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Old 06-07-2007, 02:05 AM
Grant Stockly Grant Stockly is offline
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Join Date: Jan 2005
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From: "jack99rubin"
Date: Sun, 03 Jun 2007 12:48:32 -0000
Subject: [Altair Computer Club] Re: 88-PPG fixed with story

Thanks for another interesting chapter - hope you're writing a book
on demon chasing! Again, I'm guessing at the final resolution - you
resoldered the pin and all was well?

Another tool you've mentioned in passing is a desoldering pump - I
recently picked up a discontinued Hakko unit and it has made a
tremendous difference in my approach to these sorts of problems - it
is so easy to remove junk components like cheap or broken sockets
that I don't think twice about replacing them. I've used "the pump"
to salvage 40-pin IDE header connector of multilayer PC motherboards
so it's definitely up to the task of dealing with '70s era stuff.

I join H.E.Bob in congratulating you on repairing the PPG unit - it
sat in my attic for years until it went to Bob a while back. I never
used it but I'm glad to see it alive and well.

best,
Jack

--- In altaircomputerclub@yahoogroups.com, Grant Stockly <grant@...>
wrote:
>
> Using the data from the logic analyzer I traced out the problem
with the
> 88-PPG and fixed it. This makes me happy and I'm sure Robert
too. :
> ) Btw Robert if you are reading this please call or e-mail me! : )
>
> I've uploaded all data sheets for the ICs listed and a ton more
stuf in the
> file area. All you digital pack rats should go take a look at
it. : )
>
> There is also a picture of a 88-PMC ROM card with a ZIF socket
> installed. This allows me to swap ICs without removing the card.
>
> ===
>
> A0 is stuck high. This causes no odd bytes to be programmed and
all even
> bytes to be programmed twice with data from the associated odd
byte. : (
>
> There is a section on the motherboard where A0 is buffered twice.
The
> input to the circuit is 0-5v TTL and the output is -48-0v. A0 is
stuck
> high before this. A0 is generated by a 7400 positive NAND gate, 4Y
(pin
> 11). Pin 13 is high. Pin 12 needs to be high in order to make pin
11
> low. Pin 12 goes to a 8212, an 8 bit I/O port, pin 4 where it is
stuck low.
>
> Pin 4 on the 8212 is DO1 (data out bit 1 of 8). Pin 3 is the
associated
> DI1 (data in bit 1 of 8).
>
> There is no activity on pin 3 of the 8212. The 8212 is not bad.
>
> Pin 3, DI1, is shared on all 3 of the 8212s. If it is stuck low
then D0
> should be stuck low too. I checked the wave form and D0 NEVER goes
> high. Hmmm... : ) A0 stuck high, D0 stuck low. This makes sense
since
> the data lines are inverted compared to the data.
>
> Pin 3 on the 8212 goes to pin 2 of a 74LS14 a Hex Schmitt-Trigger
> Inverter. Pin 2 is an output, pin 1 is its input. This is one of
the
> tarnished ICs... Nothing at all on pin 1.
>
> Pin 1 on the 74LS14 connects to R11, R12, and pin 9 on the system
interface
> cable. No signal there either. There is no signal on pin 9 of the
S-100
> interface card either.
>
> The problem is in the interface card. Its pretty simple. : )
>
> Pin 9 on the interface cable goes to:
> -pin 24 on an unidentified empty socket.
> -pin 3 of IC D, a 74367 a Hex Buffer Driver with tri state output
> -pin 6 of IC G, a 8836 (quad NOR gate, sorry no datasheet)
>
> The 8836 pin is an input, so unless it is shorting then it isn't
the
> problem (I have seen this...).
>
> The legs on IC D are tarnished a nice dark black color. : ) Pin 3
is an
> output, pin 2 is the input to that buffer. Pin 3 on the IC socket
was not
> soldered very well at all. Looked like someone blew their nose on
> it. This is weird because the rest of the board is nice and
shiny. When
> it heated up it must have pulled tight against the board. I
noticed that
> the when sticking a wire into the socket it had no resistance and
when
> pulling it out it varied between 200 and 5 ohms. : )
>
> It works now! : ) I just copied the PPD ROM and used it to make
copies of
> other ROMs. : )
>
> Grant
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