DEC PDP-12 Restoration Blog starting 1/1/18

1/6/18
Back to working on the LINCtapes.
Last week we tried to "mark" a LINCtape.
The first pass blindly writes the Timing and Mark tracks to the tape.
It then turns the tape around and writes the Reverse Block Numbers and Reverse CRC.
It failed instantly on the second pass, so maybe the Timing or the Mark tracks were not written correctly.

We checked the frequency of the clock for the reel motors on the TU56 tape drive.
It measured 40.86 Hz, so we adjusted it a little closer to 40 Hz.
We checked the capacitor voltage waveform for the reel motors and compared the 'scope image to what is in the TU56 manual.
The manual shows a spike in the sine wave, where the 'scope showed a pure sine wave.
Next week I will bring my LCR meter so we can measure the reel motor capacitors and make sure that the are 100uF.

We looked at the tape head signals for the timing track and mark track when "marking" a tape.
Click on the image for a larger view.
The two 'scope channels show the voltage across the two coils in the Timing Track head.
You can see extra glitches in the transitions which could cause problems when reading the Timing Track.

Click on the image for a larger view.
The two 'scope channels show the voltage across the two coils in the Mark Track head.
There are no glitches in the Mark Track transitions.

We need to find out why there are glitches when writing the Timing Track.
Next week we will look at the TTL signals that drive the G882 modules that make the Timing Track and Mark Track signals.

2/3/18
Still running great for demonstrations today, except for the LINCtapes.

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