Hi
Congratulations on launching 18v!

Ilcecco wrote on 2025-01-30, 12:21:

on the VCC section i see sort of PWM signal (very very short) but no VCC is generated

take a look at my post from 2025-01-21, 09:33. I included a screenshot of various signals there. The 18_INT_PWM lasts 25ms, while the other PWM signals are very short; if I remember correctly, they last for several cycles, around 200us each. The voltage will not be generated because the signal lasts too briefly.

Ilcecco wrote on 2025-01-30, 12:21:

On IC5 i have a sort of flat 12V from P17 where i'm expecting to see a PWM for the N9V

Is there any signal at P4 IC5; P6, P7 IC6? Maybe you have a faulty IC6 like I do.

Ilcecco wrote on 2025-01-30, 12:21:

How does the DRV from IC3 on pis 18 19 20 should look like ? I have a flat tension

I haven't checked this, but I suspect (due to the lack of any documentation for this circuit) that there are only control pins on this side, possibly some transmission between IC3 and IC13, while the PWM signal is generated by IC3 on pins 18, 19, and 20.

Ilcecco wrote on 2025-01-30, 12:21:

Is it correct that IC3 receives a 20Mhz clock on P1 and gives back to IC13 a 10Mhz clock from its P2?

Yes

Ilcecco wrote on 2025-01-30, 12:21:

Please tell me i'm not dealing with a broken IC13 or IC3 that would mean the board in unfixable 🙁

I am an optimist; I believe that IC13 and IC3 are working fine. 😀

Making a lot of additional checks i found that IC13 is receiving feedback from the attempt to pull up the buck converters with the exception of P12V on IC13 PIN 30. (P29,31,32 are receiving "something" back). in other words IC6_P5 is not receiving anything visible.

What i don't understand is that the V12 main is passing through R35 10k and then apparently much reduced. Even if V12_EN is pulled up as DRV and P12V_PWM_DRV is generated and received on IC5 with a duty cycle of 50% (that is usually the starting point i guess) the P12V_PWM is a signal very very low at around 2V.

If i got everything right : the PWM voltage should be higher with IC5 pulling it down to ground according to the DRV duty cycle and frequence. If the voltage normally present on P12V_PWM is very low from the beginning, even pulling it to ground is not looking sufficient to properly drive Q22 and Q23. I see no other source of voltage for P12V_PWM other than the 12V main via R35 but i was expecting something around 5V to pass though and supposed to be pulled down, no?

also the signal on P12V_EN is present at R121 R13 but very very weak in terms of voltage difference, but i struggle in understanding again what i should find given it's the 18V after the 47K resistance.

Ic6 is fine (replaced like yours already)...

If you don't have a PWN signal at the Q4 gate, then the only things left to check are diode 12 and transistors Q22, Q23, Q4, assuming that the resistors and connections are functioning properly.

While reading carefully the whole guide from scratch i found this sentence : "Furthermore, the push-pull transistors driving the DC-DC converter MOSFETs are of a different type, resulting in different values for the accompanying resistors."

My board is a FIVPS4, does it mean that the kind of transistors of the schematic is different from the original ones on my board ? I replaced some of them with the kind reported on the schematic, did i mess up ?

Guys, i'm really happy to say that i see the light at the end of tunnel!

My power supply now generates all PWM signals, all mosfets are on (last i had to replace was Q6) and now the it's turning on with solid LED light !!!!

It's draining about 0.11A @12v from my bench power supply.

I verified the voltages output and they look right.

Now the bad news after the fantastic ones : the problem is now moving elsewere (as expected) given that once connected to the motherboard the led blinks and the machine doesn't fire up (no floppy, no HDD).

Time for a new challenge!

Let me thank you all for everything you did so far 😀

Let's say i got 50% success : the PC is alive but i was able to turn it on only once. The PSU is unstable and can't always boot correctly. Trying offload it starts quite more often and when it does it doesn't turn off.

I think the problem is in the push-pull couples where i had to replace some of the transistors but with new ones that are probably not exactly equal to the the original ones of my FIPVS4. It's struggling to overcome the first attempt to turn on after the initial 25ms.

PWM signals are always generated but look like IC13 is often not satisfied of the result. Few times the PSU also emitted a small "vibrating" (inductors?) noise like something out of sync.

At least i know the motherboard and display are alive and that once the PSU will be stable i will only miss the HDD and FDD to connect back!

Any suggestion that can explain the current behavior? I would exclude broken traces given it's sometimes able to turn on and when it does doesn't turn off anymore...

Hello, congratulations on reaching the next stage 😀 First of all, I would check the correctness of all the voltages generated by the power supply. Does the power supply always behave unstably, or only when the motherboard is connected? If it's the latter, perhaps one of the circuits on the motherboard or the display is causing too much load on the power supply.

Hello Piotrosz,

the PSU is unstable regardless if connected to the pc mainboard or not. If it succeed to turn on (often but not always) after the first 25ms then it works and stays up, otherwise it shuts down after the 25ms and that's it.

That's why i'm thinking there is something in the push/pull transistors i replaced... like if they are barely able to do their job and/or no always being quick enough to properly drive mosfets.

Now i'm waiting for the delivery of some 2SC3325 (i used equivalents so far), what i'm worried about is if a FIVPS4 revision was using such transistors or not. The original ones have nothing written on them so i'm not exactly sure of their kind but only if they're PNP or NPN...

Hi,
well, only the measurements are left. If there is a PWM signal, look for a circuit with inappropriate voltage. I managed to power up the motherboard – it turned out that I damaged the RTC chip, and the board was stopping at code 03. What's left to do is fix the lack of backlight for the display, a non-working hard drive, and a floppy disk drive that is not functioning correctly (it reads disks, but the motor keeps spinning even when there is no disk in the drive). On a positive note, I managed to boot the MS-DOS command prompt from the floppy disk drive. If I find the time after finishing the repairs, I will upload a video of the restoration of my T1600.

Update:
1. The signal for controlling the backlight is provided through transistor Q9 on the mainboard, which is located next to the support sleeve for the hard drive controller. The cause of the backlight not functioning was a lack of ground at the emitter of the transistor; the through-hole on the left side was damaged.
2. The improper operation of the floppy disk drive was caused by excessive resistance on microswitch SW1; in this case, cleaning the contact with chemical agents helped

Hello guys!

Congrats Piotrosz for your progress, i'm playing catch up 😉

After the replacement of the 2sc3325 my power supply looks like stable in turning always on/off and i managed to turn on the pc!

I then discovered that with the HDD connected the machine doesn't last on for more than 2 seconds, I fear the disk is stuck and unable to spin (or is requiring too much power at the moment to try it). I will look into it.

In the meantime i disconnected the HDD and after the first message about bad cmos battery (i removed it) i get a CMOS checksum error that prevents me to move on regardless if i press any key or not.

Did it happen to you as well ?

Cool to see you got it working!
Without the HDD it will sit at that screen for a long time but it will eventually continue to boot when HDD detection times out.
Give it a couple of minutes and it should continue.

Hello Fust!!!

Of course you're right as usual, after a long time the machine moves forward.

After some slaps also the hdd started to spin even if with some hiccups now and then, The machine is now almost boosting but...

But as soon i press F5 to load default values then is shuts down, the feeling is like the psu is barely keeping it on and when the hdd tries to do something the voltage drops and the board got reset. The dc light then flashes until i plus off the 12v power supply and connect it back.

Do you have any hint about what can be a little "weak" in the psu despite the full recapping ?

Maybe it would like to have batteries power as well instead of just the external 12v ? I aldo tries to rise voltage a little bit but as far i understood the psu doesn't care much about main voltage given it's "rebuiling" its own voltages regardless the main input (battery or dc ps).

I feel really close to the goal but so far i never seen anything beyond the F5 and load default confirmation...

Thanks!
Stefano

I missed to say that the last thing it does after the F5 default confirmation is a single beep like they usually do just after POST process finish...

Have you measured the voltages and do you actually see them drop?
I'm asking because I ran in to issues with the setup defaults on the /40 as well, there was something odd with it but I'll have to look in my notes.

The PSU does indeed create it's own voltage rail from either the 12V bruck or the batteries. It also doesn't require any battery to run, not even the CMOS/clock battery. Most likely is a cheap or wrongly specced cap, aMOSFET being marginal and not keeping up, a damaged trace or a bad inductor. That last one being very unlilkely, which is good 'cause I couldn't find new ones that would work.

If you're going to replace the batteries be careful to use the exact same chemistry (mostly NiCd in this case) and do not swap in lithium cells.

Hello Fust!

I'm thinking about a certain "weakness" of the PSU because when the pc requires a little bit more current (peaks around 1.5A @ 12.5v on my bench power supply) I can see the backlight to spike down. Sometimes it shuts down at the first attempt to spin the HDD that looks like an heavy task.
And in any case when I set the cmos values with F10 or i press F5 to load defaults it turns off. With defaults it shuts down immediately, when selecting one of the three HDD options it checks memory first and turns off as soon as reaching 1024KB and one beep.

I just disassembled my T1600/40 that i didn't work on at all and the PSU looks amazing (FIVPS3 instead of FIVPS4 of my first psu), I will test it after a full recap and check but I'm confident I will have less to work.

Swapping display or hdd doesn't change anything, both display work and both HDD spin on my T1600.

Thanks
Stefano

Sorry, one more question : do you know the value of C38 ? Looks like a bit cracked but I can't read anything on it...

Hi Stefano,
great to hear about your progress. I haven't solved the problem with the hard drive, meaning my two JVC drives seem to be damaged. However, in the second T1600, I had a Conner drive with a standard IDE 40 interface, and that drive is also damaged. Nevertheless, I connected the IDE-40 -> IDE-44 -> CF interface and managed to use FDISK to create a 20MB partition on the CF card (128MB), and now the drive is normally recognized in the system.
In the schematic, C38 it is marked as 300nF.

I'm not entirely sure it actually is 300nF, I vaguely recall measuring it in-circuit so that might be completely off.
Can't imagine it being a terribly important component though, it seems to be a decoupling/debounce cap for the reset signal for IC13. A couple 100 pF will probably do the trick.

The display dimming sounds like either not enough current from your bench PSU or insufficient capacitance on the output rails of the laptop's PSU board. Too high ESR caps can also cause this.
I'd start by checking your bench PSU is up to the task first, 1.5A sounds like a lot but it really isn't for these old things. The backlight uses completely different rails so it dimming has me suspect of something further upstream.