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5116 'scope LVPS checkout / query


Tim Phillips
 

from Tim P (UK)
Having another go at fixing my 5116 - the one that burns the 5.1 ohm
'sacrificial' resistor on the HV board. Naive question - am I right to
assume the LVPS needs loading before it will regulate ? Like having the
boards connected and plug-ins installed? All voltages except the +5V are
off spec:
+200V is +225V, +30V is +37, -30V is -34V and won't adjust.
I still suspect the HV board but, as is oft repeated here, first check LVPS
Its the version with the row of fuses, as in the 5111A.
many thanks
Tim


Ke-Fong Lin
 

Hi Tim,

The 200V is unregulated so 225V should be ok if unloaded.
As for the +30V, -30V, and +5V they should regulate even without load.
Can you basically disconnect everything from the LVPS and check?
Start the debugging with the -30V as it is independent with the other low voltages.

Best regards,


robeughaas@...
 

The + and - 30 Supply have current-sharing resistors across the pass transistors, so the supplies will not regulate without a load. R841 and R861

--
Bob Haas


Ke-Fong Lin
 

Ok, my bad. I thought it was the usual voltage divider feedback, and these resistors were for current limiting purpose.