Trio CS-1560 Oscilloscope Repair - Page 1
Forum section: electronics / repair

author: Cyderize
author_rank: Newbie
timestamp: February 23, 2018, 12:15:17 am
content: Hi, this is my first ever post here (and it's going to be a long one) - I've been doing electronics as a hobby since early high school and I'd always wanted an oscilloscope, but couldn't really afford one. But I recently picked up an old Trio CS-1560 (the original, not the CS-1560A or CS1560A II) for next to nothing, with the catch being that it didn't work. So now with a couple of years experience in electronics I figured I could have a go at repair. Beware that I am an amateur, and I know that the voltages present here are potentially deadly, and this thing scares me a lot, so if someone decides they want to follow along with their scope, it is at their own (high!) risk. I've already gotten it to a mostly working state, and I'll document my progress here. I'm using the service manual for the CS-1560A II which I found online. The scopes seem identical (at least in topology, if not down to the component level) electrically.1. Cleaning and inspection: It was pretty dirty when I picked it up. I took it home and cleaned it - I removed the knobs and cleaned them separately (and haven't put them back on yet). I also removed the front panel bezel and the panels with the control labels and cleaned them with dish soap. I removed a lot of the grime on the case with baking powder as a mild abrasive and a lot of elbow grease. I also noticed that the power cord was it pretty bad condition, so I'll have to replace that eventually. And also, annoyingly, while I have the bottom and top+sides chassis panels, the back one is missing so I guess I'll have to make a replacement somehow. The photos are all post-exterior cleaning. The focus pot seems to have been replaced by a previous owner since it's now a knurled type rather than the D shaft type like all the others - its rod is also missing so I made a new one out of 1/4" dowel (I filed the end of the rod down to look like a flat tip screwdriver to engage with the slot in the knurled pot shaft) which seems to work well. There is evidence of some kind of past repair attempt, although whoever did this seemed to have been a bit heavy handed - several of the pads have been lifted of the board and destroyed so there components whose leads were soldered to the PCB tracks themselves (with the solder mask scraped away) rather than to the pad. I also noticed a transistor with one pin flapping around in the breeze. I had to bodge a wire from the lead to the destination pad since the pad had been completely destroyed.2. Powering it up: The seller said that it would 'light up' but there was 'no sine wave'. Plugging it in and turning it on, indeed the graticule light and front panel power LED worked, but there was no trace to be seen. After a little while though, I noticed some magic smoke escaping from a resistor in the power supply. This was R338, which seems to feed the primary of the flyback transformer. I figured that either C326 or Q317 had shorted (which would explain the lack of at trace since the CRT would be missing its 2kV acceleration voltage). Sure enough, the diode test on Q317 did not at all resemble a working transistor. I replaced the old (2SD381, as in the photo) one with a TIP31C I had lying around (I don't want to have to buy anything it possible, so I'll try to use stuff I have in my parts bin). I don't see why it wouldn't work (but tell me if I'm eventually going to set my TIP31C on fire). The resistor still measured fine despite its now charcoal appearance, but I replaced it anyway since I had some 2R2 resistors in my parts bin.3. Take number two: Powering it up with the new switching transistor on the flyback was successful - a trace appearedand it looks like the horizontal time base works. However, neither vertical input seems to work at all. According to the service manual, this indicates that Q123, Q124 or IC103 aren't working - and upon inspecting the board, I noticed that IC103 wasn't even there at all! Someone had removed it and not replaced it.So of course the vertical amplifiers wouldn't work. The original should be an RC733T in a metal can package, but this is no longer available. I found the uA733 from TI, which looks to be the correct replacement, but it's not available in that metal can package. Instead, I ordered some samples of the 14-pin SO package from TI (while I could buy the PDIP one, I didn't want to spend money, so I went with the package that had free samples available). They arrived a couple of days later, and I bodged the SMD package in there with wire wrap wire and a lot of hot glue (just to make it obvious its a complete bodge).4. Third times the charm: Power it up now for the third time, it seems to work - the calibration square wave appears and looks pretty good and both channels operate. The horizontal timebase and trigger circuit seem to work fine, as well the dual trace mode. Unfortunately the vertical sensitivity pots seem to be pretty flaky, jumping all over the place. I don't have any contact cleaner, so I'll have to buy some.This brings us to the current day. Any recommendations for contact cleaner? I've never used the stuff before so I want to buy something good since I'll probably be using it in the future. I haven't yet replaced the electrolytic caps, but I should definitely do that (but those I'll have to order since I don't have the right kinds or high enough quantity in the parts bin).Hopefully this has been informative or interesting (or hopefully both), and I look forward to any advice on further restoration (and also advice on what to do about the missing back panel).