[sdiy] The fatman is finished...

[Scott Gravenhorst]

Gabriel Lindeborg <gabriel.lindeborg at sverige.nu> wrote:
>Great Scott!
>
>I think your on to something, the AC voltage over the 7805 output 
>(GND-Output/Pin2-Pin3)(the 7912 and 7808 reads 0V) reads ca 10V as does 
>AC from Vss to Vcc on the uP. Then is it the regulator or a cap failing?

Vss to Vcc ?  Do you mean Vcc to ground?  Noise should be rather low, well under
0.5 volts AC.

Capacitor probably, maybe (I hate to say this again) not well soldered? 
Backwards?  Too small?  Perhaps even a bad electrolytic cap...  The resistors
that feed the regulator wrong value?  Too much noise on 5 volts can easily cause
the MCU to behave oddly (and not work at all!)

While regulators can fail, they usually don't and bad new ones are extremely
rare, though possible.  Normally this kind of thing is caused by poor filtration
or excessive load.  But normally when the problem is corrected, the regulator IC
begins to behave correctly.  A bad regulator doesn't usually have a noisy output
as a failure mode.  Rather it would have the wrong DC output and it might change
voltage value as the load changes (which might look like noise actually if the
load current changes erratically).

Another thing might be a resistive solder connection to the diodes in the
supply.  Reflow should fix that.

To have a scope would answer many questions, but you're doing fine so far.

>
>
>//Gabriel
>
>Scott Gravenhorst wrote:
>> There's more inline, but I do have one important question:  Are you sure the
>> 5 volt supply is 5 volts, AND that it is properly regulated/filtered?  A
>> supply with noise can have an _average_ DC voltage that could appear correct
>> on a DMM, however if there's a problem with it, you might have 60 Hz
>> appearing on the rail from bad filtering, wrong resistor size, or excessive
>> load.  If your signal tracer has a series cap, try listening to the 5 volt
>> (to ground) supply rail.  Or you could try to measure AC volts with the DMM.
>>  There should be very little AC volts on the supply.  I don't have a spec for
>> that, but measure that and let us know what it says...  This is another place
>> where an oscope is wonderfully superior to a DMM.  If you've got a serious
>> problem with 60 Hz noise, you'll hear it with the tracer and you'll see it on
>> the DMM set to AC volts.
>> 
>> I've put other comments inline below...
>> 
>> Gabriel Lindeborg <gabriel.lindeborg at sverige.nu> wrote:
>> 
>>>Hi Crow!
>>>
>>>
>>>>The Old Crow wrote:
>>>>It sounds like the 8031 is not running.  Check to see if there is a
>>>>processor clock at IC1 pin 18 (or 19, whichever is the internal osc
>>>>output)...
>>>
>>>The up seems to be running as I get totaly different power up behavior 
>>>when I remove the crystal.
>>>
>>>
>>>>or check for the ALE strobe pulses at pin 30.
>>>
>>>The ALE output stays at a constant 0V as far as I can tell with the 
>>>simple multimeter I got at hand.
>> 
>> 
>> ALE (Address Latch Enable) will toggle with each bus access, so this will
>> toggle very fast (megahertz rate) and you won't see it with a multimeter. 
>> Here's where you need a scope.
>> 
>> 
>>>>Are C2 and C3 the
>>>>listed 33pF values?
>>>
>>>Yep, marked 33 anyway...
>>>
>>>
>>>>Check for solder bridges, cracked traces, etc. in the 
>>>>IC1/IC2/IC3 === MCU/latch/ROM connections.  Check IC4 connections as well.
>>>
>>>The soldering and traces has been scrutinezed w. a loupe, over and over 
>>>again (I feel pretty confident w. the soldering as I have soldered for a 
>>>living some years ago).
>>>
>>>I'll try to reburn the 2764... but then again the power up behavior 
>>>should be constant if thats the problem, shouldn´t it?
>> 
>> 
>> If you can burn, you should be able to verify.  No?
>> 
>> 
>>>//Gabriel - I wish I had a scope at home...
>> 
>> 
>> Borrow one ?
>> 
>> ---------------------------------------------------------
>> - Where merit is not rewarded, excellence fades.
>> - Hydrogen is pointless without solar.
>> - What good are laws that only lawyers understand?
>> - The media's credibility should always be questioned.
>> - The only good terrorist is a dead terrorist.
>> - Governments do nothing well, save collect taxes.
>> 
>> -- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
>> -- Linux Rex         | RedWebMail by RedStarWare
>> -- FatMan: home1.gte.net/res0658s/FatMan/
>> -- NonFatMan: home1.gte.net/res0658s/electronics/
>> -- Autodidactic Master of Arcane and Hidden Knowledge.
>> 
>> 
>

---------------------------------------------------------
- Where merit is not rewarded, excellence fades.
- Hydrogen is pointless without solar.
- What good are laws that only lawyers understand?
- The media's credibility should always be questioned.
- The only good terrorist is a dead terrorist.
- Governments do nothing well, save collect taxes.

-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
-- Linux Rex         | RedWebMail by RedStarWare
-- FatMan: home1.gte.net/res0658s/FatMan/
-- NonFatMan: home1.gte.net/res0658s/electronics/
-- Autodidactic Master of Arcane and Hidden Knowledge.