Topics

130 LC meter

Bob Albert
 

Is there anyone familiar with this unit? I have one that isn't working and have traced the problem to the variable oscillator not oscillating.

It's plate voltage is too low but everything seems good otherwise. Too low for enough gain to oscillate. The tubes are good and the transformer resonates. There is less than one volt on the plate of V4A.

Craig Sawyers <c.sawyers@...>
 

Have you checked the smoothing capacitor on the power supply? Mine was open circuit.

Craig

Is there anyone familiar with this unit? I have one that isn't working and have traced the problem
to
the variable oscillator not oscillating.

Jack Wills
 

My Tek 130 had several Sprague Black Beauty capacitors in the oscillator
circuit

All were bad. I've had enough trouble (and been shocked enough) with these
caps
that I replace them on sight.

On Sat, Oct 19, 2019 at 1:05 AM Craig Sawyers <c.sawyers@...>
wrote:

Have you checked the smoothing capacitor on the power supply? Mine was
open circuit.

Craig

Is there anyone familiar with this unit? I have one that isn't working
and have traced the problem
to
the variable oscillator not oscillating.




Bob Albert
 

I have spent lots of time and lost lots of sleep over this.  I got up in the middle of the night last night and discovered that the problem appears to be a defective tube socket.  Not the best news but I'll probably bite the bullet after a few more tests.
I measure a variable resistance from one tube pin to ground, and see a spiky noise coming from the next stage.
Bob

On Saturday, October 19, 2019, 01:05:56 AM PDT, Craig Sawyers <c.sawyers@...> wrote:

Have you checked the smoothing capacitor on the power supply? Mine was open circuit.

Craig

Is there anyone familiar with this unit?  I have one that isn't working and have traced the problem
to
the variable oscillator not oscillating.

John
 

Bob: have you sorted the problem? Was it the tube socket?
John

Bob Albert
 

John, I replaced the socket and it's not working.  However there seems to be a weak oscillation which I can adjust with the transformer.  It's not much amplitude and insufficient for the unit to operate.
I took it off the bench in frustration.  Not sure what to do now.  I saw this oscillator has a 1 meg resistoro not present in the other one so I shorted it but no difference.  Maybe I made a wiring error.
Bob

On Monday, November 4, 2019, 02:22:14 AM PST, John <john@...> wrote:

Bob: have you sorted the problem? Was it the tube socket?
John

Ed Breya
 

Did you see Jack's advice earlier, about the black-beauty or other brand paper caps? I'd recommend looking closely at the few that are in there. If they get leaky, it can upset the DC grid bias on a tube stage, forcing it away from proper level and action. I think your OP mentioned the oscillator plate DC was very low, which could be a symptom of bad grid bias.

I have a pristine 130, still sporting the original caps, and working fine, but have a set of modern plastic ones ready to go in for when the time comes.

Ed

Bob Albert
 

Thanks for the heads up Ed.  I will take another look at the parts but as I recall there are no paper capacitors in that part of the circuit.  A few ceramic discs and a couple of mica.
When I get motivated I will measure the voltages again and see what's what.
Bob

On Monday, November 4, 2019, 02:56:32 PM PST, Ed Breya via Groups.Io <edbreya=yahoo.com@groups.io> wrote:

Did you see Jack's advice earlier, about the black-beauty or other brand paper caps? I'd recommend looking closely at the few that are in there. If they get leaky, it can upset the DC grid bias on a tube stage, forcing it away from proper level and action. I think your OP mentioned the oscillator plate DC was very low, which could be a symptom of bad grid bias.

I have a pristine 130, still sporting the original caps, and working fine, but have a set of modern plastic ones ready to go in for when the time comes.

Ed

Ed Breya
 

Ceramic caps and even micas can get leaky after many years - not often, but I've seen it. Ed

Craig Sawyers <c.sawyers@...>
 

Mica caps are a known weak point in the Racal RA17 receiver. They are not a replace-on-sight because
there are lots of them, but worth bearing in mind that they can occasionally be problematic.

Craig

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Ed Breya via Groups.Io
Sent: 05 November 2019 00:04
To: TekScopes@groups.io
Subject: Re: [TekScopes] 130 LC meter

Ceramic caps and even micas can get leaky after many years - not often, but I've seen it. Ed

Bob Albert
 

Inthis part of the 130 therre are only two micas I believe.  I will direct my attention there to see if they are causing a problem.  Somehow I don't think that's it, but we shall see.
I am wondering why the two oscillators are different.  The variable one has a few extra components and I don't know why.
Bob

On Tuesday, November 5, 2019, 12:11:04 AM PST, Craig Sawyers <c.sawyers@...> wrote:

Mica caps are a known weak point in the Racal RA17 receiver. They are not a replace-on-sight because
there are lots of them, but worth bearing in mind that they can occasionally be problematic.

Craig

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Ed Breya via Groups.Io
Sent: 05 November 2019 00:04
To: TekScopes@groups.io
Subject: Re: [TekScopes] 130 LC meter

Ceramic caps and even micas can get leaky after many years - not often, but I've seen it. Ed

Richard Knoppow
 

There are two kinds of mica caps, silvered mica and plain mica. Silver mica of a certain period suffer from something called silver mica desease, probably due to moisture penetration through the Bakelite casings. More modern epoxy dipped caps do not seem to have this problem and it is only occasionally found on the Bakelite type. It results in the cap becoming unstable, sometimes jumping in value and sometimes having "scintillation" or rapid variations. One should be aware of it but mica caps do not have the predictable degradation with time of paper caps. The other type of mica caps, usually found a HV transmitting caps, are made of stacks of mica dielectric and foil pressed together and held under pressure. They seem to be nearly perfectly reliable but silver mica have better HF characteristics.
Ceramic caps depend on the type of ceramic. Those made with high K material do age and change value. They also have considerable voltage coefficient of capacitance. The ones made with low K such as zero TC caps, are extremely stable. Some of these caps fail, particularly the disc type, because the leads have been stressed where they enter the body and have cracked it allowing some air and moisture to enter. Disc caps can fail so should never be left out of trouble shooting routine but neither mica or ceramic need to be shot gunned as do paper or electrolytic caps.

On 11/5/2019 12:10 AM, Craig Sawyers wrote:
Mica caps are a known weak point in the Racal RA17 receiver. They are not a replace-on-sight because
there are lots of them, but worth bearing in mind that they can occasionally be problematic.
Craig

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Ed Breya via Groups.Io
Sent: 05 November 2019 00:04
To: TekScopes@groups.io
Subject: Re: [TekScopes] 130 LC meter

Ceramic caps and even micas can get leaky after many years - not often, but I've seen it. Ed
--
Richard Knoppow
dickburk@...
WB6KBL

Bob Albert
 

Yes I am aware of many of the failings of various capacitor types.  However, I am not convinced that the problem in this circuit is caused by a poorly behaving capacitor.  I will once again attempt to measure voltages and even a few components to see where something isn't right.
For now, though, my attention is directed elsewhere.  I have other means of making measurements that the 130 can do, so my interest is only to have some fun with the unit.  If I never get it working, it still is a vehicle for education.  The lowest range, 0 - 3 pF, intrigues me but I can make those measurements with the Q meter if need be.  I have a few 2 pF, I think, capacitors that I want to characterize.  They are the ceramic dogbone style.
I also have a box of variable capacitors but those are easy.  It pains me to have these, as I have no projected use for them and they are almost works of art, with the lovely bearings and shafts and other moving parts.  Back in the day, one might build an amplifier or tuner, but I don't need either.  So I will stuff them into the closet and let my heirs dispose of it all, as they will be obliged to do with all my precious possessions.  It might be prudent for some local enthusiast to make friends with me and my family in anticipation of this event.
Bob

On Tuesday, November 5, 2019, 10:26:50 AM PST, Richard Knoppow <dickburk@...> wrote:

  There are two kinds of mica caps, silvered mica and plain
mica. Silver mica of a certain period suffer from something
called silver mica desease, probably due to moisture penetration
through the Bakelite casings. More modern epoxy dipped caps do
not seem to have this problem and it is only occasionally found
on the Bakelite type. It results in the cap becoming unstable,
sometimes jumping in value and sometimes having "scintillation"
or rapid variations. One should be aware of it but mica caps do
not have the predictable degradation with time of paper caps. The
other type of mica caps, usually found a HV transmitting caps,
are made of stacks of mica dielectric and foil pressed together
and held under pressure. They seem to be nearly perfectly
reliable but silver mica have better HF characteristics.
    Ceramic caps depend on the type of ceramic. Those made with
high K material do age and change value. They also have
considerable voltage coefficient of capacitance. The ones made
with low K such as zero TC caps, are extremely stable. Some of
these caps fail, particularly the disc type, because the leads
have been stressed where they enter the body and have cracked it
allowing some air and moisture to enter. Disc caps can fail so
should never be left out of trouble shooting routine but neither
mica or ceramic need to be shot gunned as do paper or
electrolytic caps.

On 11/5/2019 12:10 AM, Craig Sawyers wrote:
Mica caps are a known weak point in the Racal RA17 receiver. They are not a replace-on-sight because
there are lots of them, but worth bearing in mind that they can occasionally be problematic.

Craig

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Ed Breya via Groups.Io
Sent: 05 November 2019 00:04
To: TekScopes@groups.io
Subject: Re: [TekScopes] 130 LC meter

Ceramic caps and even micas can get leaky after many years - not often, but I've seen it. Ed




--
Richard Knoppow
dickburk@...
WB6KBL

Rajesh VS
 

Bob,in case it helps..my LC130 was misbehaving when I acquired it, and
the cause was a broken solder joint around the Zero adjust controls (front
Panel) . Don't recollect if it was in the fine or coarse control.
Location is top left in the pic -
http://w140.com/tekwiki/wiki/130#/media/File:Tek_Type130_Inside-1.jpg



On Tue, Nov 5, 2019 at 11:00 AM Bob Albert via Groups.Io <bob91343=
yahoo.com@groups.io> wrote:

Yes I am aware of many of the failings of various capacitor types.
However, I am not convinced that the problem in this circuit is caused by a
poorly behaving capacitor. I will once again attempt to measure voltages
and even a few components to see where something isn't right.
For now, though, my attention is directed elsewhere. I have other means
of making measurements that the 130 can do, so my interest is only to have
some fun with the unit. If I never get it working, it still is a vehicle
for education. The lowest range, 0 - 3 pF, intrigues me but I can make
those measurements with the Q meter if need be. I have a few 2 pF, I
think, capacitors that I want to characterize. They are the ceramic
dogbone style.
I also have a box of variable capacitors but those are easy. It pains me
to have these, as I have no projected use for them and they are almost
works of art, with the lovely bearings and shafts and other moving parts.
Back in the day, one might build an amplifier or tuner, but I don't need
either. So I will stuff them into the closet and let my heirs dispose of
it all, as they will be obliged to do with all my precious possessions. It
might be prudent for some local enthusiast to make friends with me and my
family in anticipation of this event.
Bob
On Tuesday, November 5, 2019, 10:26:50 AM PST, Richard Knoppow <
dickburk@...> wrote:

There are two kinds of mica caps, silvered mica and plain
mica. Silver mica of a certain period suffer from something
called silver mica desease, probably due to moisture penetration
through the Bakelite casings. More modern epoxy dipped caps do
not seem to have this problem and it is only occasionally found
on the Bakelite type. It results in the cap becoming unstable,
sometimes jumping in value and sometimes having "scintillation"
or rapid variations. One should be aware of it but mica caps do
not have the predictable degradation with time of paper caps. The
other type of mica caps, usually found a HV transmitting caps,
are made of stacks of mica dielectric and foil pressed together
and held under pressure. They seem to be nearly perfectly
reliable but silver mica have better HF characteristics.
Ceramic caps depend on the type of ceramic. Those made with
high K material do age and change value. They also have
considerable voltage coefficient of capacitance. The ones made
with low K such as zero TC caps, are extremely stable. Some of
these caps fail, particularly the disc type, because the leads
have been stressed where they enter the body and have cracked it
allowing some air and moisture to enter. Disc caps can fail so
should never be left out of trouble shooting routine but neither
mica or ceramic need to be shot gunned as do paper or
electrolytic caps.

On 11/5/2019 12:10 AM, Craig Sawyers wrote:
Mica caps are a known weak point in the Racal RA17 receiver. They are
not a replace-on-sight because
there are lots of them, but worth bearing in mind that they can
occasionally be problematic.

Craig

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Ed
Breya via Groups.Io
Sent: 05 November 2019 00:04
To: TekScopes@groups.io
Subject: Re: [TekScopes] 130 LC meter

Ceramic caps and even micas can get leaky after many years - not often,
but I've seen it. Ed





--
Richard Knoppow
dickburk@...
WB6KBL






--
/Rajesh

Bob Albert
 

Thanks but that's not it.  The zero adjust controls work okay and the problem is simply that the variable oscillator doesn't oscillate.  I have a tiny signal from it whose frequency can be varied a bit with the tuning slug.  But it's nowhere near enough to drive the mixer.
Bob

On Tuesday, November 5, 2019, 11:54:04 AM PST, Rajesh VS <rajeshvs@...> wrote:

Bob,in case it helps..my  LC130 was misbehaving when I acquired it,  and
the cause was a broken solder joint around the Zero adjust controls (front
Panel) . Don't recollect if it was in the fine or coarse control.
Location is top left in the pic -
http://w140.com/tekwiki/wiki/130#/media/File:Tek_Type130_Inside-1.jpg



On Tue, Nov 5, 2019 at 11:00 AM Bob Albert via Groups.Io <bob91343=
yahoo.com@groups.io> wrote:

  Yes I am aware of many of the failings of various capacitor types.
However, I am not convinced that the problem in this circuit is caused by a
poorly behaving capacitor.  I will once again attempt to measure voltages
and even a few components to see where something isn't right.
For now, though, my attention is directed elsewhere.  I have other means
of making measurements that the 130 can do, so my interest is only to have
some fun with the unit.  If I never get it working, it still is a vehicle
for education.  The lowest range, 0 - 3 pF, intrigues me but I can make
those measurements with the Q meter if need be.  I have a few 2 pF, I
think, capacitors that I want to characterize.  They are the ceramic
dogbone style.
I also have a box of variable capacitors but those are easy.  It pains me
to have these, as I have no projected use for them and they are almost
works of art, with the lovely bearings and shafts and other moving parts.
Back in the day, one might build an amplifier or tuner, but I don't need
either.  So I will stuff them into the closet and let my heirs dispose of
it all, as they will be obliged to do with all my precious possessions.  It
might be prudent for some local enthusiast to make friends with me and my
family in anticipation of this event.
Bob
    On Tuesday, November 5, 2019, 10:26:50 AM PST, Richard Knoppow <
dickburk@...> wrote:

    There are two kinds of mica caps, silvered mica and plain
mica. Silver mica of a certain period suffer from something
called silver mica desease, probably due to moisture penetration
through the Bakelite casings. More modern epoxy dipped caps do
not seem to have this problem and it is only occasionally found
on the Bakelite type. It results in the cap becoming unstable,
sometimes jumping in value and sometimes having "scintillation"
or rapid variations. One should be aware of it but mica caps do
not have the predictable degradation with time of paper caps. The
other type of mica caps, usually found a HV transmitting caps,
are made of stacks of mica dielectric and foil pressed together
and held under pressure. They seem to be nearly perfectly
reliable but silver mica have better HF characteristics.
    Ceramic caps depend on the type of ceramic. Those made with
high K material do age and change value. They also have
considerable voltage coefficient of capacitance. The ones made
with low K such as zero TC caps, are extremely stable. Some of
these caps fail, particularly the disc type, because the leads
have been stressed where they enter the body and have cracked it
allowing some air and moisture to enter. Disc caps can fail so
should never be left out of trouble shooting routine but neither
mica or ceramic need to be shot gunned as do paper or
electrolytic caps.

On 11/5/2019 12:10 AM, Craig Sawyers wrote:
Mica caps are a known weak point in the Racal RA17 receiver. They are
not a replace-on-sight because
there are lots of them, but worth bearing in mind that they can
occasionally be problematic.

Craig

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Ed
Breya via Groups.Io
Sent: 05 November 2019 00:04
To: TekScopes@groups.io
Subject: Re: [TekScopes] 130 LC meter

Ceramic caps and even micas can get leaky after many years - not often,
but I've seen it. Ed





--
Richard Knoppow
dickburk@...
WB6KBL






--
/Rajesh

Rajesh VS
 

if you lift one leg of C22 (towards V15A), it will isolate the variable
oscillator, and then you can troubleshoot it without involving other parts
of the circuit.
Same with C17 - the amplifier for variable oscillator - where you can
disconnect it from mixer and check its input and output.
Check V4 heater voltage, and I hope C402 was checked. I cant recollect
precisely, but I had issues around these components as well.
if you want I can check the DC bias voltages for V4 and update you over
weekend.


On Tue, Nov 5, 2019 at 1:48 PM Bob Albert via Groups.Io <bob91343=
yahoo.com@groups.io> wrote:

Thanks but that's not it. The zero adjust controls work okay and the
problem is simply that the variable oscillator doesn't oscillate. I have a
tiny signal from it whose frequency can be varied a bit with the tuning
slug. But it's nowhere near enough to drive the mixer.
Bob
On Tuesday, November 5, 2019, 11:54:04 AM PST, Rajesh VS <
rajeshvs@...> wrote:

Bob,in case it helps..my LC130 was misbehaving when I acquired it, and
the cause was a broken solder joint around the Zero adjust controls (front
Panel) . Don't recollect if it was in the fine or coarse control.
Location is top left in the pic -
http://w140.com/tekwiki/wiki/130#/media/File:Tek_Type130_Inside-1.jpg



On Tue, Nov 5, 2019 at 11:00 AM Bob Albert via Groups.Io <bob91343=
yahoo.com@groups.io> wrote:

Yes I am aware of many of the failings of various capacitor types.
However, I am not convinced that the problem in this circuit is caused
by a
poorly behaving capacitor. I will once again attempt to measure voltages
and even a few components to see where something isn't right.
For now, though, my attention is directed elsewhere. I have other means
of making measurements that the 130 can do, so my interest is only to
have
some fun with the unit. If I never get it working, it still is a vehicle
for education. The lowest range, 0 - 3 pF, intrigues me but I can make
those measurements with the Q meter if need be. I have a few 2 pF, I
think, capacitors that I want to characterize. They are the ceramic
dogbone style.
I also have a box of variable capacitors but those are easy. It pains me
to have these, as I have no projected use for them and they are almost
works of art, with the lovely bearings and shafts and other moving parts.
Back in the day, one might build an amplifier or tuner, but I don't need
either. So I will stuff them into the closet and let my heirs dispose of
it all, as they will be obliged to do with all my precious possessions.
It
might be prudent for some local enthusiast to make friends with me and my
family in anticipation of this event.
Bob
On Tuesday, November 5, 2019, 10:26:50 AM PST, Richard Knoppow <
dickburk@...> wrote:

There are two kinds of mica caps, silvered mica and plain
mica. Silver mica of a certain period suffer from something
called silver mica desease, probably due to moisture penetration
through the Bakelite casings. More modern epoxy dipped caps do
not seem to have this problem and it is only occasionally found
on the Bakelite type. It results in the cap becoming unstable,
sometimes jumping in value and sometimes having "scintillation"
or rapid variations. One should be aware of it but mica caps do
not have the predictable degradation with time of paper caps. The
other type of mica caps, usually found a HV transmitting caps,
are made of stacks of mica dielectric and foil pressed together
and held under pressure. They seem to be nearly perfectly
reliable but silver mica have better HF characteristics.
Ceramic caps depend on the type of ceramic. Those made with
high K material do age and change value. They also have
considerable voltage coefficient of capacitance. The ones made
with low K such as zero TC caps, are extremely stable. Some of
these caps fail, particularly the disc type, because the leads
have been stressed where they enter the body and have cracked it
allowing some air and moisture to enter. Disc caps can fail so
should never be left out of trouble shooting routine but neither
mica or ceramic need to be shot gunned as do paper or
electrolytic caps.

On 11/5/2019 12:10 AM, Craig Sawyers wrote:
Mica caps are a known weak point in the Racal RA17 receiver. They are
not a replace-on-sight because
there are lots of them, but worth bearing in mind that they can
occasionally be problematic.

Craig

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of
Ed
Breya via Groups.Io
Sent: 05 November 2019 00:04
To: TekScopes@groups.io
Subject: Re: [TekScopes] 130 LC meter

Ceramic caps and even micas can get leaky after many years - not
often,
but I've seen it. Ed





--
Richard Knoppow
dickburk@...
WB6KBL






--
/Rajesh






--
/Rajesh

 

The old silver mica caps were used in lots of antique AM broadcast radios (1930s -1960s), particularly in the IF stages. They were known to cause a phenomenon called "thunderstorming", which, even when receiving a strong station, caused a random loud static that sounded exactly like a nearby thunderstorm. This was caused by the silver deposits on the surfaces of the mica layers actually migrating toward the edges of the mica slabs. The high voltages used in those old tube radios drew the silver atoms toward the edges of the mica, and when enough silver migrated to the edges, there was enough current leakage in the capacitor to cause the "static" noise.
Of course, the only solution was to replace the old micas with new ones and retune the IF stages.

Spoken from experience {:>)
Dave M

Craig Sawyers <c.sawyers@...>
 

Agreed - that was my experience with the RA17. Of course I have a 130 LC meter (no surprise there!)
and the only thing I had to do was replace the smoothing electrolytic, which had gone down with the
"Sprague twist-lock disconnection issue". Haven't fired it up in quite a while; must do so and make
sure it is still alive.

Craig

The old silver mica caps were used in lots of antique AM broadcast radios (1930s -1960s),
particularly in
the IF stages. They were known to cause a phenomenon called "thunderstorming", which, even
when receiving a strong station, caused a random loud static that sounded exactly like a nearby
thunderstorm. This was caused by the silver deposits on the surfaces of the mica layers actually
migrating toward the edges of the mica slabs. The high voltages used in those old tube radios drew
the silver atoms toward the edges of the mica, and when enough silver migrated to the edges, there
was enough current leakage in the capacitor to cause the "static" noise.
Of course, the only solution was to replace the old micas with new ones and retune the IF stages.

Spoken from experience {:>)
Dave M

Dave Wise
 

Not the OP's problem but it's about the 130. I want to document some interesting symptoms and their causes.

I calibrated it using Albert Otten's 2016 topic "Tpe 130 L-C meter calibration using only a 300 pF cap", https://groups.io/g/TekScopes/topic/7656928#126515 .
It's much easier than acquiring an S-30, all you need is one good close cap and a frequency counter. Few labs had counters when the 130 was born.
Since I had the counter connected, I decided to try calibrating C7, Resistance Compensation.

SYMPTOM: Not possible to match readings 1M vs 100K, C7 at minimum still too big.
FIX: Reducing R8 helps. I put in 680K instead of 1M.

NOTES:
Carbon film and metal film have less parasitic C than carbon comp at these values and frequencies.
This helped, but still some tubes work in V4, some don't. I can't predict it.
Fewer fail after the mod.
You can substitute 6GH8 and 6EA8 in place of 6U8.
All my 6EA8s work; the Amperex 6GH8 fails. So does at least one 6U8.
It doesn't matter how it tests; some weak tubes work and some strong ones fail.
I measured my 1M/100K parasitic C with GR 1656 at 10kHz.
It's about the same as 1kHz.
I used Conductance mode with AC excitation, and an external variable cap on the bridge arm to balance out the parasitic.
Then measured the variable cap. You have to push it through an equation to get the true C instead of a multiple due to the ratios.
Old black Welwyn 1% spiral-cut carbon? film resistors in my junk box which happen to be the type used in early production of S-30: 100K is 0.4pF, 1M is 0.5pF .
So I adjust R8 so 1M reads 0.1pF higher than 100K.

SYMPTOM: C99 and C100. Tek installed them backwards, they're
drawn backwards on the schematic too.
FIX: Probably EVERY 130 needs them replaced, with anode facing positive voltage.

SYMPTOM: 10pF reads high on 30pF range.
CAUSE: Dielectric Absorption in C92 .0047/400 charge pump cap.
This is the first failure I've ever seen that was traceable to DA.
It makes the cap appear larger than it should be, and changing with frequency,
so readings that are low on the scale pump too much charge and get skewed upwards.
NOTES:
You can measure C92, C93 etc in-circuit if the 130 range switch is OFF.
The cap is not leaky.
It's a Sprague 160P DiFilm. (Plastic/paper dielectric.)
Use GR 1680-A to measure C at 1kHz and 400Hz.
Should read 4.7nF, but I see 6.5nF at 1kHz, 7.5nF at 400Hz. Yep.
An ideal cap will be constant C, a cap with DA will increase with decreasing f.
Later I tested it using the soak-discharge-wait-measure method. Significant rebound,
over 10V after 100V soak. That will mess up a charge pump!
FIX: Replaced with polypropylene. Also replaced C93 .015/400 10pF range because
it had some DA too.
Joke's on me - now 10pF on 30pF range reads a bit low where it used to read high.
I bet those 160P caps had some DA when new, and Tek drew the meter scale to fit.
Maybe I should have used something with more DA. Oh well, |error| is smaller now at least.
LESSON: Replace all paper/plastic caps in positions sensitive to Dielectric Absorption,
even if they test good for leakage.

HTH,
Dave Wise

Albert Otten
 

Hi Dave,

Nice to read that at least one person was content with that frequency based calibration method.
Some remarks follow below.

Albert

On Thu, Jan 2, 2020 at 09:21 PM, Dave Wise wrote:


Not the OP's problem but it's about the 130. I want to document some
interesting symptoms and their causes.

I calibrated it using Albert Otten's 2016 topic "Tpe 130 L-C meter calibration
using only a 300 pF cap", https://groups.io/g/TekScopes/topic/7656928#126515 .
It's much easier than acquiring an S-30, all you need is one good close cap
and a frequency counter. Few labs had counters when the 130 was born.
Since I had the counter connected, I decided to try calibrating C7, Resistance
Compensation.

SYMPTOM: Not possible to match readings 1M vs 100K, C7 at minimum still too
big.
FIX: Reducing R8 helps. I put in 680K instead of 1M.
It looks like my C7 is also at the in one extreme but I don't take the risk (of breaking) to adjust his trimcap.

NOTES:
Carbon film and metal film have less parasitic C than carbon comp at these
values and frequencies.
...
Old black Welwyn 1% spiral-cut carbon? film resistors in my junk box which
happen to be the type used in early production of S-30: 100K is 0.4pF, 1M is
0.5pF . > So I adjust R8 so 1M reads 0.1pF higher than 100K.
I didn't try to measure parasitic capacitance of a 1M or 100k resistor. But I did several measurements with resistor clamped in the fixture, excessive lead length cut away. I inserted a resistor, set the offset for a reading at 1.5 pF, the quickly removed the resistor and noted the drop in the reading. Essentially always the reading dropped by 0.25 to 3 pF, for different R values and different ratings (1/2, 1/4, 1/8 W), metal film and cc. So I should be content with the setting of C7 I think. The big Welwyn resistors (body over 1") give a drop of 1 pF or slightly more.
There is also a remark in the manual about line voltage sensitivity when C7 has not enough range. You might look at this.

SYMPTOM: C99 and C100. Tek installed them backwards, they're
drawn backwards on the schematic too.
FIX: Probably EVERY 130 needs them replaced, with anode facing positive voltage.
This seems so obvious that we might guess that Tek had a special reason for this. At full scale deflection the (reverse) voltage is 0.75 V. That might or might not be high enough to cause damage? Could it give some meter protection when the multivibrator frequency approaches zero?

SYMPTOM: 10pF reads high on 30pF range.
CAUSE: Dielectric Absorption in C92 .0047/400 charge pump cap.
This is the first failure I've ever seen that was traceable to DA.
It makes the cap appear larger than it should be, and changing with frequency,
so readings that are low on the scale pump too much charge and get skewed upwards.
NOTES:
You can measure C92, C93 etc in-circuit if the 130 range switch is OFF.
The cap is not leaky.
It's a Sprague 160P DiFilm. (Plastic/paper dielectric.)
Use GR 1680-A to measure C at 1kHz and 400Hz.
Should read 4.7nF, but I see 6.5nF at 1kHz, 7.5nF at 400Hz. Yep.
An ideal cap will be constant C, a cap with DA will increase with decreasing f.
Later I tested it using the soak-discharge-wait-measure method. Significant rebound,
over 10V after 100V soak. That will mess up a charge pump!
FIX: Replaced with polypropylene. Also replaced C93 .015/400 10pF range because
it had some DA too.
I measured C91 to C95 both with the GR 1656 (1 kHz) and with a cheap Aldi DMM. The 2 readings were remarkably close, within 0.5%, except for 3.5% in C91. Also the differences from nominal were <10%, except again for C91. The measurement principles are very different. Hence I would trust C92 to C95 but question C91.

Joke's on me - now 10pF on 30pF range reads a bit low where it used to read high.
I bet those 160P caps had some DA when new, and Tek drew the meter scale to fit.
Maybe I should have used something with more DA. Oh well, |error| is smaller now at least.
LESSON: Replace all paper/plastic caps in positions sensitive to Dielectric Absorption,
even if they test good for leakage.
The scales are non-linear anyway since the frequency deviation is a nonlinear function of the capacitance under test. This is most pronounced at the 300 pF range but hardly visible at the 10 pF range. The 3 pF range can be considered as linear. I doubt that Tek corrected the scale for other effects. I didn't check my 130 yet with 10 pF on the 30 pF range but this should be easy using the frequency counter. Maybe later.

HTH,
Dave Wise