LINCtape/DECtape Head Alignment

[Jay Jaeger]

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.

JRJ


On 12/26/2016 12:08 PM, Michael Thompson wrote:
> The RICM is working on the skew adjustment on a TU56 tape drive on a
> PDP-12. We only see a 5mV signal from the head, so when we flip the tape
> over we will only see 1mV. This is below the capabilities of my 'scope.
> 
> The DEC skew adjustment procedure talks about using a DEC amplifier to
> boost the head signal to several volts. We are planning to make an
> equivalent amplifier using a modern Op-amp. It would be really convenient
> to have one of the Amphenol 133-022-03 connectors from a G851 Relay module
> on our amplifier so it would plug directly into the head cable.
> 
> Does anyone have a DEC G851 module that we could remove the connector from?
> 

BCC:  Original poster