DX100 improvements

[John K5PRO]

I picked up a free DX100 yesterday from a dusty 20 year rest in a loft. Removed the knobs and cleaned it, and vacuumed and brushed down the chassis. It is acutally in really good shape physically. Maybe i can actually get a sig on AM (as I never seem to get finished my other two AM rigs, one a Junkston 500 restore and the other a Continental 1kW BC rig).  

Now I have questions for the experts:

Are many DX100s on the air, any idea? I ran an Apache as a novice and new general back in the early 70s, loved it back then. Gads, even used the SB10 for SSB! It is a direct descendent of this rig, easy to see.

I have read Timtrons' list of audio mods, as well as those by Ed AA8TV. Fixing the modulator for wider response and adding neg feedback seem like excellent goals. I noted that the original Heath design drove the modulator into clipping and then filtered it with build-out LPF capacitor across the transformer secondary, sort of a crude modulation maximizer.

Where does one find 1625's anymore? Is there an easy sub? The apache used the 6CA7s i think, which were expensive hi fi tubes.

The book and description show the DX100 (not the DX100B but the original two tone silver pyramid-faced rounded-corner box)  having a very wide range of output loading up to 500 ohms or so, with a set of switched capacitors for coarse loading and a fine loading variable cap on concentric shafts. Mine doesn't seem to have this at all, the front panel is marked for the coarse and fine controls, but only the variable cap exists, I don't even see the location or components for the switched Caps. Anyone have a clue about this mystery?

Finally, I need to replace the plastic grey knob for the drive control pot, and the outer concentric knob for the tuner on the same. They are missing/cracked. Anyone got a dead DX100 that they would part these out from?

I hope to get this big box on the dummy load today, assuming that the electrolytics will reform OK. Then will consider which of the mods to do for audio. I prefer to do outboard processing, with balanced 600 ohm circuit. I assume that the first 12AX7 stage may be unncessary then.

Thanks for any advice your collective wisdom can give!

[W3SLK]

John, Ed Seib, VA3ES compiled a nice booklet of mods and hints for both the Apache and DX-100. Interestingly enough, I just found it today when looking for some 'wall paper'. Also, if you can, contact Ray, WD2AFJ. He did the mods on my DX-100B and the reports I get are very good. Email me at w3slk@uplink.net and I'll send you Ray's address. I'm sure he can help you out.

[Steve - WB3HUZ]

Ed's compilation includes/features the Tron mods. I wouldn't mess with most of the other audio mods (w/the exception of the feedback mods) as many are half measures or just plain incorrect.

Tron's "turbo" mods reduce or eliminate the modulator clipping due to an impedance mismatch. The will allow you to run more audio without get W-I-D-E like most stock DX-100s do. I've heard an Apache with this "turbo" mod and some other Tron mods done to it and it was the best sounding Apache I've ever heard - hands down.

I've seen 1625s at fests. Fair Radio used to have them. They are identical to 807s except for the 12v filaments. I would think they would be fairly common as the mil bought tons of them and few amateur radios used them other than the DX-100 (there was 50's/60's vintage linear that used 4 of them).


In my experience, the DX-100 is the most commonly used AM rig. There are tons of them on the air!

[w3jn]

Where does one find 1625's anymore?

Lemme know if you're good on QRZ.com, and you can expect a package.
I have a couple dozen or so moldering in my attic and I'm happy to donate a pair to the cause.

73 John

[John K5PRO]

I'm good in QRZ.com. Thanks a bunch.  I tested the 1625s and both are the 1959 vintage, one has 1/2 the mu of the other, so i though might as well put in a closer matched set.

73
K5PRO

[KL7OF]

I've not tried this on a DX a hundred but it works well on a viking 2 and a 32 v.......use your outboard audio processing gear to drive the grids of the modulators thru an audio transformer......bypass the stock speech amp completely...no drilling and blasting and good audio.......good luck

[w3jn]

John, that knob you need, is it the same as what was used on 50's Heathkit test equipment?  Gray plastic, skirted, one-piece molded?  If so I'll include a couple with the toobs.

73 John

[John K5PRO]

Thanks guys. The knobs I am looking for is the concentric pair, the outer one is 1/4 inch shaft, grey fluted, and the inner one is a smaller diameter (3/16?) shaft which is used for RF drive I believe.

I glued the outer one back together last night, will see if it holds. The inner one is missing altogether so I am using something off an old Scott Hifi which doesn't look OEM but fits. If you have the authentic knobs around, would love them. Thanks again. Tonight I will finished getting the 'lytics replaced or reformed, and find out why there is one broken resistor inside. Looks like the audio stuff is well covered.

BTW, I looked in the 1959 ARRL HB, and the 807 modulator is all there, with the 12AX7 preamp and all. Only they used a 6C4 triode for the driver while Heath chose to use a 12BY7 triode connected in the DX100.

Wonder why Heath switched to the 6CA7/EL34 for the TX1 Apache? It is a more expensive and lower plate dissipation pentode. Maybe they wanted to get away from using plate caps and get all the HV under the deck or inside the RF cage?

[Jim, W5JO]

Somewhere I remember Tek scopes used little red knobs.  Maybe they would fit or you could adapt.  I think these are hard to find.

[W3SLK]

Wonder why Heath switched to the 6CA7/EL34 for the TX1 Apache? It is a more expensive and lower plate dissipation pentode. Maybe they wanted to get away from using plate caps and get all the HV under the deck or inside the RF cage?

Because they could get the 1625's for a pittance. 1625 were manufactured in such quanity and were pretty cheap on the surplus markets, made them a good candidate. Somewhere in my shack, I have a bag with about a dozen new ones. Originally, they outfitted the Apache's with Mullard EL34's whether they were less expensive than their American counter-parts remains a mystery. MY TX-1 has the Mullards installed. I do have some Tesla EL-34 and GE 6CA7's yet to try. My hanging question is why not the venerable 6L6??

[John K5PRO]

The 6L6 had similar plate dissipation rating, but had only a 360 or 500 volt plate rating depending on whether it was 6L6 or 6L6GC. The EL34 had 800 Volt rating. Apache ran the modulator off the same B+ as the pair of 6146s.

Marshall used the EL34s in many guitar amplifiers. They had a peculiar distortion when overdriven, which created their distintive Marshall sound.
The Marshall wall of sound.

Vacuum Tube Valley magazine (issue #2 and #16) ran nice articles on the reasons for EL34/6CA7, and one of them was to get around RCAs beam power tube patents.

http://www.vacuumtube.com/

[K6IC]

RE the missing fixed laoding caps:


I think that it was common to replace the switched fixed caps/one section variable with the DX 100B's triple section loading cap.   My second, recently acquired DX 100 is the original non-B model.  It has some mods,  including no fixed loading caps,  and the B's triple-section loading cap.

1625's were dirt cheap on surplus market,  and Heath often used good parts from the surplus market.

Here in the West,  the DX100 is quite common,  there were a ton made,  and they seem to be well designed ... go forever.

73  GL    Vic   KF6RIP[/quote]

[John K5PRO]

Thanks for the tip on the loading cap revision. That makes sense. What threw me was the front panel labeling still showed COARSE and FINE where the old knobs would have been.
John

[w3jn]

John - I'll get those 1625s out tomorrow.  Try as I might I can't seem to escape work to get to the post office.

73 John

[W3SLK]

Just a thought aloud: What was the major difference between the DX-100 and -100A? Now there's a rig you don't hear too often.

[Pete, WA2CWA]

Just a thought aloud: What was the major difference between the DX-100 and -100A? Now there's a rig you don't hear too often.

If you heard one, it would be in your dreams. They never made a DX-100A.

DX-100 1955-1958
DX-100B 1958-1960

[N3WWL]

There was also a version made for European export.  I believe the letter designater was "U".  I'm unsure of the circuit difference in the "U" model.

[John K5PRO]

W3JN

Thanks for the 1625's - they arrived yesterday! I am on my way to getting this DX100 on the air. Got the electrolytics replaced/reformed yesterday.

This is a great gang of AMers on this list.
73
John

[w3jn]

Any time, John... glad to help.  Better you use them than they molder in my attic for another 10 years.

73 John

[John K5PRO]

UPDATE on DX100 REFURBISHMENT:

Replaced the HV electrolytics with 2 x 210 uF 450 VDC
Reformed the Med Voltage electrolytics (20 uF x 3)
Replaced the Bias electrolytics

Found about 4 unsoldered and same # of poorly soldered connections, some buried deep near the chassis underside. The guy who build this thing, bless his soul, had bad vision or a bad week. Seems like some areas were fine, then others were a mess. (Esp under the 6146s and near the AM/CW mode switch - lots of melted wire insulation)

Replaced the line cord with 3 wire, and added a single fuse to hot side, and replaced the 0.005 uF RF bypasses on the AC line with UL approved bypass caps.

One 1625 was low mu (1800 vs 3000 on the other). When fired up, one tube had a very hot spot on the edge of its plate, glowing brighly orange. . Thanks to a generous contribution by John W3JN, I was able to find another good tube, and match both of them.
 
It tested out with plenty of RF, 120-130 watts easily, also lots of drive to the 6146s. The audio was abismal. Response was flat 500-2000 Hz and below 400 Hz was non existant. Distortion bad (7% THD) when driven above 50% mod. The S/N is about 48 dB when the AF gain is zero, and some background hum is evident at higher levels. Not too objectional though. The S/N became 70 dB when filters switched in on the Tek AA501 audio analyzer, when the AF gain is turned up - the noise is dominant from the 12AX7 mike preamp stage as expected. Carrier shift is 4-5% at full mod.

I spent the day yesterday ripping out the audio section, which is buried behind a little copper divider plate and the front panel mike jack. Recapped the coupling and cathode bypasses, and added a feedback loop from the 500 ohm tap of the mod tranny back to the second 12AX7 cathode. Tried the AA8TV feedback mod (has a voltage divider and then a series electrolytic to break the DC path and pass audio).

Audio response was better, but still not stellar. At least it could modulate at 100 Hz now. Removed the above feedback mod and replaced it with a 1 Meg series R, and a large cathode bypass capacitor (47 uF) per Timtrons posted modification. No series cap or shunt 200 Ohm resistor in this feedback circuit.

Final audio response is very good. It is about 3 dB down at 100 Hz, using 1 KHz as reference 0 dBm. 400Hz to 3 KHz is flat. Out to 6 KHz about a dB drop and it goes out futher. However, there is some ugly distortion on the modulated envelope on scope. Seems like opening up the audio and adding the feedback has worsened this aspect. Carrier shift is much better, maybe 2% at full mod.

I plan to go ahead and fix up the 6146 screen circuit, adding a fixed screen voltage source in addition to the resistor to the modulated plate voltage. On the web I found the 60:40 ratio circuit for an Apache. After this, I may consider replacing the 5R4's with solid state rectifiers as I have a pair of Semtech SCH7500 devices. However, there is something sentimental keeping me from doing it, seeing those gorgeous rectifier tubes (OK - they don't have the blue glow of 816s). I think the carrier shift and the ability to get a bit more out of the DX100 would benefit from SS rectifiers for higher and more stable 800V.

I might neutralize it, a la TX1 Apache scheme. It does seem to have a bit of quirky response to final output tuning, on the grid side. I haven't been up to 10 meters yet, still playing into a load on 80 and 40.

Its been a fun project, as I never anticipated having a DX100, it sort of fell into my lap (ouch!). I hope to get it on air this week once I figure out the source of audio distortion.

Does anyone have the section of the manual on operating the DX100, and the voltage charts for the tube elements? These seem to be missing on the BAMA manual.

[Jim, W5JO]

What is your email address John?  I have a voltage chart and can scan.  Will also scan the tune up procedure for you.  What the manual says about tune up is 5 ma on the grids, and 250 ma on the plates unless you plan to operate phone.  On phone load to less than 250 ma.  My manual doesn't specify, so I load to about 230 ma.


Are you good on QRZ.com?

[John K5PRO]

Jim
Yessir, I am OK in QRZ.com. I downloaded the TX1 apache manual from BAMA, as it appears very similar. I finally realized that Apache advertized 150 watts AM "input" and DX100 was rated 100 "Output". Marketing trick. THey both have same plate voltage and current. It appears that the TX1 had a different mod transformer, as it had 6CA7 which needs about 11K ohms versus 6K for the 1625s.

73
John

[John K5PRO]

Can anyone tell me about the Electric Radio article in April 1993, on the DX100? Who wrote it, what was the content (modifications or just history)?
THanks.

[Jim, W5JO]

John, I looked at QRZ and it did not list your email.  Whazzup with it?

[Pete, WA2CWA]

Can anyone tell me about the Electric Radio article in April 1993, on the DX100? Who wrote it, what was the content (modifications or just history)?
THanks.

Here's where to find the complete ER Index. Have patience, it takes some time to load:
http://www.qsl.net/n9oo/ersearch.html