Tektronix 2465B oscilloscope teardown

[MarkL]

...
Thanks Mark.  I tried CH1 trigger and LINE trigger and the waveform is exactly the same.  Also there are tiny spikes in the waveform, do I measure those or just the center band of the waveform?

Attached are some example waveforms and my results below:
J119 Pin    Vdc   p-p ripple (with spikes, without spikes)
1        -14.95   8mw, 4mv
2        4.98   35mv, 20mv
4        9.96   8mv, 4mv
5       -4.96   10mv, 7mv
6       14.92   8mw, 5mv
8        86         10mv, 8mv
9       41.7   8mv, 6mv
11       -7.99   8mv, 4mv
12        4.96   15mv, 5mv

On your photo of pin 2, it looks like about 30mV p-p total ripple and maybe 10mV 2x line ripple.  On pin 4, about 7mV total and I can't see any 2x ripple.

The total p-p ripple would have components from the AC line and also the switching power supply which runs at approx. 20kHz.  The tiny spikes in your pin 4 photo don't correspond to that, or 2x the line frequency.  But even if you count them, you are still well under the total p-p ripple.

It's a somewhat subjective series of tests, as you are trying to visually average the trace on the screen.  A digital scope could do it for you now-a-days, but I don't think that's what the authors had in mind.

Another approach is to use a DMM set on AC volts.  This will measure the total p-p ripple up to the frequency limit of the DMM.  As long as that's 20kHz or higher, it will catch any poorly performing filter caps that are need of replacement.

The tiny spikes might be common mode noise.  You can check by probing pin 7 or 14 on J119.  This is GND and if you still see the same noise, it's going to be visible in all your measurements.

[MarkL]

I said:

...
Another approach is to use a DMM set on AC volts.  This will measure the total p-p ripple up to the frequency limit of the DMM.  As long as that's 20kHz or higher, it will catch any poorly performing filter caps that are need of replacement.

A clarification on this... The DMM on AC will remove the DC and show you a measurement of the remaining AC signal which would be the total ripple and noise, but it will be displaying the RMS of that AC signal (assuming a true RMS DMM), and not the p-p value that you would get off the scope screen.  The p-p value would be 2.8 * DMM reading for an ideal sine, which admittedly this is not, but it would at least provide a ballpark estimate to compare with the table and tell you if something was radically wrong filter-wise.

[tonedeak99]

...
Thanks Mark.  I tried CH1 trigger and LINE trigger and the waveform is exactly the same.  Also there are tiny spikes in the waveform, do I measure those or just the center band of the waveform?

Attached are some example waveforms and my results below:
J119 Pin    Vdc   p-p ripple (with spikes, without spikes)
1        -14.95   8mw, 4mv
2        4.98   35mv, 20mv
4        9.96   8mv, 4mv
5       -4.96   10mv, 7mv
6       14.92   8mw, 5mv
8        86         10mv, 8mv
9       41.7   8mv, 6mv
11       -7.99   8mv, 4mv
12        4.96   15mv, 5mv

On your photo of pin 2, it looks like about 30mV p-p total ripple and maybe 10mV 2x line ripple.  On pin 4, about 7mV total and I can't see any 2x ripple.

The total p-p ripple would have components from the AC line and also the switching power supply which runs at approx. 20kHz.  The tiny spikes in your pin 4 photo don't correspond to that, or 2x the line frequency.  But even if you count them, you are still well under the total p-p ripple.

It's a somewhat subjective series of tests, as you are trying to visually average the trace on the screen.  A digital scope could do it for you now-a-days, but I don't think that's what the authors had in mind.

Another approach is to use a DMM set on AC volts.  This will measure the total p-p ripple up to the frequency limit of the DMM.  As long as that's 20kHz or higher, it will catch any poorly performing filter caps that are need of replacement.

The tiny spikes might be common mode noise.  You can check by probing pin 7 or 14 on J119.  This is GND and if you still see the same noise, it's going to be visible in all your measurements.

Thanks Mark. Unfortunately I still don’t understand how to make the proper measurements but on the bright side if I overestimated and still within spec of the p-p then Im a happy camper.  I guess Ill pass on the 2x line test.

[factory]

Thank you all so much.  I have my order ready.  It's definitely more than I was hoping to spend.  I checked the hours with EXER 05 and the scope has 5000.  Is there a particular point when a full recap is truly necessary?  Or can I get away with just changing the SRAM battery and the RIFA caps?  Is checking the ripple important to make this decision?  I also took a video of EXER 02 as recommended by many here.  Also any help on the proper/best points to attach a temp battery to the SRAM will be greatly appreciated.

The problem with the electrolytic caps in these is their age, seals can still perish if they are unused parts that are have been stored for 25+ years, this affects many well known brands of capacitor, dating from the 80s & 90s.

Someone I know on another forum bought a supposedly recapped scope from an ebay crook, then it failed some time after, because only the RIFAs had been changed. 
The PSU failure damaged many parts on several boards, including some expensive hybrids and corrupted the contents of the EAROM, it was repaired but it cost quite a bit for the parts and a lot of time to fix.
Of course your scope might last many years before this happens, or it could fail quickly. It may cause no damage, or could write it off.

David

[tonedeak99]

Thanks factory.  Decisions...decisions....

I am very confident I can replace the caps and do a good job, but you hear so many horror stories of working scopes that then die after recapping.  Just trying to weigh the pros and cons.  Which brings me to a story.  After doing the ripple and Vdc checks I put the scope case back on and left it at that.  The next day I turn on the scope and I get a dreaded error message: CT TEST 84 FAIL 0C.  I didn't even touch or disconnect any components.  All I did was remove the case, probe J119, and put the case back on.  Even with the case off I had a fan blowing on the A1 board the whole time.  Luckily I powered it down and back up and the error went away, but it does concern me that something as trivial as removing the case could trigger an error.

[AMR Labs]

....I get a dreaded error message: CT TEST 84 FAIL 0C.  I didn't even touch or disconnect any components.  All I did was remove the case, probe J119, and put the case back on.

The "CT" part in the error message indicates this is coming from the Counter-Timer option of the scope. You can look it up in the 2465 options service manual where the CT option is also included. In the past I've heard about others resolving this error by replacing the CAL jumper and doing the CT calibration only, and this did resolve the problem. CT cal procedure is also explained in detail in the same options manual, and if I remember correctly all that is needed is a known accurate 1MHz or 10MHz signal to be fed into CH1 input and follow the procedure. Its been a while since I fitted the CTT board into my 2465B. The error you got might be due to a marginal CT calibration that may be drifting in and out of spec just on the limit, hence you only got the error once, but doing the cal should get you back on spec and get rid of that error for good.

Can't remember now if your scope is a 2465B, in which case this is the options manual that you want:
https://w140.com/tekwiki/images/1/17/070-6864-02.pdf

If its a 2465A version use this options manual:
https://w140.com/tekwiki/images/3/36/070-5857-00.pdf

Otherwise for the plain 2465 this is the options service manual:
https://w140.com/tekwiki/images/8/88/070-4632-00.pdf

[tonedeak99]

Hope this isn’t too much of a newbie request but Im having a hard time finding any good instructions online with videos or pictures on how to check p-p ripple and 2x line frequency ripple using an analog scope. I think the p-p ripple is just the measurement of the max top and min bottom of the waveform across the screen. See my zoomed in picture of pin 2 with annotations. But I can’t seem to figure out the 2x line frequency number which is a very small number for most pins in table 5-1. I also did check the ground pins 7 and 14 for noise and here is the result.  Seems like minimal noise of less than 1mv.

[tonedeak99]

Took another shot of pin 4 but increased the timebase so the waveform wouldnt flicker.

[David Hess]

Use line triggering to make the ripple measurement.

[MarkL]

Hope this isn’t too much of a newbie request but Im having a hard time finding any good instructions online with videos or pictures on how to check p-p ripple and 2x line frequency ripple using an analog scope. I think the p-p ripple is just the measurement of the max top and min bottom of the waveform across the screen. See my zoomed in picture of pin 2 with annotations. But I can’t seem to figure out the 2x line frequency number which is a very small number for most pins in table 5-1. I also did check the ground pins 7 and 14 for noise and here is the result.  Seems like minimal noise of less than 1mv.

I think it's a good question.

It's difficult to make good 2x line measurements with an analog scope and no filter to eliminate or reduce frequencies higher than 120 Hz.  I can't argue with the numbers in the table, but I'm actually not in agreement with Tek's measurement technique for this.  I think the best that can be done when following their instructions is to look for 120 Hz variations in the overall envelope of the mess on the screen.

Below is a screen shot of J119.2 on a 2465 using a DSO and a lot of averaging to filter out the other noise.  This is what you're trying to measure.  If the 2x ripple is low, like this 2465 (approx 0.625mV), it's going to be impossible to see on an analog scope when mixed in with the other noise.  So, I'd say if the 2x ripple doesn't make itself obvious, it's probably ok.

I've also included some shots of the same pin on an analog scope (actually the same 2465 measuring itself).  It's a subjective call where to place the p-p cursors and I'm not including the extreme outliers.  For the 2x line ripple, it may help to set up the cursors as shown for 120 Hz, so you know what periodicity you're looking for in the envelope.

Everything is line triggered.  Sorry for the shaky photos; I didn't get out the tripod.

[tonedeak99]

Use line triggering to make the ripple measurement.

Thanks and yes all my tests were done with the LINE trigger per the service manual.

Hope this isn’t too much of a newbie request but Im having a hard time finding any good instructions online with videos or pictures on how to check p-p ripple and 2x line frequency ripple using an analog scope. I think the p-p ripple is just the measurement of the max top and min bottom of the waveform across the screen. See my zoomed in picture of pin 2 with annotations. But I can’t seem to figure out the 2x line frequency number which is a very small number for most pins in table 5-1. I also did check the ground pins 7 and 14 for noise and here is the result.  Seems like minimal noise of less than 1mv.

...I've also included some shots of the same pin on an analog scope (actually the same 2465 measuring itself).  It's a subjective call where to place the p-p cursors and I'm not including the extreme outliers.  For the 2x line ripple, it may help to set up the cursors as shown for 120 Hz, so you know what periodicity you're looking for in the envelope.

Wow thank you so much for the explanation.  I'm almost there.  Would my attached picture be a proper yet subjective interpretation of your 2x frequency ripple?  I added two black lines comparing the two peaks at 120hz apart.  They are about <1mv apart.

[MarkL]

...
Wow thank you so much for the explanation.  I'm almost there.  Would my attached picture be a proper yet subjective interpretation of your 2x frequency ripple?  I added two black lines comparing the two peaks at 120hz apart.  They are about <1mv apart.

You're welcome!  Sorry I didn't get a chance to post back sooner.

I think that's a good call (and verified by the DSO).  The 120 Hz period is definitely visible in that photo.  And in this instance for pin 2, you're looking for 2x line ripple >30mV, so it would be quite visible.

[tonedeak99]

You're welcome!  Sorry I didn't get a chance to post back sooner.

I got it!!! You are a great teacher Mark!

[MarkL]

I've been thinking more about the measurement specification "p-p Ripple at Two Times Line Frequency".  I've been interpreting that very literally in that it's asking for the maximum difference in peaks of the waveform spaced 120Hz apart.  And that's the example I posted.

While some problems may manifest themselves that way, such as a weak half in the AC rectification stage, they may be asking for the p-p ripple *OF* two times the line frequency.  I would interpret that more broadly as any AC line related ripple, and would encompass failures in the output filter capacitors.

In other words, the measurements table for each rail is asking for the peak low frequency ripple (AC line related) and also the total ripple, which includes both the low frequency ripple and the high frequency ripple from the switcher stage.

In this case, I want to amend what I posted about the "2x line" measurement points.  Using the 2465 from before on J119.2, below is the averaged waveform on the DSO, and how that appears on an analog scope.  In the case of the analog display, instead of picking out two peaks, you're trying to pick out a consistent peak and a trough on one side (upper or lower) of the waveform envelope, and as before with the scope line triggered.

Again the measurement on the analog scope is very subjective, but in the photo it's in the right ballpark and maybe the upper cursor should be a little higher.  If the ripple was near the limit, in this case 30mV, you would clearly see the deviation in the envelope.  With some of the other rails on J119, it's just going to be impossible to see their limits of a mV or two.

It would have been nice if Tek provided more than a half a sentence or an example of what they intended.

[tonedeak99]

...Again the measurement on the analog scope is very subjective, but in the photo it's in the right ballpark and maybe the upper cursor should be a little higher.  If the ripple was near the limit, in this case 30mV, you would clearly see the deviation in the envelope.  With some of the other rails on J119, it's just going to be impossible to see their limits of a mV or two.

It would have been nice if Tek provided more than a half a sentence or an example of what they intended.

I too have been thinking about this test and what the Tek engineers really wanted.  I want to propose another interpretation.  Could they have wanted us to measure the p-p ripple at 2 points 120hz apart and subtract the two voltages?  See the pic attached.  Point A has a p-p of 24mv.  Point B (120hz later) has a p-p of 23mv.  So the p-p ripple at 2x the line frequency would be 24-23=1mv. 

[tonedeak99]

Here is the parts list for a 2465A A2A1 and A3 boards.  This list was originally compiled by Condor Audio over on the Tek group, but I updated the OOS parts and some capacitors for ones that were closer to the original specs.  Mouser part # was the one provided by Condor Audio.  An "x" under available means I replaced that part with another.  The BOM column is the final Mouser part#:
Resistors               Yachad's recommendation      
Original ---------------------------------------                     
Part#   Value   Watts         Mouser Part#   available   BOM
R1010   15 Ω              0.5         660-PCF2C150K              660-PCF2C150K
R1019   15 Ω       0.5         660-PCF2C150K              660-PCF2C150K
R1020   270K Ω      0.5         594-5093NW270K0J      594-5093NW270K0J
Capacitors                     
Part#   Value   Voltage   Type            
C1016   .068 µF   250   PLSTC      594-222233810683       594-222233810683
C1018   .068 µF   250   PLSTC      594-222233810683       594-222233810683
C1021   290 µF   200   ELCTLT      647-UPW2E331MRD   x   232-200TXW330MEFR18X
C1022   290 µF   200   ELCTLT      647-UPW2E331MRD   x   232-200TXW330MEFR18X
C1025   100µF   25   ELCTLT      647-UHE1E101MED      647-UHE1E101MED
                     
C1034   4.7 µF   10   TANT            505-MKS2B044701KJC00      505-MKS2B044701KJC00
C1020   .0022 µF   250   PPR           594-2222-336-60222      594-2222-336-60222
C1051   .0022 µF   250   PPR            594-2222-336-60222      594-2222-336-60222
C1052   .01 µF   250   PAPER      594-2222-336-60103      594-2222-336-60103
C1066   4.7 µF   35   ELCTLT      647-UPW1H4R7MDD   x   647-UPW1H4R7MDD1TD
C1065   .056 µF   250   MTLZD      594-2222-372-41563      
C1072   3.3 µF   350   ELCTLT      647-UPW2V3R3MPD      647-UPW2V3R3MPD
C1112   4.7 µF   35   ELCTLT      647-UPW1H4R7MDD   x   647-UPW1H4R7MDD1TD
                     
C1101   100µF   25   ELCTLT      647-UHE1E101MED      647-UHE1E101MED
C1102   100µF   25   ELCTLT      647-UHE1E101MED      647-UHE1E101MED
C1110   250 µF   20   ELCTLT      647-UHE1H331MPD      647-UHE1V271MPD
C1111   250 µF   20   ELCTLT      647-UHE1H331MPD      647-UHE1V271MPD
C1113   180 µF   40   ELCTLT      647-UHE1H331MPD      647-UHE1H181MPD
C1114   250 µF   20   ELCTLT      647-UHE1H331MPD      647-UHE1V271MPD
C1115   250 µF   20   ELCTLT      647-UHE1H331MPD      647-UHE1V271MPD
C1116   180 µF   40   ELCTLT      647-UHE1H331MPD      647-UHE1H181MPD
                     
C1120   10 µF   100   ELCTLT      647-UPW2A100MED   x   647-UPW2C100MPD
C1130   10 µF   100   ELCTLT      647-UPW2A100MED   x   647-UPW2C100MPD
C1132   10 µF   160   ELCTLT      647-UPW2C100MPD      647-UPW2C100MPD
C1220   10 µF   100   ELCTLT      647-UPW2A100MED   x   647-UPW2C100MPD
C1240   10 µF   100   ELCTLT      647-UPW2A100MED   x   647-UPW2C100MPD
                     
C1260   100 µF   25   ELCTLT      647-UHE1E101MED      647-UHE1E101MED
C1280   100 µF   25   ELCTLT      647-UHE1E101MED      647-UHE1E101MED
C1300   100 µF   25   ELCTLT      647-UHE1E101MED      647-UHE1E101MED
C1330   100 µF   25   ELCTLT      647-UHE1E101MED      647-UHE1E101MED
C1350   100 µF   25   ELCTLT      647-UHE1E101MED      647-UHE1E101MED
                     
C1400   47 µF   25   ELCTLT      647-UHE1E470MDD   x   647-UPV1H470MGD
C1402   47 µF   25   ELCTLT      647-UHE1E470MDD   x   647-UPV1H470MGD
C1274   1 µF   50   nonplzd      505-MKS2C041001FJC00   x   505-MKS2D041001KKI00
C1291   1 µF   50   nonplzd      505-MKS2C041001FJC00   x   505-MKS2D041001KKI00
C1292   1 µF   50   nonplzd      505-MKS2C041001FJC00   x   505-MKS2D041001KKI00
                     
RT1010   7.0 ohm   NTC Thermistor         995-SG210   x   
RT1016   5.0 ohm   NTC Thermistor         995-SG200      995-SG200

Also I'm still debating if I should replace the 100uf capacitors with 150uf as done by @FireDragon earlier in this thread to improve PS ripple and noise.

[MarkL]

...Again the measurement on the analog scope is very subjective, but in the photo it's in the right ballpark and maybe the upper cursor should be a little higher.  If the ripple was near the limit, in this case 30mV, you would clearly see the deviation in the envelope.  With some of the other rails on J119, it's just going to be impossible to see their limits of a mV or two.

It would have been nice if Tek provided more than a half a sentence or an example of what they intended.

I too have been thinking about this test and what the Tek engineers really wanted.  I want to propose another interpretation.  Could they have wanted us to measure the p-p ripple at 2 points 120hz apart and subtract the two voltages?  See the pic attached.  Point A has a p-p of 24mv.  Point B (120hz later) has a p-p of 23mv.  So the p-p ripple at 2x the line frequency would be 24-23=1mv.

I think the end result of that measurement would be the variation in the high frequency amplitude.

By subtracting the top point from the bottom point, you're left with just the amplitude of the high frequency component at that point in time.  And when you do  it at two different times, you get two high frequency amplitudes that you're then subtracting.  The low frequency information has been subtracted out, so I don't see how it relates back to the table.

[tonedeak99]

Thanks Mark.

Ive come across 3 new issues with my scope that I wanted to share with everyone:

1.  The voltage readings appear a bit off from my other scope and a tektronix signal generator I have. If I set the generator for a sine or square wave with 20v amplitude (the max for the generator) my other scope reads them as 20v but the 2465A reads 21.5V. Unfortunately the 2465A doesn’t have the cool measurement function that the 2465B has to double check the voltage reading.  Btw any way of adding that feature to a 2465A?

2. The trace comes and goes as the unit warms up. Sometimes it disappears completely. This seems to fix itself after it warms up. Still testing to make sure. Adjusting the readout knob doesn’t help.

3. My focus knob is set to 3 oclock in order to be in focus. I would think a properly functioning scope would be closer to 12 oclock.

[MarkL]

...
Ive come across 3 new issues with my scope that I wanted to share with everyone:

1.  The voltage readings appear a bit off from my other scope and a tektronix signal generator I have. If I set the generator for a sine or square wave with 20v amplitude (the max for the generator) my other scope reads them as 20v but the 2465A reads 21.5V. Unfortunately the 2465A doesn’t have the cool measurement function that the 2465B has to double check the voltage reading.  Btw any way of adding that feature to a 2465A?

Have you checked the other vertical ranges for accuracy?  Are they ok or off by the same percentage?  Do the cursor readings match the graticule?

I have not heard of any way to upgrade an A to a B.  The A1 board in the B has some additional hardware dedicated to the parametric measurements.

2. The trace comes and goes as the unit warms up. Sometimes it disappears completely. This seems to fix itself after it warms up. Still testing to make sure. Adjusting the readout knob doesn’t help.

Is it fading out and then back in, or does it suddenly disappear and reappear?

The readout knob?  Does the readout fade, or did you mean the trace intensity knob?  Or do you mean everything disappears?

Maybe it's heat related?  Perhaps a session with a can of freeze spray (or inverted canned air) might provide some clues.

3. My focus knob is set to 3 oclock in order to be in focus. I would think a properly functioning scope would be closer to 12 oclock.

I have a 2465 and a 2445A and neither of my focus knobs sit at 12:00.  They're both around 2:00.

There are additional adjustments you can try, and all in combination with each other because they interact.  The front panel ASTIG for one, and on the HV board there is HIGH DRIVE FOCUS, and depending on your serial number, there might be an EDGE FOCUS.  This is covered in the adjustment procedure.

It takes some patience to iterate for the best focus.  I've found it difficult to get everything on the screen in perfect focus and it usually ends up being a compromise.

I'd recommend marking the current pot positions with a fine Sharpie before tweaking.

[tggzzz]

I have not heard of any way to upgrade an A to a B.  The A1 board in the B has some additional hardware dedicated to the parametric measurements.

There's a fleabay seller that allegedly converts 2445b into 2465b. Allegedly comes complete with "do not tamper" stickers, so that if you spot his perfidy he can refuse to refund money.

[MarkL]

I have not heard of any way to upgrade an A to a B.  The A1 board in the B has some additional hardware dedicated to the parametric measurements.

There's a fleabay seller that allegedly converts 2445b into 2465b. Allegedly comes complete with "do not tamper" stickers, so that if you spot his perfidy he can refuse to refund money.

Yeah, he's still selling on ebay, and even more is that the "conversion" is not 100%, as unhappy buyers found out way after the purchase.  Have a read:

  https://groups.io/g/TekScopes/topic/7658884#140202

[tonedeak99]

Have you checked the other vertical ranges for accuracy?  Are they ok or off by the same percentage?  Do the cursor readings match the graticule?

The generator is a Tek CFG250.  It was a switch for either (i) up to 2V or (i) up to 20v amplitudes, so I tried both and maxed the anplitude setting in hopes it would read exactly 2V and 20V.  Both voltages have the same problem by the same percentage and even across different vertical ranges of the scope.  If it wasn't for my other scope reading the generator as accurate at 2v and 20v, I would probably assumed it was the generator that was off.  The cursor readings match the graticle.  To make matters more confusing the calibration point on the scope does read accurately at .4v.

Is it fading out and then back in, or does it suddenly disappear and reappear?

The readout knob?  Does the readout fade, or did you mean the trace intensity knob?  Or do you mean everything disappears?

Maybe it's heat related?  Perhaps a session with a can of freeze spray (or inverted canned air) might provide some clues.

The trace dims in and out many times in no discernable pattern.  Sometimes the dimming is light, sometimes it is accompanied by a slow flicker while dimming and then other times it continues until it's disappeared.  Mostly happens during warming up but I have also noticed it after the 20min warm up cycle.

I meant that when adjusting the readout intensity knob it didn't affect it.  The trace intensity knob does correct it temporarily, but it then corrects itself.  Meaning if the trace starts dimming and/or disappears
 if I immediately set the trace intensity knob to compensate for the dimming it will brighten up, but then in a few seconds it corrects itself and become super bright.  Maybe the pot for this knob needs to be cleaned?  With regards to your recommendation what components do you recommend I freeze?

I have a 2465 and a 2445A and neither of my focus knobs sit at 12:00.  They're both around 2:00.

There are additional adjustments you can try, and all in combination with each other because they interact.  The front panel ASTIG for one, and on the HV board there is HIGH DRIVE FOCUS, and depending on your serial number, there might be an EDGE FOCUS.  This is covered in the adjustment procedure.

It takes some patience to iterate for the best focus.  I've found it difficult to get everything on the screen in perfect focus and it usually ends up being a compromise.

I'd recommend marking the current pot positions with a fine Sharpie before tweaking.

If your scopes are also in this position then I will just let it be.  Thanks.

[alan.bain]

Have you checked the voltage on the screen electrode around the distributed Y-plates? If this drifts (it is regulated by a simple zener circuit) then the focus position also drifts and ultimately getting a very good sharp focus becomes impossible.  As I recall it should be around the output level from Y amps with no signal, but think it is marked in the SM around 30V I think (no SM here!)

Alan

[AMR Labs]

I meant that when adjusting the readout intensity knob it didn't affect it.  The trace intensity knob does correct it temporarily, but it then corrects itself.  Meaning if the trace starts dimming and/or disappears
 if I immediately set the trace intensity knob to compensate for the dimming it will brighten up, but then in a few seconds it corrects itself and become super bright.  Maybe the pot for this knob needs to be cleaned?

Be aware that all rotary controls on this scope are "fly by wire" meaning there is only a plain DC voltage on them that gets "Muxed" over to an ADC, which I believe is what then actually talks to the CPU, which in turn then carries out the desired action. But the CPU itself is grossly under powered for all the stuff it has to constantly handle.  This sometimes, depending on the CPU load, makes some controls feel like they are slow or sluggish to react, or don't react at all until one reaches past the intended point in their range and then suddenly things jump to a new value, and which to some extent might be what you are seeing. Lubing of the pots will never hurt, and would after all this time even be recommendable, however I understand it is not so trivial to get to the back of some of those controls. I've been lucky in that sense and so far never had to actually try to get to them, so this might not apply to all of them. Even pulling some of the knobs might require carefully heating them up a bit with a hot air gun so allow them to come off without breaking. Not even sure if all the pots actually have openings that would admit some fluid into them, but others here will know for sure. In any event use only Deoxit Fader Lube (green fluid), and not the cherry red one which is meant for mechanical switch contacts. If nothing else I'd try some 70-90% alcohol, but Deoxit is king.

[MarkL]

Have you checked the other vertical ranges for accuracy?  Are they ok or off by the same percentage?  Do the cursor readings match the graticule?

The generator is a Tek CFG250.  It was a switch for either (i) up to 2V or (i) up to 20v amplitudes, so I tried both and maxed the anplitude setting in hopes it would read exactly 2V and 20V.  Both voltages have the same problem by the same percentage and even across different vertical ranges of the scope.  If it wasn't for my other scope reading the generator as accurate at 2v and 20v, I would probably assumed it was the generator that was off.  The cursor readings match the graticle.  To make matters more confusing the calibration point on the scope does read accurately at .4v.

I would make sure the CFG250 was accurate before diving in.

I would start with the scope and set up a stable DC power supply for 2V, as measured by a DMM.  Then probe it with the scope directly using either no probe or a 1x probe and see if you get the expected voltage for both polarities.  Then repeat for 20V.

As for the CFG250, there's several ways to check it.  Probably the easiest is to set up a sine wave output of 2V or 20V, say at 100Hz, and then measure it with your DMM.  If you're saying it's supposed to be outputting +/-2V (4V pk-pk), then your meter should be reading the RMS of that which would be 2V / sqrt(2) = 1.41V.  You can repeat that for 20V.

This looks like a 100% analog function generator, so I wouldn't be surprised if it was off.


I will look into some possibilities for the dimming.  As AMR points out, the CPU is in the middle of the intensity control (and most others), so the issue could be in a number places.

EDIT: Sorry, you said 2V and 20V *amplitude*.  So the RMS calculation is the same, but use 1V and 10V for the peak, so you should get 0.707 volts and 7.07V on the DMM.