This power supply is not straightforward to repair because there are three different versions of the power supply, and none of them match the current circuit drawing.

The most current circuit board is labelled Rev. F, which is the same as the current schematic, but substitutes a 3.9Ω resistor for a 25kΩ thermistor.  The same modification is made on the Rev. E circuit board, but not on the older units.

A big change is in the circuitry for the low voltage, high power DC supply. In the current version, the control circuitry is isolated from the regulator circuit. 

Various boards have other modifications.  For example, resistors are jumpered and diode bridges are added.

The power supplies have a low current, midrange voltage supply switched with the high voltage supply.  This section will probably be disabled since it is unused to prevent potential problems caused by a faulty switch.  One indication of a problem in this area is that a 10kΩ resistor in the voltmeter switching circuit is commonly burned.

#2 (Rev E. circuit board)

Manufacturer #CV-25, serial number 0232

Foothill/DeAnza 64932

No circuit diagram available for Rev. E.  As found condition has a 10kΩ resistor in the voltage divider to supply the 2N5550 transistor base and a 22kΩ resistor to establish the circuit output.  The 10kΩ resistor is jumpered out and the 22kΩ resistor shows signs of burning.  Replacing these resistors to match the Rev. F. configuration with a 2.7kΩ resistor in the base voltage divider and a 100kΩ resistor at the circuit output.  (Note:  10kΩ and 22kΩ resistor values agreed with the values printed on the circuit board.)

#7 (No revision - original?)

Manufacturer #CV-25, serial #170

Foothill/DeAnza 64937

This unit appears to work as designed after replacement of the power cord.  There is some transformer noise.

Power supply repair

#6 (Rev F)

No manufacturer model number or serial number

Foothill/DeAnza 64933

Replaced BU204 and 2N3771 power transistors, replaced 10kΩ resistor for meter indicating LED.  Added a heat sink for the BU204.  The as-found version had no heat sink and the underlying circuit board was discolored.

0 - 80VDC and 0 - 20 VDC power supplies OK, 0 - 500 VDC power supply puts out 7.5V (~Vbe?)

Replaced transistor and 500 VDC output is less than 1 VDC.  Contrary to the published diagram, the resistor value at the output of the BU204 is 57Ω rather than 62Ω.

Found a resistor of unknown value on the trace side of the circuit board that I didn’t notice before.  Apparently, this is  a voltage divider to assist in establishing the base voltage.  This modification may be in conjunction with the replacement of a 25kΩ thermistor with a 3.9Ω resistor.  This revision is past Rev. G, which is the only readily available diagram.  Another board with the same modification has a 10kΩ resistor in a similar location.  The burned resistor on this board appears to be a 1/4W resistor and the resistor on the other board looks like a 1/2W resistor.  Both resistors show signs of overheating.  Replaced resistor with 10kΩ, 1W resistor.  This is not ideal because the 500VDC power supply now goes to over 600VDC.  Leaving circuit as is, since the next 1W or above resistor I have is 100kΩ.

#3 (No revision - original?)

Manufacturer #CV-25, serial number 0193

Foothill/DeAnza 64936

No repairs or modifications on the trace side of the circuit board.  20 VDC output was 3VDC.  Output resistor found disconnected.  Soldered wire back and checked resistor and continuity.    (Note:  220Ω 2N3771 output resistor is soldered to the panel ground banana jack.)

  

#4 (Marked “Rev. D?”)

Manufacturer model CV-25 serial #179

Foothill/DeAnza 64931

Replaced 2N 3771 transistor for 20 VDC power supply.

Replaced 10kΩ resistor with a higher wattage resistor in base supply voltage divider.

The HV transformer is not working.  Problem isolated to bad fuse holder.

0-500 VDC power supply goes to 700 VDC.  10kΩ resistor in base supply voltage divider is probably too low with the 25kΩ thermistor changed to a resistor.

  20 VDC power supply goes to 45 VDC.

Note:  220Ω 2N3771 output resistor is soldered to the back of the circuit board.