2007 09 11 -- More Diagnostics

We were able to get the tape rocking back and forth. This was done by cleaning the PDP-8/M that the TD8E card is hosted in :-) [The problem with this particular PDP-8/M, that I now realize in hindsight, is that it was never properly restored. I got it a long time ago, powered it up, and saw that it worked, so I've just been using it since. But it was starting to get a little tired, so we did a full cleanup. Things work much better now!]

The tapes still don't "read", but at least dumptd8e is now writing something to the console.

2007 09 26 -- Success!

Concident with this whole TD8E controller puzzle was another fault. When you pressed EXTD ADDR LOAD, the MA would increment. I just put this down to an annoying, but not important, problem. Turns out, the M8310 "Major Register Control" board was damaged, and replacing it with one from spares made the serial port work reliably (there were some issues) AND made the TD8E work! So, I dumped a bunch of tapes to the serial port.

Now all that's left to do on the TU56 is the front faceplate physical cleanup.

2007 09 27 -- On to the next project: TU60

I have two TU60 units. One is a TA8, and the other is a TA11A. Since they're both identical (at the TU60 end of things) I decided to do them both at the same time.

As usual, the power supply was the first to be done. Each power supply consisted of 4 x 14mF @ 20V capacitors. I wasn't going to do 8 capacitors individually! So, I set up all 8 capacitors with their own series resistors, and reformed them all at once.

To make a long story short, the TA8 seems to work, with a slight problem with the right hand side (RHS) drive. If you press and release the rewind button on the front panel, it rewinds, and then stops a few seconds later. If you press and release it again, same thing. If you press and hold it, it rewinds until a few seconds after you let go. The LHS drive is fine; press and release and it rewinds all the way. That'll be the first thing I fix on the TU60 internal controller.

The M8331 card that goes into the 8 is, however, hosed. With the card in place, doing a LOAD ADDR from the front panel shows a stuck MA11 bit. Also, once RUN is on, you can't stop it, unless you power off. Without the card, of course, the 8 runs fine.

2007 09 28 -- Dead Phototransistor

Looks like the reason it's not doing a rewind-until-leader is because it thinks the leader is already present. Checking the circuit diagrams for the M7761 board (sheet 4 of 5, in the section bounded by A-D and 4-5) I measured the voltages at TP18 and TP6. With a tape mid-way through in each drive, I discovered different voltages. The failed drive indicated that it was "CLR LEADER B". Swapping the two drives caused the same problem; the problem travelled with the drive. So, I'm looking for a new phototransistor assembly (the light works fine, so I'd be willing to trade with someone who has a burned-out light).

Huh. Turns out it wasn't a dead phototransistor after all. The tape I'm using is ever-so-slightly less opaque than DEC's tape, or has a bit more shine to it, or something physical like that. Turns out, the phototransistor was just being overly sensitive to light. So, I adjusted R51 by adding a 68k in parallel with it, bringing down the resistance to 29k, thus requiring a little bit more current (8V over 29k or 275 μA instead of 156 μA) and hence more light.