i've gotten nerd sniped into helping fix the IBM 729...

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i've gotten nerd sniped into helping fix the IBM 729 tape drive at the Computer History Museum...

half my face in front of an IBM 729 tape drive. i'm standing on the left side of the machine with the side panels removed. visible are several clutches and drive belts that cause the reels to turn.

Like 18 May at 0:42 | Open on mastodon.social

22 comments

Tube❄️Time

this basically means i'm buried in IBM schematics that use a totally weird representation of logic gates.

schematic diagram in a large-format binder. the diagram is full of square boxes with mysterious alphanumeric codes instead of the more familiar logic symbols.

18 May at 0:44 | Open on mastodon.social

Tube❄️Time

this particular version of the 729 has a whole bank of SMS cards -- IBM's Standard Modular System which uses germanium transistors. these came out before their (equally odd) SLT cards and modules.

an array of around 30 PC boards, each about the size of a playing card, plugged into a backplane. parts are visible on the cards, mostly resistors with color coded stripes.

18 May at 0:48 | Open on mastodon.social

Tube❄️Time

here is one of the cards. this one is an "MH" card containing 4 PNP driver transistors.

a brown circuit board with around 20 parts on it and some jumper wires. it is about the size of a playing card, and it has an edge connector sticking out the side.

18 May at 0:50 | Open on mastodon.social

Tube❄️Time

if anyone wants to see the back of the card, it looks like this.

the back of the card with just solder joints. the card has gold plated edge fingers. it also seems to have a conformal coating applied to insulate it.

18 May at 0:53 | Open on mastodon.social

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i want to build a breakout system to make it easier to test these cards. they use a 16-pin connector with 0.125" spacing between contacts. at the local electronics store, i could only find this 44-pin connector.

18 May at 0:55 | Open on mastodon.social

Tube❄️Time

my idea is to remove a few pins and cut the connector down.

18 May at 0:55 | Open on mastodon.social

Tube❄️Time

the final product. you can barely see the glue seam. bonus points to anyone who knows why the connector pin alphabet is missing a few letters. further bonus points to anyone who knows why each contact has two wires connected to it.

an IBM sms card plugged into my customized socket. the socket pins are wired up to a set of 16 test points marked "A B C D E F G H J K L M N P Q R"

18 May at 0:58 | Open on mastodon.social

prozacchiwawa

@tubetime the obvious answer with the letters is they're gonna leave out the ones that can be confused with numbers.

i'd imagine one of S or Z or both is left out as well.

oh -- or they're the symmetric ones that don't tell you if you're reading it upside down :-)

18 May at 1:05 | Open on functional.cafe

William D. Jones

@tubetime I and O too easy to confuse with 1 and 0?

I feel like I've seen this before in e.g. Game Genie codes.

18 May at 1:12 | Open on mastodon.social

Darryl Ramm

@tubetime

I != 1
O != 0

Two wires? Because you wanted to be able to insert boards either way? But why?

18 May at 1:15 | Open on hachyderm.io

Emily Velasco

@tubetime I have a front panel from some old electronic device that has the same letters removed

18 May at 1:25 | Open on social.afront.org

JeffG

@tubetime the missing letters can be mistaken for numbers!

18 May at 2:13 | Open on mastodon.social

Zack Stern

@tubetime I don’t see any traces on top of this card but my entry is: to get continuity on the top and bottom of the card at the same time. The leads go to teeth on one each of the top/bottom?