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Debugging an TU56 on a TD8E in a PDP-8/e

jackrubin

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Mar 12, 2004
Messages
831
Location
Chicago, IL and Buchanan, MI
@Mike - I may have a spare TU56 head assembly - I'll contact you off list.

@AteEM re Dynaloy - very cool stuff. Is it available in small quantities at something like affordable pricing? There are lots of people in the antique radio and instrument world who would love to learn more!

Jack
 

m_thompson

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Jul 8, 2014
Messages
1,637
Location
Rhode Island, USA
I started digging the epoxy out with a small chisel point soldering iron.
The process seems to work well, but it is time consuming.
The crater that I dug is about 1/2" deep so far.
A picture of the carnage is here.
I broke the solder connection between the orange wire and the MT(1) wire.
You can see a really tiny piece of wire just to the left of where the MT(1) arrow is pointing.
The yellow MT(G) wire looks like it goes deep into the epoxy and then loops back up.
If so, then I probably broke the MT(G) wire off the yellow wire, but maybe I can fix that.
The brown MT(0) wire goes deep into the epoxy, and I have not found the end if it yet.
 

AteEm

Experienced Member
Joined
Aug 1, 2013
Messages
111
snip...
@AteEM re Dynaloy - very cool stuff. Is it available in small quantities at something like affordable pricing? There are lots of people in the antique radio and instrument world who would love to learn more!

Jack

Hmm, good question. IIRC, at work I had purchasing buy a single quart from a distributor... I'll try and track that down tomorrow. It's been a couple of years, thankfully, since I've had to de-pot something:)
 

m_thompson

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Jul 8, 2014
Messages
1,637
Location
Rhode Island, USA
We borrowed a DECtape head from another TU56 tape drive that is connected to the unused TC11 controller in the collection. Both DECtapes on the PDP-8/e now work. We started formatting DECtapes in the collection so we can see which ones are still good. The success rate is about 50% so far, even for tapes that are probably unused.

Bob Vines volunteered to X-ray the broken DECtape head. That will show where the tiny wires are embedded in the epoxy so maybe we will have a chance of fixing the broken connection without further damage to the wiring.

We made a bootable DECtape that configures a new DECtape to boot from a TD8E tape controller. After it finishes doing some magic it asks for a copy of the OS/8 installation tape. We haven't found an image of that DECtape yet.
 

Marty

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Joined
Jul 26, 2009
Messages
3,141
Location
Boulder , Colorado USA
Hi All;
You might want to try all of the Failing tapes in what ever tape transport You are not using, and see if any will pass in the other transport.. Since the head alignment might be slightly different..
Or maybe doing something like heating them up in the oven for a little while, But AL would be a better expert than I am on that..

THANK YOU Marty
 

bobaboba

Experienced Member
Joined
Sep 7, 2013
Messages
387
Location
Scotland
Hmm - it's difficult to see the detail of the fine wires. I suppose your best chance if you can dissolve the potting is to find a broken connection or dry joint to the coil wiring from the I/O wires.
 

m_thompson

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Jul 8, 2014
Messages
1,637
Location
Rhode Island, USA
After removing the 8/e from the rack for a long project with FPGA Omnibus peripheral emulation, the 8/e is home and reassembled. I even installed the RK05 drive that I recently received in the rack. Unfortunately neither the TU56 or the RX01 work. The TU56 will go forward/backward under local control, but will only go forward under computer control. Time to get the 'scope out and check the behavior of the SN7474 E34 that holds the F/R bit and the SN7417 E39. There is also an inverter in the paddle card that plugs into the TU56 that takes the F/R signal from the TD8E and splits it into two separate control signals from the TU56. It should not be difficult to debug and repair this fault.

I ran a simple program that displays the DECtape block number in the MQ register. It looks like there is a stuck bit when displaying the sequential block numbers, so there may be another problem in the Data Register logic.
 
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