9825A Tape board repair
- Frits Jalvingh
After adding the tape drive I got from Rik Bos I tried to send the rew command to it- but that hangs the computer. So something seems wrong.
First step is to see whether we have communications working with the tape board. I measured U4 pin 12 (INT_, yellow) and U4 pin 8 (purple). U4 is a 74LS20, dual 4 in NAND gate of which pin 8 is an output and pin 12 is one of the inputs. When I start the machine (and when I send the REW command) I see this:
U4 pins measured:
Pin | Signal | Color | Value |
|---|---|---|---|
p9 | PA3_ | blue | 0 |
p10 | PA2_ | green | 1 |
p12 | INT_ | yellow | 1 pulsed down |
p13 | PA1_ & ! PA0 | purple | 0 |
The P8 output of U4 will be low ONLY when PA_0..3 is 1110 & INT_ is 1, i.e. address 0x1 and no int poll.
In our case PA3_ being 0 and p13 also being 0 seems to be an issue.
Next measure: PA1_ (U5P1, purple) and PA0_ (U10P11, blue):
PA1 is zero’ish, but PA0 (blue) is hovering in the middle.
Next measurement: on CPU board, measure U1P4 (PA0 cpu, yellow), U1P6 (PA0 buffered out, purple) and Tape board U10P11 (PA0 directly from CPU U1P6 (resistance 1.6 ohm):
All nice voltages and all nicely 1, oddly enough, so the earlier measurement must have been in error 8-(
Sending a “REW” with the same assignments does drop PA0 and this percolates proper:
Remember logic is inversed, so addr 0x1 would indeed need to drop PA0_- as it seems to do.
And of course- when I do the “rew” now it reports “Error 41” - No cartridge in tape unit, sigh.
Cause: bad contact on the flat cable connectors, most likely.