DX 40 Question

[w9bea]

Hi All:

I picked up a pretty nice DX40 at Dayton this year.  I took the case off to note the condition of the underside and all told, it appears the guy who built this did a pretty nice job at it.

I am going to replace the Power Supply Electrolytics (40 ufd at 450 volts X2).  I figure this is cheaper than smoking a power supply transformer.  The date codes on the tubes all appear to be 1958, so I am guessing the other components are pretty close in age unless they got replaced later in life.

I saw a plastic cased 2 mfd capacitor wired in across a 12K 2 watt resister that comes off of pin 9 of the 6DE7 Carrier Control/Modulator Tube.  Yet, the schematic shows these to be 10K and a .5 ufd capacitor.

Do you think this was an audio mod, or was there a later (or earlier) circuit revision?  If I elect to replace this capacitor with another exact in value, do I use an orange drop, or do you recommend another type/family of capacitor?

Also, after I get it up on the air for awhile and am reasonably satisfied with it's performance, is there any audio mods that I oufght to consider making, or more importantly, and components that are undersized and in need of replacing?

What controls the mic gain?  I dont see a mic gain control like my Johnson or 32V1 and I am wondering what controls the mic gain so the audio doesnt flat-top?

Thanks for the info.

73--Wally W9BEA

[K6IC]

Hi Wally,

I do not have a schematic for the DX-40,  and have not used one on  AM.  You might look at the AM Window,  linked from this site.  There are a number of mods for the DX-60,  which I believe is very similar circuit:
http://www.amwindow.org/tech/htm/mods.htm

Scroll down the list and look.  I know nothing of the R/C that you mentioned.  Others who actually KNOW something should reply soon.  73  GL  Vic

[WD5JKO]

Hi All:
I saw a plastic cased 2 mfd capacitor wired in across a 12K 2 watt resister that comes off of pin 9 of the 6DE7 Carrier Control/Modulator Tube.  Yet, the schematic shows these to be 10K and a .5 ufd capacitor.

   I just downloaded the manual from amforever, and I see the R-C you are mentioning. Looks like the R is a screen dropping resistor, and the C bypasses it for the audio. The change looks like they lowered the screen voltage a tad, and boosted the C to bypass that resistance for lower audio frequencies.

  The schematic looks a lot like a DX-60 to me...

Jim
WD5JKO

[WD8KDG]

Had a Dixie 40 way back when. The little beast is a controled carrier TX, no mic gain control. Gotta set your lips & tongue for the correct modulation level. Don't expect much in the way of AM, but it is fun to play with. Not going to hold the frequency on 75 meters, the big boys will run you off. Does a little better on 40.

Craig,

Addition: May 31, 2011

I'll have to add; the Benton Harbor gang designed this TX as an entry level "Novice" rig. That time period meant it was a crystal controled, 75 watt DC input TX. CW was its primary funtion.

Find a decent HG-10B VFO for the DX-40 and it makes a nice fun time rig. My brother, KN8THH, made plenty of DX contacts on 40 meters, CW.

[KC2ZFA]

I've fixed a busted DX-40 I got cheap off I-pay. This link was useful:

http://w4btx.com/retro.html

Also, I've searched far and wide but haven't been able to locate specific
to the dx-40 mods on the innertubes.

However, I've found the following

http://www.k5dkz.com/dx60c.html

which brings up the possibility of building a dx-60 in a dx-40 chassis and then
go with all the dx-60 mods that are out there (particularly QIX's audio mods).

73 de Peter

[W3GMS]

Hi Wally,

Congrats on restoring the DX-40.  Its a great little transmitter and works quite well.  I am currently restoring one for a friend right now as an early Christmas present. 

I always start with the cosmetic restoration first.  Most of these rigs will really shine when cleaned up.  Following the cosmetic restoration I go in and start replacing components.  On this one with the resistors, if they measured ok I left them alone.  The capacitors were another story.  All of the old Mica's I replace with the newer mica types.  I find that Antique Electronic supply has good prices on the Mica's.  The power supply electrolytic were replaced with newer ones from Mouser Electronics.  On the filters in the rig, I generally bump up the capacitance a bit.  There a slew of .002 disc ceramics.  On mine I found a bunch of them had drifted off tolerance so most of them have been replaced.  A good LCR meter is good in finding the bad ones.  On mine, the slide switch was replaced with a vintage style toggle switch from the same period.  From what I read those slide switches were problematic anyway.  As far as the audio goes, I would suggest you look at a mod that Steve, WA1QIX came up with for the DX-60.  It should drop right into the DX-40 as well.  I have heard Steve's DX-60 on the air and it sounds superb. 

http://amfone.net/Amforum/index.php?topic=26603.0

Steve's design relies on high level audio being fed into the 12AX7.  You could use the other half for a mic preamp stage.  A D-104 or other suitable mic would work well.  You could mount the mic gain pot on the rear panel but you shielded wire when you do. 
Also notice that Steve's design uses a bias voltage of -125V.  The DX-40 does not provide this but you can do so very easily.  I would obtain a very small 6.3V filament transformer and connect it to the filament winding the feeds the 6146.  The seconday of this new xmfr will give you the 120V to obtain your bias from.       

As was said earlier, the HG-10 works well with the DX-40 but the one I am working on is set up with the VF-1.  When the DX-40 came out, the VF-1 was the matching VFO.   

If you need any parts for yours let me know since I have a parts rig. 

Have fun...
Joe, W3GMS

[W3GMS]

A correction in my fast typing! 

"I would obtain a very small 6.3V filament transformer and connect it to the filament winding the feeds the 6146.  The secondary of this new xmfr will give you the 120V to obtain your bias from."

Its probably obvious, but the secondary of the new 6.3V filament transformer gets connected to the 6.3V winding of the DX-40 that supplies filament voltage to the final.  The primary of the new filament transformer becomes the secondary to obtain your bias voltage from.  In other words, the new small filament transformer primary becomes the new secondary. 

Joe, W3GMS         

[w9bea]

Hi All:

I greatly appreciate all of the insight that you have shown me regarding the transmitter.

I am replacing the line cord and will get the new caps in the mail most likely tomorrow or Friday.  I also have some 40 CW rocks on the way to get on the air with.

Another Gentleman sent me a couple of complete manuals to my email address..... Much appreciated.  I will keep everyone abreast of my future findings with the radio.

Thanks again all!

73--Wally W9BEA

[wb1ead]

Hi Wally..like you I've had a DX-40 here for almost a year picking away at it occasionally..it's basically restored to like new inside and out..works FB..I also quickly noticed the absence of a mic gain but feeding it with my un-amplified D104 it seems to achieve near 100% mod on peaks so I wasn't too worried over that..I read the comments about the differences in values for the resistor and cap and would agree it may "boost" the low end response.....if you do other changes that DO add to the performance somehow please post them here or by all means alert me by e-mail on my bio..I WOULD be grateful for any additional insight Wally as I plan to have this up and running soon..many tnx and gud luk!    73 de DAVE

[KN5L]

I have started a similar project and have two DX-40s. Heathkit had two versions, one has a manual copyrighted 1958 (6.8K ohm CW screen resistor) and the other copyrighted 1960 (1K ohm CW screen resistor). Has anyone found the reason for Heathkit to change the design of the AM and CW screen supply sections. The 1958 version also has changes in the modulation circuit, the larger filter capacitor and two 10W WW resistors are the obvious differences.

[gerry_w1id]

A strange thing about the DX-40 is the series B+ arrangement with the driver and oscillator. This is why the driver has a separate filament winding.

[KM1H]

The DX-35/DX-40 is notorious for smoking the transformer even when almost new. I suggest using a bucking xfmr or Variac to drop the AC to around 110V.

My DX-35 runs so much cooler on 110V and can still push any of the linears beyond "legal".

Carl

[WQ9E]

In addition to Carl's advice I would also suggest not trying to power a VF-1 (or other VFO) from the accessory socket.