DEC PDP-12 Restoration Blog starting 1/1/17
It is possible that last week we adjusted the position of the tape head so that it was one bit time (33uS) from where it should have been.
Today we will remove the tape head shim that we installed last week.
Alex brought a bulk tape eraser, so we can completely erase a LINCtape before formatting it.
We will format a LINCtape, flip it over so the oxide side is up, and then measure the head skew.
We replaced the ceramic wear plates in the right side of the TU56 tape transport with steel ones.
The ceramic ones were known to cause problems, and DEC eventually replaced them.
We formatted just the Timing and Mark tracks on a tape, rewound the tape, put the tape in motion, and looked at the output of the G882 modules.
We could see the G882 modules oscillating when there was no tape head signal, and this is normal.
With the tape in motion we could see Mark track data, but no Timing track data.
We swapped the relay boards in the TU56, but there was no change.
We connected the same G888 setup that we used last week to see the raw tape head data.
We could not see any data on the timing track.
Oh well, we will debug this next week.
Warren suggested that we look at the signal strength of the timing track using only one Op-Amp, read a tape written by DEC, and align the head for the maximum signal.
This should get us close to the correct alignment.
We can then split the timing track coils and use the normal DEC procedure to measure the time difference between track-1 and track-10.
This should let us get the head alignment under 2 uS.
We measured the resistance of the head coils.
Fortunately the all read about 1.2 Ohms, so we can continue with debugging.
We adjusted the LINCtape head skew to about +/- 1uS by reading several LINCtapes that were written by DEC.
DEC must have had some very special equipment at the factory to adjust the head skew to 0.5uS.
Click on the image for a larger view.
The upper trace is Timing track #1, the lower track is timing track #10.
We tried to format a LINCtape, and it ran through pass 1, got half way through pass 2, recovered, got part way into pass 3 and did the "shoeshine" looking for something on the tape.
We saw the same behavior on two different LINCtapes.
We need to study the MARK12 software to see exactly what it is doing in each pass to determine what we need to fix.
We looked at the documentation for the MARK12 program that formats LINCtapes on the PDP-12 so we have a better idea of what it is doing when it fails.
We spent a few hours going through the Maintenance Manuals for the TU55 and TU56 tape drives, the TC12 LINCtape controller in the PDP-12, and any other documentation that we have on DECtapes. There are a lot of pieces of documentation available, but no single document that correlates the digital/analog/magnetic signals. We will make a document that describes the digital data from the TC12 tape controller, the analog data that goes to the tape head, and the resulting magnetic patterns on the LINCtape. We will also document the reverse process from the magnetic/analog/digital signals.
We ordered a small spray bottle of Magnetic Developer. The Magnetic Developer WWW page says "This elegantly simple formulation contains iron particles suspended in a fluid with a rapid evaporation rate. When the developer is placed onto a magnetic surface, the liquid evaporates and the magnetic particles orient themselves in accordance with the magnetic pattern on the surface of the material." This should let us see the data that is on a LINCtape.