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Pyramid PS-35G power supply problem

flapineagle

flapineagle

Member
Aug 15, 2009
13
0
11
87
Woodland, Alabama
Recently got this non working power supply. Replaced the 2N3773 x 4 transisters and all the electrolitics. Tested with a 12v headlamp and it seemed to work ok. Have not had any equipment connected to it but have turned power on with no load. there was a noise like a surge sound to which I assumed was the caps charging. Tonite thinking about connecting a load to it noticed that the voltage was low (10 volts on the meter) but volts dropped to zero when load was applied. Also noticed that no surge sound when switch turned on. Any suggestion to where I might look would be appreciated.
 

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Still haven't pinned down my problem, Does anyone know what the weak points are to cause a failure when the power supply has been turned off and on without no load. Grand kids in the radio room. any suggestions where to check so I don't have to shotgun fix it.
 
I would check the voltage before it reaches the regulator circuit and see if it isn't a constant ~18vdc or so. Like right after the diode bridge. I think it uses more than one diode pack - could be wrong (yes; it does - looked at your pictures). I know the bigger supplies do. If it is; then the regulation is the issue.

I don't know if your unit has a built in 'crowbar circuit'. If so and the output of the diode pack is constant, you will have 'fun' tracking down the problem. I think there is an IC responsible for regulation (yup - seen in the pictures) along with the pass transistors (on the heat sink). You might check those IC's and the pass transistors as well . . .
 
Thanks Robb for your input. Finally got around to checking the power supply. found the pass transistors blown (2n3772) a 20 amp and replaced with (2n3771) a 30 amp. Also noticed when power up that it had no inrush sound, not sure if the bigger transistors did that but she holds steady on a lamp load so will try it out with full load soon.
 
What you did should be fine I would check the idling voltage and then check it under a load before hooking any radio equipment up to it just to be safe ;)
 

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