Re: Just got a 475, nothing showing on display

donald collie

Is the fse on the main PCB at the back left blown? If so, it is almost
certain that the Tantalum capacitor associated with this circuit is
shorted......................Donald Collie


On Wed, Jul 29, 2020 at 9:24 PM Roger Evans via <very_fuzzy_logic=> wrote:

The high ripple you measure on the 50V and 100+ Volt supplies is usually
due to the large 'bulk' smoothing capacitors on those supplies drying out
with age and simultaneously losing their capacitance and developing high
series resistance. Both these changes render them ineffective at removing
the ripple at the output of the rectifiers. C1414 is almost certainly the
main culprit, C1412 is suspect but might be OK (it is a somewhat unusual
smoothing arrangement). You can make a temporary fix by using a modern
electrolytic capacitor and attaching it in parallel to the existing
capacitor, if necessary using some short leads to position the new
capacitor out of harm's way. The new cap needs to the same or higher
voltage rating and the same or slightly larger capacitance.

Failed tantalum capacitors usually fail short circuit and pull the suply
rail to almost zero volts. They are a perennial problem with old equipment.



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