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Author Topic: Pyramid power supplies  (Read 2249 times)
K1CJS
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« on: June 14, 2004, 09:52:22 PM »

I saw elsewhere on this site where the Pyramid brand power supplies are not recommended by some hams.  What are the discovered problems with these supplies?  I have been using them for a while now and want to avoid any problems that may come up.

I'm running a PS-21KX, 20 amp output, powering two VHF rigs, one a 2 meter rig that draws about 9 amps on transmit, and one a dual band rig that draws about the same on transmit.  I usually have only one on at a time, but have run both occassionally for ARES/RACES work.  

The other is a PS-12, 12 amp output, that supplies 13.8 volts for peripheral equipment, I'd say the total draw on that one is about 6 amps.

Thanks in advance for your replies.
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AC5E
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« Reply #1 on: June 14, 2004, 10:31:49 PM »

The major problem I have with Pyramid supplies has been the amount of money they have cost me to get things fixed after they failed.

While I have had to repair a couple of my Astrons they didn't go into "full bridge output to the load" mode and destroy the equipment they were connected to.

73  Pete Allen  AC5E
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AG4RQ
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« Reply #2 on: June 14, 2004, 10:41:44 PM »

I've owned a Pyramid Phase III PS-25 power supply since 1995, the year I became a ham. The only thing that went bad was the SCR in the crowbar circuit. That failure occurred 6 years later in 2001. It wasn't much of a problem. I obtained a replacement SCR, put it in and the power supply has been like new since. My Pyramid never burned up any of my radios.
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AA4PB
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« Reply #3 on: June 15, 2004, 06:54:23 AM »

I've had a Pyramid PS-35 here on the bench for over 10 years now without a problem. On the other hand, I've had an Astron fail and put 28V on everything, wiping out both the Rx and Tx in a digipeater system. A pass transistor shorted, the over-voltage SCR fired but it opened because it was not able to handle the large current trying to discharge the big caps. The Astron had no fuse in the line between the caps and the SCR & output.
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AG4RQ
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« Reply #4 on: June 15, 2004, 08:35:52 AM »

AA4PB, my SCR shorted and shut the power supply down. With a shorted SCR, my power supply wouldn't power anything until it was replaced.
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AA4PB
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« Reply #5 on: June 15, 2004, 08:49:51 AM »

All I can tell you is that in my case the SCR in the Astron opened as soon as it fired and allowed 28V to go to everything. After that experience I added a fuse in the DC line just ahead of the SCR so that if it fires the fuse opens and the SCR doesn't have to try and discharge the big caps.
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AA4PB
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« Reply #6 on: June 15, 2004, 09:06:41 AM »

I'm not knocking Astron power supplies, by the way. I've been using them for years and this is the only one that I've had fail. I have no idea what percentage of over-voltage failures cause the SCR to open as mine did. I simply offer as a general suggestion that it is a good idea to have a fuse ahead of the SCR to guard against that possibility. I notice that several of the power supply designs in the ARRL Handbook over the years use a fuse in that manner.
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KE4MOB
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« Reply #7 on: June 15, 2004, 10:18:51 AM »

I've had to replace the SCR in almost every Pyramid that has crossed my bench.  Usually, the owner says "I just turned it on and had no voltage whatsoever.  It was working fine the last time I used it."

The filter caps usually are only 25V...out of a 28V transformer.

The factory soldering job sometimes leaves a lot to be desired.

I've seen failures of Astrons, too, but the Pyramid has the Astron beat about 2 or 3 to one when it comes to failures.

Interesting, because they are all pretty much the same inside (with the exception of the filter caps).





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W7DJM
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« Reply #8 on: June 15, 2004, 10:38:17 AM »

Years ago, I bought a radio from a VERY technically competent person, and it came with a Pyramid 20-25 amp supply.

Took it home, hooked it up, plugged it in. NOTHING. EVERY SINGLE pass transistor (6?) were open.  The regulator was b.o.'ed  (You ALL know what "b.o'ed" means, don't you?)

I still have this supply, but haven't done anything with it since my little house fire.  However, seems to me I've replaced at least two regulator chips in this thing--I finally installed a socket during the last replacement.

The only failure I've had on several Astron's over the years is an open, or low capacity filter cap.  
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KA3NXN
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« Reply #9 on: June 15, 2004, 11:16:10 AM »

I had a 20 amp unit a few years back and when it failed it put 28V to my gear costing me a pretty penny to get everything fixed. NEVER AGAIN!!

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AA4PB
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« Reply #10 on: June 15, 2004, 01:04:07 PM »

Well, if the transformer is 28V center tapped to a full wave rectifier then it is outputting 14V RMS or 19.6V peak. The capacitors will charge to 19.6V with no load and less under load so it is within the working range of a 25V capacitor (although a bit closer than I would like to see).

If the symptom is no output and replacing only the SCR fixes it then the SCRs are failing shorted for some reason (perhaps too low of a voltage rating).
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CROOKIE
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« Reply #11 on: June 15, 2004, 01:18:02 PM »

Or perhaps to low a peak repeatative current rating.  OTOH does the turn on time increase w/ increased current rating? A large but very fast fuse between the caps and the SCR can't hurt either.
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KE4MOB
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« Reply #12 on: June 15, 2004, 01:48:19 PM »

Pyramids use a full wave bridge rectifier rather than a conventional full wave rectifier.
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AA4PB
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« Reply #13 on: June 15, 2004, 02:42:16 PM »

Pyramids use a full wave bridge rectifier rather than a conventional full wave rectifier
-------------------------------------------
If thats the case and they use a 28V transformer then they are charging the 25V caps to 39.2V under conditions of no load. IF that's true they'd be having shorted caps all over the place rather than shorted SCRs. If they are using a bridge rectifier then more than likely the transformer voltage is closer to 14V RMS. It wouldn't make sense to develop 39.2V and then generate all the heat in the series regulator dropping it back down to 13.8V

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KE4MOB
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« Reply #14 on: June 15, 2004, 02:56:57 PM »

Correct.  It is a 14 V center tapped secondary...28 V to the filter caps.  (I was remembering meter readings rather than looking at the schematic. Oops.)

BTW the schematics for the Pyramid 52 can be found here:

http://www.cbtricks.com/miscellanous/pyramid/ps-52kx/pyramid_ps-52kx_om.pdf

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