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L4B/L7 Power Supply Electrolytes:

An Inexpensive Drop-In Substitute

By: Evan, K9SQG

Mark, WB0IQK

Pictures by:  Mark, N8PEP


As we all know, Drake has made some excellent ham gear over the years and I regret that they are no longer making gear for hams.  Drake enthusiasts have been keeping this gear on the air for decades and this article is intended to foster that objective.  In spite of the excellent design of gear like the L4B and L7 linear amplifiers, age, power line transients, etc. can take their toll.  In particular, the electrolytic capacitors in the power supplies can fail from time to time although this is not a common failure.  For those in the know, original equipment capacitors (200 mfd, 450 volt, axial leads) cost $25 or more if and when you can find them.  I’ve developed a means for using readily available substitute (330 mfd, 450 volt, snap-in mount) capacitors that cost under three dollars each and includes the equalizing resistor for each capacitor.  If you choose not to rewire the boards by putting the equalizing resistor across the capacitor, then just leave the resistor off.  I’ve rebuilt one L7 power supply and will soon be doing others.  I hope this information is useful to you and that you can even improve upon what is presented here


  • 1/16 hobby shop brass rod; one 2 1/4” long, another 5 3/4” long.  Watch out for the sharp ends of the rod!  They fly some distance and are a hazard to your eyes when cutting them with wire cutters.  Always use eye protection.

  • Non-insulated butt connectors, for 22-18 gauge wire; from Radio Shack packet number 64-3036.

  • 330 mfd, 450 v Nichicon capacitor from All Electronics, 1-800-826-5432, $2.75 each.

  • 100K, 2 watt non-inductive resistor. Ohmite ceramic composition resistors, available from Digi-Key (1-800-344-4539) and is their part #OY104K-ND

  • Electrical tape, one roll red, one roll black.

  • 3” length of Teflon or other tubing, to fit over 1/16” brass rod.


  • Low wattage soldering iron, about 40 watts, solder, etc.

  • Wire cutters, needle-nose pliers, clamps for holding small parts, etc.


  1. Crimp one butt connector onto one end of the small brass rod; crimp another butt connector onto one end of the larger brass rod.  Solder both butt connectors to the respective rods.

  2. Take the smaller brass rod, and make a 90 deg bend 1 1/4” as measured from the butt connector’s soldered end, not the unused/open end.

  3. Take the longer brass rod, and make a 90 deg bend right 1/4” from where the rod exits the soldered end of the butt connector. 

  4. Now the longer brass rod is going to be “bent back on itself” to form a U shape.  The “bottom” or trough of the U, opposite what will be the open end of the U, should be bent such that the two sides of the U are approximately 3/8” on centers.  The exact dimension depends on the thickness of the insulating tubing.  If the tubing is thick, like fuel line from a hobby shop, then the distance between the centers should be wider.  Practice on some scrap wire or brass rod to see what is appropriate for your parts.

  5. Take the insulating tubing, and start sliding it over the non-butt connector end of the brass rod until it comes to the trough of the U, but don’t push it any further.  This will serve as added insulation between the rod and the case of the capacitor.

  6. Take the shorter of the two rods and slip the open end of the butt connector over the POSITIVE (+) terminal of the capacitor.  Orient it such that the bent lead faces outward, and away from both terminals.  Crimp the butt connector to the terminal of the capacitor, and then solder it to the terminal.

  7. Take the longer rod, and slide the open end of the butt connector over the NEGATIVE (-) terminal of the capacitor.  Orient it such that it is facing away from the two terminals with the long dimension of the rod, with insulation, along the length of the capacitor.  Crimp the butt connector to the terminal of the capacitor, and then solder it to the terminal.

  8. Take the roll of the BLACK electrical tape and use it to secure the insulated brass rod to the case of the capacitor at the NON-TERMINAL end of the capacitor.  Take the roll of RED electrical tape and use it to secure the brass rod to the TERMINAL end of the capacitor.  The tape not only secures the brass rod but also serves as an easily recognizable way of identifying the positive and negative polarities for the respective brass rods.

  9. Solder the equalizing resistor between the two terminals such that one end is near the 90 deg bend of the shorter brass rod and the other end is near the trough region of the longer brass rod.  Use standard procedure of wrapping the wire lead around the brass rod, cut off the excess, and then solder

  10. Next, orient the capacitor near the power supply board to be used as a “spacing template” and put a 90 deg bend in the longer brass rod so it faces the same direction as the shorter brass rod.  That is, ensure both brass rods are facing the same direction so that it will be a drop-in replacement for the original capacitor and equalizing resistor.  Note that after bending, the center to center distance of the rods is about 4 3/8”, the approximate distance between the brass eyelets on power supply board. (see Figure 1)

  11. The drop-in assembly can now be inserted into the power supply board, and the brass rods soldered to the respective eyelets.  After cooling, cut off the excess brass rod. NOTE that the cut off ends of the rod will be sharp and fly some distance when you cut them off!  Always wear eye protection.

This method will save quite a bit of money over the standard equipment, axial lead electrolytics that sell for about $25 each, if and when you can find them.  Note, if your supply is an early vintage with 600 ma rectifiers, it might be a good idea to upgrade them due to the higher capacity and surge current of the newer caps.  I used 3A 1 kv diodes. 

Enjoy those Drakes! 

73,   Evan