PDP-12 Restoration at the RICM (tony duell)

Jon Elson elson at pico-systems.com
Tue Oct 13 20:24:09 CDT 2015

On 10/13/2015 07:32 PM, Michael Thompson wrote:
> Tracks 1 & 2 look fine.  We swapped the probes for track 2 & 3 just to make
> sure that it wasn't a logic analyzer problem. A 'scope connected to the
> differential signals shows the same track 3 glitches. The glitches were
> present with the TU55 and the TU56 tape drives. We used different control
> and data cables for the TU55 and TU56. We swapped the G882 modules between
> tracks, and from the TC01 in the PDP-8/I and did not see a change. We
> swapped the W603 module in slot F16, the R107 in F12, and the W512 in F13.
> The LTR WRITE ON L signal is inactive and stable. We tried several tapes,
> including ones formatted and written by DEC and the glitches are present on
> track 3.
OK, so the drive must have a head amplifier and a comparator 
(sometimes called a slicer in tape technology) to convert 
the amplified analog signal to a digital one.  Have you 
swapped the read amps and comparator cards?  (Sounds like 
yes, above.) Have you looked at the analog signals?  it is 
possible a defect in the tape head is causing the read 
signal to be different than what should be seen.  Looking at 
the amplified analog signal might give some hint at the 
behavior.  (Don't bother trying to look at the raw tape head 
signal, it will be really tiny.)  I'm not sure where the 
read amps and slicer are, whether they are in the control or 
the drive. (Seems like the read amp HAS to be in the drive, 
though, for noise immunity reasons.)


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