Topics

My 2465B has arrived + A5 Board leaky caps cleanup

tekscopegroup@...
 

On Tue, Nov 6, 2018 at 08:03 AM, Chuck Harris wrote:
Hey! Don't muck with the DAC adjustment, unless you are prepared
to do a full calibration!

It is not supposed to be +1.25 and -1.25 volts, but rather a 2.500V span.

Sure, if you are a perfectionist, and diddle with the 10K reference
resistor values, and put in perfect opamps, it can achieve those values,
but that is not a necessary goal: 2.500V span when doing the calibration
routine is the necessary goal. -1.25 and +1.36V references that are *stable*
is the goal, as any errors in them get stored in the calibration constants,
and as such calibrated out, when the scope is calibrated.

Any tweaking, or replacement of DAC parts that you do will require a
full calibration of your scope. Why? Because you have no idea what they
were like when it was last calibrated.

If they were on the low (or high) side of spec when last calibrated, and
you adjust them to be right on, with a DVM that was on the high (low) side
of spec), they errors could add up to be wrong enough to perform poorly.

-Chuck Harris
OK understood. I have not touched the adjustment at R2010. Only replaced both 10K 0.1% resistors (the original ones where both open). Also replaced R2016 1K 1% because I pulled it out by mistake and one of the contacts looked a bit iffy, so that one was also needing replacement anyway. The original value of this last resistor checked on my HP3478A was 1.00248K and the substitute measured 1.00302K, so only a 0.0539% difference between them. The voltages from the +1.36 and -1.25 references did not vary very much from what the where while still using the temporary 5% resistors. I initially had +1.3848 and now it is 1.3836v. And -1.2688 went to -1.2681v. Not much of a difference.

I was looking at the procedure to check the 2.5V DAC voltage span in the service manual (page 5-3) and plan on verifying it at pin 13 of J119. But I will not be making any adjustments to it, since as explained above this might actually be detrimental and worsen the calibration state of the scope. So for now I will only record the voltage range, and they will stay as is.

I have received my new GQ-4X4 programmer, and the DS1225 from Mouser, but before I pull out my original NVRAM still need the machined 28-DIP socket which I had to reorder as I messed up the PN, and also ordered a FW16W08 FRAM to play with and an Aries SOIC to DIP converter board.

After getting the cal data rewritten on a fresh D1225 I am assuming I will have to tackle the power supply caps next. Sometimes see a very small almost unnoticeable horizontal jitter on a displayed waveform, so this is probably ripple in one or more power supplies. I last time the scope was open also took a glance into the power supply board and noticed I do have the infamous RIFA exploding caps and at least the one that sits closer to the angle I was able to peer onto the board shows the typical small cracks. I'm on 120V so I understand the risk of them exploding is less than on 220v mains, but I will replace them asap once I have all the other PS caps on hand as well. Already have the replacements for those, but since pulling out the PS boards as a pair its not exactly easy, will probably wait until I have all the required elco caps on hand as well and get it done once.

BTW can someone please explain how does one take the power supply ripple reading for 120Hz only, as opposed to the total ripple reading? By using the HF reject triggering option?

Thanks.

tekscopegroup@...
 

Added 4 pictures to my photo album showing rise time measurements of the 2465B (o.57nS) and also my 2247A (~2.0nS). BTW the 0.57nS rise time was confirmed with the corresponding parametric measurement, the result was exactly 0.57nS as well. Also included a close up of the 2N2369A avalanche pulse generator I build (last picture), and that requires around 90V to work, but I just use my Heathkit IG-4505 with DC output set to the 100V scale, and it works just fine. Much easier that having to build that cumbersome DC-DC converter used in the original pulser article.

https://groups.io/g/TekScopes/photo/76874/1?p=Name,,,20,1,0,0

BTW is it possible to reorder the pictures? I could not find a way to do it through the web interface.

tekscopegroup@...
 

Link above is only for one picture. Full album is really here:

https://groups.io/g/TekScopes/album?id=76874&p=Name,,,20,1,0,0

Harvey White
 

On Fri, 23 Nov 2018 04:26:56 -0800, you wrote:


For measuring AC ripple at 120 Hz, I'd look at the ripple, but
triggered on line. Measure anything that stands still....

Ought to get you close.

Harvey



On Tue, Nov 6, 2018 at 08:03 AM, Chuck Harris wrote:
Hey! Don't muck with the DAC adjustment, unless you are prepared
to do a full calibration!

It is not supposed to be +1.25 and -1.25 volts, but rather a 2.500V span.

Sure, if you are a perfectionist, and diddle with the 10K reference
resistor values, and put in perfect opamps, it can achieve those values,
but that is not a necessary goal: 2.500V span when doing the calibration
routine is the necessary goal. -1.25 and +1.36V references that are *stable*
is the goal, as any errors in them get stored in the calibration constants,
and as such calibrated out, when the scope is calibrated.

Any tweaking, or replacement of DAC parts that you do will require a
full calibration of your scope. Why? Because you have no idea what they
were like when it was last calibrated.

If they were on the low (or high) side of spec when last calibrated, and
you adjust them to be right on, with a DVM that was on the high (low) side
of spec), they errors could add up to be wrong enough to perform poorly.

-Chuck Harris
OK understood. I have not touched the adjustment at R2010. Only replaced both 10K 0.1% resistors (the original ones where both open). Also replaced R2016 1K 1% because I pulled it out by mistake and one of the contacts looked a bit iffy, so that one was also needing replacement anyway. The original value of this last resistor checked on my HP3478A was 1.00248K and the substitute measured 1.00302K, so only a 0.0539% difference between them. The voltages from the +1.36 and -1.25 references did not vary very much from what the where while still using the temporary 5% resistors. I initially had +1.3848 and now it is 1.3836v. And -1.2688 went to -1.2681v. Not much of a difference.

I was looking at the procedure to check the 2.5V DAC voltage span in the service manual (page 5-3) and plan on verifying it at pin 13 of J119. But I will not be making any adjustments to it, since as explained above this might actually be detrimental and worsen the calibration state of the scope. So for now I will only record the voltage range, and they will stay as is.

I have received my new GQ-4X4 programmer, and the DS1225 from Mouser, but before I pull out my original NVRAM still need the machined 28-DIP socket which I had to reorder as I messed up the PN, and also ordered a FW16W08 FRAM to play with and an Aries SOIC to DIP converter board.

After getting the cal data rewritten on a fresh D1225 I am assuming I will have to tackle the power supply caps next. Sometimes see a very small almost unnoticeable horizontal jitter on a displayed waveform, so this is probably ripple in one or more power supplies. I last time the scope was open also took a glance into the power supply board and noticed I do have the infamous RIFA exploding caps and at least the one that sits closer to the angle I was able to peer onto the board shows the typical small cracks. I'm on 120V so I understand the risk of them exploding is less than on 220v mains, but I will replace them asap once I have all the other PS caps on hand as well. Already have the replacements for those, but since pulling out the PS boards as a pair its not exactly easy, will probably wait until I have all the required elco caps on hand as well and get it done once.

BTW can someone please explain how does one take the power supply ripple reading for 120Hz only, as opposed to the total ripple reading? By using the HF reject triggering option?

Thanks.


 

On Fri, Nov 23, 2018 at 01:59 PM, <tekscopegroup@...> wrote:


Added 4 pictures to my photo album showing rise time measurements of the 2465B
(o.57nS) and also my 2247A (~2.0nS). BTW the 0.57nS rise time was confirmed
with the corresponding parametric measurement, the result was exactly 0.57nS
as well.
That's not rise time you're seeing or "measuring", it's more like slew rate and what looks like the amplitude of the pulse really isn't.
Rise time is measured between two *stable levels* (usually, between 10% and 90% levels of it), whereas your generator generates a pulse with a duration that is relatively short as compared against the 'scope's rise time. On a faster 'scope you'll see a larger amplitude than on your 2465B, more towards the actual pulse amplitude. The parametric measurement assumes you're providing a step voltage; it cannot see the shortness of your pulse.
A pulse generator is not suitable to measure rise time, unless the pulse stays on its steady levels long enough, i.e. at least 2 ns or so for the 2465B.
A realistic rise time with an infinitely fast step applied would be more like 0.8 ns for the 2465B.
Assuming a theoretical rise time of your pulse generator of 0 ns (!), your 2465B's BW would be over 600 MHz. Even with Tek's usual conservative specs, 600 MHz + is not realistic.

Likewise, 2 ns for your 100 MHz-specified 2247A is not realistic. It would imply a BW of 175 MHz.

Just for fun, compare the amplitude as shown on both 'scopes. Don't forget to put both 'scope's vertical sensitivity settings out of "variable" for that.

What you *can* conclude is that the pulse slews faster than both your 'scopes.

Raymond

Roger Evans
 

If you add 30 - 50cms of 50 ohm coax between the avalanche transistor collector and ground you will get a nice flat top pulse with duration twice the transit time of the coax. It does make the avalanche generator a little clumsy and you can find designs that etch a 50 ohm serpentine transmission line onto the pulse generator PCB.

Roger

 

On Sat, Nov 24, 2018 at 06:15 PM, Roger Evans wrote:


If you add 30 - 50cms of 50 ohm coax between the avalanche transistor
collector and ground you will get a nice flat top pulse
Yes, if you make sure that impedances match. With your 'scope, you'll have no way to check that (overshoot, undershoot, ringing) but what you'll "measure" very likely will be closer to a rise time than without the so-called "charge line" and as roger describes it, it's very easy to do.

Raymond