28V Military Power Supply

Let's take a Junk Power Supply Foundation and make a 24-28V 10-15 A Linear Power Supply for under the bench.

[Dr.Pepper]

I have fond memories of repairing power supplies. In 1967 I was a Marine based at the Futema, Okinawa Marine Corps Air Base and we did repairs on the TRC-75, which was a high powered Single Sideband transciever. The unit mounted in the back of a jeep. It put out 1, 000 watts of SSB power that would fry you if you touched the antenna during transmission. I repaired the power supply. It had a large circuit board covered with power transistors. In the field (Vietnam) often a filter capacitor would short out and blow out the entire bank of power transistors. The resulting heat would scortch the circuit board so badly that I had to replace the board, then rewire it with the 24 transistors. It was tedious work but I enjoyed the hell out of it.

[waaos]

Hi Mike, great project and thanks for sharing.

[JCWise-sfww]

Hello Mike, liked seeing you working on building a regulated power supply for a change of pace from radios.

[acestudioscouk-Ace-GACE]

Thanks for this interesting change from working on radios. I have several military radios, so the information was good for me and I'm sure, many others.

[adzbasslines]

Good to see these videos. I just designed, built and bench tested a HT PSU for a 5w Paraset CW receiver/transmitter. Those power transformer commercial oven type 12v / 0-400v toroidal transformers are great used backwards with a modern solid state square wave oscillator running a boost converter (instead of using those vibrator type spike oscillator cans), filtered and regulated -- works great!

[toddanonymous]

Nice project. You might want to hang a crowbar circuit on the output. Of course those old military radios are not as fragile as our present rigs. Maybe one of those old Kepko external crowbar devices that connected across the output terminals, and were adjustable. If those pass transistors failed short, the radio may think it got hit by a German 88 mm.

[paulphoad]

Brilliant recycling. Was thinking about the dissipation of pass transistors at current limit. Current foldback is messy but Ill throw in my 10 cent suggestion. Take the collectors of pass transistors through a couple of automotive bulbs. These, as you know, have a 10:1 resistance hot/cold. So maybe 2 to 20 ohm...dunno. trial and error, series parralel etc.
So at current limit the bulb(s) come on and drop some of the voltage appearing across the CE of transistors. It wouldnt effect regulator because the chip is connected (via the diodes) directly to 40v.
Also, the collectors, being current sources will maintain the current limit value irrespective of the bulb resistance.
Anyway....Car bulbs are getting hard to find with everything going leds.
Love your channel
Paul tech teacher Australia.

[AdamosDad]

Very good ham work, doing builds like this. I was lazy and picked up a 14volt 100amp supply from a mobile home system, for my shack.
I don't have any military equipment at this time. I realy enjoy your work and as a former military electronics tech, I like your approach.

[khalidtarawneh]

I can't Thank you enough for this, thnx for everything Mikek

[WRMD]

Great looking supply. Now you need a 400 hz 117 volt supply as well, for future projects, or just because they sound really cool! Great job including the schematics. You always do a great job approaching a project from multiple angles to see which one suits you the best. I like this approach, verses just showing one schematic and not even explaining how it works. There are designs in the old ARRL handbooks for power supplies, but a lot of the HB's I feel are written for people who "already know", particularly the older HB's. It seems as if they wanted to keep electronics privileged and arcane information.
Up here in Maine, we have a holiday in which once a year people put all of their larger "trash" on the side of the road for pick up. One year this old farm house in Rumford ran a dairy farm that had these stainless steel boxes with 24v 10 amp or so transformers and bridge retal- fryers and circuitry in them capable of making so NICE power supplies. They were in near perfect shape. Also in the pile was a Stewart Warner R-1911 (built by Knight?) AM broadcash receiver with a vibrator power supply in it. The wooden case is long since gone, but the tubes are all good. This year (about 5 years later) at the same house there was a single "ST" type 80 rectal -fryer in the pile, but no more radios or boxes. The tube tested good, for what's that's worth. Thanks for the great videos and bring this WW2 equipment back to life. You videos are always interesting and full of knowledge. 73.

[Very_Dark_Engineer]

Nice hardware. LM317 + PNP pass transistors still my best choice, this combination gives me less drop voltage so i able to use maximum voltage from transformer with good stabilisation.

[robertmeyer]

interesting re-build. LM 317T is so common. lots of ways to make that work. LM 741 op amp can be used. just more parts. LT 1083 reg IC work as well. I would have made a new LM 723 board with modern 14 pin dip one. some times you just use what is easy and parts on hand. for old 24V radio's a linear supply would be better. I made 60-65V supply with LM 723 . and 48V as well. their is a trick to float IC off ground to work higher voltage. and if you float 723 below ground. you can make bias reg for RF power transistors like MRF 454. I learned lots of tricks with the LM 723 over the years even made a 200 amp 14V with one. lots of pass transistors. 30-2N3772 . still works well I made in 1988. 240V in. some voltage sag will be fine with them old 24V radios. they run on battery system. you can use basic crowbar SCR circuit for over voltage. maybe some MOV on AC line with that filter from your old one. re use what you can. maybe you can find old working volt meter and ammeter . will be nice touch for old radios . nice project ! 73's

[stanleytimmerman]

Trying to get a ww2 aircraft radio to work. Also had to build 28 volt power supply. The 28 volt power supply works however, the unloaded dynamotor output is 140 vdc instead of rated 225vdc. Do dynamotor's permanent magnets weaken?? I have done the normal maintenance on dynamotor. Thanks for your videos.

[MrMe]

I recently bought a r392 and it came with a power supply. Sadly the PS was bad. I posted on ARF about it. It was a kepco and had a shunt on the transformer. A real odd design. So I have this big transformer I picked up to do a project like yours but I may try to see if I can power the matching transmitter as well as the transformer is at least 15lbs or more. In the mean time I took two old cb radio power supply's and put them in series. It works. Now I know the radio works.

[TheElectronicDilettante]

Isn’t it 40v differential voltage? Wouldn’t that mean if your max voltage is 40v it’s over spec?? I’m just learning and this is a genuine question.

[bazzaar]

There's no need to be so concerned about the input voltage to the LM317. The spec is for differential across the device, not absolute voltage. As long as no two pins of the LM317 have more than 40V across them you are OK. The LM317 has no connection to ground or 0V, so it never sees the full unregulated DC. You could dispense with the diodes on the input to tidy the build up a bit.
73 de G0AFV

[TheShawna]

Yep got to get the GRC-9 dy=105 Dynamotor some sort of 28 volt power supply.Thanks for the talk we had about it at at Near-fest can you let me know what size Capacitor you're using for yours?thanks, 73 Jim KB1PFL

[bradbuck]

Looking at the ammeter, it read past the 1.5 amp rating of LM317. LM338 is rated up to 5 amps, might work better?

[Capecodham]

Power supplies are not allowed to smoke unless they are 21 years old.