A Superbrain II QD Resistor Query

[jonhales]

I’m looking for help with restoring a Superbrain II QD. Note that this version of the board has power connecting via a 7-way plug. The issue should be simple to resolve with documentation, but that isn’t quite the case. It’s not that PDFs of the schematics are hard to find. But the resolution used for scanning was inadequate to produce legible documents.

A resistor on the motherboard has blown:



My query is simply “what would be the correct specification for a resistor to replace it?” Two clues remain: a possible red band to the left and a more certain gold band to the right

This is from a layout diagram of the QD motherboard. At the top right, we find that the resistor may be R39, but the layout is not an exact match to the first photo.



In the table of resistor values on the same PDF page, R39 isn’t clear and it seems possible it (and R40, 41 and 42) doesn’t have a specific value.




There’s a schematic for the PSU of the II QD. This time, the relevant part is in the top left of the page, where J2 is illustrated. We can just about make out that the blown resistor connects to pin 3 and has -12 volts:




Lastly, this may be a clue – or it could be a 24 ohm resistor with a completely different function. It’s from an earlier version of the motherboard, the type with the wide ribbon cable carrying all signals. Note the trace to the left has been cut.




Finally, a further request: If anyone has access to clean printed copies of the schematics please scan them and make them available with a link to Dropbox, Google Drive or similar.

With thanks in advance.

 

[Agent Orange]

Hard to tell the value without a schematic reference. But, whatever the value is, something caused it to short. That resistor drew a significant amount of current which was probably the result of another problem up or down the line. Caps can fail just setting on the shelf, not so much with resistors (although there is an outside chance that it could happen, but I wouldn't bet on it). You might want to look for some discoloration of components in that area.

 

[harry]

Hello jonhales,
I have a motherboard in the workshop, the resistor on my board is 24ohms ( Red, Yellow,Black, & gold( 5% tolerence) But as suggested, would pay to check out the circuit to see if there is a cause for it to burn out, best of luck...

Harry

 

[jonhales]

A further thought on the issue of the schematics. The lack of resolution could well be due to conversion of images to PDF.

The following two images are the details of the schematic drawings from which the other extracts were taken:

 

[Chuck(G)]

It's going to be a very low-range resistor--might even be of the fusible variety, where resistance increases to accommodate a heavy load.

24 ohms would be a good starting point, given the size of the traces that it's connected to. It could be lower, but probably not higher than that.

But first you have to find out why it took up smoking.

 

[jonhales]

Many thanks everyone. All your points are well taken. In particular, thanks to Harry for confirming the match between the resistors on the older and newer versions of the board. And thanks Chuck for suggesting a fusible resistor.

I thought of adding some background to the original post, and perhaps this is appropriate for the record.

This Superbrain II QD looks pristine and appears to have had very little use, judging by the lack of wear on the keyboard and the casing in front of it. The levers on the Tandons don't appear to have had any wear. The PSU looks as clean as when it left the factory, despite the tendency of these machines to hold on to dust. All this points to a failure at an early stage, and presumably someone put it aside.

We tried switching on and found the start of a sign-on message on the screen, but got no response to the keyboard and no reset by pressing the two red keys on either side of the keyboard. The Tandons have no index pulse and don't step - which explains why there was no disk read. A minor point is that the board was presumably at a stable stage of manufacture - there are no wires or modified chips. The BIOS ROM is version 4.2, which is later than the copy of 3.1 on Bitsavers.

Clearly, the restoration will involve a bit more (!) than a suitable resistor. One step at a time.