Hameg 203-6 repair: we have a lead! VB#4

So we have found the problem as to why its not working! Turns out my original assumption of thinking that because there was the right potential difference between the -1877V line and the -1900V line that there was indeed those potentials there.

But after thinking about it, I could measure the -1900V line by actually measuring the individual voltages across the potential divider network for the focus adjustment:

Turns out it came out A LOT lower than expected: 95V. That's way lower!

The problem only occurred due to a lapse in concentration. Usually, given that there are are high voltages flying round the timebase PCB and the back of the CRT PCB, I turn off the scope altogether and disconnect the mains whilst I have a think, take notes, and read the schematic. That's a pretty safe way of working. But this time I'd left it on and started to smell burning dust: that smell you get when the radiators are first turned on in your house after summer (ok people in temperate climates might not know what I'm on about).

Turning off the power, disconnecting the mains and then using the finger test found that IC501 - an opto-isolator - was very hot. Pulling it out and diode testing the diode emitter on it found it was very poorly, reading more ohmic than a semiconductor!

With the opto out, I thought why not test to see if I get the right voltages on the focus resistor network, and BINGO! Readings matched the calculations!

So next step is to order a replacement, double check that nothing else could have been taken out, and fit the new component.

Its very odd though - on day the opto's working and the next its dead!