SWTPC 6800 issues again

[Chris Elmquist]

On November 28, 2016 1:37:54 PM CST, "Ian S. King" <isking at uw.edu> wrote:
>On Mon, Nov 28, 2016 at 8:22 AM, Brad H
><vintagecomputer at bettercomputing.net
>> wrote:
>
>> Hey guys,
>>
>>
>>
>> You might recall a while back I was having issues where I'd have to
>power
>> up/power off multiple times before I'd get the 6800 to start up
>correctly
>> and give me the SWTBUG prompt.  What I did was remove an overwrought
>> modified RAM board and replaced it with a more basic 4K board set to
>the
>> $A000 range.  That worked great for a while, but not we're getting
>back to
>> the situation where I power up many times and get either ? marks, a
>string
>> of 4s, or some other random character.  I have to power off and on
>several
>> times before I get the $ prompt.
>>
>>
>>
>> I figured out how to run a proper RAM diagnostic and no errors came
>back.
>> I
>> wasn't sure how to properly test the $A000 board -  I assumed I
>couldn't
>> let
>> the test test the address space used by the test program itself, so I
>set
>> it
>> to run from A07F to AFFF (I think I did that right, I set the MSB in
>A002
>> to
>> A0 and LSB in A003 to 7F, and for the upper limit MSB in A004 to AF
>and
>> A005
>> to FF).
>>
>>
>>
>> I'm wondering now if this is really a RAM problem or maybe something
>else.
>> I don't think it's the serial card.. I've tried both the MP-C and
>MP-S and
>> no change.
>>
>>
>>
>> I have an NOS MP-B2 motherboard here.  The 'check pins' on the molex
>> connectors for the cards haven't even been cut.  I could set that up
>for
>> testing although, being totally unused I'm hesitant about altering
>it.
>> What
>> do you think on that?
>>
>>
>>
>> Brad
>>
>>
>My first question (and pardon me if this was addressed in your earlier
>thread) is, have you checked the power supply?  If possible, use a
>scope,
>but even a good DVM will tell you if you're maintaining voltage.  And
>if
>the filter cap is the original, just replace it - they have a limited
>lifespan.  I bought one from Digi-Key for $19.  Hope that helps -- Ian

Also, confirm that the connector on the power supply board (next to the big cap Ian references), that leads off to the motherboard, still has nice clean *unburned* pins.  Those Molex pins were marginal at best for a heavily loaded backplane and I found mine had been heating and were pretty much toast.  This results in huge drop of the +8V before it gets to the motherboard even if it looks all nice and happy across that big filter cap.

Chris


-- 
Chris Elmquist