UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Telequipment/Tektronix D75 No trace on Power Up.

peakfour · 24th Apr 2018, 5:55 pm

Well I got rained off from laying a path behind the new garage, so had an hour or so for another play with the scope.

Re. The non-functional Intensity control and voltage values on PC128.
Volts in from the PSU are about correct
The +0.7v at the base of TR351 measures at 0.59v, which is the same as the volts drop across a new D351, so I'm guessing OK (ish)
TR351 was dead and I've replaced it with a 2n5551, as per recomendations elsewhere on this forum, similarly TR352 is now sporting a nice new BC107.

The other transistors on the board seem to test OK with the diode test function on my DMM, and also light the telltale lamp on my old Eagle transistor tester.

The two +62v test points both measure at +95v, but when I test the 43v zener, out of circuit, using an external PSU and 1K resistor, it checks out at about 42.7v, so that seems OK. The other diodes round there, all seem to be OK using the DMM to measure volts drop and reverse resistance.
All the resistors are about in tolerance.

As I rotate the intensity pot, the voltage on the wiper side of R353 varies from 105 to about half that, which seems fair, but the other side of R353 remains stable at +95v.

D358, the 75v zener, only has about 17v across it, but I've no easy way to test it a higher voltages at the moment; the diagram seems to show a 6v square wave there too, but it's a bit hard to be sure, due to the join in the page.

Anyone any flashes of inspiration please?

24th Apr 2018, 5:55 pm	#20
peakfour Triode Join Date: Apr 2018 Location: Buxton, Derbyshire, UK. Posts: 21	Re: Telequipment/Tektronix D75 No trace on Power Up. Well I got rained off from laying a path behind the new garage, so had an hour or so for another play with the scope. Re. The non-functional Intensity control and voltage values on PC128. Volts in from the PSU are about correct The +0.7v at the base of TR351 measures at 0.59v, which is the same as the volts drop across a new D351, so I'm guessing OK (ish) TR351 was dead and I've replaced it with a 2n5551, as per recomendations elsewhere on this forum, similarly TR352 is now sporting a nice new BC107. The other transistors on the board seem to test OK with the diode test function on my DMM, and also light the telltale lamp on my old Eagle transistor tester. The two +62v test points both measure at +95v, but when I test the 43v zener, out of circuit, using an external PSU and 1K resistor, it checks out at about 42.7v, so that seems OK. The other diodes round there, all seem to be OK using the DMM to measure volts drop and reverse resistance. All the resistors are about in tolerance. As I rotate the intensity pot, the voltage on the wiper side of R353 varies from 105 to about half that, which seems fair, but the other side of R353 remains stable at +95v. D358, the 75v zener, only has about 17v across it, but I've no easy way to test it a higher voltages at the moment; the diagram seems to show a 6v square wave there too, but it's a bit hard to be sure, due to the join in the page. Anyone any flashes of inspiration please?