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P6201 dc current measurements

John
 

I have a P6201 active FET probe which I suspect is damaged. I bought it when I got a 7934 mainframe. Anyway I have measured the DC voltages and they are all OK. However I am concerned that R298 on the DC-LF Amplifier board gets very hot to the touch. It drops the +15V to +7V and I lifted the leg of one end and it has 98 ma running through it and that equates to almost 3/4 of a watt. That seems excessive. So I checked C240 and it is OK. I then lifted the +7V line going to the probe and it is drawing about 21 ma. So the question is a) is this current draw normal? And b) if not is there an easy way to separate the boards so I can find out which section is drawing the unusual amount of current? There is nothing in the manual or the schematics to indicate what the right current draw should be. Given that the +7v line is correct the R298 seems to be the right value (78 ohms) and has the right current flowing through it but the fact that it gets hot indicates its power dissipation is under rated for the job it is doing. Or is this Tektronix way of demoisurising the housing (joke here).

John
VK2DLP

george edmonds
 

Hi John

A 6201 draws approximately 105ma from the +15vdc supply and 110ma from the -15vdc supply with a total power dissipation of 3.2W . These figures are from the Tektronix manual.

73 George G6HIG On Wednesday, July 17, 2019, 10:46:20 AM GMT+1, John <@VK2DLP> wrote:

I have a P6201 active FET probe which I suspect is damaged. I bought it when I got a 7934 mainframe. Anyway I have measured the DC voltages and they are all OK. However I am concerned that R298 on the DC-LF Amplifier board gets very hot to the touch. It drops the +15V to +7V and I lifted the leg of one end and it has 98 ma running through it and that equates to almost 3/4 of a watt. That seems excessive. So I checked C240 and it is OK. I then lifted the +7V line going to the probe and it is drawing about 21 ma. So the question is a) is this current draw normal? And b) if not is there an easy way to separate the boards so I can find out which section is drawing the unusual amount of current? There is nothing in the manual or the schematics to indicate what the right current draw should be. Given that the +7v line is correct the R298 seems to be the right value (78 ohms) and has the right current flowing through it but the fact that it gets hot indicates its power dissipation is under rated for the job it is doing. Or is this Tektronix way of demoisurising the housing (joke here).

John
VK2DLP

Craig Sawyers
 

Looking at the schematic, the input FET runs at Idss, which is in the range 5-15mA. So the drain is at
0V. That sets the next transistor current to (7-0.7)/510 = 12mA and the second transistor to
(7-1.4)/300 = 18mA (rounding numbers). So the probe body will pull 35-45mA.

But the 7V also runs the output amplifier Q300/Q320, which I calculate roughly draws 14mA.

So I would expect the total drain on the 7V line to be between 49mA and 59mA, so the dissipation in
R298 should be 0.47W maximum. That resistor is a wirewound 1W rated part and so should be OK, but will
certainly run hot - probably too hot to touch, and should be spaced above the board to allow air
circulation.

Craig

Subject: [TekScopes] P6201 dc current measurements

I have a P6201 active FET probe which I suspect is damaged. I bought it when I got a 7934 mainframe.
Anyway I have measured the DC voltages and they are all OK. However I am concerned that R298 on
the DC-LF Amplifier board gets very hot to the touch. It drops the +15V to +7V and I lifted the leg
of
one end and it has 98 ma running through it and that equates to almost 3/4 of a watt. That seems
excessive. So I checked C240 and it is OK. I then lifted the +7V line going to the probe and it is
drawing
about 21 ma. So the question is a) is this current draw normal? And b) if not is there an easy way
to
separate the boards so I can find out which section is drawing the unusual amount of current? There
is
nothing in the manual or the schematics to indicate what the right current draw should be. Given
that
the +7v line is correct the R298 seems to be the right value (78 ohms) and has the right current
flowing
through it but the fact that it gets hot indicates its power dissipation is under rated for the job
it is
doing. Or is this Tektronix way of demoisurising the housing (joke here).

John
VK2DLP

Craig Sawyers
 

Not in my manual it isn't.

Craig

Hi John

A 6201 draws approximately 105ma from the +15vdc supply and 110ma from the -15vdc supply with a
total power dissipation of 3.2W . These figures are from the Tektronix manual.

73 George G6HIG On Wednesday, July 17, 2019, 10:46:20 AM GMT+1, John
<@VK2DLP> wrote:

I have a P6201 active FET probe which I suspect is damaged. I bought it when I got a 7934
mainframe.
Anyway I have measured the DC voltages and they are all OK. However I am concerned that R298 on
the DC-LF Amplifier board gets very hot to the touch. It drops the +15V to +7V and I lifted the leg
of
one end and it has 98 ma running through it and that equates to almost 3/4 of a watt. That seems
excessive. So I checked C240 and it is OK. I then lifted the +7V line going to the probe and it is
drawing
about 21 ma. So the question is a) is this current draw normal? And b) if not is there an easy way
to
separate the boards so I can find out which section is drawing the unusual amount of current? There
is
nothing in the manual or the schematics to indicate what the right current draw should be. Given
that
the +7v line is correct the R298 seems to be the right value (78 ohms) and has the right current
flowing
through it but the fact that it gets hot indicates its power dissipation is under rated for the job
it is
doing. Or is this Tektronix way of demoisurising the housing (joke here).

John
VK2DLP

george edmonds
 

Hi Craig

Try this link,

http://w140.com/tek_p6201.pdf and look at page 8

73 George G6HIG

On Wednesday, July 17, 2019, 12:24:26 PM GMT+1, Craig Sawyers <c.sawyers@...> wrote:

Not in my manual it isn't.

Craig

  Hi John

A 6201 draws approximately 105ma from the +15vdc supply and 110ma from the -15vdc supply with a
total power dissipation of 3.2W . These figures are from the Tektronix manual.

73 George G6HIG    On Wednesday, July 17, 2019, 10:46:20 AM GMT+1, John
<@VK2DLP> wrote:

  I have a P6201 active FET probe which I suspect is damaged. I bought it when I got a 7934
mainframe.
Anyway I have measured the DC voltages and they are all OK. However I am concerned that R298 on
the DC-LF Amplifier board gets very hot to the touch. It drops the +15V to +7V and I lifted the leg
of
one end and it has 98 ma running through it and that equates to almost 3/4 of a watt. That seems
excessive. So I checked C240 and it is OK. I then lifted the +7V line going to the probe and it is
drawing
about 21 ma. So the question is a) is this current draw normal? And b) if not is there an easy way
to
separate the boards so I can find out which section is drawing the unusual amount of current? There
is
nothing in the manual or the schematics to indicate what the right current draw should be. Given
that
the +7v line is correct the R298 seems to be the right value (78 ohms) and has the right current
flowing
through it but the fact that it gets hot indicates its power dissipation is under rated for the job
it is
doing. Or is this Tektronix way of demoisurising the housing (joke here).

John
VK2DLP

 

John:

I have 4 of these P6201 probes and in all R298 runs HOT! When I have the covers off of the Amplifier Box for adjusting and every time I touch R298 accidentally I jump a foot because of the temp of that resistor. I know it is HOT but accidentally touching it is a surprise and my reflex reaction is to pull back.
So you don't have a problem from R298 running hot.
Bill

John
 

Thanks for the experienced based info. I did the same and when I am looking at electronics hot usually means something is wrong. I guess I should just go through the calibration procedure and make sure everything is in spec. Thanks to all who responded. Experience beats conjecture every time.

John
VK2DLP