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

WME_bill · 24th Apr 2018, 8:59 pm

D75/D83.
It appears to be the Bright-up board which is in trouble. A very frequent fault.
Working in sequence:
1. I assume you have confirmed the +105v rail and the -24v rail from the power supply board PC149.
2. The voltage at emitter of TR353, below the 43v zener should be a fixed +62v, so it seems you must solve the 95v.
I really wonder about the -24v. Put that at zero-ish and you get near your +95v. Check voltage at say PC128/7.

3. I would strongly recommend that you measure the voltages around the transistors and diodes (with the TB not operating - see below), rather than keep removing components and testing them, as removal and resoldering can create more faults.
And semiconductors sometimes test fine at low voltages, but when put under 50v to 100v, they will fail. So this operating voltages test is better.

4. Transistor base should be about 0.6v positive (for npn) above the emitter. The collector 10v or more positive of the base. Diodes about 0.6v cathode positive of the anode if conducting, or much greater in reverse.
5. Once you have cured the 95v instead of 62v, then I suggest you check the voltage at the output of the PC128 bright-up board - the junction of R366 and R367 or the terminal 128/8 or 148/18.
6. Stop the Timebase operating, by switching off any trigger: single sweep, external, AC mode, with no signal input, External X.
7. The Intensity control should vary the output voltage from below +20v =dim to +26v = bright. If this does not happen, then work by stages looking for a variation with the Intensity control movement, from TR351 collector, collector TR353. The variation on base of TR353/TR352 is too small to measure and is a few volts only at collector of TR353.
wme_bill.

24th Apr 2018, 8:59 pm	#21
WME_bill Octode Join Date: Oct 2007 Location: Exeter, Devon, UK. Posts: 1,554	Re: Telequipment/Tektronix D75 No trace on Power Up. D75/D83. It appears to be the Bright-up board which is in trouble. A very frequent fault. Working in sequence: 1. I assume you have confirmed the +105v rail and the -24v rail from the power supply board PC149. 2. The voltage at emitter of TR353, below the 43v zener should be a fixed +62v, so it seems you must solve the 95v. I really wonder about the -24v. Put that at zero-ish and you get near your +95v. Check voltage at say PC128/7. 3. I would strongly recommend that you measure the voltages around the transistors and diodes (with the TB not operating - see below), rather than keep removing components and testing them, as removal and resoldering can create more faults. And semiconductors sometimes test fine at low voltages, but when put under 50v to 100v, they will fail. So this operating voltages test is better. 4. Transistor base should be about 0.6v positive (for npn) above the emitter. The collector 10v or more positive of the base. Diodes about 0.6v cathode positive of the anode if conducting, or much greater in reverse. 5. Once you have cured the 95v instead of 62v, then I suggest you check the voltage at the output of the PC128 bright-up board - the junction of R366 and R367 or the terminal 128/8 or 148/18. 6. Stop the Timebase operating, by switching off any trigger: single sweep, external, AC mode, with no signal input, External X. 7. The Intensity control should vary the output voltage from below +20v =dim to +26v = bright. If this does not happen, then work by stages looking for a variation with the Intensity control movement, from TR351 collector, collector TR353. The variation on base of TR353/TR352 is too small to measure and is a few volts only at collector of TR353. wme_bill.