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Hello to all.
Taking some measurements on my TEK 2465 using another oscilloscope, I see something strange.

I read for CHN5B signal -1,2 Volt versus -3,6 Volt. This only with BW LIMIT off.
Also for CHN 5A I read signal 600mV p-p versus -6 Volt as look at waveforms on service manual.

Am I doing something wrong, or I have problem somewhere in my TEK scope?

Waveforms 28 and 46 in pages 238 and 239 in service manual. These appears in diagram No 5 in page 244.

Hi Chuck,
Yes I have a problem about the trigger in the vertical amplitude in 200 mV and below, AC or DC input coupling at any horizontal timing setting. This will disappear only if I set Noise or HF Rejection coupling. On screen it look like a second sweep identical to the main sweep during the trigger in a signal. This happen at any of the two first input channels 1 or 2.

Initially I did not understand what exactly was the problem, because I was using the calibrator of oscilloscope with my x10 probe as a source.
So during the triggering I was seeing the upper and lower faces of the square wave sweep signal to roll like a straight line left or right on the screen.
I have checked the output of my calibrator on another (Protek) oscilloscope, and the trigger working perfect in all horizontal settings.

So I thought to feed the same amplitude signal 400mV p-p in my oscilloscope from my CFG253 external generator, using a sawtooth or sinewave signal at 1KHz.
Τhen the second waveform was clearly visible.
This second waveform will be moved left or right on screen using the trigger level, but never disappear except if I using the Noise or HF Rejection coupling setting.
It looks like it can't work correctly the trigger level, or better off having a poor trigger.

Maybe i will be better to do a video about this as you will better understand what I mean.

As a memo must I inform you that in this oscilloscope all the PSU capacitors have been replaced with alternate Nichicon type, and also have been calibrated before 15 days from QService.

Panos

Hi Chuck,
Yes I have a problem about the trigger in the vertical amplitude in 200 mV and below, AC or DC input coupling at any horizontal timing setting. This will disappear only if I set Noise or HF Rejection coupling. On screen it look like a second sweep identical to the main sweep during the trigger in a signal. This happen at any of the two first input channels 1 or 2.

Initially I did not understand what exactly was the problem, because I was using the calibrator of oscilloscope with my x10 probe as a source.
So during the triggering I was seeing the upper and lower faces of the square wave sweep signal to roll like a straight line left or right on the screen.
I have checked the output of my calibrator on another (Protek) oscilloscope, and the trigger working perfect in all horizontal settings.

So I thought to feed the same amplitude signal 400mV p-p in my oscilloscope from my CFG253 external generator, using a sawtooth or sinewave signal at 1KHz.
Τhen the second waveform was clearly visible.
This second waveform will be moved left or right on screen using the trigger level, but never disappear except if I using the Noise or HF Rejection coupling setting.
It looks like it can't work correctly the trigger level, or better off having a poor trigger.

Maybe i will be better to do a video about this as you will better understand what I mean.

As a memo must I inform you that in this oscilloscope all the PSU capacitors have been replaced with alternate Nichicon type, and also have been calibrated before 15 days from QService.

Panos

Yeah maybe it's better to do so. Nevertheless I thought about making an effort, since after the calibration the oscilloscope is working properly, apart from this detail.
To be fair opposite to QService, the calibration cost did not include any minor repairs to the machine.

In the meantime as I looking the reason for this problem, I made some progress. As I check the waveforms around the U500-U700-U800 according to the service manual, I see some instabilities on the signal during the interference.

Specific from the Inverted A Trigger output of U500 pin 18 to Trigger In of U700 pin 2.
After the same from the ramp output of U700 pin 35 to the A Sweep Input of the U735 pin 9.
Finally the same from the U735 pin3 to U800 A Sweep Input pin18.

That gave me the idea to look before the U500 the the trigger level lines A and B. There in A5 digital board, is a double operational amplifier marked as TL072, which drive the A and B trigger lines to U500.
Measuring the amplifier input and output with a voltmeter, I was surprised by the very low voltage across the full range of the trigger level control knob. The measurement was lower than +/- 500 mVolt across the range and in the trigger status about 200 mVolt in line A, contrary to 2.5 volt and 600 mVolt of line B.

I replaced the TL072 operational amplifier and now things was improved! As for the level of the voltage during trigger moment came at the same level for both A and B lines. About 600 mVolt.
Although it seemed that the problem was completely solved, something strange appeared using a different type probe.
As a remind, the interference was appeared when the volt/div selector was in the range of 20 mVolt and below (200 in my case using a x10 probe).

But enough with the written. I have to make a video.

Probes shouldn't make any difference to the ability
of the scope to trigger.

However, depending on whether the probe is a 1X, or
a 10X probe, different attenuator sections in the scope
will be used. The attenuator relays get thin films of
corrosion on their gold leads, and sometimes won't switch
reliably.

If the scope wasn't working properly, QService should not
have calibrated it until it was... It really isn't possible
to complete calibration with problems like you are describing.

I fix a lot of my customer's scopes that were supposedly
working perfectly, and were ready for calibration, but
had problems found during calibration. This is normal.

Bear in mind that the opamps you were probing are intended
to be sample and hold cells. The opamp is a unity gain
follower, with essentially infinite input impedance. It
is measuring the charge put in the hold capacitor by the mux.
The hold capacitor is only about 0.1uf, and will discharge
almost instantly when loaded by the input impedance of a DMM
or a scope probe.

-Chuck Harris

Panos wrote:

Yeah maybe it's better to do so. Nevertheless I thought about making an effort, since after the calibration the oscilloscope is working properly, apart from this detail.
To be fair opposite to QService, the calibration cost did not include any minor repairs to the machine.

In the meantime as I looking the reason for this problem, I made some progress. As I check the waveforms around the U500-U700-U800 according to the service manual, I see some instabilities on the signal during the interference.

Specific from the Inverted A Trigger output of U500 pin 18 to Trigger In of U700 pin 2.
After the same from the ramp output of U700 pin 35 to the A Sweep Input of the U735 pin 9.
Finally the same from the U735 pin3 to U800 A Sweep Input pin18.

That gave me the idea to look before the U500 the the trigger level lines A and B. There in A5 digital board, is a double operational amplifier marked as TL072, which drive the A and B trigger lines to U500.
Measuring the amplifier input and output with a voltmeter, I was surprised by the very low voltage across the full range of the trigger level control knob. The measurement was lower than +/- 500 mVolt across the range and in the trigger status about 200 mVolt in line A, contrary to 2.5 volt and 600 mVolt of line B.

I replaced the TL072 operational amplifier and now things was improved! As for the level of the voltage during trigger moment came at the same level for both A and B lines. About 600 mVolt.
Although it seemed that the problem was completely solved, something strange appeared using a different type probe.
As a remind, the interference was appeared when the volt/div selector was in the range of 20 mVolt and below (200 in my case using a x10 probe).

But enough with the written. I have to make a video.

On Sun, Dec 22, 2019 at 04:31 PM, Panos wrote:

because they have very low in ohm resistance (about 130 ohm) when I measure
these with forward polarity. Conversely they have a complete cut.

Sorry I must to said forward 80 ohm and conversely 2 kohm.

On Sun, Dec 22, 2019 at 04:31 PM, Panos wrote:

because they have very low in ohm resistance (about 130 ohm) when I measure
these with forward polarity. Conversely they have a complete cut.

Sorry I must to said forward 80 ohm and conversely 2 kohm.

because they have very low in ohm resistance (about 130 ohm) when I measure
these with forward polarity. Conversely they have a complete cut.