Drake AC-4 Power Supply Rebuild

The Drake AC-4 power supply was used to power a number of transmitters and transceivers in the drake "4" line. I had not seen any pictures of the AC-4 I obtained prior to purchasing it. I figured, it is just a power supply, how much can go wrong. When I received the AC-4, and went over it, I did find a number of questionable things.

This page describes my experiences rebuilding the AC-4 to be placed into service.

It is worth noting, I used replacement capacitors from the Hayseed Hamfest Company as I was looking for a more original rebuild. While the AC-4 capacitor kit is not yet on the Hayseeed web site, you can send them an email for all the details.

An alternative to the Hayseed kit is The Heathkit Shop AC-4 Power Supply upgrade. There is a good page describing the Heathkit Shop upgrade installation here.

As an aside, I am planning to use the AC-4 with my TR-3, as the AC-4 power supply has a connector for the ALC, which I intend to hook up when I get my L-4B all ready to go. I got the AC-4 so I could just hook everything up without having to "work on" the connector on the AC-3. Well, that was the plan...

The starting point

An I mentioned, I did find some things which, for me, needed attention. The first was the already replaced capacitors. The second was a loose wire from the power transformer secondary.

While the capacitor replacements would have worked, I thought they just looked ugly. So I figured I'd replace them. Looking at the various options, the Heathkit Shop upgrade probably being the most well known, I also checked various suppliers of multi-section capacitors.

It was during this search I sent Hayseed an email to see if they had an AC-3 or AC-4 set. (They have replacement capacitors for a number of Drake units.) The response said they did have some available, but the information had not yet been put up on their web site.

After thinking it over a bit, I decided to order two power supply kits and a TR-3 kit; since, in addition to the AC-4, I have a TR-3, RV-3, and AC-3. The picures below show the rebuild as it progressed.

Here are a couple of pictures of the AC-4 as it arrived; needing some TLC.

Prior to starting disassembly, I took several pictures of the original wiring and layout for reference. This is something I strongly recommend to anyone attempting to rebuild just about anything: take lots of pictures.

Not pictured is the metal cover to the AC-4.

After disassembling the power supply, I was left with a bare chassis. Here are some pictures of the chassis after I have installed the capacitors, gromets, terminal strips, and some other miscellaneous bits. I decided to replace the gromets protecting the power transformer leads, as the original ones had gotten brittle, and had started to get some significant cracks in them.

I cleaned the chassis with simple liquid detergent and warm water. I did not make it perfectly clean, as I wanted some of the "character" to remain. I used paper towels and compressed air to make sure the chassis was dry.

Here is a picture after installing all the components and finishing up the wiring. While I did reuse many of the original components and wiring, I did opt to:
  • Replace the original two-wire power cord with a three-wire grounded cord in the interest of safety.
  • Replace all the diodes with 1N4004 diodes. No major reason other then I had them around.
  • Replaced the 10k 1/2w resistor as the leads of the original were too short with the replacement capacitor.

One thing I did discover when wiring everything back up was the color coded wire in the multi-contuctor cable (to the TR-4) did not match what was listed in the TR-4 manual. Not a real big issue, but it did mean I had to check and verify all the wires to the connector to identify the color of the wires being used.

Here is picure of the rebuilt AC-4 from the top. You can compare this with the first picture to see the differences.

Phase 2 Work

Before placing the newly rebuilt AC-4 back into service, I wanted to make one more change. I wanted to add a relay in the power transformer primart circuit to reduce the amount of current being handled by the On/Off switch on the TR-3/4. As a point of reference, prior to the relay upgrade, the load on the On/Off switch contacts was a minimum of 325 mA (power supply unloaded) and after the load was just over 10 mA. This is a sgnificant reduction in the load on the contacts.

The materials I used to add the relay were:

I had originally planned, and ordered, a socket for the relay. Unfortunately, the dimensions of socket are not given, and when I received it, I discovered it was much too wide to fit in the AC-4 chassis. Fortunately, connections to the relay can be made using standard 0.25 inch quick disconnect (crimp) connectors; available just about everywhere.

The picures below show how I added the relay.

The first step consists of rerouting the wire going from L1/C11 to the power transformer. In the picture, this is the wire skirting the foam mounting tape.

The second step was to make a mounting pad/cushion for the relay. This was originally planned as just a cushion, but that changed; which will be described below.

Here is the relay ready for mounting. I used black wire for the relay coil connections, and brown wire for the contact connections. In addition to the cushion on the chassis, you can also see some of the mounting tape on the top of the relay.
Here is the updated schematic, showing the circuit changes used to add the relay. Here is a list of the changes:
  • Disconnect the brown wire from the terminal strip having the connections to the power transformer, and add a quick disconnect connector. This will go to one side of the relay coil.
  • Make a small (4 inch) wire with a quick disconnect connector on one end, and strip and tin the other end. Solder the tinned end to the power transformer terminal strip, where the (Drake) wire from L1/C11 is connected. This will go to the other side of the relay coil.
  • Make a small (4 inch) wire with a quick disconnect connector. Solder the tinned end to the fuse connector, at the same point as the (Drake) blue wire. This is connected to the common relay contact.
  • Make a small (4 inch) wire qith a quick disconnect connector. Solder the tinned end to the power transformer terminal strip, where the (Drake) brown wire was removed at C10. This will connect to the normally-open relay contact.

It is worth noting my AC-4 wiring differed a bit from the schematic. Instead of C12 being connected to L2 at the same place on the fuse holder, it was connected to the other side of the fuse, along with the blue wire.

Here is the relay, all connections made, in place with some foam mounting tape. I also put unused quick disconnect connectors over the unused lugs on the relay. No sense having possible failure points expose.

Some of you may notice a problem with this installation. More below.

Here is the relay, the same as the previous picture. Except... it has been moved away ffrom the chassis to allow the base plate (lip) to fit between the relay and the chassis. This was the problem mentioned in the previous picture.

This also necessitated a slight change in the relay mounting. Instead of just using some foam mounting tape to hold the relay to the side of the chassis, I had to remove that tape and remove some of the backing on the cushion, making it the new mounting pad for the relay.

Here is a close-up of the relay and wiring. I've added some arrows to show most of the connections. The original (Drake) brown wire connection to the relay coil is hidden under all the other wires.

You can see the new brown wire I added, and the two black wires I added. I also identified the original (Drake) blue wire.

Here is everything set to be closed up. In addition to the mounting pad on the chassis for the relay, I also added some mounting tape on the cover to misimize movement of the relay. The tape appears to be offset because the cover will be flipped over to be attached to the chassis.

I hope you have enjoyed hearing about this project. It was a fairly straight forward, and easy, project. I'm sure it will last through many more years of service.


Mark, N1VQW

January 8, 2012