Kenwood CS-4125 oscilloscope focus & intensity

[Jarvik7]

Over the weekend I just rescued a Kenwood CS-4125 20MHz scope that had been thrown out and sitting in the rain.
After drying it out and powering it up, it works perfectly aside from the focus and intensity controls.

Focus works to a point, but at its sharpest the trace is still about 2-3mm thick.
Intensity doesn't affect the trace brightness at all, but will worsen the focus somewhat when adjusted.

I checked the control pots with my dmm and getting expected readings throughout their travel.
I'm guessing that either an internal adjustment pot for focus needs adjustment, or a resistor etc has gone out of spec/died.
Popping the case reveals no obviously burnt components.

Unfortunately I only have an owners manual with zero useful information, no service manual.

Anyone have any experience with one of these units, or maybe the service manual or schematic?

[Jarvik7]

Found this snippet in a catalog:

Auto-focusing
The auto-focusing function corrects the focusing error
automatically even when the intensity is varied.

Which implies that the focus control is fine, but just the intensity control is broken and is throwing off the focus.

[tautech]

Start with downloading this document to help you with scope problems and repairs.

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

[Jarvik7]

After a couple hours of Googling I've only managed to find some sites where you can pay for the manual, but I did find a CS-4025 service manual. While it's not the same model and some controls are in different locations, the theory behind the power supply unit should be similar I hope. I've emailed Texio and am waiting for a response...

The only hint I found in that troubleshooting file for intensity/focus problems is that being unable to reduce the intensity (exactly my problem) may be due to a "weak DC restorer diode" in the control grid supply. Unfortunately I don't have a schematic but I suppose I can test all the diodes on the power supply unit board. In the CS4025 schematic there are 9 in circuit with the intensity control input.

I'm guessing here, but would a failed restorer diode cause blanking to not function altogether?
I cannot see the beam return so that's not an issue I have...

[mij59]

Are sure all the parts are dry ?
Any sings of residues from the rain ?
Please post some photo's

[Jarvik7]

Are sure all the parts are dry ?
Any sings of residues from the rain ?
Please post some photo's

The behavior will probably be easier to understand with a video, so I'll try to upload something to youtube when I get home.

I opened the case (top and bottom) and directed a large fan at it for about 5 hours before confirming there was no moisture and powering it up. Inside is pretty clean and it otherwise works fine, so I am assuming that the focus/intensity was an existing problem when it was thrown away.

[tautech]

After a couple hours of Googling I've only managed to find some sites where you can pay for the manual, but I did find a CS-4025 service manual. While it's not the same model and some controls are in different locations, the theory behind the power supply unit should be similar I hope. I've emailed Texio and am waiting for a response...

The only hint I found in that troubleshooting file for intensity/focus problems is that being unable to reduce the intensity (exactly my problem) may be due to a "weak DC restorer diode" in the control grid supply. Unfortunately I don't have a schematic but I suppose I can test all the diodes on the power supply unit board. In the CS4025 schematic there are 9 in circuit with the intensity control input.

I'm guessing here, but would a failed restorer diode cause blanking to not function altogether?
I cannot see the beam return so that's not an issue I have...

Blanking is the return of the sweep and not directly related to the HV DC restoration.

If the scope got very wet there may still be moisture in the controls.
Time in a sunny warm location should dry it out.

[Jarvik7]

Sorry about the lack of pics so far. Work suddenly got busy and I haven't had a chance yet.

Focus has improved quite a bit after scrubbing some residue from the bottom of the amplifier board with a white eraser.
The scope was sitting upside down on the rubbish heap so I guess some rain and dust got in there and created some capacitance or something.

I've beeped out some of the circuit path and, after some thought, it seems likely to be a dead component.
With the intensity knob on full, it is giving 0V at the wiper, increasing to 5v when intensity is at minimum. Something must be open or transistor gain weak (resulting in 0v or close to it) and forcing the rest of the circuit to think the knob is at full intensity. The feedback into the autofocus circuit is working as tweaking the intensity knob will change focus slightly but not influence the brightness at all. Sending a signal through the Z-axis input also doesn't do anything to the intensity.

I still haven't gotten a response from Texio, so lacking a schematic with some test points I think over the weekend I'll just start pulling out components in the path and testing them, starting with the transistors.

Hopefully I can get this fixed before my Tektronix 2430a shows up and I lose interest

[Jarvik7]

I've replaced 2 diodes (1SS83) in the HV circuit that I found to be leaking (conducting roughly 50% in reverse direction) with same part no., but it has had no effect on the functionality of the intensity knob. I've also checked the large-value resistor (R58 in this diagram).

I've attached the circuit diagram for CS-4025, which is similar though does have some differences.

Test point 7 seems fine.
Test point 8 couldn't be checked as I do not have a 100x probe
Test point 9 is correct.
I don't fully understand test point 10. Does this waveform show the sine from TP7 combined with TP14? In any case, I do not get this shape.
Test point 11 couldn't be checked as I do not have a 100x probe.
Test point 12 is a lower range (around -80 to -530v if I recall, instead of -100 to -700).
I haven't found test point 13 yet.
Test point 14 has the correct shape but seems to have a lower DC offset (maybe by 40v).
+-8v, +140v rails to transistors are correct.

The difference in DC offset could be simply explained by the fact that this is a different model oscilloscope, but as TP7 and TP14 have at least the correct shapes, the waveform must be getting mangled in the HV circuit. I will try to upload some testpoint waveform pictures soon (TP10 in particular).

I'm starting to think the neon lamp (NL1/2 in this circuit, though the CS-4125 only has a single lamp) may be the problem. What would the effect on the circuit be if the lamp was faulty?

Regarding the related focus blurriness:
It seems to get worse the longer the unit is turned on, so that appears to be heat related.
Adjusting the focus pot only makes it worse (VR3).
The large value resistors (R41-R43) at the cascaded focus transistors all match their markings.

[mij59]

Hi,

If you vary the INT level does the dc level of TP14 vary ?
Check TP6, and the -10V supply.
Check the resistors R31..R36.

[Jarvik7]

Hi,

If you vary the INT level does the dc level of TP14 vary ?
Check TP6, and the -10V supply.
Check the resistors R31..R36.

Thank you for the reply.
TP14 DC level does vary with INT knob position.
I will try to locate and measure R31-36, TP6 and -10v supply and get back to you.

The TP6 area I think might be very different, as that side of the transformer only has 3 pins on the CS-4125 vs 4 pins on this diagram. I haven't looked at that area of the circuit yet.

[tautech]

I don't fully understand test point 10. Does this waveform show the sine from TP7 combined with TP14? In any case, I do not get this shape.

Refer to the frequency of TP 6. The switcher is running @~20uS period.
Now take that frequency and impose it on TP 14 waveform @~5 mS period.
That is how TP 10 waveform is arrived at.

IME suspect any high value resistors and HV caps in this part of a CRO.

While it would be great if the manual is close, keep in your mind it is not the right manual and many things may be different.

[Jarvik7]

Hi,

If you vary the INT level does the dc level of TP14 vary ?
Check TP6, and the -10V supply.
Check the resistors R31..R36.

TP6 and +-10v supplies check out fine. Still need to locate R31 ~ R36 (component numbers are all different on CS-4125).

I've attached some waveform pictures:

focus.jpg: The CS-4125 at it's best/clearest focus. Some nice burn-in from someone leaving X-Y mode on too.

TP6: Looks fine aside from jitter
TP7: Looks fine aside from jitter
TP10: Totally wrong (there is no sine superimposed on it). The brightness difference of one segment is a camera artifact.
TP14: Shape is fine but DC offset is lower than diagram

[Jarvik7]

Here is another waveform from the point between R24 and C22.

The shape looks strange to me. Zooming in you can see that there is a sinewave superimposed on it.

[mij59]

Here is another waveform from the point between R24 and C22.

The shape looks strange to me. Zooming in you can see that there is a sinewave superimposed on it.

The signal from TP7 is not present at TP10, check the components around TP10.

[Jarvik7]

I've already checked the diodes (replaced 2), HV caps (in-circuit with a DE-5000 - capacitance and ESR seem fine) and resistors (in-circuit) and everything seems to check out. I also held a new 1000pF resistor against the C22 contacts but there was no change.

The only thing I haven't tested is the neon lamp (NL1, NL2, though the CS-4125 only has one). From what I can glean from Google it functions to absorb voltage spikes, so it should have no effect on the circuit (and stay unlit) under normal operation?

[mij59]

I've already checked the diodes (replaced 2), HV caps (in-circuit with a DE-5000 - capacitance and ESR seem fine) and resistors (in-circuit) and everything seems to check out. I also held a new 1000pF resistor against the C22 contacts but there was no change.

The only thing I haven't tested is the neon lamp (NL1, NL2, though the CS-4125 only has one). From what I can glean from Google it functions to absorb voltage spikes, so it should have no effect on the circuit (and stay unlit) under normal operation?

In-circuits measurement can be tricky, did you disconnect one of the resistor leads ?
Yes i think the neon lamp is an over voltage protection.

[Jarvik7]

My bench multimeter (Advantest R6441C) has an in-circuit ohm mode that seems to be pretty dependable, but if I can't find anything else I may need to start lifting legs. The plating is pretty poor quality so I'm afraid of lifting pads (lifted one so far).

I just recreated the HV circuit in LTSpice (generating approximate waveforms for TP7 and TP14 using a voltage source) and could get roughly the expected TP10 (and TP11 HV) waveform. The waveform between R24 and C22 is very different than my actual oscilloscope.

Totally deleting the HV caps (thus removing them and everything to the right of them from the circuit) does not effect TP10 at all, so I can eliminate them from suspicion.

I tried deleting the electrolytic at C14 and managed to get almost exactly the same R24/C22 waveform. I will try remeasuring it out of circuit tonight.