(Solved) When oscilloscope repair goes from bad to worse (kikusui cos5040)

[fezder]

Initial problem with this oscilloscope, was gone channel 2. It was super dim at best, barely seen as best described "overshot noise". This happened just few days ago.
Now today, I took vertical amplifier board out to troubleshoot components. Suspected components were ok (tested out of circuit one-by-one.

Now then, when I placed board back in place, BOTH channels are gone, beam is stuck on place with full intensity (intensity knob doesn't do anything). Obviously I thought bad contact and started fiddling knobs (vertical intensity knob on channel 1 did make beam bigger in vertical direction, but time/div and horizontal position were stuck). Making matters worse, smoke started rising somewhere....no idea where, but smoke is smoke.

'scope in question is kikusui cos5040, and I do have schematics available. Although I have bad feeling about this repair attempt!

 

[Nigel Goodwin]

Now today, I took vertical amplifier board out to troubleshoot components. Suspected components were ok (tested out of circuit one-by-one.

Although I have bad feeling about this repair attempt!

I'm not surprised, as I've posted here repeatedly you DON'T repair things by randomly removing components and attempting static tests on them - you're unlikely to cure the problem, and MUCH more likely to cause further damage.

 

[fezder]

It wasn't completely random, since I compared measurement results between working channel and faulty one and isolating faulty section by backtracking.
I didn't see any feedback from crt vertcial deflection part back to amplify part, so that's why I focused on vertical amplifier section.
Parts I measured were faulty side's rectrifier diodes used in channel selection, as well as transistors related to vertical position.

 

[fezder]

Ok, slight progress.
Channel 1 back online. Didn't take anything out of circuit, but took pcb's out for visual inspection. After putting all together, channel 1 works.
Loose connection?
Channel 2 is still dead, both visual side and triggering.

 

[fezder]

This is in xy-mode

and small video, with small operation tests:

 

[fezder]

More odd stuff:
vertical=dual
Up to 0.5ms (until tv-v/tv-h) no dual traces, but channel 2 position also affects channel 1. Offset caused by channel 2 position knob is always one reticule with some noise on top, not affected by volt/div settings.

 

[fezder]

Bit more info: since triggering doesn't work either with channel 2 to display channel 1, measured points at vertical amplifier section, where also is internal trigger output. Diodes CR108, 208, 107 and 207 are acting as switches that are controlled by U301 (pins 5 & 6). Channel 1 triggering signal comes just fine, but channel 2 is way too much attenuated.

U202 input from attenuator resistor thingy (...) comes at pin 9, both channels show same signal there. Using external function generator feeding 'scope in question. Signal is attenuated between channels when comparing voltages at bases of Q205, Q206, when comparing channel 1 Q105, Q106. There's around 50% difference. Difference with Q208 and Q108 collector voltages are even more, 52mV vs 540mV.

No idea if these help anything...

All voltages are measured against 'scope earth

 

[fezder]

Ok, previously mentioned 0.5ms time/div problem is not tied to time/div, but alt/chop. In dual operation mode, chop mode causes channel 2 position knob to affect channel 1 position. There is something really weird stuff going on.

 

[fezder]

Found good source of information, which has part discussing vertical part too.

https://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

it mentions commoning push-pull amplifier section and by using that trick of shorting bases of push-pull pair, I got second channel on screen (channel 2 position knob however still affects channel one position on chopping mode, but progress anyway)

Shorted first Q301 and Q302 bases, beam centered, so fault is before those. Previous push-pull is made by Q205 and Q206, again centered.
I also noted quite high voltage differences between right side pins of U202 and U201, both in DC and AC region.

Also, when short in place, XY-mode works as intended too, even if combined with line-trigger. Line and/or dot can be moved with position knobs.

 

[fezder]

Found one faulty part, and even it is only partially broken. U202 pins 1 and 2 show infinite, tested both DMM and octopus tester with another 'scope. All other pins function. Makes me wonder what has killed those pins. I tested it out of circuit.

https://inst.eecs.berkeley.edu/~ee105/fa15/labs/CA3086_datasheet.pdf

Reason why I took U202 into inspection was because amplifier commoning trick which hinted fault being between U202 pin 9 and Q205/Q206 pair. (since after that It's onlyvertical amplifier section) and only basic resistors and capacitors, which in my understanding fail rarely(ish). And that there was voltage difference between pin 1 and 5 as I stated before. Better to check neighboring components too if they have caused U202 pins 1 and 2 to open.

I also compared results with working channel's CA3086

And of course, got no that part on hand, except if I take it from one of my linear PSU's, but those are working units.

 

[throbscottle]

If you haven't done so already, you need to check all the internal power supplies, both at their output caps and also after any on-board filtering in the suspect areas. Don't just check the voltage, check them with another oscilloscope too. I mention it because this is a known issue with my oscilloscope and causes weird stuff to happen.

Another issue I came across getting my own 'scope working (I bought it as "spares or repairs") is transistors that check ok with a dmm, but have lost their gain. Though this may just due to their great age - your oscilloscope is a lot more modern than mine.

Also check the high voltage circuits. The fact that you can't adjust the brightness suggests something has failed there, possibly in the tube's cathode or grid supplies. A x10 probe for your meter is a good idea for doing this, and not hard to build your own. Sort this problem out and you might find some of the weird behaviour just disappears.

 

[fezder]

Oh yes, you mean AC ripple? I'll double check that, but I didn't notice any bad ripple on supply rails. Main filter caps were already reaplaced in 2014, forgot to mention that detail...

Hmm, haven't heard of transistors losing gain? I thought they either work, or short/open.
Intesity problem sorted itself out (poor connection?), since channel 1 came back alive. Channel 2 only comes as horizontal line when those transistor bases are shorted, what I mentioned earlier.

 

[throbscottle]

Not just ripple. I had a bad rectifier a while ago. My 'scope is 50+ years old, and stuff just dies. The transistors were a real surprise to me too, but there you go. They were sufficiently old to have gold plated leads! Had a film capacitor where it's capacitance had just drifted away, too..

It might be worth going through the suspect boards and re-work all the joints with your soldering iron. If you had one bad connection, there are probably others. You can't actually see them, they just go noisy/intermittent/open.

So it sounds like channel 2 beam is missing either because there is no trigger pulse getting to it, or it is being pushed right off the screen by a DC voltage occurring before those 2 transistors.

I think you are right about the problem being related to alt/chop.

It might be worthwhile simply testing all the resistors, capacitors diodes and BJT's in-circuit with a multimeter before spending more time doing proper fault finding. It costs you maybe an hour but potentially could save days or weeks if you are lucky. If you do it, don't be concerned with values that just look wrong - you are looking for outright short or open circuits (or very high value resistors).

 

[fezder]

I'll take video tomorrow of current situation. (just had exhausting day; thesis presentation seminar and maturity test, almost graduate EE. Believe or not lol.)

 

[fezder]

Ok, here's current situation:

 

[Inquisitive]

That scope is upside down... Pommie can you assist?

 

[fezder]

That scope is upside down... Pommie can you assist?

Haha! I was going to flip video around before upload. During troubleshooting it also had to be upside down causing slight confusion using controls. Thankfully isolation transformer switch is nearby if things go sour. When you see second channel, that's when I short those amplifiers. I checked that busted part and thankfully It's not expensive. It's back in place now to hopefully prevent furter mishaps.

 

[throbscottle]

Ok so what does pulling/pushing the position knob supposed to do? Seems like a nice clearly defined fault now.

 

[fezder]

Pulling postion knob changes chop sweep only with dual trace operation. Normally It's either chop or alternating, but position knob can override that setting, so only chop is used. Without it, chop/alt is chose from time/div setting (<0.5ms is alt).

 

[KeepItSimpleStupid]

Hmm, haven't heard of transistors losing gain?

That happens. Annoying when it does.