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I bought a broken 7603, the fan was working but nothing else.

I opened the power supply and started looking but did not really find anything.

I did not find on the power supply board the TO transistors that Stefan mentioned (referenced below). After moving to the LV Regulator Board, they were there - 6 of them. Re soldered all those connections and now I have graticule lights and power led on.

After I reconnected the power supply to go to the LV Regulator I noticed something was smelling. Turned the power off and decided to change the position of the cable that goes to the Z-Axis Amplifier board and supplies 15V unregulated. P870 on the PS has 3 pins but the cable only two wires. The smell got stronger and I noticed that the big transistors connected to the HV board were getting hot. So that was a bad move I made.

After accessing the High voltage board I noticed that it was just floating, the one metallic post and the 3 plastic ones were not secured. It seemed to me that the contacts of the transformer could have been making contact with the metallic enclosure.
I also noticed that the big HV disk cap had some melting on both sides.
I secured the board with the metallic post (don't have plastic screws for the other 3)and made sure that there was a good separation between the big HV disk cap and adjacent caps.

At this moment nothing smelling burned so I suspect the transformer was making contact with the metallic enclosure.

However, Q1152 on the Z-Axis board which seems to supply 130v to the HV board on line B overheats. When I remove the cable that supplies 15V unregulated the problem goes away.

At this point I am not sure how to continue. I only have a Multimeter
to check continuity. I may get a cheap old Radio Shack Transistor tester and a cheap Chinese ESR tester like this one:


or this one:


How do I go about testing the transformer on the HV board?

>> Stefan Trethan wrote:
>>The 7603, unlike the 7623A, has a linear power supply.

>>There are big TO-3 series pass transistors on a heatsink, the >>problem
>>is that they are in sockets that combine an unfortunate choice of
>>plating with an unfortunate mounting arrangement. Thermal expansion
>>and contraction stresses the solder joint between socket pin and >>PCB,
>>which eventually causes even the leaded solder to fail. The failure
>>line is very thin and difficult to see even if you are used to >>looking
>>for this. Usually the driver transistor has failed from carrying the
>>load current, but the TO-3s are still good once you reestablish
>>contact.

>>Another common failure is big Sprague capacitors with internally
>>corroded terminations (reading no capacitance / excessive ripple on
>>the supply).

>>As you can see I know more about the 7603 than the 7623A, my 7633 >>has
>>been working fine so I was never in there.

>>ST