Kenwood Oscilloscope timebase x2 errror

[physicsgeek]

Hi,

I am new to the forum and am hoping that someone may be able to help. I have acquired a Kenwood CS 4125 which appeared to be working fine. I took it to the school (where I teach physics) and tested it using our sig gens. It gives measurement of the timebase which is exactly half that expected and half that of the other scopes that we have. I have checked the time base control setting carefully and am certain that it was not set incorrectly. (When set at 5ms/div the measurement between subsequent peaks was 2 divisions).

Any suggestions as to what the problem might be would be most welcome.

Thanks in anticipation!

[cmjones01]

Here's a simple idea: has the timebase knob moved round on its shaft by one position, so it's pointing to the wrong setting? You can check this by turning it all the way to one end, then the other, and seeing if it lines up with the markings on the front panel. I note also that the timebase has a 'variable' adjustment, the little grey one above and to the right of the main timebase knob. This should be turned all the way clockwise, I think, and will probably have a click to indicate that it's in the CAL position, which is what you want.

Chris

[MrBungle]

Also are you sure the signal generator is accurate?

[physicsgeek]

Hi,

I checked that the knob wasn't misaligned by counting back from both ends so I am confident that is not the problem. Also, all other scopes were in agreement with the Sig gen (and with mains frequency which my scope showed as 100 Hz) so I really think this is a problem with my scope.

Cheers for the rapid responses!

[physicsgeek]

Also, I have checked that the cal switch is in the correct position.

[physicsgeek]

A further check of the sweep speed clearly shows that it is at half the indicated value, i.e. when set at .5 sec/div (maximum setting - control goes no further) it actually takes 10 s for a full sweep of 10 div. Similar results on all other settings. Before I go looking, can anyone throw any light on whether there is a means of adjusting this or is there a something like a frequency divider in the sweep control circuit that might produce a X2 error such as this?

[Diabolical Artificer]

Service manual here - https://elektrotanya.com/kenwood_cs-.../download.html

Andy.

[dave cox]

what about when it is set to 0.2S / div ?

dc

[dave cox]

Is the timebase 'variable' in the cal position ?

dc

[WME_bill]

Kenwood oscilloscope CS4125.
So we can think about your problem, can you clarify please that this X2 effect occurs at all timebase speeds.
If a resistor chain is faulty, then the effect will occur in steps, every 4 speeds. See S201 table on the circuit diagram. Are those switches closing and opening the circuit as they are meant to do.
If a capacitor, then the effect is with an adjacent group.
But if at all speeds, then it suggests a fault with the charging mechanism, or voltage available, or the Time base hybrid integrated circuit IC201.
But so much seems hidden in that hybrid IC. You might have to see if you can get a replacement.
A less likely alternative is that the amplitude of the scan is badly out, and someone has twiddled the X Amplifier gain to try to correct it. Though your effect is the opposite I think, of what that would do. You naturally will have checked the TB x10 switch.

Would a few direct checks with another scope help to find where the effect is occurring.
Others have commented on the obvious troubles:
Switch pointer wrongly placed.
Calibrator incorrect - have you checked against the 50Hz mains. Lots of things from the mains can appear at 100Hz if there are some diodes and capacitors around.
wme_bill

[physicsgeek]

Thanks for the additional responses since my last comments. With respect to the above:

The fault is appearing on all time base settings. I will re-confirm this when I go back after half term but I did check 0.2 s/div which gave a sweep time of approximately 1 sec rather than 2.

As previously indicated I have checked that the cal switch is in the correct position.

I will now check against our other scopes to see exactly where the fault is occurring. All the others gave "correct" readings, including against the 50Hz mains which mine read as 100 Hz. As soon as I have the results I will post them so that you can advise me further.

Thanks very much for the service manual, it will be invaluable!

[dave cox]

50Hz mains @ 5mS 10mS and 20mS would be good