Kikusui Oscilloscope Repair

[davenz]

Hi team and first let me thank everyone for what has become a valuable resource for me. I am more a lurker than a contributor simply because I don't know squat about anything as far as consumer electronics go. Actually, that's not strictly true, as an avionics technician I can repair some stuff but aircraft electronics don't show up here very often!

Anyway, I inherited a Kikusui Model 537 scope. It's a little retro beauty and very, very well made. The biggest problem is that it doesn't work properly. I opened it up and noticed a resistor 'flying' from one of the circuit boards; obviously someone had replaced it before as there was a burnt spot on the board under where the resistor should have been. Instead whoever had performed the previous repair had inserted the resistor from underneath and placed it with a gap, probably for air in case it got warm again. The air gap wasn't enough because this resistor was smoked and open circuit. The colours were history so I guessed going by the first and last colours. It turns out I was way off, but more on that later.

I replaced the resistor with what I thought it should be and fired it up. With much joy I noted it working. That is, I noted a trace appear on the screen. It was short-lived, however and lasted about thirty seconds before I got smoke, accompanied by crackling and popping and it stopped working. The resistor was smoked again. I resorted to Google but could find nothing specific to my model. I did find the hompeage of Kikusui and emailed them asking about it. They offered a repair agent in Australia; not an option. I asked (more like begged) if they had a circuit and after a lot of toing and froing and me agreeing not to redistribute it, they sent it to me (big kudos to Kikusui there.) I discovered that the 100 Ohm resistor should have been 100K. I replaced it and carefully switched it on. More crackling and popping and a hot resistor. Long story short I tracked it down to a shorted capacitor (.022uF at 1500V) in the 506v/HT circuit on the PSU board and duly replaced it. I also replaced the On/Off/Intensity switch/pot as this was the one doing the popping by arcing internally.

To my relief the thing fired up OK and I got a nice trace on the screen. I followed the instructions in the user manual (freely available from Kikusui.com) and set about calibrating it. That's as far as I got and this is where I need your help (if you read this far!). I get two dots on the screen, one above the other, which I assume is the test voltage (1v peak to peak) which should display as a square wave. When altering the multiplier switches, the dots move closer or further away from each other, as would be expected. I have up and down positioning, horizontal gain, intensity, focus, variable and all other controls all manipulate the trace, but I get no actual waveform displayed. It looks to me like a static reading without any sweep, if that's the right terminology (as you can tell, I don't know scopes very well). The sweep frequency switch has no effect at all on the two dots; they just stay put where they are and it is in this area where I think the problem lies. I eyeballed the circuits as best I can and can see no obviously damaged/smoked components. There are three circuit boards inside the thing, all interwired. I don't think the power supply issues I had before would have touched any of this circuit but I could be wrong. All the voltages (which are marked in various test points on the board) are good and within limits.

It is all made with discrete components so I could easily replace everything on the board but I thought I might get some ideas from the board before resorting to this type of repair.

Sorry for the War and Peace post and thanks again for the resource.

Dave.

 

[bountyhunter]

The problem is no sweep: that means trouble in the circuit that generates the sweep (sawtooth) voltage used for the sweep or possibly the horizontal output stage.

A scope has a vertical output stage and a horizontal output stage to cause the electron beam to move. You are getting correct vertical delection but no horizontal deflection. In some scopes the sweep circuits and horizontal are on the same board. They should be near each other because the sweep directly drives the horizontal stage.

good luck.

 

[davenz]

Cheers

Thanks for that, I suspected it might be something like that but it is good to get an expert opinion. I will let you know how I get on.

Dave.

 

[davenz]

Right, progress report.

I went through the sweep circuit as suggested and tested all 10 transistors and three diodes. I found two dead trannies and a dead diode. They are typical Japanese components; 2SC458 for the trannies and 1S1586 for the diode. I replaced them with BC549C's and a 1N4148 diode. On switch on, I now get some movement, but not right across the screen and not the nice square wave I was expecting with the calibration signal. Mind you, I don't know how to use a scope or what I am doing but am following the calibration suggestions in the scope's user manual. I certainly don't get an image like the one in the manual, more like (at 1/1 on the attenuator switches) a series of parallel dashes, like a square wave, but with the risers missing. I cannot get it to show anything else (intensity when turned full bore just mushes out the trace) out to the sides of the screen either; it runs out of control at about 25% of the screen to go (both left and right; up and down scrolls out of view. In light of this, are the substitutes I have used suitable for the purpose and OK for components and I am replacing? They all have very similar specs. but there might be something I am overlooking.

Something is still not quite right but I am getting there and I will pull every transistor and diode in the scope and test them if I have to.

I will report back if/when I find anything else.

Thanks

Dave.

 

[bountyhunter]

Right, progress report.

I went through the sweep circuit as suggested and tested all 10 transistors and three diodes. I found two dead trannies and a dead diode. They are typical Japanese components; 2SC458 for the trannies and 1S1586 for the diode. I replaced them with BC549C's and a 1N4148 diode. On switch on, I now get some movement, but not right across the screen and not the nice square wave I was expecting with the calibration signal. Mind you, I don't know how to use a scope or what I am doing but am following the calibration suggestions in the scope's user manual. I certainly don't get an image like the one in the manual, more like (at 1/1 on the attenuator switches) a series of parallel dashes, like a square wave, but with the risers missing. I cannot get it to show anything else (intensity when turned full bore just mushes out the trace) out to the sides of the screen either; it runs out of control at about 25% of the screen to go (both left and right; up and down scrolls out of view. In light of this, are the substitutes I have used suitable for the purpose and OK for components and I am replacing? They all have very similar specs. but there might be something I am overlooking.

Something is still not quite right but I am getting there and I will pull every transistor and diode in the scope and test them if I have to.

I will report back if/when I find anything else.

Thanks

Dave.

a series of parallel dashes, like a square wave, but with the risers missing.

That may be normal as often the rise times of the waves are so fast you don't see them unless you stretch the sweep out in time and zero in on the edges.

intensity when turned full bore just mushes out the trace

That's also typical.


As for components substitutions: the most important specs on transistors is current/voltage ratings and ft (bandwidth) which is critical for the ones that process signals.