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
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