Re: Tek 7D20

Roger Evans

The -5V and main +5V supplies come from the switching regulator on the power board in the 7D20. Since you mention only the -5V I assume the +5V is OK and the switching supply is at least partially working. The feedback to the switching device regulates only the +5V, the -5V has a separate regulator using U660, Q660 and Q650. You need to find if the -5V is low because of a fault in the regulator circuit or too much current drain due to a fault elsewhere and this probably means stripping the 7D20 to remove individual boards (not too difficult). I had a power supply problem on mine and several electrolytics were 'bad' some had high ESR and I think three were almost open circuit with no measurable capacitance. If C451 C550 or C650 are failing that could be your problem, otherwise it is a fault on another board and you have to remove boards one at a time to isolate the problem. You don't say if the problem has now gone away or remains intermittent, it would be surprising to have an aging capacitor behaving intermittently but others may have more or different experience.

If the power supply board is removed you can power it just from +15V and -15V (and ground) on the edge connector to the mainframe. You need some sort of load to test the -5V regulator, I am guessing 50mA current draw would be a good starting point, the +5V can supply 1-2 amp. If the -5V supply fails several parts of the analogue circuitry don't work and this could cause the shrunk display. You will also get a cascade of errors so it is likely you have just this one problem.

Best of luck,

Roger Evans

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