7904/Repairs

7904 PSU

Morris Odell wrote:

I had to make a test load box to run it outside the scope - the box includes a voltmeter and LED indicators for each voltage. As anyone who has had to work on one of these supplies will know, the power for the control IC is derived from a current transformer in series with the primary of the 25 kHz power transformer so you need a minimum load for the IC to be powered properly and get the supply to work. For anyone going down that path, here are the loads I used (courtesy of the junque box):

+130 .. 2.2K 10 watts + and -50 .. 250 ohms 20 watts each + and -15 .. 82 ohms 10 watts each (I would have used lower values, perhaps 33 or 27 ohms 10 watts if I had them ) +5 .. 10 ohms (probably could use 4.7 ohms)

The total load is therefore 35.6 watts.

The faults I found in this supply were:

Two shorted diodes in the +5 volt lamp supply - replaced with 40 volt 3 amp Schottky diodes. Several tantalum caps with high ESR but none shorted - many replaced anyway. A faulty dual transistor in the +15 supply.

Göran Krusell wrote:

When I repaired my 7904 some years ago I replaced [the inverter] transistors with BU208A in TO-3 package, with good result.