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Odd 2247A problem, looking for insight


 

I have a nice looking Tek 2247A scope here that has been sitting on the shelf for a while, due to a bad Channel i input. the signal is a horribly attenuated and differentiated replica of the actual input. Everything else is perfect. In a moment of covid-induced boredom, I felt I had to get to the bottom of this. I had a hunch that this was the input resistor being broken or damaged (there is always a flying resistor from the input BNC to the vertical input on 2200 series scopes). It was the only thing I could think of that could give this result.

It takes some effort to de-can the scope, and remove the complex shield over the vertical inputs, but sure enough, when opened, the barely visible input resistor (62 ohm carbon film) was BURNED. I replaced it, and ta-da, everything is working. What the heck?

Here's what I just can't figure out, this scope has no 50 ohm input function, so no low impedance path exists to help burn this resistor due to over-driving, SO HOW DID it get burned and leave the scope undamaged?

I am totally mystified by this, and keen to hear any opinions. I just can't see how this fault happened without vaporizing the vertical input channel.

all the best for the new year,
walter (walter2 -at- sphere.bc.ca)
sphere research corp.


John Gord
 

Walter,
I suppose it is possible that U112 has beefy input protection (diode clamps to the supplies?) that might survive enough current to burn a small 62 ohm resistor. At maximum sensitivity the attenuators would be switched out of the circuit.
--John Gord

On Sun, Jan 3, 2021 at 11:35 AM, walter shawlee wrote:


I have a nice looking Tek 2247A scope here that has been sitting on the shelf
for a while, due to a bad Channel i input. the signal is a horribly attenuated
and differentiated replica of the actual input. Everything else is perfect. In
a moment of covid-induced boredom, I felt I had to get to the bottom of this.
I had a hunch that this was the input resistor being broken or damaged (there
is always a flying resistor from the input BNC to the vertical input on 2200
series scopes). It was the only thing I could think of that could give this
result.

It takes some effort to de-can the scope, and remove the complex shield over
the vertical inputs, but sure enough, when opened, the barely visible input
resistor (62 ohm carbon film) was BURNED. I replaced it, and ta-da, everything
is working. What the heck?

Here's what I just can't figure out, this scope has no 50 ohm input function,
so no low impedance path exists to help burn this resistor due to
over-driving, SO HOW DID it get burned and leave the scope undamaged?

I am totally mystified by this, and keen to hear any opinions. I just can't
see how this fault happened without vaporizing the vertical input channel.

all the best for the new year,
walter (walter2 -at- sphere.bc.ca)
sphere research corp.





Harvey White
 

Let's see, 62 ohm resistor (that feels like a 1/10 watt just to start), input capacitance of some sort (20 pf, but you'd think that wouldn't be much), feed the scope RF.  27 Mhz, 20pf gives about 294 ohms, 144 mhz gives about 55 ohms.  I'll suggest someone was trying to look at the output of a transmitter directly.  I'm surprised more didn't go....

Harvey

On 1/3/2021 2:35 PM, walter shawlee wrote:
I have a nice looking Tek 2247A scope here that has been sitting on the shelf for a while, due to a bad Channel i input. the signal is a horribly attenuated and differentiated replica of the actual input. Everything else is perfect. In a moment of covid-induced boredom, I felt I had to get to the bottom of this. I had a hunch that this was the input resistor being broken or damaged (there is always a flying resistor from the input BNC to the vertical input on 2200 series scopes). It was the only thing I could think of that could give this result.

It takes some effort to de-can the scope, and remove the complex shield over the vertical inputs, but sure enough, when opened, the barely visible input resistor (62 ohm carbon film) was BURNED. I replaced it, and ta-da, everything is working. What the heck?

Here's what I just can't figure out, this scope has no 50 ohm input function, so no low impedance path exists to help burn this resistor due to over-driving, SO HOW DID it get burned and leave the scope undamaged?

I am totally mystified by this, and keen to hear any opinions. I just can't see how this fault happened without vaporizing the vertical input channel.

all the best for the new year,
walter (walter2 -at- sphere.bc.ca)
sphere research corp.









teamlarryohio
 

Any clue as to its provenance? Harvey's xmtr shot was the first thing that crossed my mind. One of those little resistors could go before you finished saying 'Oh sh**!'
-ls-


 

some more data on this odd fault:

1. the resistor is 1/4w, fairly big to burn up.
2. when set to DC coupling, both inputs measure almost exactly 1Mohm with my 34401A.
3. vertical cal is good on the channel that had teh burned resistor.

on a related topic, this 2247A unit has the same terrible frequency cal issue (on screen counter) I have seen in other units. the adjustment is incredibly sensitive, and no matter how you set it, once closed up and run for a while, it drifts low by about 50 Hz. I am going to put a hole in the case to set it, it's just too frustrating the way it is.

all the best,
walter

--
Walter Shawlee 2
Sphere Research Corp. 3394 Sunnyside Rd.
West Kelowna, BC, V1Z 2V4 CANADA
Phone: +1 (250-769-1834 -:- http://www.sphere.bc.ca
+We're all in one boat, no matter how it looks to you. (WS2)
+All you need is love. (John Lennon)
+But, that doesn't mean other things don't come in handy. (WS2)
+Nature is trying very hard to make us succeed, but nature does not depend on us.
We are not the only experiment. (R. Buckminster Fuller)


Harvey White
 

1/2 watt transmitter power at the right frequency gives you 1/4 watt per resistor.  If there were any snubbing diodes in the amplifier, then frequency becomes less important.

Then there's likely no damage (guessing) to the input circuitry, the capacitor is not shorted.

may be another example of "part is also a fuse" given the correct circumstances.  Again, we're looking at finding the name of the cow (if any) while examining a Big Mac.....

Harvey

On 1/3/2021 7:17 PM, walter shawlee wrote:
some more data on this odd fault:

1. the resistor is 1/4w, fairly big to burn up.
2. when set to DC coupling, both inputs measure almost exactly 1Mohm with my 34401A.
3. vertical cal is good on the channel that had teh burned resistor.

on a related topic, this 2247A unit has the same terrible frequency cal issue (on screen counter) I have seen in other units. the adjustment is incredibly sensitive, and no matter how you set it, once closed up and run for a while, it drifts low by about 50 Hz. I am going to put a hole in the case to set it, it's just too frustrating the way it is.

all the best,
walter


tekscopegroup@...
 

On Sun, Jan 3, 2021 at 04:38 PM, Harvey White wrote:
on a related topic, this 2247A unit has the same terrible frequency
cal issue (on screen counter) I have seen in other units. the
adjustment is incredibly sensitive, and no matter how you set it, once
closed up and run for a while, it drifts low by about 50 Hz. I am
going to put a hole in the case to set it, it's just too frustrating
the way it is.

all the best,
walter
Hi Walter, if you make that adjustment hole and it aligns successfully, could you please post the information so as to be able to duplicate it? I still need to make that same adjustment on my 2247A and was also considering making a small access hole to the trimmer, but I am terrible at matching things up and it will probably end up being way off. Thanks -Alex


SCMenasian
 

Walter,

I'm not familiar with this instrument; so, I may be way off base --but -
Rather than drilling a hole in the case, why not the following?

1) Adjust as normal
2) Close the case
3) Measure, as accurately as possible, the amount of drift.
4) Open and readjust - this time high by the measured drift.

It might drift down to the correct value when closed again.

Stephen


Colin Herbert
 

I have just had an idea for aligning holes in cases where access to some internal item might be required (and I wish I'd thought about it some time ago). It occurred to me that lots of simple little devices (such as laser pointers, laser measurers, laser torches, IR thermometers and the like) have a laser beam incorporated. If the item that needs the access-hole and the laser-pointer device are aligned without the casing being present, making as sure as possible that the laser-beam is aligned horizontally and vertically with the item to access, then when the casing is in place the laser-spot will mark on the casing where the hole needs to be made. This necessitates that neither the laser-pointer nor the device that needs external access are allowed to move between initial alignment and replacement of the casing, but that should not be beyond the wit of man. I may not have made myself clear and I haven't personally tried this idea out. I think it might just work. It would certainly be better than the bodges that I have done in the past. Who wants to be the guinea-pig?
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of tekscopegroup@miwww.com
Sent: 04 January 2021 14:21
To: TekScopes@groups.io
Subject: Re: [TekScopes] Odd 2247A problem, looking for insight

On Sun, Jan 3, 2021 at 04:38 PM, Harvey White wrote:
on a related topic, this 2247A unit has the same terrible frequency
cal issue (on screen counter) I have seen in other units. the
adjustment is incredibly sensitive, and no matter how you set it, once
closed up and run for a while, it drifts low by about 50 Hz. I am
going to put a hole in the case to set it, it's just too frustrating
the way it is.

all the best,
walter
Hi Walter, if you make that adjustment hole and it aligns successfully, could you please post the information so as to be able to duplicate it? I still need to make that same adjustment on my 2247A and was also considering making a small access hole to the trimmer, but I am terrible at matching things up and it will probably end up being way off. Thanks -Alex


Roy Thistle
 

On Mon, Jan 4, 2021 at 08:26 AM, Colin Herbert wrote:


Who wants to be the guinea-pig
You can use a laser level (they make dots, and lines)... mounted on a mini-tripod. The disadvantage (compared to measuring from an origin point) are... the scope with the case off can not move (once you position the dot on the pot... you can't really pick up the scope, to put the case on... and then put it back in position again)... and the laser level must not move either. If the laser level is too far away... very small changes in angular position will affect the accuracy of where the dot is... when the case is replaced.
It's doable, but not easy.
Usually, I just make measurements from an origin point... making sure the axis is square, using a level if necessary.
With a 400, and 2200... and some 500 and 7000, after the case, or the panel, is removed, the parts are recessed, and there is a "frame" remaining the supports the panel, or case... you can use "paper" placed/stretched over that...to make a pattern... and if you can avoid parallax, you can mark the spot where the hole needs to be... and transfer that to the case.
Making a hole in a case, to adjust a trim-pot, unless you make a big one...or more than one... until you get it right... it might not be easy... and too good looking, once it's done.


Roy Thistle
 

On Sun, Jan 3, 2021 at 11:35 AM, walter shawlee wrote:


barely visible input resistor (62 ohm carbon film) was BURNED
Hi Walter:
I'm pretty sure that 62 ohm is a damping resistor for the relay switches, to prevent oscillations when they switch.
If someone had the channel one set to ground that leaves that input floating. (The center pin of the BNC is connected through the 62 ohm resistor.... and through a NC relay... and then to a NO relay... so the center pin is not grounded... it's floating.)
My guess is that someone had channel one hooked up through a 1X probe to SMPS... either the input caps... or a transformer... and got a high voltage/high energy pulse they didn't expect.
Either there is a spark gap on the PCB, or it arced through to the relay coil.
A further guess is that it arced through the first relay. (Those potted Panasonic relays probably don't have enough isolation.) So the relay might be damaged.
When the scope is set to ground, all the vulnerable goodies (like the attenuators) are disconnected from the input when channel one is set to ground. But the 62 ohm carbon film resistor is still there. Carbon films are not good at handling high energy pulses (they blow up)... carbon composition is best (the entire body of the resistor can absorb the impulse, instead of just the carbon ribbon), but carbon comps don't have the stability of carbon film, or metal film resistors.