UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Scopex 4D-25 Oscilloscope Repair Help

OldTechFan96 · 10th Oct 2017, 1:01 pm

With the mention of tunnel diodes I thought that I should test the 1N3719 at position D407. Diode check showed it to have a forward voltage of 0.003V each way so I think it is bad. I reinstalled it and double checked to make sure it was the correct away around.

I turned on the scope and nothing came on the screen. I measured the voltages on the +/-20V and +110V rails. The positive rails were fine but the -20 rail only measured 1.6V at the negative side of C684.

I decided to measure the ECB voltages of TR682 (BD157):
E - -33V
C - 1.5V
B - -32V

The low collector voltage prompts me to investigate TR683, TR684 and TR681. Looks like that this problem could be similar to the one were the 20V rail was low.

There is never a dull moment with this scope!

10th Oct 2017, 1:01 pm	#111
OldTechFan96 Octode Join Date: Jul 2017 Location: Newcastle Upon Tyne, Tyne & Wear, UK. Posts: 1,186	Re: Scopex 4D-25 Oscilloscope Repair Help With the mention of tunnel diodes I thought that I should test the 1N3719 at position D407. Diode check showed it to have a forward voltage of 0.003V each way so I think it is bad. I reinstalled it and double checked to make sure it was the correct away around. I turned on the scope and nothing came on the screen. I measured the voltages on the +/-20V and +110V rails. The positive rails were fine but the -20 rail only measured 1.6V at the negative side of C684. I decided to measure the ECB voltages of TR682 (BD157): E - -33V C - 1.5V B - -32V The low collector voltage prompts me to investigate TR683, TR684 and TR681. Looks like that this problem could be similar to the one were the 20V rail was low. There is never a dull moment with this scope!