Another C1-112 for repairs - Page 1
Forum section: electronics / repair

author: Thilo78
author_rank: Regular Contributor
timestamp: October 28, 2018, 09:55:08 am
content: Hey folks,I recently found a C1-112 which is in good nick optically and has all the accessories, but is not working, sadly.When powering up the first time, it briefly gave me a reasonable picture, including a trace on the scope side and the numbers on the multimeter side. Then the picture faded away.I went through and recapped the whole thing and already changed the transistors in the deflection amplifier to the back of the unit.I got the +/- 12 V power supply working again and adjusted it properly within the +/-0.1V range.In the meantime I managed to get a dot on the CRT again, meaning that the 2kV supply and the CRT work.But when measuring what is marked as the +5V rail, it sometimes shows around 5.66V, then it returns to the 10.2V coming from the rectifier to the pass transistor VT48. Thinking that VT48 and its neighbour VT46 could be faulty, I swapped them for more modern replacement transistors, just like I did with my C1-94s. But the behaviour doesn't change. For the moment I disconnected the multimeter boards to avoid exposing them to the +10V until I figure out what is happening.Edit: Found the mistake. I'm not sure if the original transistor VT46 was in the wrong way, but my replacement certainly was. Now I have the 5V rail back.Funny enough: The indicator light on the front is said to be powered from +5V, but when I measure there, I have no voltage.Edit: It seems that the 5V don't get delivered away from the power supply board. But I can't find how it's supposed to be routed. The ribbon cables weren't conclusive so far.Does anyone here know their way around the C1-112 to give me some advice where to go next?I'm still in the process to Google-translate a Russian article on the C1-112, so it'll be some time until I get some useful instructions on troubleshooting.

author: Thilo78
author_rank: Regular Contributor
timestamp: October 28, 2018, 10:01:59 am
content: I did find a thread here in the forum that offers some advice:https://www.eevblog.com/forum/testgear/c1-112a-oscilloscope-problem/msg463915/#msg463915I'll check this info as a next step and report back.

author: Thilo78
author_rank: Regular Contributor
timestamp: October 28, 2018, 01:26:55 pm
content: I followed the +5V through the scope and found the culprit:At the scope/meter selector switch the voltage wasn't passed through to the other boards.Must have been a broken solder joint, as after reflowing the switch, the scope and meter section came to life.The scope is good now, showing a clean signal and being adjustable for external signal and the internal calibrator.But the multimeter section still doesn't seem to work as it only shows zeros no matter which mode is selected.I'll have to dig deeper there, it seems.The character generator board seems to work fine. I guess the second multimeter board has some kind of fault which I need to track down.But I'm pretty happy with the result so far.

author: Thilo78
author_rank: Regular Contributor
timestamp: July 01, 2019, 02:33:28 pm
content: It's been a while, and I'm still working on the C1-112.(It takes that long as I can only tinker on the weekends, and even then schedule is tight. And also I had a bunch of other repair projects coming in.)Since my last post, the multimeter portion didn't change much. This weekend, I did pull the ICs on the ADC board, though.MC34, one of the NAND ICs, tested bad. It did apparently recover, but I suppose it's an intermittent fault and decided to order new ICs.A bunch of K155LA3 and K155TM2 are on the way for testing.Meanwhile, I'm still searching for a German or English version of the service manual. I do have the original operator's manual, but that's Russian and hence not much help to me.So, if anyone has these manuals, I'd be more than happy to get a copy.Thanks in advance!