CRT oscilloscope died - Page 1
Forum section: electronics / repair

author: ainfirdaus11
author_rank: Newbie
timestamp: March 21, 2022, 02:22:29 am
content: Hi,I have an oscilloscope problem. The CRT died.I have checked, the switching transistor of HV board is short circuit. I replaced it, shorted again.The oscilloscope type is Iwatsu SS 7802I have checked at the green mark and all are ok. The red mark, resistor value become 8K and I already replaced it. But, the transistor switching shorted again.Thanks

author: Renate
author_rank: Super Contributor
timestamp: March 21, 2022, 01:27:14 pm
content: Could you show a bit more of the power section and how IC1 is getting its feedback?Is IC1 a comparator or an op-amp?Depending on how this circuit is, I might try disconnecting the bias from the resistor/IC1, use a higher resistance and see if you can just get it to oscillate a bit to narrow down whether there is a short or that the control is busted and it's running wide open. Just an idea.

author: ainfirdaus11
author_rank: Newbie
timestamp: March 21, 2022, 04:33:18 pm
content: It is the whole HV board scheme

author: Renate
author_rank: Super Contributor
timestamp: March 21, 2022, 05:00:44 pm
content: Sorry, that screen shot tells me nothing. The resolution isn't there.Do you have another oscilloscope?When you powered it up and it blew up did it sound like it was making voltage? Did it blow up immediately or take a few seconds? You didn't hear HV arcing or anything, did you?You also want to make sure that swapping transistors and parts so often that you don't hurt the PCB.Do you have a part number for that IC1?

author: ainfirdaus11
author_rank: Newbie
timestamp: March 21, 2022, 10:21:46 pm
content: The IC is 4558 dual op ampI have no other oscilloscopeWhen I powered it up, it blew up after a few second. I heard nothing.

author: Renate
author_rank: Super Contributor
timestamp: March 22, 2022, 12:23:15 am
content: Ok, then IC1 is actually providing bias. I thought 22k was pretty small.These things can be a little tricky and you don't want to blow up any more of those transistors.Not having a scope complicates things too. And the secondary has lots of high voltage on it.Ok, the easiest (and lowest voltage) test point to see if anything is happening is the junction of D4 and C22 (being fed off pin #8 of the xformer).I don't see any voltage indication, but it should be showingminusseveral hundred volts.You can attach a meter to that point and to ground.I think that I'd pull the right side of R6 (2.2k) from pin 7 of IC1. Connect an alligator clip from the loose end of R6 to ground. This should hopefully give you a reasonably safe operating point.I'd turn on the power for 1/2 a second. Watch if the DVM on D4 jumps. Check (with power off!) if your transistor is getting hot.If everything is cool, try a second, then two seconds. Still cool? Steady reading on the DVM? If it doesn't feel good, stop.It would be nice to see what pin 7 of IC1 is doing. It's probably at 12V if things have worked out so far.Is there an adjustment procedure that tells what voltage at what point you should adjust R15? (to the right of IC1)Edit: This is presuming that IC1 is not in a socket. If it is in a socket you can just remove it and ground the "right side" (as in the schematic) of R6.