Help to identify missing parts in 286 board

[Rauli]

I have a 286 board, but it has some missing parts (3 resistors and 2 transistors). I would appreciate if somebody has the same board and tell me exactly the values/ID for these parts.

The board itself has no brand, but it has 2 stickers: "103FS" and "LM910901722". The 1st one is probably the board model, but the 2nd one could be just the serial number. This board is found in TH99 under the name "Unidentified LM-103".

It is a late 286 board manufactured in 1991 week 20.

On the back side there is a square logo with "HS" or "HCS" letters folowed by "-A-200".

It has a 16MHz 286 and the chipset is a "Headland HT12/A". It came with a 287XL

I have searched for photos of this board (to try to see the missing resistor colors) but I obtained no results.

The board:


Detail of the missing parts (R5, R6, R7, Q1 and Q2) position:


ID stickers:

 

[pietja]

It looks like the battery circuit, with is very generic.
If no one has the same motherboard it is not so hard to reverse engineer but then we need a picture from the other side of the board.

Here you can see the battery circuit of the IBM 5170 http://www.minuszerodegrees.net/images2/5170_battery_circuitry.jpg
And here is the same for a RTC card http://www.vintage-computer.com/vcf...-XT-RTC-Level-of-interest&p=347584#post347584

If you have a multimeter and some time you can find out how the circuit is connected.
The transistors (Q1 and Q2) are almost guaranteed to be the 2N3904 and 2N3906, the resistor values are not that critical.

 

[Rauli]

Thanks.

I tried to track where the parts connect to. I did it with an older 286 board (to figure out what jumpers were for) looking at the tracks in the front and back side of the board. Unfortunately, this late board seems to have internal layers, as several pins aren't soldered to any track neither on the front, nor in the back side. But, just in case, I upload a photo of the back side.

 

[Rauli]

I used the multimeter and found new connections:
Q1E is connected to PS connector +5V
Q1B is connected to R5 left side
Q1C is connected to external battery connector pin 3
Q2E is connected to R7 left side
Q2B is connected to R6 right side
Q2C is connected to R5 right side
R7 right side is connected to R6 right side
R8 right side is connected to external battery connector pin 3
R9 right side is connected to external battery connector pin 3
CMOS chip (HM6818A) pins 16 (-STBY) and 20 (CKFS) are also connected to external battery pin 3
Barrel battery + is connected to external battery connector pin 2
Barrel battery - is connected to external battery connector pin 4

"Left" and "right" according to the photo showing the missing and existing parts on the front side of the board.

 

[Rauli]

I managed to make a schematic diagram of the battery circuit. I don't know a lot of electronics, but I suppose D3 is for discharge current (for the barrel or external battery), and D1 for recharge current (only for the barrel NiCd battery).

As far as I can see, all the missing parts belong to the recharging circuit, don't they?

Assuming the external battery does not need to be recharged (because I think they are not rechargeable)... are the missing parts needed at all?



"Normal operation" means the barrel battery is being used. To connect external battery, the "normal operation" jumper is removed and voltage is applied probably (I'm never sure about thos) to pins 1 (+) and 4 (-).

Note: Maybe I missed some connection...

 

[pietja]

I think the connection between D1 and R6 goes to the 5V supply and the line on the very bottom is going to 0V ground.
If you then compare your drawing with the one of the IBM 5170 you see that all the the parts left from the red line are missing on your board.
This is not the charging circuit but power the RTC when the system is on to save battery power.
D1 and R10 are probably responsible for recharging the on board battery, witch you should replace before it leaks.

So i would say these should be you missing parts:

Q1 2N3906
Q2 2N3904
R5 10KΩ
R6 51KΩ or 47KΩ witch is easier to get
R7 10KΩ

 

[retrogear]

Yes, I agree with pietja that you need 5v between D1 and R6 and bottom line to ground. Q2 should be NPN and
Q1 is PNP

 

[Rauli]

You were right about the missing connections. I have updated the drawing with those connections and the values of existing resistors. One capacitor have the marking "10μ 16" (the "16" is under the "10μ") and the other "472Z". On the diodes I just could see a part: "N" on one, "48" on the other.

Compared to the IBM 5170 link you provided, the circuit with the transistors seems to be identical to the missing parts in my drawing.

So thanks, you solved my problem!

PS: Inspecting other boards I could find 3906 and 3904 transistors in the battery circuit, too.

 

[pietja]

Just for completion:
C3: 10µF 16V Tantalum
C48: 4,7nF 50V Ceramic
D1 and D3: 1N4148

The circuit is a very basic voltage monitor that powers the RTC from the main 5V supply when the voltage is above a certain value.
If the main 5V supply is below this value (or when the parts are missing) the RTC will be powered from the battery.

You can change the value of R6 to change the voltage level at with the RTC will be powered from the main supply. (only valid when R7 is 10KΩ, R6 and R7 are a voltage divider)
R6 = 56KΩ = 4.62V
R6 = 51KΩ = 4,27V
R6 = 47KΩ = 3,99V

 

[archeocomp]

Funny. I just got the same MOBO with the same components missing. It is also type 103FS, absolutely identical to the OP one. And very similar to this http://minuszerodegrees.net/at_clone_bios/LM-103S%20(103FS-16)%20motherboard.jpg but little different. It uses single HEADLAND HT12 chip, instead of three HT101/102. To me it looks like the components OP mentions had been removed at the factory. On my board exactly the same components are missing. Second option, it was world wide known repair/hack to these motherboards. Why would otherwise the same surgery had been done to the (at least) two distinct but identical boards in distant countries? Have a look at the pictures.

 

[Stone]

I have a 286 board, but it has some missing parts (3 resistors and 2 transistors).

Funny. I just got similar MOBO. The same parts missing. Look at the pictures. My board has slightly different layout. To me it looks like the parts had been removed at the factory.

Why do you assume that these components have been removed or are missing?

Do you have the boards' schematics?

Do you know that the designs actually have these components installed?

Just because a component is identified on a PCB doesn't mean it was intended to be included in that particular revision.

Look at some of your other boards. How many of them also show some 'missing or removed' components?

I have lots of boards with these so-called missing components. Some even have 'missing' ICs. FWIW... they all work!

I think you're making assumptions that are unjustified.

 

[archeocomp]

Those "missing" components have been cut out, after they have been soldered to the board. There are still remains of them in the holes. It is better visible from beneath, but very first picture in this thread gives good impression.

 

[mediasponge]

Based on the last schematic posted, if Q1, Q2, R5, R6, R7 are all missing, then R8, C48 do nothing. If all the components are there, I'm not sure what the function of R8, C48 is, perhaps just to soften the rise of the +5 at the CMOS. D3, R9 provide power to the CMOS from the battery when power is off. D1, R10 recharge the NiCd when +5 is on. If it was my system, I'd restore the missing parts, but swap out the NiCd for a non-rechargeable Lithium, and remove R10, D1. In that mode, D3 would block the +5 from trying to recharge a Li battery.

 

[archeocomp]

I suspect that R8, C48 were left there just because they do nothing. No need to do unnecessary work for the factory.

I already have holder for 3xAAA (alkaline)batteries. I plan on using them connected to pins 1 and 4. That way I could leave the board as it is. On Saturday I will test the board, maybe it is not working at all. Just inspecting it (today it arrived) and finding missing components brought me here.

 

[mediasponge]

I suspect that R8, C48 were left there just because they do nothing. No need to do unnecessary work for the factory.

I already have holder for 3xAAA (alkaline)batteries. I plan on using them connected to pins 1 and 4. That way I could leave the board as it is. On Saturday I will test the board, maybe it is not working at all. Just inspecting it (today it arrived) and finding missing components brought me here.

I was wondering about the purpose of R8, C48 when all the missing components were installed. No matter. If you connect a battery from pin 1 (+) to pin 4 (-) with pin 2 open, the battery will be under load all the time. The whole point of the complete circuit is to let the +5 power the CMOS when the power is on, to extend the battery life.

 

[archeocomp]

You are right about R8, C48, they seem to have no other function than delay line. What makes me scratching my head, is the fact that NiCd battery was on the board with jumper across pins 1,2 installed. It must have had very little leak current, as it obviously was never charged.

BTW the board is working. I just have tested it. And CMOS holds data as expected. I will not populate missing components. Three AAA alkaline batteries hold charge for ages. I have them in another 286 since approx. three years and voltage is at 4.1V. And NiCd is not good for board's health anyway.


I have just found in manual for another 286 HT12 board (can be found on web as ht12a_user_manual.pdf) that external battery 6V can be used, that means four batteries would be even better. That is info related to this thread:
http://www.vcfed.org/forum/showthread.php?35576-3-6v-battery-on-286-motherboard

 

[Stone]

BTW the board is working. I just have tested it. And CMOS holds data as expected. I will not populate missing components.

Good decision.

If it ain't broke... don't fix it.

 

[archeocomp]

I have just found (but did not win) another exemplar of this particular motherboard in local classifieds. This one still has discussed components. I just want to share the pics for reference.

More detailed photo can be seen here
https://drive.google.com/open?id=1_48eymgpdUEsRnzcSHuveqwqe6GsoNfS