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Old 06-07-2007, 02:06 AM
Grant Stockly Grant Stockly is offline
Join Date: Jan 2005
Posts: 447

From: Dan
Date: Sun, 03 Jun 2007 12:50:47 -0400
Subject: Re: [Altair Computer Club] 88-PPG fixed with story

what I like to know is somebody there transcribing this while you troubleshoot ?
you must have a secretary hiding back there
I'm always too consumed to type while troubleshooting

Good job !,

[ My Corner of Cyberspace ]

Grant Stockly 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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