LINCtape/DECtape Head Alignment

Michael Thompson michael.99.thompson at
Sun Mar 19 13:07:26 CDT 2017

> Date: Sat, 18 Mar 2017 21:46:21 -0500
> From: Jay Jaeger <cube1 at>
> Subject: Re: LINCtape/DECtape Head Alignment
> Curious:  How are you measuring the signal from the head?  Do you have
> an honest to gosh differential probe, or are you using some other
> technique?  (If you have a differential probe, then the TU56 manual
> indicates that you should see 10mv-12mv (the addition of the two paired
> heads together), so as a first guess I am guessing you are looking at
> the coils one at a time.
> The reason I ask is that the TU56 that I use most often has gotten a bit
> cranky over the years.  Generally I can read and write, but I do
> typically see some errors - unacceptably many, and it *seems* that the
> longer the machine is on, it seems the more mark track errors I get when
> running the ZTCC?? diagnostic (test 3).
> I don't have a differential probe, and the A-B math function on my Rigol
> DS2072 scope is not anywhere near fast enough (though maybe a firmware
> patch which I have downloaded will help, but I doubt it will help
> because their is a lot of HF noise on the signals when measuring
> voltages this low).  However, if I apply a 50KHz low pass filter on the
> signal on the scope, then sometimes I can see a 5mv per coil signal
> using an ordinary probe.  I say sometimes because the scope seems to
> have some firmware problems so it isn't consistent in its behavior.  (I
> have downloaded a firmware update that *might* help).
> I don't really doubt my heads at this point - certainly nothing is open
> - I can measure each coil at about 1.5 ohms (3.0 ohms across both), but
> it is something I would like to make sure I know how to do.
> Also, have you degaussed your heads?  If so, how?  I ask because some of
> my symptoms could point that way (I have yet, for example, to test with
> a tape, have it get worse, then go back with the machine "cold" and see
> if it gets better - and if it doesn't, that could point to demagnetized
> heads.)
> Thanks.

We used the procedure in the TU56 maintenance manual, and used two G888
modules to make the equivalent of the G500 described in the manual. The
G888 modules really cleanup the high frequency noise mixed with the head

Michael Thompson

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