Help with Pyramid PS52KX 46 Amp power supply - Page 1
Forum section: electronics / repair

author: N8AUM
author_rank: Regular Contributor
timestamp: March 13, 2015, 03:56:23 pm
content: Hello all and tnx for reading this. Any ways, I just purchased a " Refurbished" Pyramid PS52KX 46 Amp power supply from fleabay and 1st thing I did was to open it up. I found C7 swollen which was probably caused when the SCR (crowbar) blew open causing 25 volts across it. Not a big deal but I am trying to understand the purpose of D4, C7 & C9. Also cant wrap my head around the way the 723 is being powered. I'v built supplies around the 723 and had it's V+ from a separate source but this one has me stumped.Any ideas ?

author: Seekonk
author_rank: Super Contributor
timestamp: March 13, 2015, 04:11:29 pm
content: Being such a high current supply, they want as little voltage as possible across the power transistors. The 723 supply just stacks a additional voltage on top of the main power source allowing sufficient drive. If you have more than expected on the output there is a problem with the drive. D4 is reverse voltage protection, the capacitors are for normal output filtering for surges.

author: German_EE
author_rank: Super Contributor
timestamp: March 13, 2015, 05:01:20 pm
content: D4/C7/C9First Guess - Reverse polarity protection.Second Guess, when the SCR fires you could get ringing on the output line, this diode is to prevent the ringing going to negative with respect to chassisThe 723 power supply arrangements are unusual but will still work. They join the 723 supply to the positive rail rather than chassis but it will still work. Maybe done this way load fluctuations do not affect the 723 as much, I do not know.

author: AG6QR
author_rank: Frequent Contributor
timestamp: March 13, 2015, 06:21:13 pm
content: Quote from: German_EE on March 13, 2015, 05:01:20 pmI'll put in my somewhat uneducated guesses on both topics.D4/C7/C9 are all in parallel across the final output. The capacitors aren't necessarily there for the same purpose as the diode. Consider the capacitors first. Any time you see high value electrolytic and low value ceramic capacitors, there's a good chance it's for noise filtering of different frequencies. The big electrolytic provides some surge capability for load transients. C9 can absorb higher-frequency noise. Since this is a radio power supply, there's a chance that a radio transmitter could inappropriately send a bit of RF back along its power leads.The diode could be there to absorb ringing that went negative, as you suggest.The 723 supply is interesting. I don't know what the AC voltages are of the transformer secondaries. But note that the big secondary of the transformer feeds a bridge rectifier and filter cap to powering the main V+ rail. There's another bridge rectifier (circled in red) and C6 that provide a rail a bit below the main V+ rail (how much below V+? I wish I knew). That rail is connected to the 723's input power pin, P12, through R11. But 723's pin 12 is also connected to the main regulated 12V output of the supply through D3. Why? I'm going to display my ignorance here and confess that I don't know. It seems like it would work if they shorted out C6 and eliminated the bridge rectifier circled in red. They should also be able to eliminate D3, so that the 723 gets all its supply directly from the top rail.This may be totally off the mark, but my wild idea is that, by powering the 723 with the regulated 12V supply through D3, the output of the 723 is more stable than it would be if the 723 were powered directly via the unregulated top rail. But that can only work after the supply has started up. To "bootstrap" the supply, the 723 needs to get some power directly from the transformer, so that it can then turn on the pass transistors, and provide regulated 12V output.I welcome correction. I'm not at all confident that I'm on the right track here, but I find it interesting...

author: N8AUM
author_rank: Regular Contributor
timestamp: March 13, 2015, 08:25:16 pm
content: Thanks for all the responses. After Seekonk initial response I did some poking around. the second bridge adds 8 volts to the rail so the 723's V+ is now 32 volts which also supplies the driver Q1. I dont see how that would help regulation since the 723's V+ will still fluctuate with the load unless the higher voltage helps ? And now I remember back in the old days we use to add different flavored caps on the output to help with noise. As far as D4 goes, seems like everyone agrees that its there for reverse voltage protection. D1, D2

author: AG6QR
author_rank: Frequent Contributor
timestamp: March 13, 2015, 08:45:54 pm
content: Quote from: N8AUM on March 13, 2015, 08:25:16 pmOops, had misread the direction of the bridge rectifier, thinking it was dropping the voltage below the main V+ rail, but it's clearly adding more voltage. Thanks for the voltage measurement, which makes that point obvious. Like you, I'm not sure why they boosted the supply, though increasing the supply voltage to provide plenty of margin for dropout probably plays a part.

author: N8AUM
author_rank: Regular Contributor
timestamp: March 13, 2015, 09:24:19 pm
content: Quote from: AG6QR on March 13, 2015, 08:45:54 pmI totally missed it but it's been over 25yrs since I played with one.Now to figure out D1 & D3. D1, another reverse polarity just for the 723 ?D3

author: Seekonk
author_rank: Super Contributor
timestamp: March 13, 2015, 09:35:35 pm
content: One is a zener and the other protects the 723 from being destroyed should power exist on the output when power is off.

author: N8AUM
author_rank: Regular Contributor
timestamp: March 13, 2015, 10:01:46 pm
content: Quote from: Seekonk on March 13, 2015, 09:35:35 pmI only see 1 zener for the OVP with SCR.With power off and if you apply power to the output the 723 would get power via D3 ?

author: Seekonk
author_rank: Super Contributor
timestamp: March 13, 2015, 10:16:10 pm
content: Yes, otherwise it would get power from the external transistor drivers and the reverse voltage would destroy the output transistor in the 723. This the reason you often see a diode across a 78XX regulator when input power has the potential of decaying faster than the output.