[Boatanchors] Thoughts on Reviving the Heathkit DX-100

Wed Jan 6 20:18:46 EST 2016

Decades ago, when I lived about 35-miles from the Heath plant in St. Joseph, Michigan, (Heath's mailing address was Benton Harbor but the actual plant was on Hilltop Road in St. Joseph), after getting my driver's license, I used to drive up to the plant to "drool over" all the kits.  There was a showroom just to the left after entering the "H" shaped building where they had all the current products assembled and on shelves.  There was a "counter" there and one could actually purchase the kit and get it in less than 5-minutes!

If someone had any questions about any product that Heath had ever made, the counter clerk would call back into the plant and, within a very few minutes, someone from the engineering department, often the lead engineer on the project, would come out and answer your questions.  It was not uncommon for the engineer to draw, on a piece of paper, a schematic to modify the kit to improve operation.  Then, he would go back, into the plant, and come out with a paper bag with all the parts to make the modification and there was never a charge for the advice or for the parts!

At the time, I had my first DX-100 and, sometimes, asked about that transmitter.  On one of my trips to St. Joseph, the engineer gave me an in-house paper that had the voltages, at every tube pin in the DX-100.  Over the decades, this sheet has become very dog eared.  But, I have scanned the page and have included it, with a number of other information sheets from Heath, some of which were not, generally, given widespread distribution.  The voltage chart, for the DX-100, appears on Page-6 at the following URL:

 http://nebula.wsimg.com/a299627e256e4c19a81224d1bd62bb46?AccessKeyId=5DDC3F25F0398F58962E&disposition=0&alloworigin=1

Note that the heater / filament voltages are a fair amount BELOW the "standard" voltage especially the 6.3 VAC readings.  These measurements were made with a 117 VAC line voltage.  As such, the DX-100 can easily withstand the higher line voltage without any problems.  There is also a note concerning the high voltage on the 6146 plates.
 Glen, K9STH 
 Website: http://k9sth.net

      From: Bill Cromwell <wrcromwell at gmail.com>
 To: boatanchors at mailman.qth.net 
 Sent: Wednesday, January 6, 2016 1:35 PM
 Subject: Re: [Boatanchors] Thoughts on Reviving the Heathkit DX-100

Hi Bry,

I suspect they can handle the voltage. In fact they have been. What 
about the pair of electrolytic cpas each rated at 450 volts for a totla 
of ...um..900 volts. Too close to the bone. The heater supplies will be 
only a few per cent higher than they were before. That's okay too if the 
original voltage wasn't already at the high end of tolerances.

73,

Bill  KU8H

On 01/06/2016 02:08 PM, Bry Carling wrote:
> 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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