6 Dec
2021
6 Dec
'21
9:37 p.m.
On Mon, Dec 6, 2021 at 7:39 AM Michael Thompson <
michael.99.thompson at gmail.com> wrote:
On Sun, Dec 5, 2021 at 11:46 PM Josh Dersch <derschjo at gmail.com> wrote:
Looks like I lucked out. Testing the head signals at the backplane on the
TU55 revealed that pin BD1 was a flat line at about -0.5V. Turned out to
be a bad relay on the G851 relay board. (There was also some gnarly black
gunk on the pins on the head connector which somehow wasn't the
problem...) I don't have any spare relays, but I swapped a G851 in from my
TU56 and the glitches have gone away. If anyone has any spare relays for
these, or a G851 lying around, let me know.
The Basic Search diagnostic still isn't finding any blocks on the tape, and
I can tell by the lights on the panel that the LPB register isn't doing
much of anything, so I have some more debugging to do. But I'm really glad
that the TU55 is OK. Thanks for the help!
- Josh
I swapped boards around in the datapath for these
bits on the TC01 side
last night and the problem doesn't move. On a whim I put my thumb on the
tape as it passes over the head and it seems to affect the glitchiness of
the data, at times making it appear to go away entirely. Doesn't appear to
be a tape tension problem -- it seems adequate and putting my thumb on the
supply reel to increase tension a bit does not have the same effect. This
would seem to point to the TU55 being the issue, and quite possibly the
heads. I haven't had time today to look at the signals coming off the
heads but I plan to soon.
Thanks for the help!
- Josh
I would measure the resistance of the head coils at the relay board
connector. Hopefully they all measure about the same. Figure 2-2 in the
maintenance manual shows how the MARK and DATA tracks are wired, and the
extra connection available for the TIMING track. We found a head on the
PDP-9 where the +D0 connection was open.
--
Michael Thompson