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2465 Trace focus


david
 

Hey guys, Got a 2465 that appears to work OK so far. I noticed the trace is kind of thick, even with no probes attached. The only way to get a thin, clean trace, is with the 20 MHz BW limit on, Is this normal?
David


n4buq
 

Enabling the 20 MHz BW Limit does tend to make for a more sharp trace in that model series. How much fuzzier the trace is with that control disabled may be an issue but that would need more information (that others can do a much better job of explaining that I).

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "david via groups.io" <davide_us=yahoo.com@groups.io>
To: TekScopes@groups.io
Sent: Friday, October 9, 2020 11:59:43 AM
Subject: [TekScopes] 2465 Trace focus

Hey guys, Got a 2465 that appears to work OK so far. I noticed the trace is
kind of thick, even with no probes attached. The only way to get a thin,
clean trace, is with the 20 MHz BW limit on, Is this normal?
David






Tom Lee
 

It's completely normal, and the cause is fundamental: Thermal noise. I'm betting that you're in the 1M impedance mode during the experiment. You'll see a sharper "focus" in the 50-ohm state, as well as the grounded-input state.

Here's a homework problem for you (can't help it -- it's an occupational hazard): Compute the rms thermal noise of a 1 megohm resistor over a 400MHz brickwall bandwidth (ok; here's the answer: It's roughly 2.5mV). You'll perceive a thickness that is a few times the rms value.

Engaging the 20MHz bandwidth limit shrinks that down by a factor of root-20 (about 4.5).

Shorting the input or engaging the 50 ohm mode will similarly knock down the noise to the point where the trace thickness will be controlled mainly by the electron optics of the jug itself.

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 10/9/2020 11:10, n4buq wrote:
Enabling the 20 MHz BW Limit does tend to make for a more sharp trace in that model series. How much fuzzier the trace is with that control disabled may be an issue but that would need more information (that others can do a much better job of explaining that I).

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "david via groups.io" <davide_us=yahoo.com@groups.io>
To: TekScopes@groups.io
Sent: Friday, October 9, 2020 11:59:43 AM
Subject: [TekScopes] 2465 Trace focus

Hey guys, Got a 2465 that appears to work OK so far. I noticed the trace is
kind of thick, even with no probes attached. The only way to get a thin,
clean trace, is with the 20 MHz BW limit on, Is this normal?
David







david
 

Thanks guys, I thought it was probably normal, but it doesn't hurt to ask. Another problem I'm having is front panel Focus adjustment. Went through the adjustments in the service manual but still cannot focus waveform across entire width of CRT. I can focus in center, or on both edges of CRT but can't get sharp focus across entire width. Wonder if I missed something, Has anyone had this problem?
David


Gregor
 

Hi David!
Have tried to adjust the Edge Focus (R1864 on the HV board) as described in the service manual?
-Gregor

On Tue, Oct 13, 2020 at 09:08 PM, david wrote:


Thanks guys, I thought it was probably normal, but it doesn't hurt to ask.
Another problem I'm having is front panel Focus adjustment. Went through the
adjustments in the service manual but still cannot focus waveform across
entire width of CRT. I can focus in center, or on both edges of CRT but can't
get sharp focus across entire width. Wonder if I missed something, Has anyone
had this problem?
David