[sdiy] Error on the SoundBite board!

[Eric Brombaugh]

mikko.helin at jippii.fi wrote:
> Hi,
> 
> First, the hex inverter on SoundBite is 74AC04 which is buffered vs. 
> 74HCU04 which is unbuffered. Anyway, it may be that there are other 
> reasons for oscillator problems than the component values used (unless 
> they have been verified). In this thread is similar case, here the 
> problems are relaxation oscillations (unfortunately the problem was not 
> solved, anyway the same 100k feedback resistor was used here as well):
> 
> http://www.burnyourbonus.info/sci.electronics.design/thread717.html
> 
> There the problem seems to be the hysteresis that keeps up the 
> relaxation mode oscillations.
> 
> Suggestions to decrease relaxation oscillations in this Intel app note 
> for 8048 (!!):
> 
> ftp://download.intel.nl/design/mcs51/applnots/23065901.pdf
> 
> I think simply adding a 0.1 uF bypass cap directly to pins 7-14 would 
> propably help, maybe the 1 Mohm to Vcc as well. There are propably 
> reasons why so low Rf was choosen in the first place (cause it's cheap 
> demo board they propably wouldn't like to use an extra HCU gate).
> 
> Noticed just the Eric's earlier post, maybe the reason the mod did work 
> was because the board was cleaned and not the the component swap itself?

Interesting theory.

Sorry if I gave the impression that the board was dirty as I received 
it. It wasn't - the solder joints were clean and shiny. When I stated 
that I wicked off the excess and cleaned the pads I meant that I removed 
the normal solder fillet to make removal of the SMD resistor more easy.

Actually, since I didn't clean off the rosin the board looks a lot worse 
now than when I started. But it works better.

Eric