6 Dec
2014
6 Dec
'14
12:58 p.m.
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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