Topics

7904A checkout + calibration


Chris Wilkson
 

All of this 7904(A) talk has inspired me to get my 7904A (SNB011071) checked out and calibrated.
But I don't have the equipment to do it. Does anyone here do scope calibrations for hire within say 200 miles of Detroit?


Eric
 

Hey Chris,

    What you need will vary greatly based on the plugins you need calibrated. I have not done a 7904A mine did not need adjustment but I have done 7603's and 7704A's to do one "right / by the book" it is a pretty involved process at a minimum you have 3 calibration documents you have to work through, The frame then vert amp then time base. Also depends on the vert plugins you needs calibrated. 1 plugin or both or more. For the most part the signals needed are pretty modest up until you do the high frequency calibration of the vertical amps. Anything over 200 Mhz the sources can get a little exotic. Tunnel diode pulses. So the limits of my bench at this point are 500Mhz I cant do a 7104. Before I did my own I got one quoted from Tek If I remember correctly they will still do a factory calibration on a 7K frame but I think the 7704A was $1,100 for the calibration. It inspired me to do my own.


Eric

On 6/1/2020 11:14 AM, Chris Wilkson via groups.io wrote:
All of this 7904(A) talk has inspired me to get my 7904A (SNB011071) checked out and calibrated.
But I don't have the equipment to do it. Does anyone here do scope calibrations for hire within say 200 miles of Detroit?


 

On Mon, Jun 1, 2020 at 06:35 PM, Eric wrote:


What you need will vary greatly based on the plugins you need calibrated.
... and on your requirements for the performance and calibration of the mainframe.

If you are going to use your 'scope with the same horiontal and vertical plugins all of the time and at relatively low edge speeds (lower frequency, longer rise/fall times, your requirements will be modest.
If you intend to use the mainframe with higher requirements and/or several different plugins however, some sort of standardization/normalization is required, especially as regards vertical amp behavior.
The best way to achieve a normalized mainframe would be the use of at least one, preferably two 067-0587- 01 or even one -02 Calibration Fixture/Signal Standardizer. Unfortunately, these are difficult to find and expensive. The -02 would only be really needed for the vertical system (edge speed) of a 7104.
Most importantly, these normalizers allow you to check and adjust the step response (high frequency behavior) of the horizontal and vertical system, avoiding the influence of any (non-calibrated) plugin, and the mainframe's horizontal and vertical sensitivity, linearity and distortion.

A traditional and still very convenient calibration set would further include a TG501 Time Mark Generator, which produces calibrated marks (needles) for time base plug-in adjustment, a PG506 for vertical amp amplitude sensitivity, LF behavior and plug-in step response adjustment (up to about 2ns edge speed).
A sine wave generator (like an SG503) would be nice for trigger-offset and -sensitivity adjustment.

Very nice and convenient all but you'll definitely exceed what you pay for the 7904A with this lineup. Any one of the instruments would be useful, especially the PG506.

Official calibration documents state different requirements.

Raymond


Chris Wilkson
 

On Mon, Jun 1, 2020 at 09:35 AM, Eric wrote:


I think the
7704A was $1,100 for the calibration. It inspired me to do my own.
I also got a quote for something like that a few years ago when I got this scope.
And a couple of $200-300 quotes just to EVALUATE the frame...no cal or repair included.
That's why I'm hoping to find someone here to do it for me. I can pay, and gladly.
Even just getting the mainframe checked and standardized would be a great start.

I need one good scope to fix the rest!


Chris Wilkson
 

PS. I'm leery of shipping these things. I've had a few frames destroyed by UPS "keg throwers". That's why I said close to Detroit so I can drive them myself. But I'm open to other options too.


Eric
 

If I remember correctly if your goal for the frame is evaluation, then really all you need a scope for is ripple measurement on the power supplies. If I remember correctly. For the most part everything is done through the calibration fixture. You do need to send some signals to the scope for things like geometry adjustments and the like. The plugins are typically adjusted after the frame is finished. I have a second calibrator the -01 But it needs a cam switch wheel it is missing out of the unit. So at the moment non functional. Also what plugins do you have? I am a ways to the south of you but not cross country. You can shoot me an E-mail off list if you would like more info.

 Before you even do anything Cal related do you know the unit even needs adjustment? Of the frames I have done they were all only slightly out. Actually less then the 3% and only on some of the adjustments not even all of them. You can sanity check the scope against a square wave generator even the calibrator on the scope is good for “is it in the ball park” comparisons. If you have to go in the 7904A I took some pretty good high res photos in there but there are ceramic hybrids ALL over in that one. If you wanted to try tackling the adjustment on your own I would get a 7603 or a 7704 or 7704A to try first 7603 can be had on E-bay for around 100 some times even with plugins. Mine came with a time base and a 7A18.

Eric

On 6/1/2020 2:30 PM, Chris Wilkson via groups.io wrote:
PS. I'm leery of shipping these things. I've had a few frames destroyed by UPS "keg throwers". That's why I said close to Detroit so I can drive them myself. But I'm open to other options too.


 

On Mon, Jun 1, 2020 at 09:36 PM, Eric wrote:


If I remember correctly if your goal for the frame is evaluation, then really
all you need a scope for is ripple measurement on the power supplies. If I
remember correctly. For the most part everything is done through the
calibration fixture. You do need to send some signals to the scope for things
like geometry adjustments and the like. The plugins are typically adjusted
after the frame is finished. I have a second calibrator the -01 But it needs a
cam switch wheel it is missing out of the unit. So at the moment non
functional. Also what plugins do you have? I am a ways to the south of you but
not cross country. You can shoot me an E-mail off list if you would like more
info.

Before you even do anything Cal related do you know the unit even needs
adjustment? Of the frames I have done they were all only slightly out.
Actually less then the 3% and only on some of the adjustments not even all of
them. You can sanity check the scope against a square wave generator even the
calibrator on the scope is good for “is it in the ball park” comparisons.
If you have to go in the 7904A I took some pretty good high res photos in
there but there are ceramic hybrids ALL over in that one. If you wanted to try
tackling the adjustment on your own I would get a 7603 or a 7704 or 7704A to
try first 7603 can be had on E-bay for around 100 some times even with
plugins. Mine came with a time base and a 7A18.
I think that among other things, Eric says "don't change anything unless you know it's necessary". That is very good advice. It's very tempting e.g. but very wrong to adjust things like (power supply) voltages when they're only slightly off. Quite often, it makes things worse with regard to other aspects, like horizontal or vertical sensitivity.
You don't need a 'scope to be within spec. to check power supply ripple.
Unless you know what you're doing, don't even think about adjusting a scope like the 7904A. Also, adjusting high frequency behavior is generally not really necessary, unless changes have been made.

Raymond


Chris Wilkson
 

On Mon, Jun 1, 2020 at 10:30 AM, Raymond Domp Frank wrote:


On Mon, Jun 1, 2020 at 06:35 PM, Eric wrote:


What you need will vary greatly based on the plugins you need calibrated.
... and on your requirements for the performance and calibration of the
mainframe.
I would like to get all of the performance out of the mainframe. Because...why not? :)
Buy yes, I have a slew of different plugins that I will be switching in and out of this frame
plus the others that I intend to keep (probably a 7x34, and a 7104 at least. Maybe a 7603.)

I have some of that TM500 gear, acquired in a bulk purchase. But none of it is checked out or calibrated.
And I just received 3 flexible extender kits from John Griessen. But I don't have any normalizers....yet.
I was holding out for a -02 because I have 3 7104's and several lesser frames to do and I want to reuse plugins.
But as you said the -02 normalizers are rare and expensive. Eventually I'll try to do the plug ins myself.
Or most of them. But I need a good frame as a starting place.


Harvey White
 

There are some tests you can do on your own, especially since you're not trying to standardize the frame.

Check the power supply voltages.  Don't adjust them unless they're out of tolerance.  Adjust them and you *will* want to calibrate the scope.

If the calibrator has a DC output, measure that with a good meter.

If you have a stable frequency source that's known to be fairly decent (or a generator and counter), that's all you need to verify the horizontal calibration.

Using the calibrator, you can check the vertical attenuators to an extent.

Do the performance verification section of the manual, that'll get you close.

Much of the calibration in these scopes normally doesn't need to be done as much as you might think.  The input standardizers and the like are like making two or more watches agree with each other.  The performance checks (especially with just one scope to consider) are more like making a watch accurate at noon and midnight, and trusting to linearity the rest of the day.

Having said that, there are some voltage references you can either buy or build, and a timing generator is not hard if you have a number of TTL chips and are happy wiring them up.  You're not going to get 1 ns timing marks, but 10 could be decent (100 Mhz oscillator).

Harvey

On 6/1/2020 2:22 PM, Chris Wilkson via groups.io wrote:
On Mon, Jun 1, 2020 at 09:35 AM, Eric wrote:

I think the
7704A was $1,100 for the calibration. It inspired me to do my own.
I also got a quote for something like that a few years ago when I got this scope.
And a couple of $200-300 quotes just to EVALUATE the frame...no cal or repair included.
That's why I'm hoping to find someone here to do it for me. I can pay, and gladly.
Even just getting the mainframe checked and standardized would be a great start.

I need one good scope to fix the rest!



 

On Mon, Jun 1, 2020 at 10:38 PM, Chris Wilkson wrote:


I would like to get all of the performance out of the mainframe. Because...why
not? :)
I see.
Make sure you don't fall intro the trap of "calibrating" by employing instruments of unknown calibration status or with knowledge lacking. I don't know anything about you so please forgive me if I'm underestimating you.
It could be best to have some reliable party do the calibration of some key equipment for you, after which you'll have some references. The 7904A mainframe certainly is a candidate.

Raymond


Chris Wilkson
 

Yes, I know....if it ain't broke, don't fix it! I'm a firm believer in that!

But I have way too many of these things, all acquired with variations
on "it used to work...I'll sell it to you cheap". Seriously. I have at least 20,
maybe 30 frames and about 85 plugins. Something must be done! They're
just sitting in storage costing me money and getting even older.

The situation here is NONE of them are calibrated or even known working.
And I don't have the working signal sources needed to set the scopes up.
Plus I don't want to go through the effort of bringing all of them back to life and doing
any or all of the legendarily hard service procedures only to find out later that I set
everything to the same *wrong* values on multiple scopes.

I'm perfectly fine with "almost perfect" performance. They are analog instruments
approaching 50 years old after all. But I do want to be able to trust them for what they are.
I also have some ideas for new plugins that I want to experiment with and when I'm
designing and tuning those I want to have a known good base to design to.


Chris Wilkson
 

On Mon, Jun 1, 2020 at 01:49 PM, Raymond Domp Frank wrote:


On Mon, Jun 1, 2020 at 10:38 PM, Chris Wilkson wrote:


I would like to get all of the performance out of the mainframe.
Because...why
not? :)
I see.
Make sure you don't fall intro the trap of "calibrating" by employing
instruments of unknown calibration status or with knowledge lacking. I don't
know anything about you so please forgive me if I'm underestimating you.
No problem...we're all strangers together. For reference....I have a BSEE from MIT.
I focused on analog circuit design under Jim Roberge. Professionally I used to design
Flash memory chips (analog blocks) and amplifier ICs. Now I'm an IC applications engineer.

I'm comfortable in the lab and have no problem there. I just have a lack of appropriate
tools at the moment and WAY TOO MANY tools and I'm trying to remedy both.

It could be best to have some reliable party do the calibration of some key
equipment for you, after which you'll have some references. The 7904A
mainframe certainly is a candidate.
THIS! This is exactly what I'm trying to do. EXACTLY.


Eric
 

The plugin's could be checked in with a DMM and a frequency counter If i remember correctly. I think a 100 Mhz scope was helpful for some wave shapes. But the main reference for the PG501  is DC and the TG501 is a frequency counter.

On 6/1/2020 4:59 PM, Chris Wilkson via groups.io wrote:

Yes, I know....if it ain't broke, don't fix it! I'm a firm believer in that!

But I have way too many of these things, all acquired with variations
on "it used to work...I'll sell it to you cheap". Seriously. I have at least 20,
maybe 30 frames and about 85 plugins. Something must be done! They're
just sitting in storage costing me money and getting even older.

The situation here is NONE of them are calibrated or even known working.
And I don't have the working signal sources needed to set the scopes up.
Plus I don't want to go through the effort of bringing all of them back to life and doing
any or all of the legendarily hard service procedures only to find out later that I set
everything to the same *wrong* values on multiple scopes.

I'm perfectly fine with "almost perfect" performance. They are analog instruments
approaching 50 years old after all. But I do want to be able to trust them for what they are.
I also have some ideas for new plugins that I want to experiment with and when I'm
designing and tuning those I want to have a known good base to design to.




 

On Mon, Jun 1, 2020 at 11:26 PM, Eric wrote:


The plugin's could be checked in with a DMM and a frequency counter If i
remember correctly. I think a 100 Mhz scope was helpful for some wave shapes.
But the main reference for the PG501  is DC and the TG501 is a frequency
counter.
Checking the TG501 (Time marker Generator 501)'s frequencies can be done using a frequency counter but there's a bit more to it, like amplitude peaking at the higher frequencies. A 100 MHz 'scope is useless for that.

Checking and adjusting the PG506 involves two main things: Amplitude of the 1 kHz nominally square wave and rise- and fall-edges for the 100Hz - 1MHz fast-edge square wave.
To adjust the rise and fall edges a well-adjusted wide-bandwidth 'scope is needed. 100MHz is unusable for that; the edge speed is <1ns with very little overshoot and no ringing.
Both types of PG506 outputs are supposed to have very flat horizontals.
Many PG506 units have a small button to the lower right of the word "Period" on the front to set the amplitude calibrator output to DC but not all.

Raymond