Subject: M220B (PDP-8/I) schematics available

From: "Robert Krten" <[email-masked]>
Date: Mon Jul 7 20:17:37 2003

For those of you who missed the announcement in alt.sys.pdp8,
I've now almost completed the M220 version B schematic.  It's
available at:

http://www.parse.com/~pdp8/pdp8i/m220b.html

The trials and tribulations of the PDP-8/I restoration can be
found at:

http://www.parse.com/~pdp8/pdp8i/restore.html

Plenty-o-dead 7474's in that one :-)

Cheers,
-RK

--
Robert Krten, PARSE Software Devices +1 613 599 8316.
Realtime Systems Architecture, Consulting and Training at www.parse.com
Looking for Digital Equipment Corp. PDP-1 through PDP-15 minicomputers!

---

From: Ethan Dicks <[email-masked]>
Date: Wed Jul 9 18:44:16 2003

> http://www.parse.com/~pdp8/pdp8i/m220b.html

Cool!  Thanks!  I have a few M220 cards that will eventually benefit
from your efforts (I borrowed them from a parts machine to keep the
main machines running).
 
> Plenty-o-dead 7474's in that one :-)

That's been my experience, too.  I have replaced as many as 8 7474s
at a go to get an -8/L back on its feet.  I don't recall replacing
any other chip, as a matter of fact.

-ethan

---

From: Jim Davis <[email-masked]>
Date: Wed Jul 9 18:45:40 2003

>any other chip, as a matter of fact.
7474's Dead d flops? whats the story on that?
Jim

---

From: Ethan Dicks <[email-masked]>
Date: Wed Jul 9 19:11:27 2003

> 7474's Dead d flops? whats the story on that?

Dunno.  I can say categorically that they were failed chips.  They
did not pass muster in a handheld TTL tester, and when replaced,
the PDP-8/L worked.

There was a mention on the list of a batch of 7440s that failed in
another M-series machine (might or might not have been an -8/L)
with the same date code.

Not sure why 7474s from the mid-to-late 1960s would be more fragile
than, say, 7400s.  Perhaps the flipping and the flopping wears them
out faster than anding and oring?  :-)

-ethan

---

From: Bob Shannon <[email-masked]>
Date: Wed Jul 9 19:29:26 2003

Jim Davis wrote:

<snip>

> 7474's Dead d flops? whats the story on that?
Good question!  I've also chased down several bad 7474's in my Imlac, 
and I suspect some bad 7474's on a
few HP 1000 series I/O boards I have down in the shop.

Is the 7474 a 'dog'?  I've know a few TTL parts that were pretty bad, 
mainly counters though.  But a latch?

---

From: "Robert Krten" <[email-masked]>
Date: Sun Jul 13 19:24:01 2003

Ethan Dicks sez...
> out faster than anding and oring?  :-)

:-)

In my case, the failure modes were as follows:

2 chips failed with:  grounding the PRESET line caused the Q
to go high as expected, but when PRESET was left floating Q
went low.  [DEC leaves the PRESET and CLEAR lines floating on
various modules, so I consider this a fair test].

1 chip failed with insufficient +5 on the Q (or bar-Q, forget
right at this instant) output.  The measured output was something
like 1.6V.

Like Ethan sez, when these chips were replaced, things got better.

I think the "reason" might be that these chips were pushing the edge
of semiconductor manufacturing technology -- they are certainly one of
the more complicated chips from the early 74xx (all 14-pin only :-))
era.  That's my theory...

Cheers,
-RK

---

From: Tothwolf <[email-masked]>
Date: Sun Jul 13 20:01:06 2003

On Tue, 8 Jul 2003, Bob Shannon wrote:
> and I suspect some bad 7474's on a few HP 1000 series I/O boards I have
> down in the shop.
>

This reminds me of a similar problem in the JC80 systems, where 74170
chips would fail more often than anything else. I'd sure like to know what
the cause of that was...

-Toth

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From: [email-masked] (Tony Duell)
Date: Sun Jul 13 20:09:21 2003

> chips would fail more often than anything else. I'd sure like to know what

I can remember replacing many 7474s in the RK11-C when I was getting that 
going. Oh, and a 7482 adder, which was impossible to find [1]...

More recently, I've had a lot of 74Hxxx parts (and yet more 7474s) fail in 
HP98x0 calculators. I guess after 30 years I can't really moan...

[1] I do mean _iompossible_. I ended up using part of a 7483 that I stuck 
to the flip-chip PCB dead-bug style and wired to the holes where the 7482 
should have been. Runs fine...

-tony

---

From: Tothwolf <[email-masked]>
Date: Sun Jul 13 20:20:27 2003

On Thu, 10 Jul 2003, Tony Duell wrote:

> > chips would fail more often than anything else. I'd sure like to know
> > what the cause of that was...
>
> I can remember replacing many 7474s in the RK11-C when I was getting
> that going. Oh, and a 7482 adder, which was impossible to find [1]...
>
> [1] I do mean _impossible_. I ended up using part of a 7483 that I stuck
> to the flip-chip PCB dead-bug style and wired to the holes where the
> 7482 should have been. Runs fine...

You know what happens when someone says impossible...

I just checked my parts bins, and I have *one* 7482, a TI part with a date
code of 7541. If you still want to replace your 7483 kludge, and know of
an economical way to get small things from the US to the UK, let me know.

Btw, what was the vintage of the old chip? Chances are one of the local
surplus places I visit from time to time has some 7482 chips in one of
their bins.

-Toth

---

From: Ethan Dicks <[email-masked]>
Date: Sun Jul 13 20:36:28 2003

--- Tony Duell <[email-masked]> wrote:
> > > Is the 7474 a 'dog'?  

One of these days, I need to get my RK11-C up and working (I have a
working RK11D that's been used recently - I've had the -C since 1984
and haven't ever had the occasion to power it on).

I should probably build a KM-11 clone first.

-ethjan

---

From: [email-masked] (Tony Duell)
Date: Sun Jul 13 20:44:44 2003

> I should probably build a KM-11 clone first.

You'll need it. Actually, it's useful to have _2_ of them, since there 
are a pair of KN11 connectors with different signals on them. 

Does anyone have the full lights panel for the RK11C? It's 144 LEDs or 
bulbs that display the I/O registers, etc. I have the panel, but not the 
right bezel for the RK11-C (I only have the DX11 bezel...)

Not particularly useful, but the blinkenlights sure look pretty....

-tony

---

From: [email-masked] (Tony Duell)
Date: Sun Jul 13 20:48:01 2003

[7482 chip]
> You know what happens when someone says impossible...

'Somebody these days claiming something is impossible is apt to be 
interrupted by some fool doing it' :-)

> an economical way to get small things from the US to the UK, let me know.

Actually, I got some 7482s a couple of years later. But I never bothered 
to replace the kludge -- it works well, so I don't see any reason to pull 
the card again. If it fails, I'll try one of the 7482s I have...

> Btw, what was the vintage of the old chip? Chances are one of the local

Around 1972, when the controller was made.

-tony

---

From: Bob Shannon <[email-masked]>
Date: Sun Jul 13 20:53:32 2003

Hell yes, the blinkinlights are over half the point to vintage hardware!

When I built my HP1000 ATAPI (IDE) disk controller, the first job was 
drilling 72 holes in the rack
mount front pannel for the LED's!

Turning that into a working disk drive was rather, well, secondary to 
getting the LEDs blinking in a meaningful
way.  I even tweaked my current limiting resistors to try to match the 
LEDs brightness to those of the HP 2113
CPU, and use a real HP grey blank panel spacer so it would look 'close 
to right'.

I gotta get .JPG of that machine on-line someplace....

---

From: Jay Jaeger <[email-masked]>
Date: Sun Jul 13 22:24:14 2003

I think I may have an RK11-C bezel (no lights, just the panel with the silk 
screen on the front, a fiber back with hole for the lamps.

Trouble is, I'm not sure I want to part with it -- someday I'd like to have 
an RK11-C to go with my RK03's.

Jay Jaeger

>Not particularly useful, but the blinkenlights sure look pretty....

---	
Jay R. Jaeger					The Computer Collection
[email-masked]

---

From: [email-masked] (Tony Duell)
Date: Sun Jul 13 22:27:01 2003

> an RK11-C to go with my RK03's.

Oh, I wasn't expected to find one here. In fact it's not somethign I 
_need_ -- the LEDs are just as pretty without the right bezel.

And yes, you do want to find an RK11C. It's one of the more 'interesting' 
units to repair that I've come across....

-tony

---

From: jpdavis <[email-masked]>
Date: Thu Jul 17 12:18:55 2003

>right at this instant) output.  The measured output was something
That sounds like a design flaw.  Even with  TTL, unused inputs should be 
tied to somthing.
What was the fanout  for the Q /Q on the fried output?

Jim Davis.

---

From: "Robert Krten" <[email-masked]>
Date: Thu Jul 17 12:42:06 2003

Tony Duell sez...
> should have been. Runs fine...

Bzzt.  Wrong.  (First, you probably mean "_impossible_" :-)).
I just bought 5 of them for US$1.25 (or so) each from
Gateway Electronics.  Along with spare 7453, 7460, 7440, etc.  I think it's
something like "gatewayelex.com".

Cheers,
-RK

---

From: "Robert Krten" <[email-masked]>
Date: Mon Jul 21 13:51:16 2003

jpdavis sez...
> What was the fanout  for the Q /Q on the fried output?

This was not in-circuit, this was in a test jig.  I don't know what the
in-circuit fanout would have been, that would require massive amounts
of digging through the schematics :-)

Cheers,
-RK

---

From: [email-masked] (Tony Duell)
Date: Mon Jul 21 15:00:38 2003

> > going. Oh, and a 7482 adder, which was impossible to find [1]...

[...]

> something like "gatewayelex.com".

Notice I said 'was impossible to find'. At the time I was repairing this, 
some 10 years ago (Eeek!), no UK supplier could supply them (and I 
tried...). This was before the Internet was commonly available in the UK, 
and we didn't really know the US suppliers of old chips.

-tony