TekScopes@groups.io | Tek TDS320 Calibration (2024)

I have a TDS320 scope which has failed calibration. After I try to
do a voltage cal and follow the prompts on the screen a message
appears telling me that voltage calibration has failed.
Unfortunately I do not have a proper calibrator. I do have a CG551AP
however this will only provide positive voltages with respect to
earth and is unable to produce the required 80mV and 15mV levels
required for the calibration. Because of this I am cheating to try
and calibrate (at least remove the failed cal status for the time
being) by using an accurate DC power source verified against a
precision multimeter which appears to be able to produce a stable
voltage within 1% or so of that required not taking into account the
inaccuracy of the meter which should be relatively low given it is a
5 ½ digit DMM. Clearly this is not within the accuracy of the +/-
0.1% specified in the service manual.

I suspect that when I try to do the Cal the calibration constant
calculated by the instrument is outside the range set by the factory
as being appropriate and that this has been caused because my
reference voltage not being good enough. Is this the case or do I
have another problem with the instrument? Nowhere in the manual does
it state why it fails the voltage calibration.

david_c_partridge

What's "improper" about the CG551AP as a scope calibrator?

I think mine is the dog's whatsits!

And it should be able to create 80mV and 15mV I think.

20mV range times 4 multiplier,
5 mV range times 3 multiplier.

If you need negative volts, that's a little trickier, as the earth on
the 551AP is connected to the earth on the output socket.

The approach I would contemplate using (at my own risk) and with
appropriate precautions because there are potentially fatal risks
involved in floating any piece of mains powered equipment.

<Flame shields up>

1) Power the TM500 in which the CG551AP is installed from an isolating
transformer. This is where it get's controversial, disconnect the
earth lead of the TM500 power lead to the isolation transformer. This
means that you've "floated" the TM500/CG551AP, and prevented the
CG551AP ground from being tied to world ground (most isolation
transformers I've used pass the earth connection straight through for
safety reasons).

2) Use a BNC to bannana socket and bannana plug to BNC to reverse the
connections to the scope. I.e. earth of CG551AP to centre pin of
scope BNC, output from CG551AP to earth of scope BNC.

I *think* this would do the job, but don't promise.

Effectively what you've done is to "float" the DUT.

PS What does the cal procedure call for as a suitable calibrator if
the CG5xxxx range won't do the job without messing around?

PPS If anyone has a better/safer technique for this I'm all ears.

Dave.

david_c_partridge

I'd more likely suspect the TDS320 acquisition board :-(

I think the approach of using an external supply and a 5½ digit
meter should give >> 0.1% accuracy.

Dave

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

I have a TDS320 scope which has failed calibration. After I try to
do a voltage cal and follow the prompts on the screen a message
appears telling me that voltage calibration has failed.
Unfortunately I do not have a proper calibrator. I do have a

CG551AP

however this will only provide positive voltages with respect to
earth and is unable to produce the required 80mV and 15mV levels
required for the calibration. Because of this I am cheating to try
and calibrate (at least remove the failed cal status for the time
being) by using an accurate DC power source verified against a
precision multimeter which appears to be able to produce a stable
voltage within 1% or so of that required not taking into account

the

inaccuracy of the meter which should be relatively low given it is

a

5 ½ digit DMM. Clearly this is not within the accuracy of the +/-
0.1% specified in the service manual.

I suspect that when I try to do the Cal the calibration constant
calculated by the instrument is outside the range set by the

factory

as being appropriate and that this has been caused because my
reference voltage not being good enough. Is this the case or do I
have another problem with the instrument? Nowhere in the manual

does

it state why it fails the voltage calibration.

The entire scope is on a 4 1/2" x 4 1/2" or so sized multilayer
circuit board loaded with lots of LSI chips or that is what is in my
TDS-380 (nearly an empty box). If your calibration problems were like
mine the circuit board may need reflowing if you have a Hot Air rework
station. Mine was failing cal when I got it and careful examination of
the board revealed several contact locations with very little or no
solder on them. I could also press against some of the LSI chips and
get it to pass all self tests.

I added solder paste to the connections that looked dry and then
reflowed the entire board with my Hot Air Station and reassembled. It
has worked flawless since.

Mitch
N4MF

--- In TekScopes@..., "david_c_partridge"
<david.partridge@...> wrote:

I'd more likely suspect the TDS320 acquisition board :-(

I think the approach of using an external supply and a 5½ digit
meter should give >> 0.1% accuracy.

Dave

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

I have a TDS320 scope which has failed calibration. After I try to
do a voltage cal and follow the prompts on the screen a message
appears telling me that voltage calibration has failed.
Unfortunately I do not have a proper calibrator. I do have a

CG551AP

however this will only provide positive voltages with respect to
earth and is unable to produce the required 80mV and 15mV levels
required for the calibration. Because of this I am cheating to try
and calibrate (at least remove the failed cal status for the time
being) by using an accurate DC power source verified against a
precision multimeter which appears to be able to produce a stable
voltage within 1% or so of that required not taking into account

the

inaccuracy of the meter which should be relatively low given it is

a

5 ½ digit DMM. Clearly this is not within the accuracy of the +/-
0.1% specified in the service manual.

I suspect that when I try to do the Cal the calibration constant
calculated by the instrument is outside the range set by the

factory

as being appropriate and that this has been caused because my
reference voltage not being good enough. Is this the case or do I
have another problem with the instrument? Nowhere in the manual

does

it state why it fails the voltage calibration.

I hope not! Where do you start with this one?

I guess in theory if you have a good DC source and an accurate meter
you should be able to get that accuracy. I don't think my DC source
was good enough to give me +/- 0.1%. The meter is probably going to
measure the DC component only so I also worry about any ripple. I
don't know what the scope will use when it does its cal. Does it
just take one sample?

--- In TekScopes@..., "david_c_partridge"
<david.partridge@...> wrote:

I'd more likely suspect the TDS320 acquisition board :-(

I think the approach of using an external supply and a 5½ digit
meter should give >> 0.1% accuracy.

Dave

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

I have a TDS320 scope which has failed calibration. After I try

to

do a voltage cal and follow the prompts on the screen a message
appears telling me that voltage calibration has failed.
Unfortunately I do not have a proper calibrator. I do have a

CG551AP

however this will only provide positive voltages with respect to
earth and is unable to produce the required 80mV and 15mV levels
required for the calibration. Because of this I am cheating to

try

and calibrate (at least remove the failed cal status for the

time

being) by using an accurate DC power source verified against a
precision multimeter which appears to be able to produce a

stable

voltage within 1% or so of that required not taking into account

the

inaccuracy of the meter which should be relatively low given it

is

a

5 ½ digit DMM. Clearly this is not within the accuracy of the +/-

0.1% specified in the service manual.

I suspect that when I try to do the Cal the calibration constant
calculated by the instrument is outside the range set by the

factory

as being appropriate and that this has been caused because my
reference voltage not being good enough. Is this the case or do

I

have another problem with the instrument? Nowhere in the manual

does

it state why it fails the voltage calibration.

I think you will find that the TDS310, 320 and 350 are basically the
same thing and are virtually the same as the TDS340, 360, 380. I
think you can change the firmware by changing a couple of inductors
which govern the instruments bandwidth and you will have any of
those scopes.

All the soldering looks good as far as I can see but I may need to
do a though check! I do have a rework station but I will only try
that if I see a dodgy area.

Thanks

--- In TekScopes@..., "n4mf_sc" <n4mf_sc@...> wrote:

The entire scope is on a 4 1/2" x 4 1/2" or so sized multilayer
circuit board loaded with lots of LSI chips or that is what is in

my

TDS-380 (nearly an empty box). If your calibration problems were

like

mine the circuit board may need reflowing if you have a Hot Air

rework

station. Mine was failing cal when I got it and careful

examination of

the board revealed several contact locations with very little or no
solder on them. I could also press against some of the LSI chips

and

get it to pass all self tests.

I added solder paste to the connections that looked dry and then
reflowed the entire board with my Hot Air Station and reassembled.

It

has worked flawless since.

Mitch
N4MF

--- In TekScopes@..., "david_c_partridge"
<david.partridge@> wrote:

I'd more likely suspect the TDS320 acquisition board :-(

I think the approach of using an external supply and a 5½ digit
meter should give >> 0.1% accuracy.

Dave

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

I have a TDS320 scope which has failed calibration. After I

try to

do a voltage cal and follow the prompts on the screen a

message

appears telling me that voltage calibration has failed.
Unfortunately I do not have a proper calibrator. I do have a

CG551AP

however this will only provide positive voltages with respect

to

earth and is unable to produce the required 80mV and 15mV

levels

required for the calibration. Because of this I am cheating to

try

and calibrate (at least remove the failed cal status for the

time

being) by using an accurate DC power source verified against a
precision multimeter which appears to be able to produce a

stable

voltage within 1% or so of that required not taking into

account

the

inaccuracy of the meter which should be relatively low given

it is

a

5 ½ digit DMM. Clearly this is not within the accuracy of the

+/-

0.1% specified in the service manual.

I suspect that when I try to do the Cal the calibration

constant

calculated by the instrument is outside the range set by the

factory

as being appropriate and that this has been caused because my
reference voltage not being good enough. Is this the case or

do I

have another problem with the instrument? Nowhere in the

manual

does

it state why it fails the voltage calibration.

BTW the scope also fails the SPC but I suspect this may be to do
with it having failed the voltage cal. The instrument however
operates normally and after the SPC is done all the DC offset is
removed etc.

--- In TekScopes@..., "david_c_partridge"
<david.partridge@...> wrote:

I'd more likely suspect the TDS320 acquisition board :-(

I think the approach of using an external supply and a 5½ digit
meter should give >> 0.1% accuracy.

Dave

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

I have a TDS320 scope which has failed calibration. After I try

to

do a voltage cal and follow the prompts on the screen a message
appears telling me that voltage calibration has failed.
Unfortunately I do not have a proper calibrator. I do have a

CG551AP

however this will only provide positive voltages with respect to
earth and is unable to produce the required 80mV and 15mV levels
required for the calibration. Because of this I am cheating to

try

and calibrate (at least remove the failed cal status for the

time

being) by using an accurate DC power source verified against a
precision multimeter which appears to be able to produce a

stable

voltage within 1% or so of that required not taking into account

the

inaccuracy of the meter which should be relatively low given it

is

a

5 ½ digit DMM. Clearly this is not within the accuracy of the +/-

0.1% specified in the service manual.

I suspect that when I try to do the Cal the calibration constant
calculated by the instrument is outside the range set by the

factory

as being appropriate and that this has been caused because my
reference voltage not being good enough. Is this the case or do

I

have another problem with the instrument? Nowhere in the manual

does

it state why it fails the voltage calibration.