Re: Just got a 475, nothing showing on display


Roger Evans
 

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.

Regards,

Roger

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