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Old 04-20-2021, 06:53 PM   #21
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Default Re: Compaq PDP-121P - Baked MOV

Ignore my last post - i somehow that wrong, including the wrong transistor

The optocoupler is not shorted.

The values in normal font are when the PSU is powered on
The values in bold font are when the paper clip shorts the white wire to ground (i'm farly sure that the ATX pinout is normal, as they have just used different colours)

Secondary side

4.96V 5.28V
0.037V 1.18V
-----------------
-----------------
0V 15.4V
12.92V 13.18V

Primary side
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Last edited by socketa; 04-20-2021 at 07:57 PM..
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Old 04-20-2021, 09:14 PM   #22
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Default Re: Compaq PDP-121P - Baked MOV

The voltage values in the last post, are the pins of optocoupler as they appear in the photo, and the dotted lines is the gap between the primary and the secondary sides.
There's 130mV at Vcc of the UC3845 when the power is turned on and the 5VSB (green wire) has the correct voltage,
and when i short the white wire to ground there is 13.5V on Vcc.

Last edited by socketa; 04-20-2021 at 09:18 PM..
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Old 04-20-2021, 09:44 PM   #23
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Default Re: Compaq PDP-121P - Baked MOV

Quote:
Originally Posted by socketa View Post
The voltage values in the last post, are the pins of optocoupler as they appear in the photo, and the dotted lines is the gap between the primary and the secondary sides.
There's 130mV at Vcc of the UC3845 when the power is turned on and the 5VSB (green wire) has the correct voltage,
and when i short the white wire to ground there is 13.5V on Vcc.
So the UC3845 should be working / sending out pulses then, as according to the datasheet, minimum startup voltage is 8.4V, and minimum running voltage is 7.6V for Vcc for UC3845 and 3843 models. Check voltage on V_ref pin of UC3845, just to confirm - should be 5V. If it is, check low-resistance Source feedback resistor connected between primary ground and Source of the MOSFET - is it open? If not, check back voltage on V_fb pin on UC3845 too so we can see if the UC3845 is told to switch or just held down to not output anything.
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Old 04-20-2021, 11:51 PM   #24
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Default Re: Compaq PDP-121P - Baked MOV

V_ref pin is 5V - so that's nice to see, thanks
and i get 28K ohms from MOFET source to ground, and there is a suspect looking resistor in between, that looks like should be 0.28 hms

There is 0V on the voltage feedback pin - I'm guessing that that is because the MOSFET is not being turned on, and therefore there is no feedback from the secondary to regulate the UC3845 chip?
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Last edited by socketa; 04-21-2021 at 12:11 AM..
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Old 04-21-2021, 01:40 AM   #25
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Default Re: Compaq PDP-121P - Baked MOV

Quote:
Originally Posted by socketa View Post
(i'm farly sure that the ATX pinout is normal, as they have just used different colours
i wouldnt be so sure.
see if the pints on the board where the wires go are labelled.
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Old 04-21-2021, 06:34 PM   #26
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Default Re: Compaq PDP-121P - Baked MOV

Quote:
Originally Posted by stj View Post
i wouldnt be so sure.
see if the pints on the board where the wires go are labelled.
i don't think that drinking ale and working on PCBs go well together.
But yeah the labels on the board confirm that the connector pinout is the usual one.

Is that resistor a fusible resistor?

Last edited by socketa; 04-21-2021 at 06:56 PM..
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Old 04-21-2021, 06:49 PM   #27
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Default Re: Compaq PDP-121P - Baked MOV

Quote:
and i get 28K ohms from MOFET source to ground
This is way too high for a source resistor, they are usually less than one ohm. The resistor is likely a metal film flameproof, There is usually another resistor between the mosfet source and the drive ic's CS pin. When the source resistor goes open it is very likely the ic has been damaged.

Last edited by R_J; 04-21-2021 at 06:55 PM..
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Old 04-21-2021, 11:06 PM   #28
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Default Re: Compaq PDP-121P - Baked MOV

i could only find a 0.15 ohm heat-shrinked, grey-bodied, non-glossy, 14mm, resistor - is it OK to use that? (the 328 transistor tester shows it to be about 0.20 ohms, after subtracting 0.17 ohms when shorting the test leads)
The original looks like it's a 0.28 ohms and is about 11mm.
Is there any significant difference between the pink-bodied and the grey-bodied resistors that you find on ATX PSU boards?
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File Type: jpg 20120107_150925.jpg (117.6 KB, 6 views)
File Type: jpg 20120107_151054.jpg (194.6 KB, 7 views)

Last edited by socketa; 04-21-2021 at 11:15 PM..
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Old 04-22-2021, 02:52 PM   #29
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Default Re: Compaq PDP-121P - Baked MOV

A low value current sense resistor in a MOSFET Source circuit probably won't open up or go high value on its own. That resistor probably received really high current, such as the MOSFET shorting or the transformer core saturating. I'd check the health of the MOSFET. If the MOSFET is shorted D-G-S, then high voltage got into the Gate circuit.
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Old 04-22-2021, 06:36 PM   #30
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Default Re: Compaq PDP-121P - Baked MOV

Yeah, i've replaced the controller chip and the MOSFET, they were, as mentioned previously, faulty
Is it good to permanently replace the pink resistor with the larger sized grey resistor?
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Old 04-22-2021, 11:52 PM   #31
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Default Re: Compaq PDP-121P - Baked MOV

Quote:
Originally Posted by socketa View Post
V_ref pin is 5V - so that's nice to see, thanks
and i get 28K ohms from MOFET source to ground, and there is a suspect looking resistor in between, that looks like should be 0.28 hms
Looks like a 1.8 Ohm to me (and that's a standard value for E12/E24 series 4-band resistors.)

Quote:
Originally Posted by socketa View Post
There is 0V on the voltage feedback pin - I'm guessing that that is because the MOSFET is not being turned on, and therefore there is no feedback from the secondary to regulate the UC3845 chip?
No, the feedback pin (2) goes inside the 384x to an error amplifier. So V_fb should be around 2.5V when the IC/PSU is regulating normally. When the voltage on the secondary side goes slightly higher than what it should be, the supervisor IC sends a signal through a 2nd optocoupler, which then (attempts to) increase the voltage on the V_fb pin above 2.5V. This tell the UC384x to decrease its PWM output, which in turn lowers the power going into the primary and reduces the voltage on the secondary side. If the reverse happens - voltage are too low on the secondary - then the supervisor does not send any signal through the 2nd optocoupler. This (attempts to) decrease the voltage on the V_fb pin below 2.5V... which tells the UC384x IC to switch "harder" (i.e. increase PWM duty output.) When that happens, more power is sent through the primary side MOSFET and transformer, thus increasing the voltages on the secondary side too. So in essence, the supervisor is constantly bouncing around between telling the UC384x IC to increase and decrease PWM output... but it all happens so quickly, that it looks as if the voltages on the secondary side are constant.

And that's how regulation is achieved in these PSUs, really.

Quote:
Originally Posted by socketa View Post
Is that resistor a fusible resistor?
Likely yes.

Quote:
Originally Posted by R_J View Post
The resistor is likely a metal film flameproof.
^ This.
Source resistors are always metal film / metal oxide and flameproof/resistant. DO NOT use carbon film type. Ceramic wirewound may also be a problem due to higher inductance.

Quote:
Originally Posted by R_J View Post
There is usually another resistor between the mosfet source and the drive ic's CS pin. When the source resistor goes open it is very likely the ic has been damaged.
Could be.
Though damage most frequently comes from when the MOSFET's Gate shorts to either Source or Drain and then backfeeds high voltage into the driver output pin (6).

Quote:
Originally Posted by socketa View Post
i could only find a 0.15 ohm heat-shrinked, grey-bodied, non-glossy, 14mm, resistor - is it OK to use that? (the 328 transistor tester shows it to be about 0.20 ohms, after subtracting 0.17 ohms when shorting the test leads)
The original looks like it's a 0.28 ohms and is about 11mm.
Yeah, that should work fine.
Again, I suspect your old resistor is 0.18 Ohm (brown, gray, silver, gold) and not 0.28 Ohms (red, gray, silver, gold.) Perhaps the brown/red band on your resistor is a bit lighter, hence possibly why you think it's red.

But even if it is 0.28 Ohms... the 0.15-Ohm should work OK too. Only difference will be during an overload / over-power condition: lower resistance will allow more power to go through before the IC detects the overload condition (which may or may not be detrimental to the rest of the circuit, depending on how over-built it is.) That said, if your resistor really is 0.18 Ohms and you put in 0.15 Ohms... that's about 15% difference roughly... so you can expect the overload limit allowed by the IC to be increased by roughly just as much.

Given that we are dealing with a HiPro PSU here, I don't think that's going to be a problem, as they tend to be built quite well.

Quote:
Originally Posted by socketa View Post
Is there any significant difference between the pink-bodied and the grey-bodied resistors that you find on ATX PSU boards?
None.
I've seen them used interchangeably. Both are metal film type / metal oxide - you can tell by the non-shiny "eggshell" finish. On the other hand, Carbon film resistors are usually shiny.

Quote:
Originally Posted by socketa View Post
Is it good to permanently replace the pink resistor with the larger sized grey resistor?
Yes, should be OK.
The bigger replacement one means it's also rated for a higher power, so it should run a tiny bit cooler... though not that this should matter. I suspect the original one (11 mm) is rated for 2 Watts, and your replacement (14 mm) is rated for 3 Watts. Again, not that it should matter, but just stating this for information completeness.

Quote:
Originally Posted by socketa View Post
Yeah, i've replaced the controller chip and the MOSFET, they were, as mentioned previously, faulty
Still, run the PSU through the series bulb / current-limiting device for a bit to verify the new IC didn't get damaged from the open Source resistor... and on that note, do check if the resistor between the Source resistor and CS pin is still good.

Last edited by momaka; 04-22-2021 at 11:58 PM..
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Old 04-23-2021, 04:07 PM   #32
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Default Re: Compaq PDP-121P - Baked MOV

Thanks for that explanation
Yes, the resistor colour looks more brown than red when it's not in the sunlight, and i found another of the same value, but 14mm, which i heat-shrinked and installed.

Quote:
Originally Posted by momaka View Post
Still, run the PSU through the series bulb / current-limiting device for a bit to verify the new IC didn't get damaged from the open Source resistor... and on that note, do check if the resistor between the Source resistor and CS pin is still good.
It's covered in glue, and it measures 2Kohms


Turned on the PSU, with PS-ON grounded, and the fan twitched slightly.
Looks like some protection is kicking in?
I have to wait about 10s, in between removing the paper clip and reinserting it, to get a response
The rail voltges come up, but then immediately drop off.
My maximum observed reading are:
3.3V rail : 3.7V
5V rail : 2.3V
12V rail : 3.9V

It wouldn't be because of the replacement MOSFET that i used in post #16, would it?

Last edited by socketa; 04-23-2021 at 04:15 PM..
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Old 04-23-2021, 08:57 PM   #33
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Default Re: Compaq PDP-121P - Baked MOV

When i ground PS-ON, about 9V appears on Vcc, then it immediately drops off.
And i get about 10V on the 12V rail (and fan spins), 5V on the 5V rail, and 1.5V on the 3.3V rail, if i accidentally short the optocoupler pins on the primary side.
Could it be base voltage is initially driving a transistor that's supplying Vcc, but then it quickly falls, and so the transistor switches off?

I'm going to retest the startup(?) cap, and look for a replacement; as that might be the cause of this after all.
It's connected to a PNP transistor that's connected to Vcc

[Edit] after previously testing that cap, i have noticed that i put i back in with the wrong polarity

Last edited by socketa; 04-23-2021 at 10:23 PM..
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Old 04-23-2021, 10:37 PM   #34
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Default Re: Compaq PDP-121P - Baked MOV

Quote:
Originally Posted by socketa View Post
Turned on the PSU, with PS-ON grounded, and the fan twitched slightly.
Looks like some protection is kicking in?
Maybe.
Did you have the series bulb on the AC input? If so, what power rating? Go with 100 Watts (or thereabouts). And if the PSU still doesn't turn on, try about 150-200 Watts (two or more 40-100W bulbs in parallel.)

Quote:
Originally Posted by socketa View Post
I have to wait about 10s, in between removing the paper clip and reinserting it, to get a response
That almost looks like short-circuit protection is triggering, but since you do get output voltages on all of the rails, I don't think that can be the case.

Quote:
Originally Posted by socketa View Post
It wouldn't be because of the replacement MOSFET that i used in post #16, would it?
No, the replacement you have in there is reasonably close enough.

Quote:
Originally Posted by socketa View Post
When i ground PS-ON, about 9V appears on Vcc, then it immediately drops off.
And i get about 10V on the 12V rail (and fan spins), 5V on the 5V rail, and 1.5V on the 3.3V rail, if i accidentally short the optocoupler pins on the primary side.
Hmmmm... Maybe not enough load on the output of the PSU, then? 5V looks normal, but 12V and 3V3 are low.

So try running the PSU with two parallel bulbs as the series limiting device and put a larger load on the 5V rail (perhaps a 12V car bulb around 20-30 Watts.) Then try powering on the PSU normally through PS-ON an observe what happens. If nothing, try the shorting of the opto again and see what the output voltages look like with the now extra load on the 5V rail. If 12V or 3V3 are too high, you may have to put a small load on those as well. Try going for resistive loads (or 12V incandescent bulbs) as HDDs and fans may not present a load quick enough to allow the PSU to regulate properly.

Quote:
Originally Posted by socketa View Post
Could it be base voltage is initially driving a transistor that's supplying Vcc, but then it quickly falls, and so the transistor switches off?
Maybe.
It's hard to say, because feedback from the secondary gets sent out so quickly. But the test you did with shorting out the optocoupler for supplying power to the UC3845 IC shows that the PSU can work... just that voltage are a bit low. That voltage on the 3.3V rail is probably what's tripping it.

Quote:
Originally Posted by socketa View Post
I'm going to retest the startup(?) cap, and look for a replacement; as that might be the cause of this after all.
Yes, do check all of the caps in this PSU. It appears to be an older HiPro unit - old enough that it has green UCC KZE caps (among lower-tier brands like Ltec and whatnot), looks like (so possibly Pentium 3 / early P4 era.) I'm not 100% familiar with this particular version, though... and it may be a custom unit too, judging by the layout.

In any case, check the basics first, as always:
1) Voltage switch in the correct position and primary caps getting about 320V DC across them (about 160V DC across each.)
2) All large output filter caps are good in terms of ESR and capacity
3) 5VSB always present when the PSU is plugged in, even when shorting the PS-ON signal to ground and afterwards?
4) Enough load on the output when testing the PSU? This being an older PSU, probably will be happier with more load on the 5V rail (so an old HDD may not be enough to "do the trick".)

Quote:
Originally Posted by socketa View Post
[Edit] after previously testing that cap, i have noticed that i put i back in with the wrong polarity
Might be worthwhile to change that cap then, as it could have been damaged. Or at least pull out and reform for 30 minutes, then re-test before re-using.

Last edited by momaka; 04-23-2021 at 10:43 PM..
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Old 04-24-2021, 12:04 AM   #35
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Default Re: Compaq PDP-121P - Baked MOV

Replaced the cap
I plugged in a hard drive for the easiest option first.
Grounded PS-ON, and the fan, and the hard drive were spinning good for about 2 seconds - then the bulb came on and glowed brightly and the PSU shut down.
Have tried it again several times, but now i'm back to just the twitching fan.

1. Yes
2) Visually look OK
3) Yes
4) As above

The PSU still starts if i short the optocoupler secondary side pins; but the 60W bulb starts to glow after a second or so if a hard drive is plugged in - so now i immediately turn it off.

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Old 04-24-2021, 02:40 AM   #36
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Default Re: Compaq PDP-121P - Baked MOV

Quote:
Hmmmm... Maybe not enough load on the output of the PSU, then? 5V looks normal, but 12V and 3V3 are low.
OK, i plugged in a motherboard that i don't care about too much (as it seems to produce a smell in the vicinity of the northbridge) and also connected a hard drive - didn't plug in the 4pin CPU connector
After a few touches of the PS-ON to Ground (where the fans just twitch), the PSU fully starts up and the CPU fan spins but revs at a rate of about 1hz, voltages are good.
The PSU wont start up (it's fan just twitches once) if i also connect the 4-pin CPU connector (but that could be due to an issue with the motherboard)

Was it OK that i replaced the 10uf 50v Teapo SC with a low impedance (ESR measures about 0.7 ohms) Rubycon YXF?

Quote:
So try running the PSU with two parallel bulbs as the series limiting device and put a larger load on the 5V rail (perhaps a 12V car bulb around 20-30 Watts.) Then try powering on the PSU normally through PS-ON an observe what happens. If nothing, try the shorting of the opto again and see what the output voltages look like with the now extra load on the 5V rail. If 12V or 3V3 are too high, you may have to put a small load on those as well. Try going for resistive loads (or 12V incandescent bulbs) as HDDs and fans may not present a load quick enough to allow the PSU to regulate properly.
i'll try that in the next day or so.

Last edited by socketa; 04-24-2021 at 03:24 AM..
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Old 04-24-2021, 03:40 PM   #37
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Default Re: Compaq PDP-121P - Baked MOV

Quote:
Originally Posted by socketa View Post
Replaced the cap
I plugged in a hard drive for the easiest option first.
Grounded PS-ON, and the fan, and the hard drive were spinning good for about 2 seconds - then the bulb came on and glowed brightly and the PSU shut down.
...
The PSU still starts if i short the optocoupler secondary side pins; but the 60W bulb starts to glow after a second or so if a hard drive is plugged in - so now i immediately turn it off.
Like I mentioned, this looks like an older PSU, so it may need more load on the 5V rail to regulate properly. A hard drive *won't* do in such cases, as HDDs typically use more power from the 12V rail. 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. This will ensure that the 5V rail has enough load to run well. If the 12V rail starts looking too high (over 12.6V), then you can add a few resistor to it too, to balance out the load.

Also, you *can't* load the PSU with just the 60W series bulb, much less a motherboard (though unplugging the 12V CPU power connector may work in some cases.) Something to the equivalent of 120-150W for the series device might be the minimum you'll need if you want to run just an HDD or the bulbs I suggested.

Quote:
Originally Posted by socketa View Post
1. Yes
2) Visually look OK
3) Yes
4) As above
For 2), you can't rely on visual check. That's why I mentioned check ESR and capacitance (if you have one of those GM328 or similar testers.) I've encountered enough Ltec caps that looked "fine" but measured out of spec to know a visual inspection is not good enough with these. Same with Teapo... and many of the other crap cap brands. The UCC KZE are probably the only ones that don't need to be checked, because they are known reliable brand and series.

Quote:
Originally Posted by socketa View Post
OK, i plugged in a motherboard that i don't care about too much (as it seems to produce a smell in the vicinity of the northbridge) and also connected a hard drive - didn't plug in the 4pin CPU connector
After a few touches of the PS-ON to Ground (where the fans just twitch), the PSU fully starts up and the CPU fan spins but revs at a rate of about 1hz, voltages are good.
The PSU wont start up (it's fan just twitches once) if i also connect the 4-pin CPU connector (but that could be due to an issue with the motherboard)
So without the 12V CPU power connector, the PSU starts and the voltages are good? If so, then you were impedance-limited by the series bulb on the AC input on the other tests for sure.

And again, with the series bulb(s), you just *cannot* put too much load on the PSU and expect it to work. From what I've been experimenting on lately with low AC input / series power limiting and ATX PSUs, the general tendency appears to be that you can pull only about 1/6 to 1/8 of the power rating of the series limiting device and still have the PSU work. So with a 60W bulb, you might be able to pull about 10 Watts from the PSU, tops.

As for the fan on the motherboard revving with 1 Hz... that's probably the motherboard boot-looping and restarting, because the CPU power is not connected, so that may be normal. And of course it not starting with the series bulb in place when the 12V CPU connector is inserted is expected.

Quote:
Originally Posted by socketa View Post
Was it OK that i replaced the 10uf 50v Teapo SC with a low impedance (ESR measures about 0.7 ohms) Rubycon YXF?
100%
The ESR of the small caps is not too important - so long as it doesn't go sky-high, of course... or at least to unreasonably high levels. Just check the datasheeet. It's not uncommon at all for small 5x11 mm caps to measure over 1 Ohm ESR... but also OK if they measure less too.

Last edited by momaka; 04-24-2021 at 03:41 PM..
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Old 04-26-2021, 05:30 PM   #38
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Default Re: Compaq PDP-121P - Baked MOV

Quote:
Originally Posted by momaka View Post
The ESR of the small caps is not too important - so long as it doesn't go sky-high, of course... or at least to unreasonably high levels. Just check the datasheeet. It's not

uncommon at all for small 5x11 mm caps to measure over 1 Ohm ESR... but also OK if they measure less too.
Thanks, i measured the ESR of that cap that i put in backwards, and it's now 4 ohms, whereas it was initially 1.2 ohms, and the capacitance was about the same as before; so now i really know how to turn a low ESR capacitor into one that's not low ESR.

That 1hz revving is the Hard drive trying to spin up

I have 3 parallel series limiting bulbs 2X60W 1X75W.
When i plug it into the original compaq computer, with the 3 series limiting bulbs, the front led blinks and it means "Power supply overload" according to the documentation.
When i plug it into my test board it wont power up via the header pins, but will power up with another couple PSUs via the header pins. (so either the PSU still has some issue,

or more (or greater wattage) paralell "series limiting" bulbs are required.
The test motherboard powers up with other PSU's via the header pins, and there is a MOSFET on the motherboard that gets super hot and smells
The test motherboard doesn't power up with this compaq PSU via the header pins. It only powers up via the paperclip, and the MOSFET doesn't get hot like it does when i use a good PSU

Anyway, using the test motherboard, the Compaq PSU, and 3 series limiting bulbs ( 70W 60W 60W):
With the 3 series limiting bulbs, the motherboard doesn't start easilly if 4pin CPU connector is plugged in (have to jiggle the paperclip lots, until it fully starts)

ATX 20 pin connected. :
Bulbs-----Total power-----3.3V-----5V-----12V-----Neg12v-----PG
3 bulbs---15.0W-----------3.27-----5.05---11.47---11.79------4.39
2 X 60W---15.1W-----------3.27-----5.05---11.47---11.79------4.39
1 X 60W---15.6W-----------3.27-----5.05---11.47---11.79------4.39

ATX 20 pin connected. CPU 4-pin connected :

3 bulbs : 15.1W-----3.27-----5.05-----11.46----11.23 to 11.80-----4.39
2 X 60W : 15.2W-----3.27-----5.05-----11.46----11.26 to 11.80-----4.39
1 X 60W : 15.7Wh----3.27-----5.05-----11.46----11.21 to 11.80-----4.39 Bulb glowing

ATX 20 pin connected, hard drive connected (and trying to spin up) :

3 bulbs : 18.5W-19.9W-----3.27-----5.05-----11.46----11.23 to 11.80-----4.39
2 X 60W : 15.5W-19.9W-----3.27-----5.05-----11.46----11.26 to 11.80-----4.39
1 X 60W : 13.3W-----------5.05-----11.46----11.21 to 11.80-----4.39 Bulb glowing and PSU shuts down
1 X 75W : 20W - Shuts down

ATX 20 pin connected. CPU 4-pin connected. Hard drive connected (but not trying to spin up) :

3 bulbs : 12.6W-----3.29-----4.47 to 4.45-----10.98 to 10.92-----11.30 to 11.8-----62mV
2 X 60W : 12.7W-----3.29-----4.47 to 5.51-----10.88 to 10.92-----11.26 to 11.8-----62mV
1 X 60W : 13.3W-----3.29-----4.47 to 4.49-----10.83 to 10.90 >>> PSU shuts down

Adding a 23W 12V bulb to the 5V rail, in any of the instances above, immediately shuts down the PSU

Adding a stove element to the paralell bulbs, doesn't make the PSU any easier to start when the 4 pin CPU connector is plugged in (sill have to jiggle the paperclip, and get lucky, until it fully starts up),
neither does it help to start the Compaq computer, with or with the 4 pin CPU connector (the front panel flashing LED still indicates "Power supply overload") - i only used the front panel button because the other PSU starts it up using that button.
And connecting a molex to the hard drive makes no difference either

Each time that i push the front panel power button, the fans twitch, and i can hear that the speaker gets power. The 5VSB rail is good and at least 5V, 1V, and 0.5V appear on the 12V, 5V, and 3.3V rails when i push the power button, but they instantly get cut off.

Any more ideas?
All that i can think of is maybe the computer is sensing high ripple on one of the rails because of a bad output cap, despite good and stable voltages.
Is that a known possibility?

Last edited by socketa; 04-26-2021 at 05:32 PM..
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Old 04-26-2021, 09:53 PM   #39
momaka
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Default Re: Compaq PDP-121P - Baked MOV

I will repeat...

You CANNOT test motherboards with series power/current limiting device. Period. There is no point in doing this test.

You can use series power limiting only with very low and known loads: i.e. 12V car bulbs, resistors, and etc., but the total load from these must NOT exceed 1/6-1/8 of the series device's power rating. And that doesn't even factor in PSU inefficiencies. So a safer bet is 1/10 of series limiting device's power rating, MAX. So for a 60W bulb, don't put more than 6W of load on PSU output. Anything more, and no guarantee how the PSU will behave.

That said, you can also monitor the PSU's AC input after the series limiting device. If the voltage drops below 180-190V AC, there's a good chance it will not work with that low of an input.

Quote:
Originally Posted by socketa View Post
All that i can think of is maybe the computer is sensing high ripple on one of the rails because of a bad output cap, despite good and stable voltages.
Is that a known possibility?
Yes, that's a possibility.
Which is why I suggested to check output caps with ESR meter.

Last edited by momaka; 04-26-2021 at 09:57 PM..
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Old 04-28-2021, 02:23 AM   #40
socketa
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Default Re: Compaq PDP-121P - Baked MOV

(Please read all first)
Quote:
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.
Quote:
So for a 60W bulb, don't put more than 6W of load on PSU output
Quote:
Something to the equivalent of 120-150W for the series device might be the minimum you'll need if you want to run just an HDD or the bulbs I suggested.
I 'only' have about 180W total of bulbs connected, so i suspect that that's why the PSU didn't start when i connected only the 23W bulb - i had a feeling that the 35 watts that you suggested was too much. Feel free to correct me if i'm not seeing clearly enough here, because if ever there was an age of folk not seeing clearly, then we are in it right now.
It was after that that i just plugged the 10pin atx connector into the test motherboard, just to see what would happen - as you can see, with only the ATX 20 pin connected, the board's pulling 15W max, which is half the amount of a 30W car bulb.
According to this additional info that you gave, "10 to 35 Watts" will not necessarily work, as it would have to be "10W to 18W"

So, accordingly, it no surprise that 23W bulb (and only the 23W bulb), has no effect on starting the PSU.

[Edit]
i just realized, and figured out, that [email protected] is the same as [email protected], 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
[end Edit]



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 a motherboard ATX, but not when loading the 12V rail with any resistor. Obvioulsy something's happing within the motherboard that's causing the 12R to come into spec, that i can't 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:
Quote:
If 12V or 3V3 are too high, you may have to put a small load on those as well
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?

In case anyone's interested, this PSU's max power ratings are
+3.3V _____ 15A
+5V_____ 11A
+12V _____ 5A
+12.8V_____ 7.5A
-12V_____ 0.15A
5.05V Aux_____ 3A

Last edited by socketa; 04-28-2021 at 03:51 AM..
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