Kikusui COS5020-ST help needed - Page 1
Forum section: electronics / testgear

author: edy
author_rank: Super Contributor
timestamp: August 07, 2014, 03:04:57 am
content: Hi folks,I picked up a deal for 2 CRO's on Kijiji recently in Toronto, Canada (see photo I've attached). The first working scope I bought for a friend of mine who wanted a CRO and I told him I would try to get him a cheap one. It was for a fully working Kikusui COS5060 60 Mhz CRO for $60 which included 1 probe.While I was talking to the seller, who is a nice electronics engineer who appears close to retiring, he also showed me another Kikusui COS5020-ST 20Mhz storage oscilloscope (also pictured) which he says powers on but it doesn't appear to sweep. However there is vertical deflection and I see 2 dots on the calibration. I can't seem to get it into X-Y mode and I am not getting any horizontal movement except when I use the "position knob"... but it only goes to the right (does not go beyond middle of screen to left side).So.... I am going to be giving the $60 Kikusui COS5060 to my friend who will be paying me back (which was the main objective). I figure the other $30 I spent on the Kikusui COS5020-ST storage scope was worth the challenge.I managed to download user-manuals for both scopes directly from the Kikusui website (kikusui.co.jp) but I cannot seem to find a service-manual/tech manual/schematic for the COS5020-ST which I want to fix. I will be making videos of this scope once I have time to power it on, and show the inside of the scope, and post it on this thread. If anyone can help along I would greatly appreciate it.Does anyone have a Kikusui COS5020-ST and any schematic or diagrams that could help? Any idea why the scope will not sweep horizontally? I am getting 2 dots that change according to the voltage setting, on both channels. But it stays in the middle of the screen. I can adjust vertical positions of both channels, and I can even horizontally change the position but seems only to the right side of the display. There is no horizontal sweep at all, no matter how I dial the knob for time.Thanks in advance for any suggestions/tips.

author: edy
author_rank: Super Contributor
timestamp: August 07, 2014, 03:19:27 am
content: Here is a link to the manual. There is a block diagram near the end of the manual, but no schematics:http://www.kikusui.co.jp/kiku_manuals/C/COS5020ST_E.PDF

author: mij59
author_rank: Frequent Contributor
timestamp: August 07, 2014, 04:55:45 pm
content: Hi,Start with a visual inspection of the circuits.Check things like connectors, fuses, switches, electrolytic capacitors, solder joints, burned, broken or missing parts.Check the power supply voltages.Please post some photo's.

author: edy
author_rank: Super Contributor
timestamp: August 09, 2014, 03:46:24 am
content: Thanks for the tips. This will be a slow fix as I have piles of higher priority stuff taking all my time at the moment. I had a chance to plug in and take pictures. Nothing obvious, but you may have a look.Symptoms... powers on and I get both channels showing up as dots on screen. I can move them with horizontal and vertical positioning knobs. The storage function is a bit flakey... probably also needs fixing, but when it does work, I can use the positioning knobs to draw on the screen like an etch-a-sketch!The knob controlling sweep function does not do anything, the dots never move horizontally. I see in the pictures I can fairly easily get in and just testing one component at a time and seeing if it falls into spec or has continuity. Could it be a bleed resistor on the cap that is part of the sweep circuit? Never bleeds the cap and therefore just stays charged and deflected to one side?I am planning to make a video and post it here so you can see the behavior. When I power up the scope at first I can see the dots move from left side of screen slowly towards the right and then stop about half way. Also, my horizontal positioning knob can move the dots off the screen on the left, but just slightly over the middle half of the screen to the right. I cannot move the dots any more right than that.When I attach the calibration signal to either channel, and I quickly move the horizontal position knob back and forth, I see my square wave....Or just two dots (square at 0v and 2v). I can feed it a slow sine wave at low frequency and probably see a dot moving up and down on the screen.... but no horizontal sweep.

author: mij59
author_rank: Frequent Contributor
timestamp: August 09, 2014, 04:14:35 am
content: Thanks for the photo's.Since you can move the spots is no problem with the output amplifiers to the tube.On the time base knob is a position for the XY mode, in this mode the time base generator is not used.It could be that the switch on the time base knob doesn't work.

author: edy
author_rank: Super Contributor
timestamp: August 09, 2014, 04:24:06 am
content: Thanks. The time base knob switch does look fairly complicated with pull-in-out center for fine adjust and various settings. The X-Y setting is all the way at the far counter-clockwise position. I can hear the clicks as I turn the knob to other time base settings. I do not believe X-Y worked because I tried using both channels as X-Y (or even sending a sweep externally produced on the Y-channel) and my dots didn't move horizontally. I will have to play a bit more and report back on my results.

author: edy
author_rank: Super Contributor
timestamp: August 10, 2014, 05:25:43 pm
content: I tested some Zener diodes the trigger/storage button board using a continuity tester and all but one of them did not have continuity, as I expected since the test voltage from the multimeter would not be enough to be over the threshold forward voltage to allow current flow. However, one of the Zener diodes beeped showing continuity.So could a Zener fail in this way? Or is the circuit allowing a short in parallel with this component (I need to look at the schematic carefully)? I would have expected it to not pass current with the continuity tester.

author: mij59
author_rank: Frequent Contributor
timestamp: August 10, 2014, 05:34:39 pm
content: Quote from: edy on August 10, 2014, 05:25:43 pmWhen overloaded a zener diode tens to short, but also can become leaky.Desolder one of the legs and measure again.

author: edy
author_rank: Super Contributor
timestamp: August 10, 2014, 07:51:54 pm
content: Thanks, I will try that. I will need to start de-soldering components and testing... That is going to be a tough job.I read here that when Zener diodes fail they tend to become shorts:http://www.allaboutcircuits.com/vol_3/chpt_3/11.htmlMeanwhile, I managed to finally put together this first (of probably a series of) videos that will help explain the problem better. Seems to be related to the horizontal signal amplifier that takes either the Y signal (of the X-Y mode) or the sweep generator signal and sends it to the tube for Y (horizontal) deflection. The "position" knob seems to work otherwise, that's why I think it's the amplifier on Y.Other problems include a burned out graticule illumination light (I just have to find out where the heck the bulb/filament is located and what to replace it with) and the storage function which is intermittent. But if I can get the sweep at least working, I can worry about the rest later.Here's the video:UPDATE: I'm reading through the service manual and figured out there is a sub-focus calibration, so I should be able to easily remedy the focus issue. Also I found DS1601 and DS1602 are 14 V lamp filaments that illuminate the graticule. I don't know how easy it is to get replacements, maybe the lamps are ok but something else is burned out in the circuit to the lamps.

author: bance
author_rank: Contributor
timestamp: August 11, 2014, 02:24:15 am
content: First thing I'd do check is all your voltage rails, then a liberal application of switch/contact cleaner to all plugs/switches/pot's....I bought a tek 2213a with very similar symtoms and that was all that was required.I think you can check for a sweep signal atjumper P-13, top board, rhs as you look from front, 50-70mm (2-3") in. I think if you turn the scope upside down you should be able to probe it ok.Q425HTH Steve.