Odd problem with IBM Model F

phughes

12 Mar 2019, 11:18

Hello,

I hope someone may be able to point me in the right direction with a strange issue I'm experiencing while refurbishing my IBM Model F.

I have stripped it down completely, cleaned everything, fitted new foam and re-assembled. I am using a Teensy board running Soarer's Converter and it all works apart from the following annoying problem:

All keys work except "c" and "m".

If I gently slacken the grounding screw on the PCB (location indicated in photo), the "c" and "m" keys work - but none of the other keys! I can't get the screw at the "sweet spot" where all keys work :evil:

I took the damn thing apart several times thinking that this was a problem with the contacts underneath!

I know that Model F is quite sensitive, but this problem is driving me crazy.

Any ideas much appreciated!

Regards,
Paul
Attachments
model_f_pcb.jpg
model_f_pcb.jpg (200.56 KiB) Viewed 331 times

SneakyRobb

12 Mar 2019, 16:22

Are you using the original cable and then using the teensy? Or do you have the teensy soldered with the pins going into the blue connector of the pcb?

Can you take a closer image of the front and back of the pcb around the area with the soldered pin connectors? That row running left from your yellow area. The one with the white stuff. What is this white stuff? Do you have a layer of the clear rubber under the pcb or that sort of clear sheet?

If I recall X and C are on the same column and N and M are on the same column. Different rows for each key. The space bar and lower keys are on that row as well.

Anakey

12 Mar 2019, 16:37

hmm could it have something to do with the stabiliser for the spacebar perhaps causing slight difference in capacitance for those pads as they are near where the stabiliser would be.

phughes

13 Mar 2019, 16:51

Thanks for the reply guys!

I have the original cable with the 5-pin din. Soldered to the Teensy is a 5-pin din plug, and I have simply plugged them together. I was a bit precious about cutting the cable to solder the wires from the block connector to the Teensy!

I will take some closer photos this evening (UK time) and post them.

phughes

13 Mar 2019, 16:52

Correction - soldered to the Teensy is a 5-pin socket! The original 5-pin din plug is then plugged into the socket.

phughes

14 Mar 2019, 22:31

Here are a couple more photos. The white stuff on the pcb was some remnants of the old foam which I have now removed.

The other photo is of the Teensy running Soarer's converter and connected to the original cable using a 5-pin din socket.
Attachments
pcb2.jpg
Close-up of pcb
pcb2.jpg (1.16 MiB) Viewed 210 times
teensy.jpg
Teensy with 5-pin socket
teensy.jpg (1.07 MiB) Viewed 210 times

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Darkshado

15 Mar 2019, 02:46

I can barely see any solder on the Teensy, are the connections good?
Try grounding the plug too, the original cable's shielding is also grounded.
If you want to do an internal USB conversion eventually: solder pin headers to the Teensy then use female/female jumper wires. It's non-destructive as long as you don't lose the original cable :P

SneakyRobb

15 Mar 2019, 18:47

Darkshado wrote:
15 Mar 2019, 02:46
I can barely see any solder on the Teensy, are the connections good?
Try grounding the plug too, the original cable's shielding is also grounded.
If you want to do an internal USB conversion eventually: solder pin headers to the Teensy then use female/female jumper wires. It's non-destructive as long as you don't lose the original cable :P

Yes the plug has a ground as well good point.

DMA

16 Mar 2019, 03:13

phughes wrote:
14 Mar 2019, 22:31
Here are a couple more photos. The white stuff on the pcb was some remnants of the old foam which I have now removed.

The other photo is of the Teensy running Soarer's converter and connected to the original cable using a 5-pin din socket.
First photo without the metal, then the reverse of the same PCB area, please. From the post photo it really looks like all the PCB traces at the edge of the metal are cut :)

JBert

16 Mar 2019, 12:25

The PCB seems to contain some sticky residue of the original foam. I've seen it as well on one of my XTs that the bottom side of the foam layer was white.

phughes

21 Mar 2019, 20:06

Cheers for the replies guys - it's much appreciated.

I'll take it apart (again) this weekend and take a couple more photos. Great idea about the internal USB conversion. I'd not thought about doing it that way.

Agreed that my soldering isn't the best which doesn't help matters! :lol:

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