Yes, ...because when i use a standard 560 ohm resistor, the 3.3V rail becomes 3.32V
(5V rail is 5.177V)
Will leave it at that....
socketa
Asbelowsoabove
Last Activity: 02-27-2026, 12:04 AM
Joined: 06-18-2014
Location: hell
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I see that standard resistor values would be 560, or 620; so i replaced the 680 ohm with a 620 (actually 615) ohm, so that now the 3.3V rail voltage has decreased from 3.45V to 3.397V.
Don't know why they didn't use a 620 ohm resistor rather than a 680 ohm, which brings 3.3V rail closer to 3.30V
Anyhow, it's fixed nowLast edited by socketa; 02-14-2026, 01:21 AM.Leave a comment:
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I just used another schematic in this post, because it's similar circuitry as this Aywun 3.3 magamp. (Had to add in next post)
My PSU has TL431 reference pin voltage divider of 680 and 1800 which would give 2.39V at ref pin
internal ref for TL439 is 2.495V
... so is is possible that this PSU was manufactured with the wrong resistor 680, instead of 580 - would it be easy for the person making this PSU to use blue grey brown instead of green grey brown?
Blue could look like green in poor lighting
This higher value resistor would cause the TL431 to turn on more and regulate...Last edited by socketa; 02-12-2026, 04:47 PM.Leave a comment:
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Also notice slight noise that's coming from around standby transformer when the PSU is powered up, but it pretty much goes away as the hard drive spins upLast edited by socketa; 02-08-2026, 01:17 AM.Leave a comment:
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Reinforced corner (could have used a longer piece of PCB)
The PCB had a fair amount bend along it's length, so i managed to straighten it but applying some force and a heat gun (only took about 10s of heating). I used a zip tie to help, and then after realizing that the board responded well to the heat, and wanted to return to it's original flatness, i then just applied a bit of pressure at the ends while heating, to straighten them.
Pretty straight now.
Since this PSU would probably have being installed with the components hanging downward (Cases have the smaller screw...Last edited by socketa; 02-07-2026, 11:55 PM.Leave a comment:
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Thanks for pointing that out - thought that it was just a scratch. So yeah, right you are - i looked at the top of the PCB, and the damage is visible on the top of the PCB, on both sides of the screw hole - maybe it was dropped at some stage.
I'll get around to doing that, and post an update here.Leave a comment:
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Replaced the PWM chip with a KA7500, which seems to be equivalent except for a reference voltage of 5.0V vs 4.9V for the original chip.
PSU still shut down.
So had a look around the LM339, and it all seemed to be good
Then thought that maybe the PWM is not getting correct voltage input from the 12V or 5V rails
So traced back to them (resistor in between each rail and the chip pin) and noticed high resistance of 40 ohms coming from this 5V rail trace.
The trace is damaged, and looks like another trace is also damaged....Last edited by socketa; 01-27-2026, 02:51 PM.Leave a comment:
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Not yet
I'll test the other small elecrolytics on the next fine day, and update with results
Will start with those two that look like they provide negative bias to the main switching transistors to turn them off fasterLeave a comment:
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Good point.
It's an equivalent TL494/LM339 setup.
The chips are probably ok, since the LM339 comparator is sending signal to the PWM controller dead time control pin, and shutting down the PSU.
At first i thought that the rail voltages were all within spec using a multimeter, but i can now see on the analog scope that the 12V is initially hitting 15V, 5V is hitting 6V, and 3.3V is hitting 4V.
The main switching transistors aren't shorted, so i wonder what's causing the overvoltage on all of the rails - I guess that the PWM part of the chip could now be faulty like you said...Last edited by socketa; 01-14-2026, 04:58 PM.Leave a comment:
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That 3.3V output cap tests good, and it's rated for 16V, which i guess increases it's tolerance for heat.
There is some protection kicking in when i ground PS_ON wire - the rail voltages come up, but the PSU pretty much shuts down straight away.
The output rails aren't shorted, so i guess that the problem is more likely to be on the primary
Will start with checking the primary side...Leave a comment:
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After replacing that 36ohm resistor, the 5VSB was 22V
So thought that there could be faulty components on the secondary
Replaced the opto with another one that caused an LED to glow brighter, and then checked the output caps, which were both good
Eventually found that the TL431 had low resistance (50Kohms-70Kohms)between reference and anode
I guess that the 5VSB got so high that it exceed this this regulator's specs
So after replacing it, the 5VSB is now 5V
Will now have a look at that 3.3V output cap, and then see if the unit will start upLeave a comment:
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AYWUN A1-5000 - 5VSB nearly fixed
This PSU is the same as the one in the switchmode power supply article by c_heddge
I got it with a non-operational 5VSB
Quite a few damaged components - circled in the schematic (ignore the secondary side, i used a schematic from a different PSU, and only adjusted the 5VSB circuit)
The electrolytic cap was high ESR; so i think what happened is that it couldn't charge up enough to turn on the TO92 transistor, thus over-heating the main MOSFET and causing it to short, which, in turn, damaged the other components
The source of the MOSFET is connected to the base of the TO92...Last edited by socketa; 01-03-2026, 10:37 PM. -
OK, it looks like the problem is solved
The connections from the RAM slots to the CPU slots were all good as far as GND is concerned
So, thinking about the odd POST code behaviour, when trying RAM in different slots, and that RAM is mostly connected to CPU, i thought to try another CPU;
and low and behold, it booted smoothly all the way through to the BIOS settings screen, straight away, with RAM in either slot.
So it was either a faulty CPU, or more likely maybe a sightly bent CPU pin that eventually ceased connecting with the CPU socket pin - since the CPU didn't want...Last edited by socketa; 12-25-2025, 05:23 PM.Leave a comment:
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Whoops, the initial board view was mirror imaged, so it was pin 239, and that is OK, as it's meant to be grounded
So it's probably still the MCP; but i'll check all of the RAM pins for shorts, because the CPU paste is highly conductive, and i'm hoping that some of the paste got into the CPU slot at some point in timeLast edited by socketa; 12-25-2025, 02:53 PM.Leave a comment:
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Got the pins wrong in the last post.
It's pin 123 (DQ_D25) that is hard-shorted to ground, and according to the boardview file that was linked to in this topic, it only goes to one pin in the CPU socket.
I removed the CPU, and the short is still there,
Looking into the RAM slot, the pin looks normal and is not touching an adjacent pin. And the CPU pin looks to be reasonably normal
The visible traces on the PCB are not damaged
So it has to be either a short in the RAM slot, or the CPU slot; which seems odd, because they both look OKLast edited by socketa; 12-25-2025, 01:43 PM.Leave a comment:
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Even more heating it again, made no difference
I think that it might be a RAM slots issue, because:
RAM in slot 2 causes the board to shut down part way through POST, and when it's in slot 1 there is that cycling (as mentioned in post #1)
If i don't put in any RAM, then it gives "No RAM" beeps
With no RAM installed in any slot, on DDR2 RAM slot 1, pin 239 ("presence-detect inputs") is shorted to ground; but on slot 2 it isn't (1.3M ohms to ground)
Removing the CPU doesn't remove this short
Pin 240 ("presence-detect inputs") is also...Last edited by socketa; 12-24-2025, 03:42 PM.Leave a comment:
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Ok, so i heated it up to 230 deg C and it went through to bios settings;
but then it didn't do it again (it went back to original symptoms)
So i heated it up to 290 deg C for about a minute, let it cool down to just warm, and still the same issue (tried two or three times)
Then, after that, on next power on, it went through to bios settings screen, with no video issues
Ten successful POSTs after that
Then turned it off for half an hour and then powered on, and it's reverted back to the original issue (although once it went through all of the POST, but the video was...Last edited by socketa; 12-21-2025, 12:10 AM.Leave a comment:
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Changed to different stick of RAM, but still the same problem
Tried another different stick of RAM and this time it successfully went through the POST sequence to the BIOS settings screen
But there was an issue with the display
Rebooted and this time the bios settings was displayed correctly, but the display issue came back after selecting a different setting screen
Rebooted again and BIOS setting screens displayed OK
Put the previous RAM back in, and it worked OK
Then after that it wouldn't go through all of the POST, irregardless of any RAM that...Last edited by socketa; 12-20-2025, 01:56 AM.Leave a comment:
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post #27 should be
10 ohm resistor- not 10K ohm resistorLeave a comment:
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