[Boatanchors] Thoughts on Reviving the Heathkit DX-100

Wed Jan 6 14:08:15 EST 2016

Interesting. I am pretty sure that the 6146 tubes can handle 900V OK.

We used to routinely run our ceramic based 807 tubes at 1200V DC to get a little more out of themin our WS 19 on the CCF (Combined Cadet Force) net in Britain in the old days!

Nice red glow on a cold day.

Bry

> To: boatanchors at mailman.qth.net
> From: wrcromwell at gmail.com
> Date: Wed, 6 Jan 2016 13:45:37 -0500
> Subject: Re: [Boatanchors] Thoughts on Reviving the Heathkit DX-100
> 
> Hi Dave,
> 
> I have been experiencing the same sorts of trouble with my DX-100. I 
> recapped it and found the final plate voltage hanging at about 900 
> volts! The line voltage here gets over 125 and closer to 130 volts 
> sometimes. The series wired caps as a unit are rated at um..900 volts! 
> Whatever margin was included when they were 'rated' is all the margin 
> there is. I have disconnected the plate supply transformer and I run a 
> pair of 2E26 tubes on the "low" voltage as finals. My DX-100 no longer 
> has the modulator section (maybe I'll restore that and maybe not). It 
> was gone before I got the transmitter. I haven't missed it. Can you 
> spell CW. I run my gear on the conservative side and I even crank the 
> power down to "official" QRP levels sometimes. I do NOT need 900 VDC on 
> the final plates. The Kenwood, running on the same AC power line also 
> leans on those 6146s with ~900 volts.
> 
> I am thinking of the bucking transformer solution for the final plate 
> voltage just as you have suggested and solid state regulation of the 
> "low" voltage which is also on the high side in my rig but well withing 
> the ratings of the parts. I may put caps with higher voltage ratings in 
> the final B+ filter and maybe a solid state regulator after that. My 
> DX-100 is still using the vacuum tube regulators. Replacing those with 
> silicon would make the voltages even higher - Yikes! If your DX-100 
> still has the modulator bits you may have to be more creative finding 
> interior space for a small bucking transformer. Maybe an isolation 
> transformer and bucker for the operations table would be more appropriate.
> 
> I do have other options for running reduced power on 160 meters 
> including a Johnson Ranger. I would just as soon have most of the power 
> from the pair of 6146s available from the DX-100. Sometimes I would like 
> to have more - maybe 50 KW <evil grin>. I'm sure the guys who do have 50 
> KW wish they had more, too. C'mon now..we know they are out there. Is 
> anybody missing their local pole pig?
> 
> 73,
> 
> Bill  KU8H
> 
> 
> On 01/06/2016 09:59 AM, David Stinson wrote:
> > Morning.  I just finished restoring a Heathkit DX-100
> > for a good friend.  The information I needed was supplied
> > by the kindness and generosity of several of our members.
> > Our people are the best!.
> >
> > Many of the grounds had gone "Hi-Z." A small "tweak"
> > of the ground screws usually fixes that. It's a common problem with 
> > old radios and the first thing I address when one hits my bench.
> >
> > This transmitter had a bad LV transformer.  It had also
> > been chopped and lopped to use the SB-10 Sideband adaptor.  The 
> > previous owner had done other things "his way" and it was quite a 
> > puzzle to undo all the mayhem.
> >
> > It's a pretty transmitter, but I'm not a fan of the DX-100.
> > The DX-100 runs everything on the "hairy edge," putting
> > max voltages on modulators, PAs and even the LV stages.  One of the 
> > 5R4 HV rectifiers had a failed and
> > arcing tube base- not surprising given how hard Heath pushes 
> > everything in this rig.
> > http://home.netcom.com/~arc5/5R4arc.jpg
> > The set was designed when the AC line voltage averaged 110-112 VAC.  
> > Today's lines run 122-125 VAC.  That's better than 10% high and runs 
> > the HV north of 800 Volts.  Not nice.  Things start arcing easily at 
> > 800 Volts.
> > No wonder people report 115W out on 75 Mtr AM;
> > they're kicking the stuffings out of everything in there.
> > I'd bet breakfast that a bunch of blown-up DX-100s
> > were fried for just this reason:  High primary voltage.
> > If it were my rig, I'd buck the line down under 110V,
> > take my 80W average out on 75 AM and call it "good."
> >
> > Moreover- the terminal strips used to build the set are cheap and 
> > prone to breakdown.  I had the 6 VAC buss
> > from the transformer try to weld it's terminal post to the chassis and 
> > I'm lucky I didn't lose the replacement
> > transformer.    I think one should coat the terminal
> > strips and the chassis around them in Corona Dope,
> > just for safety's sake.
> >
> > The 5V4 "Low Voltage" rectifier: I noticed that one of the wires that 
> > crosses the 5V4 socket had a burned spot and the replacement 
> > transformer also had a burn spot in the same place.  I put shrink 
> > tubing over
> > the burned spot.  Well, I got a new burn spot because
> > the wire crosses above a filament pin and the insulation
> > was breaking-down, causing an arc! 
> > http://home.netcom.com/~arc5/5V4arc.jpg
> > I double-shrink-tubed the entire length of the wire, plus the wires 
> > going from the LV filter choke near the chassis because I thought I 
> > saw an arc to ground there as well.
> > The DX100 is the "arcing-est" transmitter I've ever seen.
> > When transmitting, one needs a good ground because
> > RF where it doesn't belong encourages arcs in this rig.
> >
> > All this leaves me with the impression of the DX-100
> > as a fragile beast, just itching to let its smoke out and roll over on 
> > you.  If you run one, I think toning the primary voltage down around 
> > 110V and accepting
> > a little less output, applying Corona Dope to all the terminal strips 
> > in the power supplies and checking all you grounds would be excellent 
> > investments.
> >
> > GL OM ES 73 DE Dave AB5S
> 
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