MS-6178 Pentium III motherboard - bizarre fault with RTC oscillator

[dicky96]

Hi guys
This is another motherboard which I put on one side about a year ago

The original thread is here, but ended up in the BIOS.Schematics requests section when things got moved around, though a lot of it is about the faults I was finding on this motherboard. The schematic is on that thread

https://www.badcaps.net/forum/showth...hlight=ms-6178

So last time I looked at this, there was too much current being drawn from the CMOS battery. I eventually found a short circuit near the SIO that I most certainly caused myself.

At that time I had already removed the FW82801AA PCH while trying to work out where the excessive current was going.

I since ordered a replacement PCH and have now fitted it. It took me two attempts as my semi-auto BGA machine fitted the replacement then at the end of the profile it lifted it from the board This turned out to be my fault also as I had forgotten how to operate the machine properly!

Anyway I reballed it and operated the machine properly this time and it is soldered in place.

I now have about 20uA draw from the CMOS battery.

The Motherboard will power on when I press the PS_ON switch, but it does not boot - PCI POST card reads a static number such as EF or C5 (it varies which number) but it will not power back off by holding in PS_ON

I checked all the System voltage regulators and they were OK 3.3V, 2.5V. 1.8V ,1.5V. Vcore is 1.51V

What I have noticed is some strange behaviour on the RTC crystal

It is oscillating when powered by CMOS battery, and when I apply Standby power from the ATX PSU, the amplitude decreases a bit.

But when I start the motherboard with PS_ON the RTC xtal stops!

Has anyone seen that before, and any suggestions what would cause it?

See attached video

https://youtu.be/AC22vblkoYE

0:06 connect scope - RTC running on battery
0:13 ATX PSU on (STBY)
0:28 ATX Off
0:33 ATX On (STBY)
0:39 PS-ON Motherboard starts
0:58 ATX Off
1:06 Xtal starts oscillating again

[dicky96]

Re: MS-6178 Pentium III motherboard - bizarre fault with RTC oscillator

Sorry there was an error in the above post

Checking the voltage on the two Chokes (schematic page 30 VRM8.4)

VCC5 is 4.93V - 100mV hf ripple
VCCP (Vcore?) is 2.01V 30mV hf ripple, not 1.51V as I stated before.

It looks like one of the capacitors has a slightly convex top and needs changing (the on directly behind one of the chokes. Also tapping the top of this capacitor with a meter probe it has a slightly more 'hollow' sound than the others which suggests to me it dried up. It looks like I already changed on of the caps for the blue one, last year. neither chokes get hot, the convex capacitor gets slightly warmer than the others.

This will just be some capacitor of the correct capacity and voltage I had lying around.

Despite the capacitor problem, the voltages are correct and the ripple is very low so I don't imagine this is related to the problem with the RTC, but I thought I should mention it

[dicky96]

Re: MS-6178 Pentium III motherboard - bizarre fault with RTC oscillator

Sorry I realised I forgot the capacitor pics on the above post - I'll add them in the morning

[dicky96]

Re: MS-6178 Pentium III motherboard - bizarre fault with RTC oscillator

These are the pics I forgot to add

[dicky96]

Re: MS-6178 Pentium III motherboard - bizarre fault with RTC oscillator

I decided to have another look at this today. For some reason, the RTC Xtal was working the first time I powered the board on. But then the next time I tried,it wasn't running, then I found it wasn't running with the Board powered off (running from CMOS battery) either, but it was previously.

As the fault seemed intermittent I checked for Bias voltage on the RTC Xtal with the board powered off - and I didn't have any, but I did once it powered on

In the end this turned out to be a dodgy jumper on J26 (CMOS normal / clear) The little spring clip inside the jumper had lost it's tension - so I fitted another one. And now the RTC Xtal behaves properly when the board is powered off, powered on, or started. The amplitude on the Xtal stays the same.

I can also now power the board off by holding in the PS_ON button for 4 seconds -so that's an improvement.

Having said all that, it still does not boot. My POST card says 0000

I had a look at the W209CH clock generator chip and all the clocks appear to be running. Referring to MS6178 schematic sheet 7/38 I have clock signals on:

Pins 1,3,4,7,8
Pins 11,12,14,15,17,18,19
Pins 21, 22, 23

I had a look at the CPURST# and the easiest place to get to that was on RN20. However I don't know what is the pinout of these resistor networks and googling 'resistor network pinout' didn't help me

So could someone tell me what order these pins are? I can see one side is all connected to VTT1.5 (which is present) but the left hand resistor reads low on the other side, near 0.1V, while the other three read high. That could be CPURST# if it is pin 2 of the package? In that case I have found something wrong. See pic.

Other than that I checked PWRGOOD. I couldn't connect to that signal at the PCH but I checked it on the SIO pin 71 and it is high there, which is OK

I also checked on the 82802 Firmware hub for RST#, because it was a good place to access that signal. It is High, so that's OK, and for CLK on the same chicp, which is clocking, so I guess that is also OK

Now it seem I have some possible ways to advance:
1. That resistor network RN20 - which is pin two? Because I may have the CPU reset signal asserted low, so it can't start

2. Those slightly dodgy looking electrolytics (see my earlier posted pics) should I change them before continuing the diagnosis?

3. I am not sure the BIOS supports my CPU - The motherboard should support my Celeron 433 but I don't know about BIOS version, or I may have a corrupted BIOS? Can I even reprogram the BIOS, it seems to be integrated in that 82802AB FWH chip

4. It's possible I damaged the PCH when I replaced it as the first attempt at soldering it failed, so I had to reball it and try again. Or it isn't soldered properly

5. Any other thoughts?

Datasheets

[reb202]

Re: MS-6178 Pentium III motherboard - bizarre fault with RTC oscillator

How are you measuring the XTAL signal that you are showing on the video?
Remember that if you measure a XTAL directly is best if you use a high impedance buffer. Oscillocope probes have parasitic capacitance that can load the XTAL and change their resonance frequency also the input impedance from the probe is not high enough to avoid loading the circuit because the XTAL signal is very weak. Try measuring as precise as posible the frequency from the XTAL at different amplitudes. If you notice a change in frequency it might be oscillator pulling which means that there is some impedance that changes. Hope it helps!

[dicky96]

Re: MS-6178 Pentium III motherboard - bizarre fault with RTC oscillator

Hi reb202
Thanks for the advice - I was using my scope probe on x1 and x10. If you read the further posts, you will see I diagnosed the problem, and the xtal now runs at a constant amplitude when the board is powered off or on.

It's nice to see I am not just talking to myself here though

Since I was replying to your comments, I may as well reports I got a little further. I could not find out the pinout for the resistor network RN20 so I figured out I could get the pinout for the PGA 370 socket, determine which is the RESET# pin, and see if it connects to RN20 on the pin that is reading 0.1V

Actually it took me a bit of time to find the PGA 370 pinout- eventually I found an intel datasheet for the PIII processors instead which gave the CPU pinout. I'll post that here next time I am at the workshop

So, anyway, RESET# (CPU pin AH4) does connect to RN20 pin 2 (CPURST#) so my CPU is being held in reset.

CPURST# is generated by the Intel 82810 GMCH Host Interface (What I would call a Northbridge) and the datasheet for 82810 tells me that:

'CPU Reset. The GMCH asserts CPURST# while RESET# (PCIRST# from ICH) is asserted and for approximately 1ms after RESET# is deasserted. The GMCH also pulses CPURST# for approximately 1ms when requested via a hub interface special cycle. The CPURST# allows the processor to begin execution in a known state'

I've attached the 82810 GMCH datasheet here. See table 2.1

So this lead me back to the input to the GMCH, RESET#. Which then leads me back to the ICH signal PCIRST# which is accessible at R287/C298 at it is indeed also asserted low.

So that was as far a I got today - now I need to study the FW82801AA ICH datasheet to find out what conditions are required for PCIRST# to go high, then see what conditions I have on my board.

By the way, I did check at R287 for a short to ground but I don't have one, I have about 9K ohms from either end. I don't have a switch connected to Reset on the front panel header - but I do have 3.3V on one of the Reset pins there. Anyway I did then try connect a reset switch but it doesn't help any.

depending on what the 82801AA ICH datasheet tells me, or doesn't tell me, it also occurs to me to lift R287, power the board on, then drive PCIRST# high with a resistor - to see what the board does.

I'll just have to figure out the correct voltage rail to use for logic 1, but I should be able to work that out from the schematic or datasheet

[dicky96]

Re: MS-6178 Pentium III motherboard - bizarre fault with RTC oscillator

Well it seems this motherboard is giving me a right run around. The next time I tried it, I still have CPURST# low (0.17V) but now PCIRST# from the PCH is high. Before PCIRST# was asserted low.

Also with a switch connected to the reset pin header on the motherboard I can see that PCIRST# goes low when I press the switch, and high when I release it. At the same time the RESET LED on my POST test card goes off and on. Interesting the RESET indicator LED works the opposite way to what I would have assumed - it is off when the board is reset and on when it isn't

So anyway, according to the Northbridge (GMCH) datasheet, when PCIRST# goes high, CPURST# should then go high 1mS afterwards. And it isn't, it never changes at all from 0.17V.

The PCH appears to now be behaving correctly. So I've removed the GMCH. I could try reballing it and resoldering it, but it looks like I can get a replacement for about 5 euros so I will probably just order one

If nothing else, trying to fix the vintage PIII board is certainly proving to be very educational, and if I can get it working it is worth enough on the retro gaming market to recover the cost of parts - so that is free education and experience for nothing!