Yeah good luck ! A friend had one a decade ago and we could not find any service manuals back then, only a very simple user guide iirc.

silvervest wrote on 2021-09-04, 09:44:

Hey folks, […]
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Hey folks,

I'm looking for a service manual or schematics for the Compaq Presario 425/433 series, specifically the power supply board. My unit seems to power on for a second then loses power to the PC, but the high voltage for the CRT is working fine.
Can't see anything obvious, so want to refer to schematics/manual to get some better ideas.

I've had what I'd consider to be a pretty good googlin' and can't find anything for this model. The power board screams Philips CRT manufacture to me just based on the layout and silk screen markings, but I don't seem to be able to find an equivalent and none of the ID numbers on the board come up with any good results.

Thanks in advance!

Hi! I have the same issue with my 425. Did you solve the problem? Thanks Börre from Sweden

silvervest wrote on 2021-09-04, 09:44:

Hey folks, […]
Show full quote

Hey folks,

I'm looking for a service manual or schematics for the Compaq Presario 425/433 series, specifically the power supply board. My unit seems to power on for a second then loses power to the PC, but the high voltage for the CRT is working fine.
Can't see anything obvious, so want to refer to schematics/manual to get some better ideas.

I've had what I'd consider to be a pretty good googlin' and can't find anything for this model. The power board screams Philips CRT manufacture to me just based on the layout and silk screen markings, but I don't seem to be able to find an equivalent and none of the ID numbers on the board come up with any good results.

Thanks in advance!

Borre79 wrote on 2025-06-12, 20:18:

Hi! I have the same issue with my 425. Did you solve the problem? Thanks Börre from Sweden

No, sorry. It's been in storage for years now, I should dig it out and have another run at it though!

Same problem here.

I've finally gotten off my butt and had another run at my unit and got it working. Turns out a BC557 in the corner was gating an SCR on the +5v rail, clamping it short on power up.
To confirm the fault, I performed voltage injection on the power rails until I found that +5v caused my bench supply to go current limiting at around 3-4v.

Here's the PSU with the SCR and its controlling diode and transistor circled

- replacing the transistor allowed the power rail to come up cleanly and everything worked after that.

Hope this helps!

silvervest wrote on 2026-02-01, 04:14:

I've finally gotten off my butt and had another run at my unit and got it working. Turns out a BC557 in the corner was gating an […]
Show full quote

I've finally gotten off my butt and had another run at my unit and got it working. Turns out a BC557 in the corner was gating an SCR on the +5v rail, clamping it short on power up.
To confirm the fault, I performed voltage injection on the power rails until I found that +5v caused my bench supply to go current limiting at around 3-4v.

Here's the PSU with the SCR and its controlling diode and transistor circled

The attachment 20260131_185529-markup.jpg is no longer available

- replacing the transistor allowed the power rail to come up cleanly and everything worked after that.

Hope this helps!

Tremendous info! Do You happen to know the components values?

Spitz wrote on 2026-02-01, 20:45:

Tremendous info! Do You happen to know the components values?

The SCR was a TYN610P, and the transistor was a BC557. Not sure on the diode, but only the transistor needed replacement

silvervest wrote on 2026-02-01, 21:34:

Spitz wrote on 2026-02-01, 20:45:

Tremendous info! Do You happen to know the components values?

The SCR was a TYN610P, and the transistor was a BC557. Not sure on the diode, but only the transistor needed replacement

Thank You!!!!

silvervest wrote on 2026-02-01, 21:34:

Spitz wrote on 2026-02-01, 20:45:

Tremendous info! Do You happen to know the components values?

The SCR was a TYN610P, and the transistor was a BC557. Not sure on the diode, but only the transistor needed replacement

Can we use them interchangeably? Some strange values keep popping up when I compare them online.

The BC557 is 100ma and 40V max - is that sufficient?

In my case there's MP2907 so can You confirm please that You Have changed it to BC557?

Can confirm, I replaced with a BC557.

However -- I was dubious about this replacement as well, as it looked to me like it'd been replaced before in my unit. There was obvious rework on the underside and the dead 557 was identical to the ones I already had on hand, which are from the local electronics store (Jaycar) in Australia. It's entirely possible that someone else has repaired this unit in the past by replacing the NPN that was there from factory with what they had on hand (the 557) and it failed again later, so YMMV!

silvervest wrote on 2026-02-03, 21:43:

Can confirm, I replaced with a BC557.

However -- I was dubious about this replacement as well, as it looked to me like it'd been replaced before in my unit. There was obvious rework on the underside and the dead 557 was identical to the ones I already had on hand, which are from the local electronics store (Jaycar) in Australia. It's entirely possible that someone else has repaired this unit in the past by replacing the NPN that was there from factory with what they had on hand (the 557) and it failed again later, so YMMV!

Thanks. Mine is looking mint bottom side... From the datasheet they vary much. I just replaced it and ... It's not wotking. Will order 29o7!

Well, this was the issue on my unit... you may want to perform the same voltage injection on the power rails and see if you have a short to reproduce my fault.

Here's the pin map of the PSU connector that I determined. Pin 1 is marked on the silkscreen:

1Pin | Wire colour | Rail
2----+------------+-------------------
31   | WHITE      | -12v
42   | ORANGE     | -5v
53   | GREEN      | GND
64   | -          | -
75   | RED        | +5v (power good?)
86   | YELLOW     | +12v
97   | BROWN      | GND
108   | PURPLE     | GND
119   | BLUE       | +5v
1210  | BLACK      | +5v

silvervest wrote on 2026-02-03, 22:53:

Well, this was the issue on my unit... you may want to perform the same voltage injection on the power rails and see if you have […]
Show full quote

Well, this was the issue on my unit... you may want to perform the same voltage injection on the power rails and see if you have a short to reproduce my fault.

Here's the pin map of the PSU connector that I determined. Pin 1 is marked on the silkscreen:

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1Pin | Wire colour | Rail
2----+------------+-------------------
31   | WHITE      | -12v
42   | ORANGE     | -5v
53   | GREEN      | GND
64   | -          | -
75   | RED        | +5v (power good?)
86   | YELLOW     | +12v
97   | BROWN      | GND
108   | PURPLE     | GND
119   | BLUE       | +5v
1210  | BLACK      | +5v

Thank You very much!

Tried to change all the component that are responsible for switching power but failed. From time to time, very very seldom it's turning on. But right after that it's not powering on back. Maybe a good idea would be to search for psu board...

Yep. Turned on after few days, working like a charm. But no worries it won't turn on once I will tun it off.