Old Compaq Armada internal PSU repair

[yuuki47]

Re: Old Compaq Armada internal PSU repair

Well, for now, I will wait for the capacitor

Meanwhile, I realized that I don't have a hard disk to install an operating system into when I do get this thing booting.

I'm aware of SD card adapters but laptops use 2.5 inch IDE and not the normal size one, so I'd have to get both 2.5inch adapter and an SD card to IDE adapter. Which starts to make things really pricey...

I found this CF to 2.5 IDE adaptor but it looks kind of dodgy. The connector doesn't seem to be keyed, for one. How can I trust that I won't plug it backwards?

On top of that, it seems to directly connect the compact flash card. Is compact flash IDE in disguise and I don't know it????

I mean, this is a power supply board so I should post this elsewhere, but this is what I need to figure out now...

[stj]

Re: Old Compaq Armada internal PSU repair

compact-flash is ide.
but that adapter is no good - you need pins not a socket

if you want to be retro, install FreeDos
https://freedos.org/
it works really well - i use it sometimes
runs all dos software but also recognises larger drives and supports fat32

[stj]

Re: Old Compaq Armada internal PSU repair

https://www.aliexpress.com/item/33026043666.html

or

https://www.aliexpress.com/item/1005004733494391.html

[yuuki47]

Re: Old Compaq Armada internal PSU repair

But you need pins not a socket

Oh, crap. Even though I've been messing with retro tech for years this completely skipped my mind. God...

I've never bought from Aliexpress but they are cheap as hell, almost alarmingly cheap. They seem to guarantee that I'll get my items within 15 days if I buy over £9 of things.

My problem with CF is that the adapter is cheap, but the CF cards themselves are expensive as balls. £15 for 4GB of memory? You can get a 32GB SD card for £5...

Also, I did some research, and apparently CF cards aren't very reliable for storing OSes because the SWAP/PAGE file Win98 and the like use to temporarily store memory data causes constant writes that wear down CF very fast.

Well, eBay has a single SD card adapter for £22 while Ali has it for £5. I was able to get two SD card adapters plus 2 compact flash adapters for £17. That's less than eBay, and I get three extra things for future vintage projects

From my research, Aliexpress seems okay, so I'll buy and hope that all goes well...

By the time the 15 days pass, I should hopefully be able to get the PSU repaired. If not, I'll resort to the barrel plug. Thanks again!

[stj]

Re: Old Compaq Armada internal PSU repair

one advantage over ebay is aliexpress gives you shipping options.

never use super cuano - they are slow as fuck.
use aliexpress standard or saver and to the u.k. can usuallly be about 2 weeks

[yuuki47]

Re: Old Compaq Armada internal PSU repair

Well, I replaced the startup capacitor today — still nothing.

I also checked the voltage across the big brown cap next to the startup cap and the voltage was 350v. Makes a pretty impressive bang when you discharge it too. Something is telling that this isn't the problem...

[stj]

Re: Old Compaq Armada internal PSU repair

then take some close pictures

[yuuki47]

Re: Old Compaq Armada internal PSU repair

My camera sucks and it really can't focus up any closer than this, but here are some photos from different angles.









That M-Tester I have can test all kinds of things, let me know what you want to test and I'll take it out!

[stj]

Re: Old Compaq Armada internal PSU repair

oooo......
look at the black coated hybrid circuit closely - does it have a few burns on the left side?
it looks darker in places

[yuuki47]

Re: Old Compaq Armada internal PSU repair

Definitely not. It might be the camera making it look that way but the colour looks pretty consistent IRL. I don't see any burns, BTW There's two of them...

[stj]

Re: Old Compaq Armada internal PSU repair

check all the diodes with a multimeter for shorts

[yuuki47]

Re: Old Compaq Armada internal PSU repair

Found a short at the diode besides the output caps

But even after removing it to double check, the short is still there. And the diode itself forward biases fine once I removed it, so its not the diode causing the short... its something else.

There is a tiny SMD diode underneath the PCB (D7) where the output caps are. But I'm not sure how to test that.

[momaka]

Re: Old Compaq Armada internal PSU repair

That black output toroid by the output caps... check if the the windings from the two wires on it aren't shorting out. I believe that's a common-mode output choke (meant to suppress HF EMI/RFI.) I also see a bunch of Zener diodes (Z2, Z5, Z7, and etc.) on the secondary side. Check those too (just like you would a regular diode.)

Another one: ceramic cap C16 - check for short-circuit. I don't think that's connected to anything high power, but check in just in case. Feel free to check the other ceramic caps too, though I doubt they would be the problem.

Found a short at the diode besides the output caps

Is that the small red one?

If so, that might be a protection diode. If it's connected across (in parallel) with the caps, then the main rectifier might be bad.

Did you check the diode/transistor mounted on the heatsink by the output caps? That one should be the main rectifier. It's the one in the upper left corner of this picture you posted:
https://cdn.discordapp.com/attachmen...126_220859.jpg

[yuuki47]

Re: Old Compaq Armada internal PSU repair

Yes, I checked the zener diodes on the secondary side --- all good!

There are two diodes besides the output caps (should have mentioned) a regular black one and a zenner one at the edge of the board. No short on the zenner, but a short at the black diode. It's not the diode causing the short tho, its something else.

I did check the transistor besides the output caps with the multimeter. Don't those fail shorting the gate to one of the pins? Anyway, the short isn't coming from there because the Gate and Source pins have 10k ohms between them. No short at the Drain either so I'd say its good.

The ceramic C16 cap has 19k ohms across it so certainly not it.

About the filter. You mean the one with the black ring? I'm not sure how that thing works but thankfully it has positive and negative markings on the board so I probed across those. No short at all.

There is a very small disc-like capacitor by the big transistor at the heatsink... not sure you can see it from the pictures. I have a feeling that this is causing the short. It does have 0 ohms across it but other things that are on the same line do as well so that's not exactly a clue.

I mean, is there any way to find the short other than to try pulling out components at random? If it wasn't a PSU, I'd feel for heat with my hands to see what gets hot, but it is a PSU so I ain't gonna do that!

I'll pull out that disc cap tomorrow...

[stj]

Re: Old Compaq Armada internal PSU repair

maybe one of the electrolytics is shorted,
it's rare but i happens

[momaka]

Re: Old Compaq Armada internal PSU repair

Something is not quite right by that description. It sounds as if you're measuring a short-circuit on the output, but the fact that there was no short-circuit between the positive (+) and negative (-) marks of the output toroid choke suggests there is no short-circuit on the output. Therefor, I doubt the output caps would be shorted either (and you measured them already with the transistor tester, if I remember correctly.)

I did check the transistor besides the output caps with the multimeter. Don't those fail shorting the gate to one of the pins? Anyway, the short isn't coming from there because the Gate and Source pins have 10k ohms between them. No short at the Drain either so I'd say its good.

They can fail short-circuit from Gate to Drain or Source, but the most common failure mode for MOSFETs is short-circuit between Source and Drain. So I guess check for short-circuit between any of its 3 pins.

About the filter. You mean the one with the black ring? I'm not sure how that thing works but thankfully it has positive and negative markings on the board so I probed across those. No short at all.

Yup.
It's a common mode choke in this case, as it connects the negative (-) output wire of the PSU to the negative (-) side of the output caps and the positive (+) wire of the PSU output to the positive (+) side of the output caps.
Essentially, it reads like a piece of wire - i.e. short-circuit between the two "positive" ends and same between the two "negative" ends. It should not read a short-circuit between positive and negative, though, which you confirmed. This means the output of the PSU isn't shorted.

There is a very small disc-like capacitor by the big transistor at the heatsink... not sure you can see it from the pictures. I have a feeling that this is causing the short. It does have 0 ohms across it but other things that are on the same line do as well so that's not exactly a clue.

Barely, but yes.
Looks like a ceramic disc cap. Those can short-out too. But before you remove it, see what it's connected to. Often times these are connected in series with a low-value resistor (typically less than 10 Ohms), and this RC network is typically across the secondary-side output pins of the transformer. The transformer output will read like a short-circuit due to low number of turns, so checking across such ceramic disc caps usually shows the low resistance of the series resistor.

That being said, I can't make it out from the pictures how this ceramic cap is connected, so I can't say if that's what you're seeing or not.

I mean, is there any way to find the short other than to try pulling out components at random? If it wasn't a PSU, I'd feel for heat with my hands to see what gets hot, but it is a PSU so I ain't gonna do that!

Well, you wouldn't be able to feel for any hot components, because the PSU immediately shuts down when it detects a short-circuit on its output.
In terms of not removing components... well, you don't have to remove resistors, unless they test much higher resistance than the one printed on their sleeve (since resistors go high resistance or open-circuit when they go bad.) On the other hand, it's ceramic caps, diodes, and transistors that like to go short-circuit and may need to be removed out of the circuit to test properly.

There are other ways, but it's hard to tell from the two pictures of the straight top and bottom. It could be that all of the "shorted" parts you found are normal for the circuit. Or not. Sometimes when I get stuck on a PSU like this, I go back to checking things on the primary side. In the case of this adapter, if you haven't checked already, see if the primary side auxiliary winding rectifier (diode) is OK. It's usually directly connected to one of the pins on the transformer (or through a low-value resistor) and then feeds into that small "startup" cap you replaced.

But I guess let's finish with the secondary side first to make sure everything is OK. Indeed check the ceramic disc cap tomorrow and also the transistor on the heatsink for short-circuit between Drain and Source. Also, in regards to D7 - just check it in both directions between any two pins. If any two show a short-circuit, not them down. From the looks of the underside, it doesn't appear that any of the pins are connected together, so I don't think you should find any of the pins to read short-circuit between them. But do correct me if I'm wrong on that.

[yuuki47]

Re: Old Compaq Armada internal PSU repair

Good morning!



I went ahead and did more testing today. The disc cap was hard to pull out because it was behind the transistor, so I had to take this out as well. When I did, I discovered that this was actually the coil, and this was actually what had the short across it, not the ceramic cap the ceramic cap is fine.

Here are some markings to help you out:

Blue --- Transistor (by the output caps, which is the secondary side I think)
Red --- Coil windings connector 1
Light-blue --- Opposing coil windings connector 2
Brown --- Actual location of the disc cap
Green --- Black-coated PCB near transistor
Purple --- Startup Cap

Ignore the brown mess, its flux not a burn. The solder is really old so it needs tons of flux to actually melt. That's where the diode I pulled out to test was.

And now that this big heatsink and MOSFET are gone, here is a better view of the output caps.



You can see this white-looking plastic thing. That's part of the coil, it looks like the connector for the windings. So it makes sense that its shorted. But even if probe between the opposite windings, there is still a short! Is it supposed to be that way? It has two windings, gold and red, so I'm guessing not. But its probably not the coil itself causing the short, maybe another component.

That said, since I happened to take out the MOSFET, I tested it. It's good.



Mamoka --- it looks like the ceramic cap is connected in series with resistors like you said, and its right next to them too. You can see them (R42).

Strangely two of those resistors are connected to a big blob of solder. That big blob of solder connects the MOSFet's heatsink. That's weird... what's going on there? But anyway, there's no short at the disc cap after all.

Speaking of the coil, its connected in series to the black-coated PCB. But the black coated PCB is fine, no shorts and there are 1.5k ohms across the pins. I'd say its good. The white component continues to be shorted though...

About the diode directly connected to the transformer. I found it and no short!



The red markings are the connection between the cap and the diode and the green ones are between the diode and the transformer. I was able to read the model number and its a n-channel mosfet (NEC K2141)

It has 1.5k ohms between Gate and Source. I tried to discharge the mosfet to see if the drain closes properly but probing between source and drain the resistance slowly goes up... likely because of some capacitor in the line :/

There's no short though so is it reasonable to say that its okay? I really don't want to take it out, I have no de-soldering braid and the way I desolder is by stripping thick gauge wires and coating them with flux to suck the solder. Weird but works. Its a huge pain though.

It was also covered in glue underneath (you can see it in picture). Should I re-cover it?

[stj]

Re: Old Compaq Armada internal PSU repair

something thats been bugging me,
whats the large cap that is on it's side?

[yuuki47]

Re: Old Compaq Armada internal PSU repair

something thats been bugging me,
whats the large cap that is on it's side?

I think its the main voltage cap? Is that what its called? I always see these super big capacitors in old supplies like this one. Its at the primary side so I'm guessing it acts as a buffer.


Its another KMG rated for 400v 65uf. I'd say it works because every-time I discharge it, it makes a pretty big bang. Like a firecracker! But I also measured the voltage across it when the PSU is on, there are 350v there which explains the bang :P

Its not the only high voltage cap though, there is a black nichicon rated for 400v 10uf so I guess those sparks could come from that instead since they are on the same line. And by that I mean, I probed them with the multimeter and there is a direct path across both of their legs.

Should I take both out and put them on the M-Tester?

[stj]

Re: Old Compaq Armada internal PSU repair

no - dont risk the tester - those caps can probably re-constitute some charge after you short them.
the black one may be 100uf btw, if it was 10uf it would be a lot smaller i'm sure