Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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  #1   Report Post  
Norm Dresner
 
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Default Very high input impedance high voltage meter???

I have an HP200CD that needs some loving care. The manual says to measure
the difference in the voltage between two different points in the circuit --
it shouldn't be all that much but the voltages between these points and
ground is in the hundreds of volts range. The manual also says that I
should be using a meter with 220M ohm or greater input impedance to make the
voltage measurements. Any suggestions where such a beast could be
bought/rented/stolen?

TIA
Norm

  #2   Report Post  
Dbowey
 
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Default

Norm posted:

I have an HP200CD that needs some loving care. The manual says to measure
the difference in the voltage between two different points in the circuit --
it shouldn't be all that much but the voltages between these points and
ground is in the hundreds of volts range. The manual also says that I
should be using a meter with 220M ohm or greater input impedance to make the
voltage measurements. Any suggestions where such a beast could be
bought/rented/stolen?


When the HP200CD was manufactured there weren't any meters better than a VTVM,
so you can use one if you have it.

Don

  #3   Report Post  
Wild Bill
 
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I believe that the FET input analog meters have very high impedance, but I
don't know the specific range for any specific model.

Adding a high voltage probe to a meter's input will increase the impedance.
For example, the Beckman 300 series DMMs have an input impedance of 22 M
ohm, and the high voltage probe increases it by x 1000.
The HV probe sheet lists the input resistance as 1000 M ohm for the 50 kVDC
HV-211 probe.

There are other multiplier probes for meters to increase the DC input range
of meters to the 5 to 10 kV range. These would also produce a similar input
impedance increase.

A typical combination analog meter built into the HV probe might be a less
accurate instrument.

Cheers
WB
..................

"Norm Dresner" wrote in message
...
I have an HP200CD that needs some loving care. The manual says to measure
the difference in the voltage between two different points in the

circuit --
it shouldn't be all that much but the voltages between these points and
ground is in the hundreds of volts range. The manual also says that I
should be using a meter with 220M ohm or greater input impedance to make

the
voltage measurements. Any suggestions where such a beast could be
bought/rented/stolen?

TIA
Norm





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  #4   Report Post  
Norm Dresner
 
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Default

"Wild Bill" wrote in message
...
I believe that the FET input analog meters have very high impedance, but I
don't know the specific range for any specific model.

Adding a high voltage probe to a meter's input will increase the

impedance.
For example, the Beckman 300 series DMMs have an input impedance of 22 M
ohm, and the high voltage probe increases it by x 1000.
The HV probe sheet lists the input resistance as 1000 M ohm for the 50

kVDC
HV-211 probe.

There are other multiplier probes for meters to increase the DC input

range
of meters to the 5 to 10 kV range. These would also produce a similar

input
impedance increase.

A typical combination analog meter built into the HV probe might be a less
accurate instrument.


Come to think of it, I have a high voltage probe for my VTVM that'll
increase it to IIRC 100M ohm -- that's getting much closer and probably will
be sufficient.

Norm

  #5   Report Post  
Jerry G.
 
Posts: n/a
Default

Any good DVM with a high voltage probe, such as what is used for TV
servicing should do the job. Most are about or greater than 100 meg/Volt
when
used with the standard 10 meg/Volt DVM.


--

Jerry G.
==========================


"Norm Dresner" wrote in message
...
I have an HP200CD that needs some loving care. The manual says to measure
the difference in the voltage between two different points in the circuit --
it shouldn't be all that much but the voltages between these points and
ground is in the hundreds of volts range. The manual also says that I
should be using a meter with 220M ohm or greater input impedance to make the
voltage measurements. Any suggestions where such a beast could be
bought/rented/stolen?

TIA
Norm





  #6   Report Post  
Jim Adney
 
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Default

On Fri, 17 Sep 2004 15:58:23 GMT "Norm Dresner" wrote:

I have an HP200CD that needs some loving care. The manual says to measure
the difference in the voltage between two different points in the circuit --
it shouldn't be all that much but the voltages between these points and
ground is in the hundreds of volts range. The manual also says that I
should be using a meter with 220M ohm or greater input impedance to make the
voltage measurements. Any suggestions where such a beast could be
bought/rented/stolen?


There are such devices, but any modern DVM with a 10 MOhm input
impedance will work just fine to measure the voltages in your 200CD.
That warning is there because when the 200CD was made the common VOMs
of the day had much lower input impedances, so HP wanted to keep you
from using those.

I've worked on 200CDs with 10 MOhm meters with complete satisfaction.

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------
  #7   Report Post  
Norm Dresner
 
Posts: n/a
Default

"Jim Adney" wrote in message
...
On Fri, 17 Sep 2004 15:58:23 GMT "Norm Dresner" wrote:

I have an HP200CD that needs some loving care. The manual says to

measure
the difference in the voltage between two different points in the

circuit --
it shouldn't be all that much but the voltages between these points and
ground is in the hundreds of volts range. The manual also says that I
should be using a meter with 220M ohm or greater input impedance to make

the
voltage measurements. Any suggestions where such a beast could be
bought/rented/stolen?


There are such devices, but any modern DVM with a 10 MOhm input
impedance will work just fine to measure the voltages in your 200CD.
That warning is there because when the 200CD was made the common VOMs
of the day had much lower input impedances, so HP wanted to keep you
from using those.

I've worked on 200CDs with 10 MOhm meters with complete satisfaction.


Jim

Thanks for the encouragement. I've almost decided not to try to repair it
but to build a modern, solid state oscillator using the mechanical
components as the basis. The dual variable capacitors are an almost
impossible item to find and there's more than enough space in the chassis
for anything with transistors or IC's. And, as a bonus, the bottom of the
interior foundation should serve to isolate the incoming AC from the
sensitive circuitry in the upper compartment.

Norm

  #8   Report Post  
James Sweet
 
Posts: n/a
Default


"Norm Dresner" wrote in message
...
"Jim Adney" wrote in message
...
On Fri, 17 Sep 2004 15:58:23 GMT "Norm Dresner" wrote:

I have an HP200CD that needs some loving care. The manual says to

measure
the difference in the voltage between two different points in the

circuit --
it shouldn't be all that much but the voltages between these points and
ground is in the hundreds of volts range. The manual also says that I
should be using a meter with 220M ohm or greater input impedance to

make
the
voltage measurements. Any suggestions where such a beast could be
bought/rented/stolen?


There are such devices, but any modern DVM with a 10 MOhm input
impedance will work just fine to measure the voltages in your 200CD.
That warning is there because when the 200CD was made the common VOMs
of the day had much lower input impedances, so HP wanted to keep you
from using those.

I've worked on 200CDs with 10 MOhm meters with complete satisfaction.


Jim

Thanks for the encouragement. I've almost decided not to try to repair it
but to build a modern, solid state oscillator using the mechanical
components as the basis. The dual variable capacitors are an almost
impossible item to find and there's more than enough space in the chassis
for anything with transistors or IC's. And, as a bonus, the bottom of the
interior foundation should serve to isolate the incoming AC from the
sensitive circuitry in the upper compartment.

Norm


If you do decide to retrofit it with modern internals, a guy I know has a
book, I think it's called Analog Circuit Design, anyway he showed me a
schematic in there of a solid state oscillator using an incandescent lamp to
stabilize it just as the HP uses, he built it and said it performs very
well.


  #9   Report Post  
Norm Dresner
 
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Default

"James Sweet" wrote in message
news:_Xt3d.4555$je.2531@trnddc04...

"Norm Dresner" wrote in message
...
"Jim Adney" wrote in message
...
On Fri, 17 Sep 2004 15:58:23 GMT "Norm Dresner" wrote:

I have an HP200CD that needs some loving care. The manual says to

measure
the difference in the voltage between two different points in the

circuit --
it shouldn't be all that much but the voltages between these points

and
ground is in the hundreds of volts range. The manual also says that

I
should be using a meter with 220M ohm or greater input impedance to

make
the
voltage measurements. Any suggestions where such a beast could be
bought/rented/stolen?

There are such devices, but any modern DVM with a 10 MOhm input
impedance will work just fine to measure the voltages in your 200CD.
That warning is there because when the 200CD was made the common VOMs
of the day had much lower input impedances, so HP wanted to keep you
from using those.

I've worked on 200CDs with 10 MOhm meters with complete satisfaction.


Jim

Thanks for the encouragement. I've almost decided not to try to repair

it
but to build a modern, solid state oscillator using the mechanical
components as the basis. The dual variable capacitors are an almost
impossible item to find and there's more than enough space in the

chassis
for anything with transistors or IC's. And, as a bonus, the bottom of

the
interior foundation should serve to isolate the incoming AC from the
sensitive circuitry in the upper compartment.

Norm


If you do decide to retrofit it with modern internals, a guy I know has a
book, I think it's called Analog Circuit Design, anyway he showed me a
schematic in there of a solid state oscillator using an incandescent lamp

to
stabilize it just as the HP uses, he built it and said it performs very
well.


Yup. The book is "Analog Circuit Design" . It's published by
Butterworth-Heinemann in their EDN Series for Design Engineers. The editor
is Jim Williams and the article you're referencing is #7, "Max Wein, Mr.
Hewlett, and a Rainy Sunday Afternoon" by Jim Williams, p 43ff. The book is
on a shelf in my library already.

Norm

  #10   Report Post  
Jim Adney
 
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Default

On Mon, 20 Sep 2004 05:24:34 GMT "Norm Dresner" wrote:

Thanks for the encouragement. I've almost decided not to try to repair it
but to build a modern, solid state oscillator using the mechanical
components as the basis. The dual variable capacitors are an almost
impossible item to find and there's more than enough space in the chassis
for anything with transistors or IC's. And, as a bonus, the bottom of the
interior foundation should serve to isolate the incoming AC from the
sensitive circuitry in the upper compartment.


I'm sure that with Jim William's help you could do a good job at this,
but the 200CD was really a very nice instrument by the time HP
discontinued it. I think it had a nearly 30 year lifetime, during
which it got many significant upgrades. If you have an early one,
there might be some reason to "rebuild" it, but I suspect that it
would be hard to do better than the late versions. Plus it would take
a lot less time to fix than to repair.

I have some NOS 200CD replacement parts here (caps and light bulbs.)

If you send me your serial number I can tell you how early or late
yours is.

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------


  #11   Report Post  
Norm Dresner
 
Posts: n/a
Default

See replies in-line.

"Jim Adney" wrote in message
...

I'm sure that with Jim William's help you could do a good job at this,
but the 200CD was really a very nice instrument by the time HP
discontinued it. I think it had a nearly 30 year lifetime, during
which it got many significant upgrades. If you have an early one,
there might be some reason to "rebuild" it, but I suspect that it
would be hard to do better than the late versions. Plus it would take
a lot less time to fix than to repair.


I'm not quite sure what the distinction you're making between "fixing" and
"repairing". Please elaborate.


I have some NOS 200CD replacement parts here (caps and light bulbs.)


I think that the first step would be to replace the tubes, wouldn't it?


If you send me your serial number I can tell you how early or late
yours is.


005-28015


Norm

  #12   Report Post  
Norm Dresner
 
Posts: n/a
Default

See replies in-line

"Jim Adney" wrote in message
...

I'm sure that with Jim William's help you could do a good job at this,
but the 200CD was really a very nice instrument by the time HP
discontinued it. I think it had a nearly 30 year lifetime, during
which it got many significant upgrades. If you have an early one,
there might be some reason to "rebuild" it, but I suspect that it
would be hard to do better than the late versions. Plus it would take
a lot less time to fix than to repair.


I'm quite unclear what distinction you're making between "fix" and
"repair". Please elaborate


I have some NOS 200CD replacement parts here (caps and light bulbs.)

I'd think that the first step would be to replace the tubes, wouldn't
it?

If you send me your serial number I can tell you how early or late
yours is.


005-28015


Thanks
Norm

  #13   Report Post  
Jim Adney
 
Posts: n/a
Default

On Wed, 22 Sep 2004 00:48:27 GMT "Norm Dresner" wrote:

See replies in-line.

"Jim Adney" wrote in message
.. .

I'm sure that with Jim William's help you could do a good job at this,
but the 200CD was really a very nice instrument by the time HP
discontinued it. I think it had a nearly 30 year lifetime, during
which it got many significant upgrades. If you have an early one,
there might be some reason to "rebuild" it, but I suspect that it
would be hard to do better than the late versions. Plus it would take
a lot less time to fix than to repair.


I'm not quite sure what the distinction you're making between "fixing" and
"repairing". Please elaborate.


Oops, sorry, Mind fade. Make that fix vs. "rebuild."

I have some NOS 200CD replacement parts here (caps and light bulbs.)


I think that the first step would be to replace the tubes, wouldn't it?


Not necessarily, but I'll admit that mine needed tubes. It worked fine
on the higher frequencies, but crapped out down low. The longer period
at low frequency operation appeared to be able to completely dissipate
the space charge in the tubes, while they worked just fine up higher.

If you send me your serial number I can tell you how early or late
yours is.


005-28015


1965 That would be a middle era for the 200CD. I think HP sold these
from about '50 or '55 thru '77. Not great, but not bad either. OTOH,
none of them were awful. ;-)

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------
  #14   Report Post  
Norm Dresner
 
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Default

"Jim Adney" wrote in message
...
On Wed, 22 Sep 2004 00:48:27 GMT "Norm Dresner" wrote:

See replies in-line.

"Jim Adney" wrote in message
.. .

I'm sure that with Jim William's help you could do a good job at this,
but the 200CD was really a very nice instrument by the time HP
discontinued it. I think it had a nearly 30 year lifetime, during
which it got many significant upgrades. If you have an early one,
there might be some reason to "rebuild" it, but I suspect that it
would be hard to do better than the late versions. Plus it would take
a lot less time to fix than to repair.


I'm not quite sure what the distinction you're making between "fixing"

and
"repairing". Please elaborate.


Oops, sorry, Mind fade. Make that fix vs. "rebuild."


Still, there are published schematics for Wein Bridge oscillators with
.001% THD and the hardest parts to find to implement a continuously
variable one are dual tracking pots or variable capacitors. The dual
capacitor in the 200CD looks to be "ideal" for putting into a modern design.
If I decide that the criterion for deciding which way to go is just the
quality of the result, there's no contest, solid state wins hands-down. If
it's effort, changing a few tubes can't be beat. But if it's the ratio,
quality/effort, then it's a much harder decision. But I'm coming up on
retirement very rapidly so the equation changes drastically ...

Norm

  #15   Report Post  
Chuck Harris
 
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Default

Norm Dresner wrote:
I have an HP200CD that needs some loving care. The manual says to measure
the difference in the voltage between two different points in the circuit --
it shouldn't be all that much but the voltages between these points and
ground is in the hundreds of volts range. The manual also says that I
should be using a meter with 220M ohm or greater input impedance to make the
voltage measurements. Any suggestions where such a beast could be
bought/rented/stolen?

TIA
Norm


The only real reason that HP required you to use a 220M ohm input impedance
meter is their HP410C was a 220M ohm input impedance meter. As was their
earlier HP410B, and earlier HP412A.

-Chuck


  #16   Report Post  
Norm Dresner
 
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Default

"Chuck Harris" wrote in message
...
Norm Dresner wrote:
I have an HP200CD that needs some loving care. The manual says to

measure
the difference in the voltage between two different points in the

circuit --
it shouldn't be all that much but the voltages between these points and
ground is in the hundreds of volts range. The manual also says that I
should be using a meter with 220M ohm or greater input impedance to make

the
voltage measurements. Any suggestions where such a beast could be
bought/rented/stolen?

TIA
Norm


The only real reason that HP required you to use a 220M ohm input

impedance
meter is their HP410C was a 220M ohm input impedance meter. As was their
earlier HP410B, and earlier HP412A.


Gotcha. It's pretty much a nonstandard value these days -- and I think it
was back then too.

Related questions: The HP200CD service manual says, in part, "The DC
voltage between the cathodes (pin 3) of the 6AU5 tubes should be 1 volt or
less. If this voltage is excessive, it indicates a bad tube in the
oscillator (V1-V4) which must be replaced to insure low distortion in the
output waveform."

1. What is "excessive"? 1.5V? 1.1V? anything over 1.000V? ...

2. The cathode voltage for each tube is given as 3.5V. Clearly if one of
them is way off, it's pretty much the one that should be replaced. What if
they're both close but the difference is a little over 1.0V? Should I just
replace both?

3. Should I just replace all 4 tubes in the circuit? There's also a
rectifier tube whose output is filtered by a couple of 10 uF capacitors
[actually three sections of a single capacitor) and a 6H choke. How much
60/120 Hz ripple on the DC output of the power supply would suggest that the
tube and/or the capacitors should be replaced too?

4. The really hard question: Am I still trying to polish a turd and should
just replace all of the electronics with a good quality IC-based Wein Bridge
using the dual variable capacitors as the basis for this?

Norm

  #17   Report Post  
James Sweet
 
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1. What is "excessive"? 1.5V? 1.1V? anything over 1.000V? ...


Anything over 1.0v is excessive.

2. The cathode voltage for each tube is given as 3.5V. Clearly if one of
them is way off, it's pretty much the one that should be replaced. What

if
they're both close but the difference is a little over 1.0V? Should I

just
replace both?


Can't help you there.

3. Should I just replace all 4 tubes in the circuit? There's also a
rectifier tube whose output is filtered by a couple of 10 uF capacitors
[actually three sections of a single capacitor) and a 6H choke. How much
60/120 Hz ripple on the DC output of the power supply would suggest that

the
tube and/or the capacitors should be replaced too?


Have you tested the tubes? How much are replacements? If they're cheap then
it wouldn't hurt to replace them to see if it helps. Don't forget to change
out any electrolytic capacitors too.


4. The really hard question: Am I still trying to polish a turd and

should
just replace all of the electronics with a good quality IC-based Wein

Bridge
using the dual variable capacitors as the basis for this?


I would absolutely attempt to restore function of the original instrument,
vacuum tubes can still perform very well and that oscillator will provide
excellent performance once you get it going.


  #18   Report Post  
Chuck Harris
 
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Norm Dresner wrote:

Related questions: The HP200CD service manual says, in part, "The DC
voltage between the cathodes (pin 3) of the 6AU5 tubes should be 1 volt or
less. If this voltage is excessive, it indicates a bad tube in the
oscillator (V1-V4) which must be replaced to insure low distortion in the
output waveform."

1. What is "excessive"? 1.5V? 1.1V? anything over 1.000V? ...


What they are trying to show is the balance between the different tubes.
Anything over 1V is excessive (in their opinion). If you let the balance
get too far off, the oscillator will be prone to distortion, and hum.

2. The cathode voltage for each tube is given as 3.5V. Clearly if one of
them is way off, it's pretty much the one that should be replaced. What if
they're both close but the difference is a little over 1.0V? Should I just
replace both?


It probably doesn't matter what the voltage is exactly, as long as it is
around 3.5V, and less than 1.0V difference.

3. Should I just replace all 4 tubes in the circuit? There's also a
rectifier tube whose output is filtered by a couple of 10 uF capacitors
[actually three sections of a single capacitor) and a 6H choke. How much
60/120 Hz ripple on the DC output of the power supply would suggest that the
tube and/or the capacitors should be replaced too?


I would suggest that you pay more attention to how much ripple is on the
output of the oscillator. What I generally do is test the caps with a good
capacitor tester. If they seem a little leaky, I reform them for a while.
If they don't settle down to very minimal leakage current, or if the ESR is too high,
I replace them.

4. The really hard question: Am I still trying to polish a turd and should
just replace all of the electronics with a good quality IC-based Wein Bridge
using the dual variable capacitors as the basis for this?

Norm


Keep the 200CD! They are virtually bullet proof. They will last virtually
forever. You can always build an IC Wein bridge oscillator if you really need
super low distortion.

-Chuck
  #19   Report Post  
Norm Dresner
 
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As a semi-final coda to this saga, I've ordered a set of three [one spare]
6SH7 tubes and I'm actively looking for a good source for the 6AV5GTs as
well. I'm also looking to pick up on eBay a non-working HP 33x distortion
analyzer or similar oscillator from which to scrounge the dual tuning
capacitors so I can make a solid state low-distortion instrument as well.

I'll also look into replacing the electrolytic capacitors.

Thanks for all of your help and suggestions

Norm

  #20   Report Post  
James Sweet
 
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"Norm Dresner" wrote in message
...

As a semi-final coda to this saga, I've ordered a set of three [one spare]
6SH7 tubes and I'm actively looking for a good source for the 6AV5GTs as
well. I'm also looking to pick up on eBay a non-working HP 33x distortion
analyzer or similar oscillator from which to scrounge the dual tuning
capacitors so I can make a solid state low-distortion instrument as well.

I'll also look into replacing the electrolytic capacitors.

Thanks for all of your help and suggestions

Norm


Is there any reason you couldn't gang together a pair of ordinary variable
capacitors using a gear or belt system? You could probably get away with
some cord as used in tuning dials on old analog tuned radios too.




  #21   Report Post  
Norm Dresner
 
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"James Sweet" wrote in message
news:Leu5d.1139$ku4.699@trnddc01...

"Norm Dresner" wrote in message
...

As a semi-final coda to this saga, I've ordered a set of three [one

spare]
6SH7 tubes and I'm actively looking for a good source for the 6AV5GTs as
well. I'm also looking to pick up on eBay a non-working HP 33x

distortion
analyzer or similar oscillator from which to scrounge the dual tuning
capacitors so I can make a solid state low-distortion instrument as

well.

I'll also look into replacing the electrolytic capacitors.

Thanks for all of your help and suggestions

Norm


Is there any reason you couldn't gang together a pair of ordinary variable
capacitors using a gear or belt system? You could probably get away with
some cord as used in tuning dials on old analog tuned radios too.



I could also do the same thing with a pair of pots. The advantage of the
dual caps from a factory-built oscillator is that the hard mechanical work
is all done -- and they're presumably matched well enough. It's simply less
work. BTW, it's a lot easier to get 10-turn pots than even one-turn
capacitors these days..

Norm

  #22   Report Post  
James Sweet
 
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I could also do the same thing with a pair of pots. The advantage of the
dual caps from a factory-built oscillator is that the hard mechanical work
is all done -- and they're presumably matched well enough. It's simply

less
work. BTW, it's a lot easier to get 10-turn pots than even one-turn
capacitors these days..

Norm



Yeah, just thought there might be some particular reason you needed variable
caps.


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