Aui wrote on 2024-05-22, 07:27:

Would a 10k do as well ?

Technically no, because a tolerance interval of 5% allows an upper limit of no more than 9.56K.
9.1K resistors are quite common. They are in the E24 table.

https://www.newark.com/c/passive-components/r … istance=9.1kohm

On the other hand a rule of thumb says that you can generally replace a resistor with the next higher / lower value in the same (or higher) E-row. That would be 8.2K or 10K here.

No luck unfortunately with my attempt to fix the board. I cleaned everything and retinned traces as good as possible and also added a few new wires (+ the new replacement resistors). Continuity was ok on all damaged traces but the board is not waking up. CPU, chips and VGA chip are getting warm though. I wonder if it is missing a battery. There is a 4pin connector close to the site where the Varta was sitting and I am wondering if thats the connector for an external battery. However, in that case there should also be a jumper to set it to external. Which brings me to my question - can anyone identify the board - I had no luck.

On the backside there is a little icon that could be a company logo ? Thanks already in advance.

The writing "FTK-A253" suggests it´s some FTK mainboard.

There´s probably one of the traces / vias still interrupted. To get at least some information about any ongoing activity and where the the start up process stops, you should buy yourself an "ISA POST-card". (They cost no more than $10.)

I‘m using an external battery on 6 or 7 mainboards, 386 and 486. Only one of them has a jumper for it so you‘re probably good.

I see, but will these board wake up even without the battery?

Nearly all do, very few don´t.

Aui wrote on 2024-05-24, 04:22:

Continuity was ok on all damaged traces but the board is not waking up.

Do you have a POST card? It's very useful in repairs like this. In fact it's very difficult to diagnose a "black screen" problem otherwise, you don't know if the mobo is completly dead or just failing the initial self-test.
Also make sure that RAM is in the correct bank. These early mobos usually can't work with RAM just in bank 1, first you need to populate bank 0. I can't tell from the photo which is which.

I ordered one, but it will take time until it arrives. I will post an update from my post card once I got it.

thanks

Ok - I got the POST card and gave it a try - but I dont get any code readings (nor any beeps).
It seems all the LED's are lighting up correctly - so any idea what that means ?
Is there any meening to not receiving any POST codes (e.g. no BIOS access etc ?)
- CPU gets warm
- chips get warm
- BIOS gets warm
- Chips on VGA card get warm

any ideas ? - Thanks

First of all, the KBC socket (U36) is empty so you have no Keyboard Controller installed. No wonder you get nothing. That would be suspect 1.

Suspect 2 would be the BIOS EEPROM (U58), seeing as the battery damage is very close to the BIOS EPROM socket. I would start looking there first. Remove, spray some contact cleaner into socket and reseat the EPROM, then test continuity between each pin and the respective solder joint on the bottom of the PCB. Then check to make sure that the EPROM gets voltage on the Vcc pin.

I checked with a spare keyboard controller - nothing.

Suspect 2 - Yes - that is what I am afraid too - several traces (also inside the board on a lower layer) look quite bad and go under the BIOS EPROM socket. I thought I can get away without removing the BIOS EPROM socket as this is something a bit beyond my skill. Especially, if there really are bad traces under the socket you cant just add more wires as the socket wont sit flat any longer. There are also more chips under this socket - It's a mess! I may have to put it aside for a while to regain patience and then start over again.

Make sure to try at least a few KBCs, not all KBC ICs are equal..

Sigh - I bite the bullet and replaced the motherboard. These Varta Batteries are a real curse. The new board is very siilar and I also got it for a very unreasonable price.

Onwards...

So I made a step forward having a working mainboard now. Even more important, the board specs are in aggreement with the metallic front badge!

So the next stop I have now reached is the HDD. This PC came with an original MFM drive (first time that I have seen one in person) and it does of course not work.

The drive spins up and that sounds awesome. I can register it in the BIOS with the specs writtn on the drive (also giving m the correct size) but the drive seems not acessible. I also dont hear any sound indicating that the heads are moving, initializing or seeking a bootsecteor etc. The controller for the HDD also hostts the FDD and that works fine. At least in DOS 6.22 I cant acess the drive in any way (format, fdisk, chkdisk, scandisk etc) all do nothing. Can anyone suggest a small programm that may wake up the drive out of its sleep (perhaps it needs some low level format ?) Would the required software be specific for that drive (e.g. similar to a driver ?)

thanks for the help

Aui wrote on 2024-07-02, 05:02:

The drive spins up and that sounds awesome.

That's NEC D5146H? I've worked with those. The smaller and older D5126 are pretty durable (and seem to take RLL format quite well too), but the one D5146H I got my hands on was completly destroyed inside. Not sure which happened first but the heads were ripped off and the platters had deep groves in them (not to mention all the dust inside from the ground surface material). This pretty much locked the head arms into the grooves and prevented any movement.

These drives should perform a small head dance on power-on. WARNING: Not 100% sure about the H variants but the earlier ones are not self-park drives! You power them on, you need to treat them like eggs afterwards or you risk damage to track 0 which will pretty much kill them. So while you can run them on PSU alone (no controller), do not subject them to any vibrations (like travel/shipping) untill you can park them properly from OS level.

The head stepper motor has a lock, based on a simple solenoid. Remove the drive from PC, power it via some PSU, make sure the solenoid actually pulls the locking back to allow the motor to turn. If not you got a stuck solenoid or damaged drive PCB. If the solenoid works but still no movement from the stepper then it might be stuck stepper, or PCB issue, or sadly the catastrophic platter damage I've mentioned. There are some ways (less and more invasive) to try and free a stuck stepper. But first figure out the solenoid and report back.

Ok - the drive is a "NEC D5146H". The disks look fine (at least the upper one), no head crash. The solenoid frees the motor on startup. The problem is a bit similar to that repair video: https://www.youtube.com/watch?v=dErXDjMJ2dA. So I added some oil to the motor "gearbox"

The stepper motor moves the heads from the disk interior about 1cm then gets stuck. I can slightly and very gently push and help it along and then it moves another cm until it gets stuck again. I can also help it over that next bump and then it starts very frantically and forcefully moving back and forth while making very unpleasent squealing noises (but probably similar to a real reading / disk access movement). After that I get a c: drive error and the stepper motor does not try any longer. From that point on I can move the heads (i.e rotate the stepper motor) very lightly and freely over the disks with almost zero resistance. So it does not seem the motor is really just stiff or rusted in. Not sure how to proceed here, but thanks for the suggestions