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 Post subject: Heathkit DX 40 repair thread
PostPosted: Jun Thu 27, 2019 6:14 am 
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Joined: Jul Sat 28, 2012 9:32 pm
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Location: Charlotte 28211
Last week I was given, out of the goodness of a fellow club member's heart, a Heathkit DX40. A tad rough, but nothing I can't handle. Has the makings to turn out very nice. The first thing I need to get squared away is a cord and plug situation. Attached is an ebay image off a similar plug that came on it. Except the actual one was taped up. Removing the tape I found it in broken pieces with 3 fuses inside. Three fuses?? Anyway I'm likely to remove the entire cord from the radio and start fresh. But with what? The paperwork gives no clue as to what came on the transmitter originally. I've read about guys who modified old radios with 3 prong cords, and an equal amount of internet wisdom pointing out that taking old radios and rewiring them for 3 wire cords was a very bad idea. Hammarlund put .01 capacitors in their radios on both sides of the power line to the chassis, and I always questioned the sense in that. I've read and read and read about what "ground" is and how to best achieve perfect "ground". I've come to the conclusion that one day in the near future, Congress will mandate a 4th ground wire on electric goods for some authoritative screwball reason.
I need an idea on what to do to get a good cord and plug on this machine. And I suppose fusing the radio would be smart, if it doesn't have a fuse already (not sure).


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 Post subject: Re: Heathkit DX 40 repair thread
PostPosted: Jun Thu 27, 2019 8:25 am 
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Greetings to Henry and the Forum:

The schematic does not show any fuses; you are correct. Perhaps that's why one finds a number of the DX-20, 35, 40 etc. Heathkit transmitters with bad power transformers.

The fusible plug shown in your photo was quite common in the era as a way to add fuses to a radio without drilling extra holes. Most transmitters of the era used a 2-wire line cord; you were expected to tie the transmitter chassis to a good station ground which would also act as your safety ground.

I would recommend fusing the transmitter in some way. The fusible plug in your illustration holds two fuses, one in each lead. Thus, both hot and neutral are fused. The plug is not polarized, however; that's why the two fuses.... at least in most cases. You can use a fusible plug with a 3-wire grounding cord if you wish; I do this with my Johnson Ranger. I simply used a 3-wire cord and stripped the wall outlet end to 4 or 5 inches. I cut the hot and neutral leads short to fit into the fusible plug as they should. There is enough room around the cord entrance hole to bring the ground lead back out of the plug, even with the outer jacket in place (assuming you use round 18-3 cord). The ground lead is brought around to the front of the fusible plug and fitted with a banana plug, which just happens to be a perfect fit in the ground pin hole of a grounding duplex outlet. It's a little more cumbersome to connect and disconnect than a one-piece plug but it allows you to use a fusible plug with a safety ground 3-wire cord.

The reason I did this is that the Ranger rear chassis is very "busy" and there really isn't room to punch two holes for fuse holders. The Ranger is unusual in that it requires two fuses of different sizes in the AC line. One is a 5 amp fast blowing fuse to handle severe overloads immediately. The other is a 3 amp slo-blo fuse to handle overloads that would not blow the 5 amp fuse but could, over time, damage the power transformer or other components. The fusible plug allowed me to keep the original 2-fuse design without modifying the transmitter. (The original Ranger power cord was a 2-wire cord with a fusible plug from the factory.)

By the way, the DX-40 is specified in the Heathkit manual as requiring 175 watts at 117 volts. That's just under 1.5 amps. To allow for in-rush current, a 2 amp slo-blo fuse should do the job. If you want to use the Ranger trick, you could try a 3 or 4 amp fast fuse in one side with the 2 amp slo-blo in the other. You could even try reducing the slo-blow fuse size.... but 1.5 amps is right at the normal draw for the transmitter and I don't know if they make a 1 3/4 amp fuse.

Good Luck,

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Jim T.
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 Post subject: Re: Heathkit DX 40 repair thread
PostPosted: Jun Thu 27, 2019 12:12 pm 
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Those two fuse plugs were used on a lot of later Heathkit gear including the DX-60 which was the newer replacement for the DX-40 and I wouldn't be surprised if that style plug was a running change late in production because a lot of Heathkit gear used a two fuse plug which is likely another idea they "borrowed" from Johnson like most of their DX-100 design.

I don't like them because you can end up with a blown neutral fuse which is a safety hazard. Normally I replace these with a regular 3 wire line cord and an inline fuse to avoid having to drill any holes in the chassis.

If you absolutely have to keep these original then a reasonably safe approach is to mark one side of the plug as "hot/load" and put a proper size fuse in that side of the dual fuse plug. Then put a 20 or 30 amp fuse in the neutral side. This way the neutral side fuse isn't going to open during an overload. IF you take this approach you have to make sure that the plug is always oriented correctly AND also follow the old safety adage that the ground wire is the first wire connected when setting up a station and the last wire disconnected when taking the station apart. Personally, I think the 3 wire cord set is a much better approach.

Heathkit used 3 amp fuses in the DX-60 which would be a reasonable choice for the DX-40. You could probably fuse at 2 amps if you also use inrush current limiting to avoid nuisance blowing at initial power up.

Rodger WQ9E


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 Post subject: Re: Heathkit DX 40 repair thread
PostPosted: Jun Thu 27, 2019 12:15 pm 
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Jim,

What you can also do to keep that dual slow blow and fast blow scenario with the Ranger is to put both of those fuses in series in the hot side. This provides protection without the open neutral scenario which is also a NEC prohibition because of its safety implications.

I have my Ranger 2 setup this way, my Ranger 1 has the original plug which I kept because it has to plug into the matching outlet on the Desk KW for control purposes and there wasn't room to install a proper 3 wire outlet in the Desk KW outlet location.

Rodger WQ9E


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 Post subject: Re: Heathkit DX 40 repair thread
PostPosted: Jun Thu 27, 2019 1:20 pm 
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As Jim mentioned, using two different fuses was the norm when these plugs were common. Any radio having a power transformer can easily accept a 3 wire cord.

The reasons for the bypass caps of old were the same as they are today, to prevent RF getting out, and NOISE getting in, the radio. Don't forget, connection to a separate station ground was considered normal in those days. I've done the same type of power cord modification Jim speaks of, and it's a practical way of solving the problem.

I added a fuse to the rear apron of my Ranger by removing the Antenna "relay power" Crystal socket.

Image

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 Post subject: Re: Heathkit DX 40 repair thread
PostPosted: Jun Thu 27, 2019 11:41 pm 
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Greetings to Rodger, Mike and the Forum:

Unfortunately, there isn't room in the Ranger for even one fuse holder, let alone two unless they are mounted inside the cabinet on the chassis somewhere, necessitating disassembly of the transmitter to get at them. I can't do Mike's modification because I use the relay power socket for what it was intended: switching antenna changeover and receiver mute relays.

I am not as paranoid about fusing the neutral line as the NEC is. The purpose of the fuse is to protect the equipment, not the operator. A blown fuse in the neutral line isn't going to burn the house down, which is all I care about; operator safety is up to me.

Regards,

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Jim T.
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 Post subject: Re: Heathkit DX 40 repair thread
PostPosted: Jun Fri 28, 2019 2:43 am 
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Joined: Jul Sat 28, 2012 9:32 pm
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Location: Charlotte 28211
What research on this plug situation I've been able to find gives me the idea that this busted-up Elmenco plug was actually the one that came with the transmitter when it was new. So I guess I'll just dig up another one on ebay and put it back to original. My reasoning is that it was good enough back then when they were using stone knives and bearskins, and nobody was killed unless they were transmitting from the bathtub. In my lifetime I grew up when houses didn't even have 3 prong outlets at all. Now we have polarized plugs, 3 prong plugs on things where it is totally unnecessary. A lot of it is a matter of law for the sake of justifying the existence of the unelected bureaucrat decreeing it. But the facts are that they are still the same power company transformers on the telephone pole as they were then, and power came into the house on 3 lines just as then. Neutral is still "ground" as far as all that goes.
I suppose back then when TV's and radios had metal cabinets with one side of the line connected to the chassis, and you had 2 TV's sitting close enough together that you could touch both at the same time, and one was plugged in with the plug flipped, then that would be an unwanted surprise.
Sometimes I wonder if "grounding" isn't taken to overboard extremes of complexity sometimes.


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