Request for confirmation of my troubleshoot conclusion(Kenwood CS2100/BK1590)

[watchmaker]

My forever scope (BK 1590/Trio 2100) developed a glitch on channel 1.  Became very flaky (I know how to do all adjustments, but that is not the issue).

Repsonded to the old TV slap to stop vertical roll though.

I spent the last day and night pouring over the schematics and as I was touching VR1 the trace went haywire.  I mean barely breathe on it.  So I turned it off, and soaked it with contact cleaner. Turned it back on, was ok but out of adjustment.

As I tried to adjust, the trace went haywire.

So voltages out of IC 1 are also flaky (low). IfI leave the scope on overnight the trace settles down.

In other words, the IC1 ckt is unstable. I am hoping it is the MOSFET since the IC is likely proprietary. (ATM 4010).  Cannot get to the pins without removing both the preamp and attenuator board.

I have another story about what happened when I checked a2080 to see if I could swap attenuators.  (trials and tribulations story).

Below are the schematics.

THANKS!

[m k]

Maybe you soaked it so that some liquid was still somewhere.

My guess is that iC1 is a regular 4010.
Chop is from IC2, is it similar to Alt from IC1?

[watchmaker]

Thank you.  Yes,  chop and alt are the same.  I am watching it now.  The ATM 4010 is not a 16 DIP.  It is square like a uP.  Not afraid to take it apart, but I want part or a donor board first.

Do you think I am close in my Dx?  Been quite some time and I never did troubleshoot anything but tubes (where at least you could see the filaments!) 

[m k]

Oh, you meant IC12, I was elsewhere.
Yes, it's most likely rare, if not completely inhouse.
PreSuffix ATM is also less optimal for searches.

Your picture seems to be the same as Trio 2070, 2100 has a different layout.

If you touched a trimmer and major happening happened you most likely were the reason.
Old trimmers can be nasty, everything is fine until you touch them, then nothing is fine anymore, maybe never.

[watchmaker]

Thank you!

It is the 2100. Very nice but complicated layout.

Good to know about the trimmers  Channel 1 was acting up and doing lightning strikes until I would give it the vertical roll slap.  But I could not trust the readings. So, down the rabbit hole.

Happily, I just found another 1490/2100 scope for $75.  Did all the adjustments per manual and am very happy.  The first one will be a donor I guess unless  I get very lucky.  At least it does not eat much or whine at night.

[watchmaker]

OK. Several things. Replaced the trimmer. Stability restored!  Then as I continued with the adjustments, I could not get rid of overshoot on channel 2.  Finally looked at the 50 ohm termination resistor; black.  Replaced with 2 100s in parallel and the over shoot is gone.  Continued with adjustments.

Then my trigger section acted up.  I forget how I found it, but in the CS2100 manual, but not in the BK manual is a note that that the trigger section uses a a 3V Li battery to retain the last settings.  Replaced the coin type battery with a BR2/3 (ala Fluke) and that solved that problem.

Who wuda thunk a scope would use a battery?

Not happy with the 4 VRs for nulling the 4 channel trigger voltages.May replace them if I have the room.  Got them nulled, but I do not trust the trimmers to stay; very noisy.

But it now agrees with my Siglent.

Now I can sell my Kikusui.  I have two CS2100s that agree with my Siglent, so I have a backup.

The manual sez the 50ohm inputs can only handle 5vAC rms.  This would seem to limit the utility of this selection.  Is this normal?

[Zenith]

The manual sez the 50ohm inputs can only handle 5vAC rms.  This would seem to limit the utility of this selection.  Is this normal?

The same for my HP1744A. I think you'll find that's pretty usual for scopes. The 50 Ohm inputs on my Racal 1991 and 1992 also have 5V RMS warnings. Some 50 Ohm inputs have lower limits. The 300MHz to 3GHz input on my Racal 9921 has a 2.5V RMS limit. You just have to be careful.