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Author Topic: Pyramid Phase III Power supply issue  (Read 3731 times)
K8POS
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« on: July 02, 2011, 01:11:21 PM »

I have a 35 amp Pyramid power supply that shows +12 volt on the frame or ground.
If I hook up my station grounds it will short out, leave them disconnected and everything works fine.
This is the only schematic I have, although it is for the 25 amp model.
I am thinking that capacitor C9 or C10 is bad.  If I lift the PBA that contains the over-voltage circuit, which removes the ground potential for those two capacitors, it also work fine.
http://www.melvinweather.com/PWRSUPPLY.doc
Please have a look. 

Bob
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K9YLI
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« Reply #1 on: July 02, 2011, 02:29:24 PM »

 Not likely  c10  but   c9 or  c8..

or    the  SCR is defective..any of those should kill the output.
the  common side of the output  does NOT appear to be tied to frame ground, so  your symptom  would appear to be 
hot side to  frame  problem.
which  c9 could be  but not  c8 or 10  or the scr.
the only  frame gound appears to be at   junctin of  c9  c10..

how ever,  grounded  heat sink  mat be the problem  chec for  loose  nuts bolts  washers around the heatr sink.
if the heat sink is grounded to frame,  maybe bad isolation insulators  on one transistor, sleeves on the mounting screws .. etc.

you might be able to find this one with  ohm meter  frm  plus output to frame. start lifting  wires or  loosen transistors,, etc  to see if the  short goes away..

also  you said  +12..  did you mean that  or  +13.8
that would be a clue if its  only really  +12  with  13.8  output..
means  some resistance  in the short..

go for it.. trouble shooting is fun..
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K8POS
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« Reply #2 on: September 15, 2011, 08:30:34 AM »

Thought I would follow up.
Finally got a chance to dig into this thing.  Capacitor C9 was fried. (2A104K .1uf)
Replaced C9 and C10.  Everything works as should now.  $1.58 to repair beats over $100 to replace.

Bob
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N4NYY
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« Reply #3 on: September 15, 2011, 02:55:05 PM »

Bob,

Why was it fried? Was it the voltage rating too low?
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K8POS
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« Reply #4 on: September 15, 2011, 05:44:10 PM »

Have no clue as to why.  This is an old power supply.  I could not find any other components sorted or not properly insulated where needed.  I am assuming that the cap was going bad allowing too much current flow.  The schematic I have is for the 25 AMP model not the 35 amp that I have, but it follows what is inside the box.  I am just glad that Rat Shack had the part in stock so that I didn't have to source  it thru Mouser.

Bob
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