On Mon, Dec 6, 2021 at 7:39 AM Michael Thompson < michael.99.thomp...@gmail.com> wrote:
> > > On Sun, Dec 5, 2021 at 11:46 PM Josh Dersch <dersc...@gmail.com> wrote: > >> 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. > 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 > > -- > Michael Thompson >