The band switch is the weakest point in the E. F. JOHNSON MATCHBOX JR. 275 WATT.
This page is an email reponse to a question from a friend who owned an E. F. Johnson Matchbox Jr and was using it successfully with his Johnson Valiant, then bought a Heathkit SB-200 for his sideband station. The SB-200 had just enough additional PEP output that the E. F. JOHNSON MATCHBOX JR. 275 WATT switch failed.
The Johnson Matchbox Jr 275W will not handle the SB200 when the SWR is high in certain ways. Some kinds of SWR cause current which melts the plastic strips and ruins the coil. Some kinds of SWR cause higher voltages, which generally causes arcing, usually at the band switch. Johnson Valiant was 4 × 150 = 600 W peak, which a stock unmodified Valiant is really working hard to make 100% modulation peaks. SB200 is just enough more, I get 750, 800 PEP OUT on some bands if I push it, with good tubes, because the condenser only filter charges up and makes initial peaks higher. The Matchbox Jr was barely adequate with the Valiant under good conditions. The Johnson Valiant is rated 270 W INPUT CW (189 W OUT) or 240 W CARRIER INPUT AM (533 W PEP OUT). The E. F. JOHNSON MATCHBOX JR. 275 WATT was first offered for use with the E. F. Johson Viking II, which was only two 6146s in the RF final, roughly 100 W Carrier OUT or 400 W PEP OUT. The E. F. Johnson Valiant came out later, with three 6146s, with more power. The smaller E. F. JOHNSON MATCHBOX JR. 275 WATT was being pushed much harder, and under some conditions with open wire line, they could not cut it. That is why Timtron calls the Junkston matchboxes flashboxes. He likes the bigger E. F. Johson Matchbox KW one for 4 KW PEP INPUT for his QRP rigs (1KW carrier or less).
Did I ever mention I hate open wire line? See my articles on broadband dipoles and horizontal loops for a discussion why I abandoned the open wire.
- Do you happen to have the ceramic wafers for the 275 Matchbox in your junk piles…??
- How should I find the arcing point?? My hi-pot will only do 450VDC (old heath cap checker). I can’t see carbon tracks.
You are talking about sneaking up on 2 to 3 KV. If it continues to arc, there will be a small hair size track left that you probably cannot see. But it is there now. In the dark, if you are able to sneak up on the flash point using CW and drive control on SSB rig, maybe you can see the arc tendrils. Do NOT keep the rig keyed thru thet SB-200 for very long, it will not tolerate continuous full power output. If you use a LONG PLASTIC soda straw held near your ear, you may be able to move around and find it by sound. I used to do that with TV sets. I know no way to clean the carbon tracks off. I tried when I was repairing a friend's 32V transmitters loading switch. I actually replaced burned off switch fingers and it worked; you can get the small screws in the Radio Shack drawers. But the carbon tracks? You are up an unsanitary river with no means of propulsion.
Getting red glyptal hi voltage dope in there (if you can even get it any more) would be a watchmaker's job. You cannot just paint it over the carbon track anyway.
Did I ever mention I hate open wire line?
I would suggest going to resonant coax fed antennas, but you may only have space for one antenna. And you are an open wire fan. Lets stay friends and not discuss religion. Open wire is a matter of belief. Maybe use your coax fed loop on 75, and put up a dipole for 40 and up.
See my Multiband-Dipole and
Or a Carolina Windom. Works good 80, 40. Not so good 15 and 10.
So maybe a Palstar tuner is in your future. I have AM friends who use the AT2K with their K7DYY 375W carrier AM rigs. They even have a AT3K now I think. See:
No MFJ tuners though. Possibly a Dentron MT-2000, but do not use the internal balun, use an external current balun.
Or go to a homebrew tuner, "big moonshiner coil, big breadslicers or vac-you-you-you-youm condenshers" (Timtron pronounciation). No switch. Just heavy duty tap clips. On a big piece of wood, so you can change the tap clips. Very old buzzardly looking. Don't let your kitty near it, or it will permanently curl his whiskers!
The weak point is the band switch. The rest of the Junkston Matchbox Jr is not so bad. Now that your switch is wrecked, take it all apart, deep six the band switch, and rebuild it on a board. No reason to put it in a metal box. A grounded metal front panel is good idea for safety. W8JI has some good info on the flashbox and how to improve it. The input link turns can be changed. A 3 gang broadcash band condenshur of 1000 to 1500 pF in series with the ground side of the link (connect it so there are more turns on it). Now you have a Q adjustment and finer control of SWR, with 3 knobs to tweak. Put the series link cap on the back panel of the Junkston Flashbox or on the front panel of your frankenstein Junkston Flashbox. I did one this way that was a basket case, and it helped. Tradeoffs exist between 20 meters and below and 20 meters and up. With heavy spring test clips and wire stubs you have soldered to the coil, you can tinker with the input link taps too. W8JI articles:
Spend some time reading his stuff. He is a good engineer with a well equipped lab. You can learn a lot just sipping wine and flipping thru his web page.
Did I ever mention I hate open wire line?
Anyway that's my old buzzard rant on it. I hope you were able to read it with the correct Timtron intonation and get the spirit of the project.
By the way, take the arc plugs OUT of the Alpha Delta switches. Save them in a plastic bag stapled to the manual in your file drawer. They arc over in high SWR on peaks. Really pisses off a solid state amplifier or the K7DYY AM transmitter; multiple safety trips are not good for them. Disconnect antennas when not in use or if you hear thunder while on the air. That is better lightning protection.
Do not even think about using the E. F. JOHNSON MATCHBOX JR. 275 WATT model on the K7DYY AM rig. The E. F. JOHNSON MATCHBOX KW model is the one to use for that one, if you insist on open wire line. It is rated for 4 KW PEP INPUT to the final. That works out to around 2.8 KW PEP OUTPUT at 70% efficiency, optimistic for a Class C AM Plate Modulated Tube Transmitter. The K7DYY puts OUT 1.5 KW PEP. Under good conditions, the E. F. JOHNSON MATCHBOX KW will be OK. But under NO circumstances should you attempt to do ANY adjustments to the tuner with full power or even higher power. Use a MFJ-212 Matchmaker Noise Bridge and do all the matching off the air with a receiver. Tuning a tuner with full power stresses the tuner and the transmitter. It also makes beaucoup QRM, not cool.
See my MFJ-212 article.
73, and a tip of the Chapeau to WA1HLR, "Timtron"