Re: Compaq PDP-121P - Baked MOV
I just missed out on editing my previous post. So ignore that one, and use this one instead; as i had further thoughts, connected more dots, and corrected and tidied it up.
Seems like this PSU might not not 5V heavy (as previously thought)?
It's rating are:
+3.3V _____ 15A
+5V_____ 11A
+12V _____ 5A
+12.8V_____ 7.5A
-12V_____ 0.15A
5.05V Aux_____ 3A
Hence that's why adding the car bulb didn't have any effect, because i'm fairly sure that the problem is with the 12V rail (see below).
It was after that, that i plugged the 20pin ATX connector into the test motherboard, just to see what would happen.
One useful thing that came out of that, was seeing correct voltage on the 12V rail
So, in retrospect, it's no surprise that 23W bulb (and only the 23W bulb), had no effect on starting the PSU when connected to the 5V rail
I just realized, and figured out, that 30W@12V is the same as 12.5W@5V, or 2.5A - so that 23W 12V bulb, when placed on the 5V rail, should have worked if it required more loading for the PSU to start;
but, as i've discovered, the problem appears to be with the 12V rail, not the 5V rail
I didn't think to try it on the 12V rail at that time
After getting no joy when adding a wide range of resistors to the 5V rail, the PSU now starts if i attach a 3W 74ohm load resistor to the 12V rail to draw close to 2W, and i arrived at that value by trying decreasing resistances until the PSU started and the voltage hit it's highest value, which is 10.64V
The other rails are well with spec and stable
When i plugged the PSU ATX into the test motherboard, i got 11.8V out of it; so i know that it can do it, so it's odd that the max voltage that i can draw out of the 12V rail is 10.64V.
I removed the 25V 470uf Taicon PW cap that was on the 12V rail
It measures , according to the 328 transistor tester, 454uf and 0.42 ESR (or 0.83 ESR using a dedicated ESR tester)
The datasheet says that it should have an impedance of 0.273, so if "impedance" = "ESR" then it's out of spec.
And when i compared the 12V rail with a good PSU 12V rail, on an old oscilloscope (probably not doing it right though), without changing the previous settings, i could notice significant ripple compared to the other PSU.
It's still a mystery to me as to why i can get 11.8V out of it when plugged into that test motherboard ATX, but not when loading the 12V rail with any resistor. Obviously something's happening within the motherboard that's causing the 12V rail to come into spec, that i can't yet replicate.
Even though the other rails are good, might it be that they need extra loads to push up the 12V? - which is a bit different (voltage is too low) from the scenario that you mentioned:
Anyway, before removing the cap, i plugged it into the Compaq computer with some 5A fuse wire and got an LED code of over voltage condition, and it wouldn't start
That was no surprise
While removing the cap, i noticed that this has a +12.8 rail as well, for the 4pin CPU plug.
This PSU had an 8A fuse in it. P=IV = 1840W. That seems to be a bit high for a PSU that's rated at 220W.
I have a 5W ceramic fuse, so i think that there would be no problem replacing the original fuse with that. Correct?
I just missed out on editing my previous post. So ignore that one, and use this one instead; as i had further thoughts, connected more dots, and corrected and tidied it up.
Getting something like a 12V car light bulb or 12V halogen light bulb (anywhere from 10 to 35 Watts will do) and put that on the 5V rail only.
It's rating are:
+3.3V _____ 15A
+5V_____ 11A
+12V _____ 5A
+12.8V_____ 7.5A
-12V_____ 0.15A
5.05V Aux_____ 3A
Hence that's why adding the car bulb didn't have any effect, because i'm fairly sure that the problem is with the 12V rail (see below).
It was after that, that i plugged the 20pin ATX connector into the test motherboard, just to see what would happen.
One useful thing that came out of that, was seeing correct voltage on the 12V rail
So, in retrospect, it's no surprise that 23W bulb (and only the 23W bulb), had no effect on starting the PSU when connected to the 5V rail
I just realized, and figured out, that 30W@12V is the same as 12.5W@5V, or 2.5A - so that 23W 12V bulb, when placed on the 5V rail, should have worked if it required more loading for the PSU to start;
but, as i've discovered, the problem appears to be with the 12V rail, not the 5V rail
I didn't think to try it on the 12V rail at that time
After getting no joy when adding a wide range of resistors to the 5V rail, the PSU now starts if i attach a 3W 74ohm load resistor to the 12V rail to draw close to 2W, and i arrived at that value by trying decreasing resistances until the PSU started and the voltage hit it's highest value, which is 10.64V
The other rails are well with spec and stable
When i plugged the PSU ATX into the test motherboard, i got 11.8V out of it; so i know that it can do it, so it's odd that the max voltage that i can draw out of the 12V rail is 10.64V.
I removed the 25V 470uf Taicon PW cap that was on the 12V rail
It measures , according to the 328 transistor tester, 454uf and 0.42 ESR (or 0.83 ESR using a dedicated ESR tester)
The datasheet says that it should have an impedance of 0.273, so if "impedance" = "ESR" then it's out of spec.
And when i compared the 12V rail with a good PSU 12V rail, on an old oscilloscope (probably not doing it right though), without changing the previous settings, i could notice significant ripple compared to the other PSU.
It's still a mystery to me as to why i can get 11.8V out of it when plugged into that test motherboard ATX, but not when loading the 12V rail with any resistor. Obviously something's happening within the motherboard that's causing the 12V rail to come into spec, that i can't yet replicate.
Even though the other rails are good, might it be that they need extra loads to push up the 12V? - which is a bit different (voltage is too low) from the scenario that you mentioned:
If 12V or 3V3 are too high, you may have to put a small load on those as well
That was no surprise
While removing the cap, i noticed that this has a +12.8 rail as well, for the 4pin CPU plug.
This PSU had an 8A fuse in it. P=IV = 1840W. That seems to be a bit high for a PSU that's rated at 220W.
I have a 5W ceramic fuse, so i think that there would be no problem replacing the original fuse with that. Correct?
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