite a 120 hz buzz" is the pronouncement from my local ham buddy about 8 miles away, so the HV caps get replaced. 
A solder burn on my thumb later, the replacement caps are in place.
 I fired up the dx 100 this AM , found something curious.
Tuned and loaded tx per manual, nice cw signal, crisp, good tone, ect.
 Tried fone, I get a nice carrier, audio sounds great,BUT, the stupid thing modulates downward.
 I changed drive, loading, audio level and anything else I could think of, still no difference.
 Thought perhaps the downward power swing was an artifact of my metering, so I hooked the output directly to a lightbulb dummy load, bulb dims considerably when I talk.
 Next move was an easy one, I had some New In Box Raytheon 6293's sitting idle near by, I in they went in place of the 6146's-no change- the 60 year old finals must still be good.
 Swapped the plate caps on the 1625 mod tubes, swapped them back, after seeing no difference.
 I can whistle in the mike and make the plate mills go to zero, the bulb goes out as well. Mod meter goes up as it should also.
 Any thoughts? 73, Ed AB3HT
 

[ka4koe]

Do a search on this site "DX100 downward modulation". Apparently this problem is not unknown.

Philip

[KM1H]

Is the clamp tube pot set correctly?
Did you replace the LV PS and bias filter caps?

I just finished a DX-100 and found all the lower level RF tubes were weak. Replaced the 6AU6 with a 6AH6W and it is now stable even on 10M. Just got thru with a couple of hours on 75 with it and good reports even from the SDR with spectrum analyzer guys.

Some power meters will not show a good audio indication but when tuned properly and with enough grid drive of 4-5 ma tha light bulb should do its thing.

Carl

[KA2DZT]

Downward modulation, I would take a close look at the HV B+.  It probably is sagging with heavy modulation.  Even the 300V supply may be sagging do to the increase screen current from the mod tubes.

In my HB 6146 rig I completely eliminated any downward modulation by running the screens of the 6550s from a separate regulated supply. But, the change that made the biggest improvement was running the 650V B+ to the 6550s through a separate filter choke and filter cap.

So, the rig uses one HD 650V plate xfmr with SS rectifiers and two filter circuits, one uses a two stage filter (LCLC) for the 6146 RF output stage and a separate single stage filter (LC) for the 6550s.

You probably would not be able to do what I did in my HB rig, but your B+ HV supply may be weak for some reason.  If your supply is still using tube rectifiers I would check those tubes.

Fred

[wb3eii]

Fred, HV is good at 750vdc, no sag whatsoever. New 5r4wga's, though SS replacement may follow.

Carl, clamp tube will be looked at next, have the new caps for the LV and bias,maybe put them in this weekend.

Thanks & 73, will keep progress posted.

Ed

[AB2EZ]

As an additional suggestion:

Check to be sure that your grid drive (average grid current) is correct... and see if the grid current reading fluctuates when you modulate the transmitter. Try increasing the grid drive (average grid current) in case the meter reading is not correct. Based on the symptoms you describe, I'm thinking that the output rf tubes' grid drive is (for some reason) dropping significantly when you modulate. As Carl said... if the average grid current drops, the clamp tube will detect that and it will drop the screen voltage on the RF finals to protect them.

Stu

[wb3eii]

Quick update, I adjusted the bias to the point where I can no longer 'whistle down' the carrier.
 Lightbulb dummy load dims a bit under full modulation, but at least it is much brighter than it was previously under the same situation. I did not take any voltage readings.
 No time for further testing now, spent the day getting medical tests prior to my surgery on Monday. Lots of tasks around the house to accomplish this weekend as well.
 I'll have a month of from work following the procedure, so perhaps I can get this figured out.
 If you don't hear from me again, blame the anesthesiologist. 
73, Ed AB3HT

[KM1H]

Good luck on the procedure and if the anesthesiologist takes a liking to you then you may not be back. Just dont take a Carnival cruise

The wimpy 6AL5 bias rectifier and filter caps in many rigs is the cause of many problems as the tube goes soft due to excessive cap leakage current. No need to go SS, just fix the problems. The 1K resistor between the two 20uF caps could also be cooked. I have NIB 6AL5's (mostly mil spec) coming out of my ears if you need one.

Carl