DX-40 Servicing Part 1

The Heathkit DX-40 was a mainstay of the 1960's Novice Ham Radio Station. Let's bring a problem unit that was cast out, back to operating condition, and without modifications.

[@acestudioscouk-Ace-G0ACE]

A lot of work has gone into making this video; I really appreciate it and learnt a lot. I'm finding I enjoy the history as much as the electronics.

[@MIKROWAVE1]

A New ham is now using that rig!

[@acestudioscouk-Ace-G0ACE]

@@MIKROWAVE1 That's really good, I'm glad to hear that!

[@PapasDino]

As a teen I got ahold of the 600VDC plate supply of my homebrew Novice transmitter and found myself half way across the electronics shop...so true about thankfully letting go to survive! 73 - Dino KL0S

[@cw4karlschulte661]

Enjoyed it. USAF schools on Tropo (10 KW at UHF) HF high power rigs taught me safery earky. Have restored many xmtrs and c lassic radios to live for a new generation. You did this, part one and two, very well! 73 WA2KBZ

[@MIKROWAVE1]

Thanks - I need to get this thing back in the rack and off the bench!

[@LarryDeSilva64]

I still have my Heathkit DX60 with me that I had when I was a novice. Very good step by step diagnostic work. Thanks for sharing

[@alancordwell9759]

Great video Mike, and point very well made about the high voltage. I used to take a lot of risks and I've had some healthy 'belts' in my time including one from our UK 240V mains that I'll never forget. 40 years on I guess I'm older and wiser now; the thought of it makes me shudder. Very best 73 de Alan G0NFY :-)

[@scharkalvin]

Those of us that grew up on tube equipment learned to respect it. The highest voltage I ever worked with was a bit over 4000 volts in the plate circuit of a pair of 4-400A's in a home brew linear. I had plenty of pilot lamps to warm me that the plate transformer was energized. After shutting down the power supply (and pulling the ac plugs from the outlet), only then would I open the lid to work inside. I'd also first watch the needle on the plate voltmeter fall to the left peg, and then I'd grab a long, well insulated screwdriver and short the plate cap of one of the tubes to ground. (I never drew and arc) Some people would put in an interlock so the plate supply would not power on with the lid off, I never installed that, but there was an interlock that would not apply plate power unless the tubes had heater power, so those filaments were a WARNING that HV was present!!!! (Actually I could turn the HV off with the heaters on, but I still had the HV meter and pilot lamps). Worst shock I ever got (as a teen) was line voltage hand to hand. Didn't last more than a few milliseconds before I was spat out of it by muscle spasm. Was trying to splice a hot circuit. Stupid!!! A friend of mine once absent minded did try to short out his HV supply to work on a linear before the caps discharged (open bleeder resistor?) The size of a 3500 volt arc will put the fear of god in you (and weld a screwdriver to the chassis!). I also have witnessed large electrolytic caps under go RUD (rapid, unplanned disassembly).

[@janetwinslow2039]

I have two DX40U's. Mic socket is on the front, and no filament bulbs. Both are neons. There are other circuit differences too.

[@stephenwilliams5201]

Cheep baofeng Mike. Came to me. Bad soldering. So I took a phone camera. And photos helped out. During job. Photos. did well. Tks om . De kv4li

[@superhet7281]

Excellent overview, and nice repair job on the function switch. FYI, if the crystal door is missing (which it often is) you can find an exact replacement on vintage Heathkit “lab” scopes of that period.

[@NebukedNezzer]

familiarity breeds contempt.

[@markdonaldson5834]

Mike, great video as always. One thing that I do when bringing up an unknown HV power supply on a Variac is to substitute the tube rectifier with one of those plug in solid state rectifiers. That way I can bring the voltage up very slowly and perhaps, if I'm lucky, reform the filter cap, without having to deal with the filament coming up on the rectifier tube. I never bring the Variac to full line when doing this because I do not want to over voltage anything but it certainly helps with peace of mind when first bringing up a piece of vacuum tube gear where I can bring up the B+ to a low voltage and check for problems before parts go up in smoke.

[@MIKROWAVE1]

Mark that is brilliant. It would be great to know before you hit 50% that something is not going to pop.

[@2pi628]

Never shock by a transmitter but was knocked thoroughly on my ass while working on a heat pump. Lesson learned. Complacency kills.

[@MIKROWAVE1]

Wow. Considered one of the greatest technical folks ever at the ARRL, Ross A. Hull was electrocuted in a foolish accident with a power pole transformer under his bench!

[@NebukedNezzer]

an 80 meter command tx has a 3.5mhz calibration crystal

[@easyeagle2]

You said `One tube v1` installed with 6.8 vac on the filament? That`s about right. The voltage should come within tolerances with all the tubes installed. Also to get an exact ac voltage from a meter is difficult unless its a guaranteed 1% instrument. .

[@mcawrse]

Mike, I am curious what your filament voltage went to when you installed the rest of the tubes. I have a DX-40 in my collection of equipment for future restoration. Years ago I had 300vdc in one arm and out the other... quite a wake up call. It didn't prevent me from later getting my ham license :-) Love your videos! 73 and keep up the good work!

[@MIKROWAVE1]

It did go down to around 6.45VAC; still a touch high, but no worry.

[@W4BIN]

He can't believe they built this without a fuse, if it is anything like my DX100 the fuse/s was in the power plug. Ron W4BIN

[@walk-tall-hikes]

At the beginning of the video he made note of the fact that the AC chord had been replaced. So if there was a fuse in it it's long gone. Perhaps his surprise was that whoever went through that radio and recapped it and put the new cord on it did not put a fuse in it.

[@cw4karlschulte661]

I bet solid state VFO was used, running of the fil voltage, with rectifier in vfo. HV resistor may have been removed to avoid accidents. 73

[@MIKROWAVE1]

There were a lot of solid stae conversions. I wanted to see what I could do with taming the original valve circuit.

[@vincei4252]

The worst shock I've ever had was something stupid I wired up at shoulder height on a the kitchen outlet to power a cassette player that took 240v power on 2 prongs. Joking with my teenage cousin I was laughing and walking backwards towards the outlet not wearing a shirt. I still have the scar on my shoulder. The stupidity of youth.

[@stephenwilliams5201]

T storm. Near by bolt. Did the honors. At the club.

[@MIKROWAVE1]

Show off! That is a convincing demo (getting electrocuted by lightning) at the usually boring ham club project meeting.

[@stephenwilliams5201]

@@MIKROWAVE1 meeting called and a 40 meter line was still hooked up. To his rig. During the meeting. Pow. Zapp. That building was emptied out . Quick. Will never forget that. I have a real respect for storms to this day. Specially when viewing the remains. Hi hi. De kv4li.

[@K1OIK]

Check for sparks with lights off. Why use a digital meter? Jumps all over.

[@MIKROWAVE1]

Ha - why don't I have real test equipment? Like a heathkit VTVM? or a real generator from at least the 80's? I don't know. All I can say is I use what I have like a lot of junkbox hams do. I keep the pro stuff at work. Last week I was using a PNA-X 4 port Network Analyzer that cost 500K, to measure some filters and amps at work. Now that was boring.

[@K1OIK]

@@MIKROWAVE1 I have a Simpson 260, I trust it more than digital meters. Especially over varying voltages like you had

[@MIKROWAVE1]

@@K1OIK The 260 is a classic!