Source for DEC TC01 (and similar) bulbs?

Josh Dersch derschjo at
Sun Dec 5 22:45:34 CST 2021

On Sun, Dec 5, 2021 at 4:39 PM Michael Thompson via cctalk <
cctalk at> wrote:

> >
> >
> > From: Josh Dersch <derschjo at>
> > Subject: Source for DEC TC01 (and similar) bulbs?
> >
> > The Search Scope loop diagnostic shows block numbers going by in both
> > directions so a lot of the drive and controller are working, but there's
> > some glitchiness in bits 2, 5, 8, and 11 of the data so I need to trace
> > that down; I hope it's not the tape head.
> >
> > - Josh
> >
>  So what do bits 2, 5, 8, & 11 have in common? All bits come from the same
> track on the tape head, and share some of the path to the Data Buffer.


> If you wrote this tape on this system, I would try reading a tape that was
> written on another machine to make sure that the problem didn't originate
> with writing.

The TC01 isn't quite up for writing things yet (well, I haven't tried, but
I suspect it wouldn't go well).  The tapes I've been trying are known-good
DECtapes I have, that I've read on my 8/m with its TD8E without issues.

> Check for a bad connection where the tape head cable connects to the G851
> module in the TU55, where the G851 plugs into the TU55 backplane, and where
> the data cable plugs into the TU55 backplane, and possibly the K2 relay on
> the G851.
> In the TC01 you could swap the G888 module in slot C22, the S205 module in
> slot D05, the S205 module in slot E06, the S603 module in slot C02, or the
> R123 module in slot E08, with another one to see if the glitch moves to
> another bit.

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

> --
> Michael Thompson

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