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  Pyramid PS-15KX Evaluation
By Robert W. Meister WA1MIK
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I was perusing a popular auction site over the Memorial Day weekend and ran across a Pyramid PS-15KX linear power supply. The seller claimed it had no output, even though the front panel "NORMAL" LED was lit. The starting bid was under $8 so I put in a low bid and promptly forgot about it. Memorial Day morning I was surprised that I won it for the starting bid price. I received it today.


This was my first experience with any Pyramid power supply. The unit looked brand new to me. It's rated for 10 amps continuous, 12 amps surge. I took the cover off and noted that the LM723 regulator IC was soldered into the circuit board, not socketed. This seems to be the way Pyramids are built. The main filter capacitors were physically and electronically much smaller than on similar Astron supplies; all parts are soldered to a roomy single-sided circuit board. All the wiring was nicely bundled with ty-wraps. Although the wire insulation was thick, I suspect the conductors themselves were on the small side. Two 2N3055 series-pass transistors and a thermal cutout switch are mounted to a heat sink on the rear of the chassis. That heat sink can be removed to access the transistors, which are not socketed. A 4A recessed fuse and a permanently attached 5ft long #18-3 power cord complete the package. The negative output terminal and the cigarette lighter receptacle are connected to the chassis, which is grounded through the power cord.


I plugged the unit in and turned it on. The neon bulb inside the power switch lit, as did the green "NORMAL" LED on the front panel. A quick check with a voltmeter showed 13.9V on both the binding posts and the cigarette lighter receptacle. Seems fine to me.


I put the unit on my dynamic load and ran that up slowly. Somewhere between 14 and 15 amps the unit went into current fold-back mode. At this point the output voltage and current was reduced to nearly zero, the green "normal" LED extinguished, and the red "PROTECTION" LED lit up. I removed the load, the output voltage returned, and the LEDs reverted to normal indication. I adjusted the output voltage to 13.80V using the only adjustment pot inside the unit. I then measured the DC and AC voltage and current at several points, tabulated below.

OutputOutputOutput OutputInputInput InputEff
0.013.800.0 <11200.1 11---
2.013.7627.5 <11200.6 6342
4.013.7254.9 <11201.1 11448
6.013.6781.4 <11201.5 16450
8.013.62109.0 <11202.0 21052
10.013.59136.0 <11202.4 25553
12.013.54162.5 521202.8 30354
14.013.32186.5 7501203.2 34854

I measured the DC voltages on the LM723 regulator IC as well as the raw unregulated DC feeding the pass transistors, at no-load and full rated load currents. These are tabulated below, along with an equivalent Astron RS12M supply's voltages. Note that the PS-15KX was adjusted for 13.80V while the RS12M was adjusted for 14.00V at the output terminals.

#0A10A 0A9A
213.1413.21 13.3614.21
313.8013.76 13.9913.99
47.137.08 7.097.09
57.137.09 7.107.09
67.137.09 7.107.09
70.000.00 0.00.0
1014.9516.55 15.1216.70
1132.6123.76 29.2126.07
1233.5525.31 29.3826.70
1316.2617.90 16.4118.02
Vraw25.7019.05 23.1218.74


The output voltage drops noticeably at higher currents due to undersize DC wiring to the banana jacks and cigarette lighter receptacle. The output ripple was acceptable at 150mVAC or less up to the rated current.

Contact Information:

The author can be contacted at: his-callsign [ at ] comcast [ dot ] net.

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This page originally posted on Thursday 01-Jun-2017.

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.