Re: T922 Power Supply Problem

Mlynch001
 

On Sun, Jul 7, 2019 at 07:57 PM, Mlynch001 wrote:

Dave,

I am certainly not an expert, but if it is possible to operate the LV supply
without a load, it would help you to determine if the problem lies with the
supply or one of the other boards in that scope. I have restored a couple of
T922/935 scopes to operation, but never had any issues with the LV power
supply. Have you checked the voltages at the various components in the supply
against the schematic? How about VR746? This is a 9.1 Zener that tends to
fail quite often. Any of those many diodes failing could cause problems. I
have seen several ZENER and other diode failures in older equipment that I
have repaired, most recently in my Type 576 Curve tracer. You replaced
C738,758,778 along with C722, 742 and 743 ?

--
Michael Lynch
Dardanelle, AR
Also check VR762, which is the 5V reference for U742 and the 8V supply. Also, check to see if someone has added a 10uF 20V cap between the junction of VR762 and R762, (this "field modification" is penciled in on one of my schematics) could this cap be present and shorted? If you can test the voltages at the various points in the circuit and compare them to the schematic, it would be helpful. At this point, without some numbers, this becomes pure conjecture. Harvey is correct, either something is loading the supply or fooling the circuit into thinking that it is loaded.
--
Michael Lynch
Dardanelle, AR

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