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2mV and 5mV ranges read the same on 2236


 

I'm not sure if I'm just holding it wrong, or if there is something terribly wrong with my 2236, but I noticed that the 2 mV and 5 mV ranges on both channels read exactly the same. I first noticed it on channel #2 while calibrating my 475A. I would start the 2236 and the 475A on the same range, adjust the signal generator to give me a 4 division signal on the 2236, check and adjust the 475A based on that signal, then switch to the next range up on both scopes and repeat. I don't need the 2 mV range for the 475, whose lowest range is 5 mV, but I accidentally turned the knob the wrong way, and was surprised that the display didn't change.

I have a picture of the scope taking the same signal on both channels, one set to 5 mV and the other set to 2 mV, showing that the two signals read as the same amplitude (and another picture showing the ranges reversed, so we can see that it's the same on both channels) https://groups.io/g/TekScopes/album?id=259300&;p=Created,,,20,2,0,0

The only difference is that the 2 mV range shows slightly more noise on the signal than the 5 mV range, which is what you would expect from the higher sensitivity range. Maybe the scope is just poorly calibrated? (or, maybe "poorly" is the wrong word, maybe "intentionally miscalibrated so that both ranges read the same"? You'd really have to put some effort into making both ranges read the same during the calibration process to get this level of agreement)

-- Jeff Dutky


Dave Peterson
 

Jeff,
If the vertical pre-amp is designed similarly to the 465/475 then the pre-amp only has a couple/few range settings. If one of the ranges is "de-tuned" then this would show up at all attenuator settings, no?
Dave

On Tuesday, January 12, 2021, 08:50:25 AM PST, Jeff Dutky <jeff.dutky@gmail.com> wrote:

I'm not sure if I'm just holding it wrong, or if there is something terribly wrong with my 2236, but I noticed that the 2 mV and 5 mV ranges on both channels read exactly the same. I first noticed it on channel #2 while calibrating my 475A. I would start the 2236 and the 475A on the same range, adjust the signal generator to give me a 4 division signal on the 2236, check and adjust the 475A based on that signal, then switch to the next range up on both scopes and repeat. I don't need the 2 mV range for the 475, whose lowest range is 5 mV, but I accidentally turned the knob the wrong way, and was surprised that the display didn't change.

I have a picture of the scope taking the same signal on both channels, one set to 5 mV and the other set to 2 mV, showing that the two signals read as the same amplitude (and another picture showing the ranges reversed, so we can see that it's the same on both channels) https://groups.io/g/TekScopes/album?id=259300&;p=Created,,,20,2,0,0

The only difference is that the 2 mV range shows slightly more noise on the signal than the 5 mV range, which is what you would expect from the higher sensitivity range. Maybe the scope is just poorly calibrated? (or, maybe "poorly" is the wrong word, maybe "intentionally miscalibrated so that both ranges read the same"? You'd really have to put some effort into making both ranges read the same during the calibration process to get this level of agreement)

-- Jeff Dutky


Michael W. Lynch
 

Jeff,

What Dave is saying makes sense. I think that you might want to look at the #15 Switch Contact and the related 2/5mV circuity of both S10 and S60. There are some interrelated adjustments and components that might be causing the problem.

--
Michael Lynch
Dardanelle, AR


 

Dave, Michael,

Yes, that's what i figured. I've done the calibration on a 2215A, and I expect that the 2236 is almost identical, and the 2215 attenuator and vertical pre-amps looked similar to the 475/475A which I'm in the middle of calibrating now.

I see from the service manual that the 2mV setting on the barrel switch is a unique path through switch 15, and that the 2mV and 5mV paths are otherwise identical. That's got to be the point of failure. It just think it's odd that it would have failed on both channels at the same time, in the same way without the scope showing other symptoms of dirty contacts.

I've verified that both the 2213 and 2215A correctly switch to the 2mV range, so I'll have to have a look at the switch contacts in the 2236.

Thanks for the advice.

-- Jeff Dutky


 

I've also verified both channels of the 2263 using a voltage reference, down to the 50 mV range, and they both seem to be well calibrated. That makes it seems like it must be a pair of dirty contacts in the barrel switches.

-- Jeff Dutky


 

On Tue, Jan 12, 2021 at 09:40 PM, Jeff Dutky wrote:


I've also verified both channels of the 2263 using a voltage reference, down
to the 50 mV range, and they both seem to be well calibrated. That makes it
seems like it must be a pair of dirty contacts in the barrel switches.
Another possibility is defective R26 and R76 setting pots. Dark-grey helitrims fail relatively often, where the wiper comes loose.

Raymond


Michael W. Lynch
 

On Tue, Jan 12, 2021 at 03:22 PM, Raymond Domp Frank wrote:


Dark-grey helitrims fail relatively often, where the wiper comes loose
Yes, I have a TG501 that had a bad one. Those things are a real pain and source of lots of trouble.

--
Michael Lynch
Dardanelle, AR


 

Raymond,

I did have trouble with another of the trim pots on the sweep/timing board (R754). I thought I was going to need to replace it, but as soon as I tried adjusting it it came back to life. I'll try that here too, before start removing boards and ordering replacements.

-- Jeff Dutky


 

On Wed, Jan 13, 2021 at 01:00 AM, Jeff Dutky wrote:


I did have trouble with another of the trim pots on the sweep/timing board
(R754). I thought I was going to need to replace it, but as soon as I tried
adjusting it it came back to life. I'll try that here too, before start
removing boards and ordering replacements.
Hi Jeff,
The trimmer I was referring to is a flat model with a grey plastic wiper with a metal contact fixed underneath. The plastic part has a shaft, extending through a hole in the ceramic body. Underneath that ceramic body, the protruding part of the shaft at manufacture is molten to a flat disk, forming something like a mushroom causing the metal contact to exert pressure on the resistive trace.
I've seen that flat part broken off on more than one occasion so the wiper is no longer pressed against the ceramic resistive body.

Raymond


 

Raymond,

It's hard to tell without removing the cover, but I think that R26 and R76 are compact metal cans with a plastic insert for the wiper. I think I know that kind that you're talking about, but I don't see any of those in this machine. It's all either metal cans with a plastic screw head on top (like a TO-18 or TO-39 package) or Bourns blue box trim pots, as far as I can see.

Also, I opened the scope up and tried twiddling the 2 mV trim pots while feeding in a signal to see if the trace changed at all and got nothing. I'm guessing that I need to take out the attenuator board and clean the switches, which is more work that I really wanted to do on this scope at this time.

-- Jeff Dutky