HP 6715s - very odd issue

[spleenharvester]

Hey all. Posted this again here in the appropriate section. Other thread can be deleted.

I have a water damaged HP 6715s motherboard here.

It works and boots fine into Windows!

But ONLY IF leg 1 on Q75 is bridged with the DC jack's ground.

Full schematic here: http://www.mediafire.com/?a317h9te1hxup3e



I replaced Q75 to only meet the same problem. I thought Q75 was faulty since the multi read 1 on one leg, but it does the same to other identical transistors on the board.

So presumably, something that's meant to lead to ground, has given up?

Water damage was only confined to a very small 3x3cm square on the underside of the board, as residue. The spillage had long dried up.

[spleenharvester]

Re: HP 6715s - very odd issue

What the heck?

I just shorted Q75 with DC jack-in by accident. Giant plume of smoke comes up from mosfet Q18, I think oh crap, this thing's done for.

The board now works perfectly, while one leg on Q18 has literally blown off..?

What even?

[katzcaps]

Re: HP 6715s - very odd issue

Sounds like its fixed to me!

[spleenharvester]

Re: HP 6715s - very odd issue

Hahahaha I really have no idea what to do with it now, can't seem to get hold of the exploded mosfet anywhere either! Ah well

Are there any equivalent replacements around for this?

[Tuomo]

Re: HP 6715s - very odd issue

Deleted Message

[spleenharvester]

Re: HP 6715s - very odd issue

Cheers for the info mate.

Any idea what I could change Q18 with? I can't get hold of an FDS6673BZ at any good price!

Thanks again

EDIT: It's just struck me, maybe Q18 was at fault all along. I guess we may find out.

[Tuomo]

Re: HP 6715s - very odd issue

Cheers for the info mate.

Any idea what I could change Q18 with? I can't get hold of an FDS6673BZ at any good price!

Thanks again

EDIT: It's just struck me, maybe Q18 was at fault all along. I guess we may find out.

This is possible, I have to make sure at I understand correctly that circuit.

[spleenharvester]

Re: HP 6715s - very odd issue

It's leg 4 that's blown off for the record. I'm guessing either C43 or R75 would be the other two candidates for being at fault for the initial problem.

R75 appears fine and reads in at ~21.8kOhms, C43 reads in at 350ohms resistance, but after discharging, doesn't recharge in the opposite direction as per this thread https://www.badcaps.net/forum/showpo...45&postcount=4

[spidey]

Re: HP 6715s - very odd issue

it looks like, first it was a protection circuit error, but now q18 is short.

[ben7]

Re: HP 6715s - very odd issue

R75 appears fine and reads in at ~21.8kOhms, C43 reads in at 350ohms resistance, but after discharging, doesn't recharge in the opposite direction as per this thread https://www.badcaps.net/forum/showpo...45&postcount=4

Is Q75 connected while you are doing this?

I doubt R75 is bad - it is a 220K resistor - that would take a high voltage (hundreds of volts) to burn that sucker!

I think you should try replacing Q18. It could have been shorted D-G, and so when you shorted out Q75 (assuming you shorted it's D and G together), a large current flowed through Q18 and burnt out pin 4 (the G of the MOSFET).

[spleenharvester]

Re: HP 6715s - very odd issue

Q75 is indeed connected. It was a brand new replacement too so should be okay.

I've gone through my trashed motherboard stack and found a FDS6679!. It seems to be very similar to the FDS6673BZ. Worth a go?

EDIT: Also found a FDS6670S:

[ben7]

Re: HP 6715s - very odd issue

Q75 is indeed connected. It was a brand new replacement too so should be okay.

I've gone through my trashed motherboard stack and found a FDS6679!. It seems to be very similar to the FDS6673BZ. Worth a go?

EDIT: Also found a FDS6670S:

That could work, but it has a slightly higher on-resistance, a slightly lower continuous current (13A vs original 14.5A), and a lower pulsed current rating of 50A vs the original's 75A rating.

Your FDS6670S won't work because it is an N-Channel MOSFET, not a P-Channel!

I would say it is worth a try.

EDIT: The FDS6679 is lacking a gate-source voltage clamp! Not sure if that would be a big deal...

[spleenharvester]

Re: HP 6715s - very odd issue

Also just found an FDS6675A but it looks further off to me:

If it's no better I might give the 6679 a try.

Cheers

[ben7]

Re: HP 6715s - very odd issue

Also just found an FDS6675A but it looks further off to me:

If it's no better I might give the 6679 a try.

Cheers

It isn't better, it has a lower current rating than the 6679, and higher on-resistance.

[spleenharvester]

Re: HP 6715s - very odd issue

Hmm. I'll look through some more scrap boards later and see if I can come up with something closer, even if it's not a Fairchild FET.

Also, if anyone is in the UK, and is after a really oddly specific part, feel free to PM me - I have quite a few trashed boards.