Warning: Creating default object from empty value in /homepages/11/d132647312/htdocs/Amfone/mkportal/include/SMF/smf_out.php on line 47
DX-40 Good Grid Current Resonance, pegged plate current no resonance




 
The AM Forum
December 11, 2017, 01:43:45 PM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1]   Go Down
  Print  
Author Topic: DX-40 Good Grid Current Resonance, pegged plate current no resonance  (Read 849 times)
0 Members and 1 Guest are viewing this topic.
ns7h
Member

Offline Offline

Posts: 4


« on: June 25, 2017, 12:51:22 PM »

I functionally restored a DX-40 in reasonably good shape.  New filter/electrolytics and a few resistors and capacitors.   Got good output with crystals and with VF-1 on all bands.

Of course, I wanted to put it in its case ready for the S40B restoration to be completed as its vintage receiver.  That happened and the first time to load up into an antenna I got the symptoms of pegged meter instantly on keying with plate applied.  No dip.  What I did next:  Put it on Cantenna dummy load verified 1:1 with antenna analyzer with same result; Swapped 6146 with same result.  Oscillator and buffer appear to be workiing - I can hear in receiver the keyed signal that is pretty clean on frequency.  Next - opened her up and did what manual suggested - found loose upper nut on loading coil end, checked voltages - verified power supply as it should be and works in Tune position for HV to other tubes; checked output blocking capacitor and it was not open and within range.  6146 is getting high voltage on CW and AM switch positions (measured at cap) without tube in socket.  No arcing, sparking or releasing of smoke has been observed or any seeming stress (increased hum/vibration) from transformer when going to AM/CW keyed.  The meter seems in my recall more generous in the grid current than when I first set the dx 40 aside allowing as much as 6-8 ma, but will adjust with drive control to the 3 ma suggested.  I have never had the meter peg full scale on plate current before - though I must admit, I would have expected the transformer to complain more if the actual current was full scale and beyond on the meter.

I have verified within the tolerance of my equipment that the plate meter shunt is not shorted and close to the .67 ohms as well as the grid meter shunt about 20 ohms.  Resistance measurements from the tube socket to ground on the 6146 appears consistent with the circuit diagram and no bypass shorts to ground.  Cathode circuit is shorted through the key jack and appears to operate.

One thing I haven't done is measure if there is any RF OUTPUT that can be seen to resonate with the Tune control.  Unfortunately my equipment to do this went to heaven and I have ordered a new one.

Long winded I know, but seems like all the expensive parts work and I don't want to blow them up.....  Any similar experiences or additional thoughts??

Thanks, NS7H
Logged
Carl WA1KPD
Contributing
Member
*
Offline Offline

Posts: 1440



WWW
« Reply #1 on: June 25, 2017, 01:13:05 PM »

Hi NS7H

Are the variable plate and load caps OK? No shorts and good connection of the frame to ground? Sounds like you have a capacitance meter, are the values showing up  OK and changing? Are they wired correctly?
A friend of mine traced a similar problem in a T-60 to the original builder having swapped the plate and tune caps
Is there continuity from the coil end of the plate cap to the coax connector output?
Continuity across the parasitic choke?
Not that many things to rule out......
GL and keep us posted
Carl
/KPD
Logged

"Okay, gang are you ready to play radio? Are you ready to shuffle off the mortal coil of mediocrity? I am if you are." Shepherd
KF7JAF
Member

Offline Offline

Posts: 11


« Reply #2 on: June 25, 2017, 06:13:39 PM »

Check the meter switch - they are notorious on the DX-40 for going bad. Some folks replace the crappy slide switch with a toggle style. Just remembering from my experience with the DX-40. Also, 6-8ma of grid current is really high. I'd check the grid leak resistors to make sure the final is getting correct bias. Don't remember getting more than 3-4 ma of grid current on 80 meters, less as you go up in frequency.
Just a few things to check. It's a good little CW rig, not so great on phone, though.

good luck,

Dave, KF7JAF
Logged
ns7h
Member

Offline Offline

Posts: 4


« Reply #3 on: July 05, 2017, 05:19:48 PM »

Thank you all for the advice - sorry to take long to get back but only verified it today by testing the unit (after a lot of circuit tracing and verification of expected component and impedance values) with a new DPDT switch.  I tested the unit from the rig before and got funny results, but no opens.  With .67 ohm precision resistor for the place meter circuit, funny stuff in the switch can mess up the readings.

End result - back to full output over 50 watts.  CW note is clean and no chirp!

I had rebuilt a switch from the DX-40 and guess the repair failed.  I redid the repair plus another spare from a parts DX 40 that had tested open.  So I will replace the original switch as tested good and take data over a while.  At least the mystery is solved and nothing expensive smoked...


Thanks again for the responses - Bob NS7H
Logged
Opcom
Patrick J. / KD5OEI
Contributing
Member
*
Offline Offline

Posts: 6738



WWW
« Reply #4 on: July 07, 2017, 08:26:04 PM »

I'm glad this worked out well. I don't have one of those but did in the past and liked it very much!
Logged

Radio Candelstein - Flagship Station of the NRK Radio Network.
DMOD
AC0OB - A Place where Thermionic Emitters Rule!
Contributing
Member
*
Offline Offline

Posts: 1052


« Reply #5 on: July 13, 2017, 04:49:51 PM »

One of the Improvements one can do for the DX-40 (and the DX-35) is to decouple that series 6CL6 circuit.

Better tuning and stability is the result.



Phil


* DX-40 Improvement Schematics.pdf (293.26 KB - downloaded 43 times.)
Logged

"What kind of Koolaid do they make you drink in the Physics Department?" Charlie Epps to Dr. Larry Fleinhardt, NUMB3RS   Smiley
W3GMS
Contributing
Member
*
Offline Offline

Posts: 2830



« Reply #6 on: July 17, 2017, 10:12:19 AM »

One of the Improvements one can do for the DX-40 (and the DX-35) is to decouple that series 6CL6 circuit.
Better tuning and stability is the result.
Phil

Hi Phil,
Concerning the DX-40, are you talking about the topology where the oscillator and driver are in series with each other?  I always thought that was strange when Heath did that.  It also make debugging a bit more challenging since its hard to troubleshoot whats broken since neither stage will operate.  Heath's reason for doing that was for efficiency.  They did not want to use a dropping resistor off the HV supply in order to derive the 300V.   I like the DX-60 scheme much better!

I see from you P.S. changes that your deriving the lower voltage off the junction of the HV filter capacitor bank.  So you went with ground referenced cathodes on the oscillator and driver, correct? 

Joe-GMS     
Logged

Simplicity is the Elegance of Design---W3GMS
DMOD
AC0OB - A Place where Thermionic Emitters Rule!
Contributing
Member
*
Offline Offline

Posts: 1052


« Reply #7 on: July 17, 2017, 06:26:25 PM »


Hi Phil,
Concerning the DX-40, are you talking about the topology where the oscillator and driver are in series with each other?  I always thought that was strange when Heath did that.  It also make debugging a bit more challenging since its hard to troubleshoot whats broken since neither stage will operate.  Heath's reason for doing that was for efficiency.  They did not want to use a dropping resistor off the HV supply in order to derive the 300V.   I like the DX-60 scheme much better!

I see from you P.S. changes that your deriving the lower voltage off the junction of the HV filter capacitor bank.  So you went with ground referenced cathodes on the oscillator and driver, correct? 

Joe-GMS     

The Osc. and Buffer-Driver are referenced to the Keybias line so when in KeyDown, the Osc. and Buffer-Driver are grounded.

On KeyUp, leakeage currents through the tubes in the RF chain pull up the Keybias voltage so as to cut off those tubes.


Phil
Logged

"What kind of Koolaid do they make you drink in the Physics Department?" Charlie Epps to Dr. Larry Fleinhardt, NUMB3RS   Smiley
W3GMS
Contributing
Member
*
Offline Offline

Posts: 2830



« Reply #8 on: July 17, 2017, 06:33:01 PM »

That is what I had assumed. 

Thanks for the clarification.

Joe
Logged

Simplicity is the Elegance of Design---W3GMS
Pages: [1]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.077 seconds with 18 queries.