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On 03/13/2013 06:35 PM, Kenneth G. Gordon wrote:

?

Might anyone have a schematic, or even a users or maintenance manual for the B&K Precision model 3025 SweepFunction Generator?

Mine crapped out a few years ago, and I would really like to get it working.

B&K has so far been no help whatever.

Ken W7EKB

One thing that you might check is the ttl sync output.
If this is OK the problem is probably in the output amps,
fairly common.
if not, check out the power supplies.
I have had a 3020 for many years.
Don't know if it's different.

.

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Looks good. I am also building several extenders from John's kits. And I have a dysfunctional 7L14 to get working at point. I will be interested to see how you make out with your repair effort.

DaveD

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Cheap probes have their place. I use them because of price AND if they break it does not cause me the angst that breaking a decent probe would.
These probes are used with my 7854 & I know there are issues with them but when I feel the need for a better probe I use the HP or Tek probes that are in the box aabove my bench.

Being "financially challenged" (the word in the UK is skint) means that I have to look after the nice kit I have scrounged up over the years for when I need the peformance advantage it gives. Otherwise the cheap stuff works adequately for most of the time when I am not working with high speed / high slew rate signals.

Just my thoughts.

Phil

Hi to All,

Anybody who has this document (2 pages) and is willing to send me scans of it?

Greetings,

Egge Siert

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A line and the variable control behavior makes me think that there is
an open or short involving a bad part somewhere in the input buffer.
The collectors of Q30 and Q38 and relays K44 and K48 on schematic 1
would be a good place to start.

Q192 is not strictly necessary depending on the output drive
capability of U192. It just acts as a current amplifier and protects
the output of U192 which given that it was originally a 709 type of
operational amplifier, is almost necessary. I am not surprised if
Tektronix removed Q192 and replaced U192 with something better as soon
as they could. What is the actual 155 or 156 Tektronix part number on
yours?

In the service manual I have, U192 is a 709 type (LM709, uA709, RC709,
whatever) operational amplifier:

"The first op-amp to get real exposure in the UK was the Fairchild
uA709, designed by the renowned Bob Widlar and introduced in 1965. It
was a rather awkward item that required quite complicated external
compensation and was devoid of output short-circuit protection. One
slip of the probe and an expensive IC was gone. It was prone to
latch-up with high common-mode voltages and did not like capacitive
loads. I for one found all this most discouraging, and gave up on the
709 pretty quickly. If you're going to quit, do it early, I say."



Needless to say, there are lots of suitable and improved replacements
if it is bad. The only problem will be Tektronix used a TO-99 metal
can package but it is possible to make an 8 pin DIP fit with some
cleverness.

On Thu, 14 Mar 2013 05:42:41 -0000, "woooey" <woooey@...>
wrote:

Thanks, David.

When I set up the 7B70 in 'amplifier' mode, instead of a dot
I get a horizontal trace about 4 units wide. Turning up the
gain causes the trace to disappear off to the left of the
screen.

Since the other trigger functions work, this seems to me to
indicate the 'external horizontal amplifier' (U192/Q192 in
the manual). The circuitry in my 7B70 is somewhat different
in this area from the manual (there is no Q192 on my PCB).

U192 is a Tek-numbered opamp. I imagine I will need to find
a salvage 7B70 to fix this.

--ian

--- In TekScopes@..., David <davidwhess@...> wrote:

I took a quick look at the 7B70 schematics.

What you are describing with the variable control causing the external
X input to shift seems like a problem with the DC balance. Page 5-6
of the service manual has a procedure that involves adjusting R186
(Schematic 2 external horizontal amplifier balance) and then R37
(Schematic 1 external input amplifier balance) while rotating the
variable control until the X position does not change.

I would start there as it will likely provide further information
about the problem.

On Wed, 13 Mar 2013 19:38:21 -0000, "woooey" <woooey@...>
wrote:

I found the manual on BAMA.

Relay K780 has an o/c coil. Replacing this with one borrowed
from a 7B71 fixed the 'magnifier' problem but had no effect
on the problem with external input. Still, this is encouraging,
so I will peer at the schematic some more.

--ian

--- In TekScopes@..., "woooey" <woooey@> wrote:

In 'amplifier' (external input) mode, the variable control
shifts the trace off to the left when turned clockwise. There
is some low-level signal on the X-axis but this seems to have
no relation to a signal applied to the external input.

The Magnifier no longer operates. The numeric indicator on
the display changes correctly, but the timebase itself doesn't
change.

Is it worth tapping on relays, etc, or should I just look for
a "new" 7B70?

Thanks,

--ian

Thanks, David.

When I set up the 7B70 in 'amplifier' mode, instead of a dot
I get a horizontal trace about 4 units wide. Turning up the
gain causes the trace to disappear off to the left of the
screen.

Since the other trigger functions work, this seems to me to
indicate the 'external horizontal amplifier' (U192/Q192 in
the manual). The circuitry in my 7B70 is somewhat different
in this area from the manual (there is no Q192 on my PCB).

U192 is a Tek-numbered opamp. I imagine I will need to find
a salvage 7B70 to fix this.

--ian

I recently built the three kits kindly supplied by John Griessen and am
now using them to work on my 7L14 spectrum analyzer, including doing
open-first-mixer surgery.

Here is a photo of extender cables (built to the two foot length of
John's pre-cut wires) coming out of the right side of a 7306:



The cables are long enough to come out of the front of the scope, but I
did it this way to give me more length for turning the 7L14 on its side
and so the 7L14 would be beside the scope, rather than in front of it.

Thanks John!

- Robin

To make these two messages from the thread "Re: Isolation transformers
are not a panacea [was: Variak talk]" easier to find in the archives,
here is a summary of them, with links to the messages, under a new
Subject heading.

Steve <ditter2@...> wrote an interesting piece on differential
probes, including:



1 - Why ordinary probes are resonant at certain frequencies if the
ground connection to the device under test is not connected.

2 - The value and potential limits of differential probes.

3 - Difficulties measuring power transistor drive voltages in the
few hundred millivolt range when they are both at 900V or more.

4 - Achieving differential measurements with a pair of equal length
cable P6009 probes. Apparently the DC gain can be finely adjusted
on these 100x attenuation 120MHz 1500V probes:



David <davidwhess@...> continued the interesting discussion:



with mention of success using this dual probe technique, keeping the
cables close together, adding a ferrite core around the two cables,
modifying other probes to achieve the required close match of
attenuations, and: "As Glenn mentioned in the concurrent Cheap Probes
thread, if you run your hand along the probe cable and the waveform
changes, then you have problems."

- Robin

On 03/13/2013 07:04 AM, jerry massengale wrote:

Hi,

Really good soldering work.

If you mean the photos on Chris hayes's email, that's one I assembled. Thanks!

The edges of the edge connector need to be trimmed down a bit. As it is you cannot daisy chain the

extender as the edge is wider than the black connector can take.

That is true. I just found it out after the last order of PCBs for Tek_7K_flex.
I plan to fix it so they can daisy chain if possible while making a profit, but
for the first runs I was more worried about having minimal connector tolerance
slop so no wrong electrical connections could be made.

John Griessen
(back from Louisiana)

On 03/13/2013 07:04 AM, jerry massengale wrote:

Hi,

Really good soldering work.

If you mean the photos on Dave D's original email, that's one I assembled. Thanks!

The edges of the edge connector need to be trimmed down a bit. As it is you cannot daisy chain the

extender as the edge is wider than the black connector can take.

That is true. I just found it out after the last order of PCBs for Tek_7K_flex.
I plan to fix it so they can daisy chain if possible while making a profit, but
for the first runs I was more worried about having minimal connector tolerance
slop so no wrong electrical connections could be made.

John Griessen
(back from Louisiana)

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On Wed, 13 Mar 2013 21:19:00 -0000, "Hakan H" <hahi@...> wrote:

On Wed, 13 Mar 2013 14:45:39 +1100, Don Black
<donald_black@...> wrote:

I've never used them but the cheap probes are available up to several
hundred MHz (250-300?) for a bit more money. Has anyone tried them and
know how good or bad they are above 100 MHz. It may be more demanding on
fast pulses rather than sine waves.

Don Black.

--- In TekScopes@..., David <davidwhess@...> wrote:

I bought a pair of inexpensive x10 250 MHz probes (Texas TX5125R) for
my 2440 and they work fine. I verified their performance with my
PG506 using a coaxial connection to the probe tip.

Almost two years ago, on behalf of another list member, I made a
comparison between a cheap 500 MHz probe (Texas TX6150) and a Tek
P6139A on a couple of TDS scopes.
The result can be found here:

Fun stuff.

When I tested the Texas TX5125R probes (250 MHz) on my 2440 (300 MHz)
which is my fastest 1 MOhm input impedance oscilloscope, I used the
PG506 that I repaired and verified the fast transition outputs on my
S-4 sampling head at about 500ps with no significant overshoot. Since
then I have also gotten identical results with an S-1 sampling head.

The probes with a 50 ohm feedthrough termination and coaxial probe tip
connection gave results which were basically identical to using just a
50 ohm connection directly to the oscilloscope.

On one hand I do not currently have a faster pulse generator or
oscilloscope to test them but on the other hand, at 250 MHz the x10
capacitive probe loading is so high that high frequency content is
largely smushed so I would not see it anyway.

I took a quick look at the 7B70 schematics.

What you are describing with the variable control causing the external
X input to shift seems like a problem with the DC balance. Page 5-6
of the service manual has a procedure that involves adjusting R186
(Schematic 2 external horizontal amplifier balance) and then R37
(Schematic 1 external input amplifier balance) while rotating the
variable control until the X position does not change.

I would start there as it will likely provide further information
about the problem.

On Wed, 13 Mar 2013 19:38:21 -0000, "woooey" <woooey@...>
wrote:

I found the manual on BAMA.

Relay K780 has an o/c coil. Replacing this with one borrowed
from a 7B71 fixed the 'magnifier' problem but had no effect
on the problem with external input. Still, this is encouraging,
so I will peer at the schematic some more.

--ian

--- In TekScopes@..., "woooey" <woooey@...> wrote:

In 'amplifier' (external input) mode, the variable control
shifts the trace off to the left when turned clockwise. There
is some low-level signal on the X-axis but this seems to have
no relation to a signal applied to the external input.

The Magnifier no longer operates. The numeric indicator on
the display changes correctly, but the timebase itself doesn't
change.

Is it worth tapping on relays, etc, or should I just look for
a "new" 7B70?

Thanks,

--ian

On Wed, 13 Mar 2013 20:49:26 -0000, "Steve" <ditter2@...> wrote:

--- In TekScopes@..., Bert Haskins <bhaskins@...> wrote:

Two points that I would like to see discussed:
What do YOU do with the probe ground leads in the above two paragraphs?
I'm certainly not a newbe here ( have, use a 7A13 ), but I'm not an
expert either

Then do we really, really need need differential probes ( hard to
find and expensive ) in the above?
Yep, I know the CMRR is better....

The tek app note is OK but it really should have covered the grounding
issue to a greater depth.

Bert

The question of what to do with the ground leads often comes up. Experienced users have reported conflicting results from connecting them to a local (non-floating) ground – sometime improving the signal, other times it has no effect. I tried to model this a few years after I wrote the article to see if I could put some science into the conflicting results they reported. There is an explanation, but because a user will not know the impedances in their circuit, there is no hard and fast rule.

. . .

Since we usually do not know the relative impedances to ground of the common mode and differential mode components, we can forget the science and use an empirical method – try both and see which is better! Note that in many cases, you don't have a good local earth ground point in the circuit under test, so you can't connect the probe ground. In others, connecting the probe lead to the local earth ground creates a ground loop which you are trying to eliminate with the differential measurement!

This has been my experience. Under ideal conditions, modeling will
show what is going on but in practice the ground point impedance will
vary too much with frequency because of the physical limitations of
construction.

After taking extra precautions like forming the two probe cables to
run closely parallel for minimum loop area and maybe adding a common
mode suppression ferrite core around both cables, it is easiest to
just test with the differential probe grounds connected to "ground"
and then test with the probe grounds only connected to each other.

As Glenn mentioned in the concurrent Cheap Probes thread, if you run
your hand along the probe cable and the waveform changes, then you
have problems. This points to modeling the probe to circuit
connection as an RF balanced (one probe) to unbalanced (the circuit)
connection with an RF current on both sides of the probe cable shield
for three actual connections. Using two probes in a differential
connection should solve that problem just like using a balun but you
still have the center conductors of both probes plus a third and forth
connection comprising the inside and outside conductors of the two
shields which are now in parallel so if the ground is a poor RF
ground, you still get a current on both sides of the coaxial shields
which can affect the measurement results in some odd ways.

What about tying the two probe grounds together? Usually this will improve the high frequency response, but it may introduce a ground loop if the probe leads are not tightly twisted together.

I forget where but at least one of the Tektronix differential
amplifier manuals recommends this configuration for some measurements.
I usually lightly wire tie the cables of the two probes together so
they move as one.

Next Question - Differential probes in front of a 7A13? You are right - if you need the highest CMRR, then this is the way to go. You can calculate the rough amount of CMRR that you need if you know the common mode voltage and differential voltage you want to measure. When calculating CMRR needs, you need to determine how much common mode signal leaking into your measurement is acceptable. A good starting place is 10% or lower. Designers of motor drives and high power SWPS that have 700-900 V bus voltages need well matched probes when trying to measure upper gate drive. In this application you need to resolve a few hundred mV riding on a 900 V dynamic signal. Remember, the first order term that sets the maximum CMRR your measurement system will achieve is the relative gain (or attenuation) match between the + and – inputs.

A practical note – if you can find a pair of P6009 probes, they can be roughly matched for differential use. There is a fine adjustment on the DC attenuation in the comp box. I don't think this was intended for differential matching, but rather just to get the attenuation with spec. It was difficult to do with the divide by one hundred attenuation these probes have. The tip capacitor and resistor have minimal voltage coefficient, which more modern differential probes tend to have, so they maintain their match well over a wide common mode range. If yo go this route, be sure to get a pair with the same length of cable. I think Tek offered 3 versions – 3.5, 6 and 12 feet.

I originally suspected that the DC trim in the P6009 was used to
adjust the probe attenuation factor closer to the ideal value so that
it could support an extended temperature range and still be within its
specifications but the similar P6008 (x10) lacks the DC trim.

At some point I anticipate either modifying a pair of P6008 probes to
add a DC trim or picking up a pair of P6055 or P6135A probes.

On 13 Mar 2013 at 16:05, Bob Albert wrote:

Is the 3020 close enough?

Nope. I have already checked that one out. It isn't very close. Double the number of controls,
and less than 1/2 the frequency range, not the same inputs and outputs.

Thanks, though.

Ken

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