Kikusui COS5041 Oscilloscope - Display too bright

[Shandy]

Hi guys,

I need some help; I have a Kikusui COS5041 Oscilloscope

http://www.kikusui.co.jp/kiku_manuals/C/COS5040_5041_E.pdf
https://elektrotanya.com/kikusui_cos5040_sch.pdf/download.html

The trace starts with a very bright dot and then the normal horizontal trace.
I can move the trace with the line position control so I don’t see the bright dot, but the whole screen is illuminated, all the phosphor dots are active.
I am unable to reduce the intensity of the trace; it does varies up and down but does not reduce to minimum.

I checked the voltages on the CRO pins;

Cathode pin 2, -1760 volts.
Control grid (1st grid) pin 3, -1120 volts
Focus grid (2nd grid) pin 4, -1760 volts.

I checked the high value resistors in the brightness and focus circuit and they appear to be okay.

I'm not sure where to look next.

Please any help would be appreciated.

[stevesousa]

Hello,

This scopes have a feature were the second timebase highlights the main trace, i.e., the main trace is brigter in the section that shows magnified in the.... magnified trace, for lack of better words.
If you're not operating it incorrectly, there are some 74 logic chips that control he toggling of modes, at least that's what broke in mine with a similar failure, can't remember exactly. Also, have a look at the power supply rectifiers for open diodes.

I definitely think you're looking in the wrong place.

Cheers
Steve

[stormdog]

Not to sound like I have more experience than I do -I'm basically just starting out- but I was confused by the same issue on my HP 1741. The feature Stevesousa pointed out exists on mine too. If the delay timebase control isn't set to "off" on mine, the part of the trace that would be shown by the delayed sweep is highlighted brightly. I bought a used scope on Ebay and was worried the CRT was damaged, but it was just settings.

[james_s]

Careful while you're troubleshooting that, those bright dots will burn the phosphor off and leave you with permanent black spots in the screen.

[Shandy]

Thanks guys for the replies so far....

I've had this scope for many years and up until now it has always given me good service. I haven't used it for quite sometime and I was a little upset to find this fault when I switched it on!

I'm sure the fault lies in the tube drive circuit, but I'm not sure what voltages I should expect to see.  The circuit only shows the cathode voltage of -1800 volts. I measured -1760 volts here, using a home made 10 to 1 voltage probe on my DVM.

The voltage on the control grid, pin 3 is -1120 volts and this worries me as that means there is -640 volts difference between the cathode and grid, and I don't think that's right.

Any help would be appreciated.


   

[mzacharias]

I had this exact problem on a Tek 465. Shorted capacitor associated with the CRT grid. Traced it from the intensity control. Easier than I expected.

[woodchips]

The grid circuit is a very high impedance, voltage readings mean nothing. Check by confirming the resistors in the grid circuit are correct. Next check the brightup capacitor doesn't have the slightest leakage, it sits between the grid, -2000V, and ground, the brightup amplifier.

[Shandy]

I had another look at my scope yesterday and checked the components in the Brightup circuit and can't find any problems there 

Still hoping for a solution 

[SMdude]

Have a good check for dry solder joints.
I have a cos5100 model and have had a couple of dry solder problems.
Failing that, wave a bit of heat about the place and see if it suddenly behaves..

[Shandy]

Thanks, I've already checked for dry joints and given it a good look over for anything out of place.

[ccsupplies]

Hi there I had the same or similar problem, Ive got the Kikusui Cos 5042TM I traced the fault to to CR836 its a Diode 1SS83 its in the CRT bias circuit comes of the CRT bias control RV821 (500K and not 50K as mentioned in the circuit diagram-by the way there a lot of errors in the circuit diagram) I fitted a BAV21 diode instead of ISS83.
make sure to test the diode out of circuit on a high resistance range (Range 1K to 10K) this will show a leakage.
A good test is to adjust the CRT Bias control, you will find that it does not alter the screen brightness as the 1SS83 diode is passing to higher voltage, once you fit the new diode, you can readjust the CRT bias control.
Ive also had another fault, no channel 1 input Signal  this fault was caused by U101 is a LF13741( this could also apply to channel 2 input.
regards
Eric

[tggzzz]

I checked the voltages on the CRO pins;
Cathode pin 2, -1760 volts.
Control grid (1st grid) pin 3, -1120 volts
Focus grid (2nd grid) pin 4, -1760 volts.

Those are almost certainly wrong. Since the resistors are high impedance, it is possible that the meter is loading the chain and affecting the indirect measurement of the key value, V_gk. It would be better if you measured V_gk directly. Ob warning: be careful with those voltages! Keep one hand stuffed in your back pocket!

Based on my debugging a Tek465 that was acting as a torch[1], I would expect  to be in the range 40-150V, depending on the brightness. The V_gk waveform should be an exact copy of the LT z-axis brightness waveform, translated to the cathode voltage. Of course V_gk is difficult to measure without an HV probe, but the LT variant is easier

Typically problems with brightness are related to the DC restorer circuit, and people usually just replace the caps and diodes rather than debug specific items.

Of course, if you don't want to debug that scope, working Tek 475/2445B are available at reasonable prices; PM me if interested - I come to London intermittently.

[1] https://entertaininghacks.wordpress.com/2016/03/09/rescuing-a-broken-tektronix-465-crt/

[Shandy]

Thank you tggzzz for your reply,
I really want to get this oscilloscope working properly again. I'm hoping this is not a unique fault and someone has had a similar problem and can point me in the right direction.
I can still use the scope, everything is still working okay, I just have to wear sunglasses!

[tggzzz]

I can still use the scope, everything is still working okay, I just have to wear sunglasses!

I wouldn't use it; you can damage the phosphor and, allegedly, even crack the glass due to the bean causing localised heating.

When I was experimenting with my 465, before I realised the CRT was faulty, I defocussed the beam and kept it off the screen as much as possible.

[Shandy]

Ha-ha I was joking! 

I have a Rigol DS1052E that I use most of the time, but it would be nice to get my Kikusui working as well.

[Shandy]

Hi Eric,
Thanks for your reply; I only saw your message now. I missed it as two came in at the same time and I only read one! It sounds positive. Thanks for letting me know about the problems with the circuit diagram. I got mine off the internet and it's not a great copy. I sent an email to Kikusui America and asked them for a service manual. I must say they were very quick in replying although they said the 'scope is too old and they no longer service that model. I'll put my scope back on the bench again tomorrow and check your suggestions. I'll let you know what I find.