ELF II AP-1 power supply

Most recent revision of this page Mar 24 2025(c) Herb Johnson. This Web page describes the Netronics Elf II AP-1 power supply. Most photos courtesy of the Ed Keefe estate and some taken by Josh Bensadone. I'll build a similar unit for an Elf II I'm restoring from Will Dodson. Work in progress on his Elf II modifications are on that Web page. Check the other ELF II I worked on (don't own) at this Web page. Web search will find many pages on my site about the 1802 COSMAC, including the Lee Hart 1802 Membership Card pages - Herb

Elf II AP-1 power supply

Netronics produced the Elf II computer in the early 1980's. It's designed around the RCA COSMAC 1802 CMOS processor, and many of its components are RCA 1800 family IC's or are 8-bit microprocessor ICs (RAM, ROM, I/O devices) of the period. Here's a portion of an ad for the AP-1., a 5VDC and 20VAC power supply for the Elf II. This power supply is relatively simple but has a few features to be considered further. My intentions are 1) to document it and 2) replicate it in a visually and electrically similar fashion, here in 2024 and almost half a century later. - Herb Johnson

Operational details

Here's the assembly manual and schematic for the AP-1. It describes a simple unregulated 8V DC supply which also brings out about 18-20V (peak volts) AC from the secondary. It doesn't describe the details of operation, which need to be identified in order to replicate it 45 years later without the original parts.

Here's some photos of the AP-1. They are from a unit restored by Ed Keefe, whose estate now provided the photos and has units under repair by others including Josh Bensadon, who also took some of these photos.

The interior of an AP-1 from above, before the capacitors were replaced.

Side view, after capacitors were replaced.

Another side view.

The essential feature is a AC line to 12.6V CT (center tapped) AC transformer, which also has taps on the primary (line voltage) side. The secondary drives a full-wave bridge (four diode) rectifier into a large electrolytic capacitor. This produces a DC voltage which when lightly loaded would be around 8 volts DC. But that DC voltage depends on the AC line voltage, and on how much current is drawn from the large cap. Too much current will produce 120Hz ripples (variations in DC voltage). A note in an ad from the era says it supplies "8 volts DC at 3 amps". The manual claims 8V at 5 amps". The 8VDC is regulated on the Elf II cards and motherboard by 7805 TO-220 5V regulators, to provide regulated 5V to the digital logic. Also the AC voltage across the full secondary winding (20VAC peak, 14V average) is available to be rectified and DC regulated to +12V and -12V for other circuits in the ELF II.

That's the general description of the AP-1 supply and it's use with the Elf II (and other Netronics products such as their video terminal). MOre details are needed to replicate the unit and to understand why it performed as it does.

AC DC voltage adujustment, current capacity

8 volts DC is barely enough to supply 7805 and other analog regulators, enough voltage margin for stable 5-volt operation. (Thanks to Lee Hart for that observation.) However, because of the additional primary taps, the AC line can be attached to more or less turns of the primary winding, to change the output voltages. Less turns, reduces the ratio of primary turns to secondary turns: that increases the secondary (nominal 6.3VAC) voltage. More turns, increases the ratio, reducing the secondary AC voltage. The design includes switches to connect one of the primary taps to the AC line.

Here's a view of the back of the AC side switches. Instructions in the manual says "adjust switches to 7.5V to 8.5V DC". This image of the unassembled AP-1 transformer, shows the variety of wires - windings and taps - of the AC transformer. Colors identify the windings and taps. Josh Bensadon measured the transformer: "3.1" wide, 2.6" tall, and about 2.6" thick (with the windings), the core is just 1.1" thick." There's a number "3234" on the transformer, no other marks.

The historic reason for taps in an AC line transformer primary, was the low AC line voltage available in some parts of the USA in the 1970's and earlier. AC voltages near 100V were not uncommon. In the 21st century by contrast, 120V AC is pretty common, and so untapped "6.3VAC" transformers expecting 110VAC or lower may produce higher AC (therefore DC) voltages.

On this linked Web page I found this description of AP-1 repairs and tests from Aug 2023, with modern replacement capacitors (I hope of similar capacity). (The primary AC voltage wasn't given, it's likely 120V AC.)

Testing done at no load and with a 2A constant current load, [DC volts with ripple in RMS volts]:

Switch Settings (1/2/3) 	No Load  	2A Constant Current Load  
L/L/L 			8.26 V with 3mV 	6.57 V with 152mV
H/L/L 			9.55 V with 4mV 	7.86 V with 153mV
L/H/L 			10.5 V with 4mV 	8.72 V with 154mV
L/H/H 			12.1 V with 4.5mV 	10.45 V with 155mV 

Cabinet details

Photos courtesy of Josh Bensadon and the Ed Keefe estate.

Unassembled AP-1 kit showing unassembled cabinet, transformer and parts bag.

Unassembled front panel of AP-1.
Unassembled back panel of AP-1.

Josh measured the cabinet: "The box is 5.3" wide, 5.9" deep, 3.2" tall (with feet) 3" without feet.". It's a folded rectangular aluminum cabinet of the period. In the 1970's 80's era industrial products were painted "powder coated" or "splatter coated", which produced a specific "pebble" texture to the paint surface. I think the finish shine would be called "matte" or "semigloss". Certainly not flat or gloss.

Paint, color, texture

The color is a blue of the era, called variously IMSAI blue, DEC blue, IBM blue etc. There's lots of chatter on the Web about identifyting this color-of-blue today. I see references to "IBM blue RAL 5012". An IBM blue color is identified as "RGB 75 107 175, CMYK 75 43 0 0, Pantone PMS 2718 C". If you have another cabinet of this color you might take it to a paint-store or department for a color-match specification. Or grab paint sample chips but be sure to know your light source (incadescent, sunlight, LED color temp) when comparing.

You can chase this around and around to find a modern supply of paint by color and a means to apply it. Plan B is to buy a vintage blue cabinet. Plan C is to find locally a decent can of blue spray paint of approximate color. A popular spraycan brand is Rustoleum and they have some textured finishes. Experiment on a testing aluminum surface! Or pay someone to do it better.

master class on spatter and powder coating

By request from Jon Chapman, who reproduces MITS Altair grey cabinet covers of similar quality:

The coating on the original Optima [produced, for MITS Altair] chassis and likely what's on that ELF supply is called "spatter coat" -- it's produced by turning up the volume on a paint spray gun until it starts spitting drops with the mist spray, letting it sit a minute, then wiping the excess off with a squeegee. Sometimes it's followed with a second, non-spatter coat to keep the color even.

The powder coat on the reproduction Altair tops is a textured powder coat. It's a Sherwin-Williams product that they can not only match for color and glossiness, but also for texture. There are of course limits to the texture match, since it's a somewhat random process and each one is a little different It's not like they're laying down a repeatable pattern.

Powder coat is of course a plastic coating. It's applied by putting a HV static charge on the part to be coated, then spraying the plastic powder onto the part with a special gun. The part is then baked, which melts the plastic powder and bonds it to the base material.

Can this finish be approximately produced from a spray can for home-players?

Not possible with a spray can, at least not with any consistency. It's doable with a home shop paint sprayer, the kind that runs off an air compressor, but I'm told it requires considerable practice. - Jon Chapman

Copyright © 2025 Herb Johnson