Decmate II keyboard problem

[Fleetingthought]

Hello all,

I've got a Decmate II that has a keyboard issue. I've tested with a LK401 that I use with my Rainbow 100 system, keyboard works fine with the Rainbow but gives a error 48 with the Decmate. I've had a look at the motherboard, tracing pins 14 & 15 from the db15 connector to a series of what looks to me to be picofuses. Testing these in circuit with a multimeter, 4 of these "picofuses" show a resistance of approx. 530 ohms and the remaining 2 appear to be open. Anyone have any experience with these? Are these really picofuses? Any insights would be appreciated.

Tom

 

[dave_m]

Testing these in circuit with a multimeter, 4 of these "picofuses" show a resistance of approx. 530 ohms and the remaining 2 appear to be open.

Fuses wouldn't measure 500 ohms. Perhaps they are chip resistors, i.e., surface mounted resistors.

 

[Fleetingthought]

I agree, I don't ever recall seeing a picofuse measure 500 ohms. The attached image of a Decmate II motherboard I found on the net, I've drawn a box around the components in question, although, the indicated components on this board do not look like the same as what is on my board (mine are small green axial beads marked CM 471 JSA). The ones pictured look to me like some flavor of cap.

Tom

 

[dave_m]

I agree, I don't ever recall seeing a picofuse measure 500 ohms.

Do you have a link to the schematic? Do you know the signal names on pins 14 & 15?

 

[Fleetingthought]

I've not been able to find the schematic for the Decmate II, only the original Decmate which appears to have used a completely different circuit layout. Pin 14 is keyboard>computer and pin 15 is computer>keyboard, but no, I don't know what DEC called those signals.

 

[vrs42]

mine are small green axial beads marked CM 471 JSA

I found a CM471J described as a 470 ohm 1/4W carbon resistor, 5% in an unrelated parts list on the net, for whatever that is worth ("J" suffix means 5%). Not sure what the "SA" means. Perhaps something about the package or manufacturer?

Vince

 

[Fleetingthought]

Well, after staring at this rice grain sized part again under a better magnifier glass, it reads CM 471 J5A. Which google's up MuRata 470pf 50v capacitor. I have zero of these, so I'll order and wait a week to see if this makes corrects the problem. The ones I tested measuring 500 ohms appear to be partially failed and the ones that appeared open were likely okay. I'll report back the results.

Thanks,
Tom

 

[dave_m]

I've not been able to find the schematic for the Decmate II, only the original Decmate which appears to have used a completely different circuit layout. Pin 14 is keyboard>computer and pin 15 is computer>keyboard, but no, I don't know what DEC called those signals.

I can't find a schematic either. The only document I found states that a interface to the keyboard is a RJ telephone jack with the computer supplying +12V and ground and the serial data lines at +/- 5V. When connected do you get either +5V or -5V on the data lines?