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Tek 4051: Tape drive way out of calibration


Veteran Member
Jan 1, 2020
London, UK
I have two Tektronix 4051s, and neither has DC200 tape cartridge drives in perfect health.

On the first, the drive sounds like it's been built out of flagpole pulleys and pepper grinders. I assume I can service the transport motor etc. with some porpoise jaw oil (or some sewing machine oil if I can't find any of that). It reads and writes tapes successfully.

The second sounds better but is in a worse condition. When I first tried it, an attempt to get a tape directory listing (via TLIST) would list the first record on the tape and then a tape error. This is a bad result but not terrible: at least a lot of the components needed to read the tape are working. I thought it might be the case that the head had wandered slightly out of alignment, so why not give the adjustment screws a little tweak and see what happens...

Well, what happens is that TLIST now fails to do anything useful: either it'll give you a tape error or it'll roll the tape a bit, stop it, then hang the machine until you eject the tape (and then it's a tape error). Silly me.

Tek's published procedure for getting a tape drive back into service requires a special "calibration tape" and an accompanying tuning procedure for you and your oscilloscope. These tapes have probably all been destroyed. Has anyone managed to adjust a Tektronix tape drive into working without a calibration tape? I can use other Tek 405x machines here to create tapes containing data, for what that's worth.


Veteran Member
Mar 7, 2018
Texas, USA
I have looked but never found anyone with the Tektronix 4050 calibration tape.

I think you should use the tape that works in the working 4051 as your 'calibration' tape and adjust the second 4051 until it can read that tape.

I assume you have already cleaned the tape heads with isopropyl alcohol and a swab with a long stick?
I use Q-Tips with a long surgical hemostat to clamp the short Q-Tip with 91% isopropyl alcohol on one tip.
I then pull the hemostat out and turn the Q-Tip around and use the dry end to dry the head.

Similar issue occurs with the 4907 Floppy Drive system.
I have not found the Tektronix (shugart) 8-inch hard-sectored calibration disk.
I have two 4907 Floppy systems - one read the Tektronix CAD disk from Living Computer Museum - the other does not.

I think I saw a 4907 floppy system behind your two 4051. If it works - use it with the second 4051.


10k Member
Jun 19, 2012
UK - Worcester
I see the schematic on PDF page 104 of 228 at http://www.bitsavers.org/pdf/tektronix/405x/070-2286-00_4051_Service_Vol2_May77.pdf.

I would be inclined to measure the resistances of the two separate heads by disconnecting them one at a time and using a multimeter set to resistance. See if the two heads are of a similar resistance to each other.

Don't forget that each head consists of a centre-tapped winding - so measure from both "end to end" and from the centre tap to each end.

Look for an open circuit or a gross resistance disparity.

After that I would try and read a tape and look for outputs from the various operational amplifiers.

If you have 'tweaked' the head, perhaps you can use your oscilloscope to tweak things back by looking for a signal maximum?

Just thinking out aloud.