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2445 CH2 Amplitude is 1/3 of expected value

Jay H.
 

Hi,

I'm new here and appreciate any advice or direction you can give.

I've acquired a 2445 in excellent cosmetic condition and am at "Amplitude Check" only to find that CH2 is reduced by a factor of 3. It reads 133mV, versus CH1 properly reading the calibrated square wave as 400mV. I've tried DC Leveling and Automated Diagnostics. That multiple of three reminds me of an op-amp.

Also both the start (400 mV) and end (0 mV) of the square wave have a small, leading sine wave imposed over it. I've assumed that it's just a failing lead, but the lead is new (from an unopened package) and now I'm starting to wonder.

I have photos and will try to upload them. But I'm new here and appreciate any guidance.

--
Jay H.

Chuck Harris
 

Probes die new in the box. Don't let appearance do the diagnosis.
Try the probe on the other inputs. If it doesn't work everywhere,
try disassembling it into modules, and reassembling the modules...

The ringing on the leading and trailing edges of the square wave
indicates a mismatch somewhere... like a blown attenuator.

It is extremely popular for new scope users to blow out the input
attenuator on an oscilloscope. They do things like hook them up
to TV flyback circuits, transmitters, or HV circuitry.

Probably the second most popular thing to do (in the never ending
search for things to look at), is to connect the ground clip on the
probe to the hot side of the power line, and burn up the ground clip,
the probe shield, and the green/yellow bonding ground wire inside of
the scope...

- Chuck Harris

Jay H. via Groups.Io wrote:

Hi,

I'm new here and appreciate any advice or direction you can give.

I've acquired a 2445 in excellent cosmetic condition and am at "Amplitude Check" only to find that CH2 is reduced by a factor of 3. It reads 133mV, versus CH1 properly reading the calibrated square wave as 400mV. I've tried DC Leveling and Automated Diagnostics. That multiple of three reminds me of an op-amp.

Also both the start (400 mV) and end (0 mV) of the square wave have a small, leading sine wave imposed over it. I've assumed that it's just a failing lead, but the lead is new (from an unopened package) and now I'm starting to wonder.

I have photos and will try to upload them. But I'm new here and appreciate any guidance.

Jay H.
 

Thanks Chuck,

I got my hands on another scope and found that the probe is good. Two things cause me to think the BNC on CH2 is good: it's a perfect multiple of 3 and I had inserted and twisted the probe several different times with the same effect. One thing makes me think a board is bad: it's in superb condition with more wear and tear just from being moved than being used. It looks like someone with more enthusiasm than knowledge did something untwards with CH2, as you suggested.

Right now I haven't time for digging into it. I'm going to let someone else, with some time, play with it.

It's now listed on eBay.

Cheers,
Jay

Siggi
 

Late to the game here, but this sounds like the trouble I had with the
attenuators on my 2430. Try and measure the resistance of the input in
1M/DC coupling as you run up and down vertical sensitivities. It should be
1MOhm throughout, betcha it’s way up off the trouble range.
In my scope there was heavy oxidation on the contacts, fixed with some
deoxit...

On Sun, Jul 14, 2019 at 00:59 Jay H. via Groups.Io <jcholsap=
yahoo.com@groups.io> wrote:

Thanks Chuck,

I got my hands on another scope and found that the probe is good. Two
things cause me to think the BNC on CH2 is good: it's a perfect multiple of
3 and I had inserted and twisted the probe several different times with the
same effect. One thing makes me think a board is bad: it's in superb
condition with more wear and tear just from being moved than being used. It
looks like someone with more enthusiasm than knowledge did something
untwards with CH2, as you suggested.

Right now I haven't time for digging into it. I'm going to let someone
else, with some time, play with it.

It's now listed on eBay.

Cheers,
Jay



Jay H.
 

THanks, Siggi,
I thought about trying something like that just as a matter of isolating the problem from the outside. But I'm a bit overwhelmed with work and family right now and once I start troubleshooting I usually keep at it. And that perfect multiple of 3 really nags at me. Call it intuition, the force, upset stomach,... But for next time, I am noticing a "pots" theme with these old o-scopes which isn't surprising without gold plated contacts.
Cheers,
Jay