Acceptable Ripple

[John Robertson]

On 12/06/2014, 10:26 AM, Robert Jarratt wrote:
> Just checking a PSU in a MicroVAX 3100 Model 80. I am measuring about 10mV
> ripple on the 5V supply and 20mV ripple on the 12V supply. Does that seem
> reasonable?
>
>   
>
> I am not sure if it is a scope artefact, but there appear to be large spikes
> at the top and bottom of each sawtooth. The machine appears to partially
> work, it seems to be getting some errors during the memory test, but I don't
> know if that is the spikes I am seeing or just a bad component somewhere.
>
>   
>
> Regards
>
>   
>
> Rob
>
>
TTL Logic is notorious for creating spikes, so you may have some (many?) 
bad 0.1ufd (or 0.01ufd) caps on the logic bus. typically there is one 
0.1ufd cap per TTL device.

Check any tantalums too if present on the bus.

The 'rules' from Don Lancaster most excellent book "TTL Cookbook" are: 
(quote)

Here is a list of rules for capacitor placement. But the cardinal rule 
is this: Use enough for your needs - and then add some more!

Use one 0.01 - 0.1uf short-leaded disc capacitor for every four gate 
packages.

Use one 0.01 - 0.1uf short-leaded disc capacitor for every two MSI 
packages (Medium Scale Integration).

Use a separate  0.01 - 0.1uf short-leaded disc capacitor for every 
package separated more then 3 inches from the nearest bypass capacitor.

Use a 10uf - 6 volt tantalum capcitorwhere the +5V Line leaves the 
circuit board.

(end quote).

John :-#)#

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