Otherware/Otherware_1_SB_Development.iso/amiga/info/scram_50.lzh/SCRAM500.lzh/docs/trouble.doc

Text File | 1992-06-20 | 6.8 KB | 183 lines

TROUBLESHOOTING
===============

I hope most people will have no trouble with the SCRAM 500, but
obviously some boards will struggle to life or one day myseriously die.
The tips described here should be used as a guide, however, tricky
problems will come up that need a reasonable amount of diagnostic skill.
Hopefully, someone in your city or User Group or on the Nets will be
able to help. Armed with the Schematic Diagrams, just about any
AmigaTechnoPerson should be able to locate the fault. At worst they
might need an oscilloscope and some beer.

Stay cool, think positive and DON'T PANIC!


Catastrophe - smoke!
--------------------

Let's deal with the really scary scenario first - you blow up the Amiga.
This isn't as bad as it might seem. I haven't seen this happen often and
when it does happen it usually is only the 68000 chip that is dead. That
is easily fixed, and probably gives you an excuse to upgrade Agnus or
KickStart any way. If you do replace the 68000, you might put in a
HC68000 device - HC uses less power and is a lot cleaner. Before you
blow it up again, try to work out why it happened. Here are some ideas.

  o You plugged/unplugged the SCRAM with power on.
  o You zapped the system with static - watch out for carpets.
  o Your SCRAM board has a serious short (power to signal, for example)


Next on the gloom scale, the SCRAM starts to smoke. This can be serious,
but usually it can be fixed. You probably haven't checked the board
thoroughly for shorts and bridges. Now the problem will be highlighted
for you in carbon! If the SCRAM has been checked out with a multi-meter,
then you should be able to avoid this unhappy circumstance. The other
likely cause of smoke is an IC back to front. This will usually require
replacing the IC with a new one.


The SCRAM stops the Amiga from working
--------------------------------------

You plug the SCRAM on and the Amiga is dead - just a black screen. This
means the SCRAM is seriously interfering with some vital Amiga function.
Go through the following check list.

  o remove all socketed chips from SCRAM - any change?
  o check power rails for normal voltages
  o check expansion bus signals for funny levels (logic probe / Scope)
  o recheck SCRAM for solder bridges and gross faults
  o last resort - start cutting TTL out (avoid if possible)

Fixing this type of fault requires some diagnostic skills - the cause is
likely to be different depending on whether the board has just been
built or has failed in service. If you pull out all socketed devices and
the problem remains, it is almost certainly a solder bridge.


The Amiga only gets to a coloured screen
----------------------------------------

This is a slight advance on the previous fault. This is more likely to
be an address or data bus fault (typically a short). Remove all socketed
chips and look for shorts.


The SCRAM Config LED flashes
----------------------------

Things are getting better. The SCRAM Config LED is turned on when
KickStart goes out to AutoConfig expansion boards. This means the 68000
is getting a fair way along, and the SCRAM is mainly working. Again,
remove socketed chips and see if the Amiga functions normally. If it
does, then the fault is probably not an address/data short, but some
more benign fault. Try the following techniques:-

  Disable the RAM part of the board.

   o Pull out all DRAMs (they shouldn't be in yet anyway)
   o Set jumpers for 0Megs RAM
   o Remove Humphrey and Griswold chips

   The RAM section of the board will now be inactive. Only a gross fault
   (ie solder short) can be causing trouble.


  Disable the SCSI controller

   o Pull out Boot ROM, Cyril and SCSI chip.
   o Disable AutoBoot switch

   The SCSI section is now inoperative and should not be causing
   problems.


  With Bertie in place, you should be able to get the Config LED to come
  on and stay on when you start the Amiga. This should now be your goal.
  If you study the schematic (sheet 1/3) you will see that the
  AutoConfig logic is fairly simple - most stuff is happening inside
  Bertie. Once you get AutoConfig happening, you can proceed with the
  other sections by populating them and testing them.


SCRAM configs, but RAM appears to be faulty
-------------------------------------------

If RAM only partially works, it is likely to be one of the following:-

  o solder faults in the ZIP array - dry joints and bridges!
  o bent RAM pin in socket
  o error with the LS257 multiplexors
  o faulty RAM chip

To diagnose RAM faults try RAM TEST programs, Monitor programs and a bit
of detective work. For example, if you can get a Monitor working, you
can tell a lot by writing patterns of 00 and FF to all the RAM

  o one of the 4 nibbles not working -
      ? faulty DRAM chip
      ? missing +5, GND, RAS, CAS

  o repeating blocks not working or duplicated
      ? RAM mux address line missing or shorted
      ? solder fault on LS257 or ZIP array

  o RAM corrupts with time (refresh problem)
      ? missing E clk
      ? check 393 device
      ? check REFRAS\ signal from Griswold to Humphrey

If you can't get the RAM to do anything, then look at the signals
supposedly emenating from Griswold and Humphrey. You should see
reasonable RAS, CAS and REFRESH signals. If you don't, then suspect one
of these chips.  The easiest way to debug the Custom Chips is swapping
with known good chips. Having access to a working SCRAM is also useful
for debugging.


SCSI Problems
-------------

The SCSI circuit is simple and socketed. Just about any problem will
look the same - the disk won't talk!

Make sure your SCSI drive is working and the cables are good. Don't use
25 way ribbon to connect to SCSI - scrambled wires or 50 Way Ribbon is
important to avoid crosstalk problems. Some of the new SCSI devices have
very high transfer rates and can be cranky with long or noisy cables.

Look for all the normal solder problems - dry joints, bridges etc.
Sometimes PLCC chips can have damaged or bent pins so check this.
Use a Monitor program to try to read the EPROM and SCSI registers.
Try to activate the Disk LED - prove Cyril is working.
Replace SCSI chip (static sensitive device)

It should be fairly easy to write a bit of code that goes out and does
some diagnostics on the SCSI chip to find problems. All the
documentation on the DP8490 chip is available in the National
Semiconductor Mass Storage Handbook. Some of this is SCRAM 500 User's
Manual.


SUPPORT
-------

I hope that as people get SCRAMs going they become sources of support
for others. There should be a reasonable amount of healthy discussion on
the nets which may provide encouragement for the owners of non-working
SCRAMs.

If you have a serious problem you can send me an EMAIL, and I will try
to solve it (the problem), but try other more direct avenues first.

EMAIL [e-mail]

In Australia, we have a resident SCRAM expert who can fix all things
Amiga.

Chris Ralph, Sydney - Phone [phone]