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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Default This has me puzzled (resistor change)

There is a video on youtube, where a guy re-caps and repairs and tests
out a late 1940's Precision Apparatus E-200 signal generator.

Video name:
"Repair of a late-'40's Precision E-200-C RF signal generator"

This sig gen has a built in audio tone modulator (oscillator). He
replaced a resistor in that circuit, which was out of tolerance. He
replaced it with a modern film type resistor. The audio modulation did
not work afterwards. After testing everything, he went and replaced that
resistor (again) with an old style carbon resisstor, and after that, the
modulation worked.

What the heck?????

I was always under the impression that film resistors are identical
(electrically), and their only difference is appearance. Now I learn
that is not true.....

WHY?????

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Default This has me puzzled (resistor change)

On Sun, 20 Jan 2019 16:33:41 -0600, tubeguy wrote:

I was always under the impression that film resistors are identical
(electrically), and their only difference is appearance. Now I learn
that is not true.....

WHY?????


Really need to see a schematic. AFAIK the only functional difference
between film and carbon is that latter are noisier.





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Default This has me puzzled (resistor change)

On 20-1-2019 23:50, Cursitor Doom wrote:
On Sun, 20 Jan 2019 16:33:41 -0600, tubeguy wrote:

I was always under the impression that film resistors are identical
(electrically), and their only difference is appearance. Now I learn
that is not true.....

WHY?????


Really need to see a schematic. AFAIK the only functional difference
between film and carbon is that latter are noisier.





A bulk carbon resistor has less self-inductance.
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On 1/20/19 5:50 PM, Cursitor Doom wrote:
On Sun, 20 Jan 2019 16:33:41 -0600, tubeguy wrote:

I was always under the impression that film resistors are identical
(electrically), and their only difference is appearance. Now I learn
that is not true.....

WHY?????


Really need to see a schematic. AFAIK the only functional difference
between film and carbon is that latter are noisier.


They also have a large negative voltage coefficient.

Cheers

Phil Hobbs

--
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Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

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Default This has me puzzled (resistor change)

On 1/20/2019 6:24 PM, Phil Hobbs wrote:
On 1/20/19 5:50 PM, Cursitor Doom wrote:
On Sun, 20 Jan 2019 16:33:41 -0600, tubeguy wrote:

I was always under the impression that film resistors are identical
(electrically), and their only difference is appearance. Now I learn
that is not true.....

WHY?????


Really need to see a schematic. AFAIK the only functional difference
between film and carbon is that latter are noisier.


They also have a large negative voltage coefficient.

Cheers

Phil Hobbs

Also, Carbon composition resistors will take spikes and surges that will
render a film resistor open circuit, and are more tolerant of short term
overloads.

Regards,
Tim



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Default This has me puzzled (resistor change)

On 2019/01/20 5:23 p.m., Tim Schwartz wrote:
On 1/20/2019 6:24 PM, Phil Hobbs wrote:
On 1/20/19 5:50 PM, Cursitor Doom wrote:
On Sun, 20 Jan 2019 16:33:41 -0600, tubeguy wrote:

I was always under the impression that film resistors are identical
(electrically), and their only difference is appearance. Now I learn
that is not true.....

WHY?????

Really need to see a schematic. AFAIK the only functional difference
between film and carbon is that latter are noisier.


They also have a large negative voltage coefficient.

Cheers

Phil Hobbs

Also, Carbon composition resistors will take spikes and surges that will
render a film resistor open circuit, and are more tolerant of short term
overloads.

Regards,
Tim


If I am not mistaken that is why film resistors are used where you want
flame-proof behaviour.

Carbon comp let out lots of magic smoke...

John :-#)#
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Default This has me puzzled (resistor change)

On Sunday, January 20, 2019 at 9:35:52 PM UTC-5, John Robertson wrote:
On 2019/01/20 5:23 p.m., Tim Schwartz wrote:
On 1/20/2019 6:24 PM, Phil Hobbs wrote:
On 1/20/19 5:50 PM, Cursitor Doom wrote:
On Sun, 20 Jan 2019 16:33:41 -0600, tubeguy wrote:

I was always under the impression that film resistors are identical
(electrically), and their only difference is appearance. Now I learn
that is not true.....

WHY?????

Really need to see a schematic. AFAIK the only functional difference
between film and carbon is that latter are noisier.

They also have a large negative voltage coefficient.

Cheers

Phil Hobbs

Also, Carbon composition resistors will take spikes and surges that will
render a film resistor open circuit, and are more tolerant of short term
overloads.

Regards,
Tim


If I am not mistaken that is why film resistors are used where you want
flame-proof behaviour.

Carbon comp let out lots of magic smoke...

John :-#)#



And when they don't, they change value in a big way when overheated.

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Default This has me puzzled (resistor change)

On Sunday, 20 January 2019 22:33:45 UTC, wrote:

There is a video on youtube, where a guy re-caps and repairs and tests
out a late 1940's Precision Apparatus E-200 signal generator.

Video name:
"Repair of a late-'40's Precision E-200-C RF signal generator"

This sig gen has a built in audio tone modulator (oscillator). He
replaced a resistor in that circuit, which was out of tolerance. He
replaced it with a modern film type resistor. The audio modulation did
not work afterwards. After testing everything, he went and replaced that
resistor (again) with an old style carbon resisstor, and after that, the
modulation worked.

What the heck?????

I was always under the impression that film resistors are identical
(electrically), and their only difference is appearance. Now I learn
that is not true.....

WHY?????


So IIUC he changes the R value and it no longer oscillates. Presumably he's thereby changed the loop gain. Where is the surprise?


NT
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Default This has me puzzled (resistor change)

On Mon, 21 Jan 2019 00:10:42 +0100, Sjouke Burry
wrote:

On 20-1-2019 23:50, Cursitor Doom wrote:
On Sun, 20 Jan 2019 16:33:41 -0600, tubeguy wrote:

I was always under the impression that film resistors are identical
(electrically), and their only difference is appearance. Now I learn
that is not true.....

WHY?????


Really need to see a schematic. AFAIK the only functional difference
between film and carbon is that latter are noisier.





A bulk carbon resistor has less self-inductance.


I kind of thought that might be the case....

I prefer using the old carbon resistors in vintage tube gear, but they
are getting hard to find and expensive. They are all NOS now. i dont
think there is any place making them anymore.

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On Monday, 21 January 2019 08:22:47 UTC, wrote:
On Mon, 21 Jan 2019 00:10:42 +0100, Sjouke Burry
wrote:
On 20-1-2019 23:50, Cursitor Doom wrote:
On Sun, 20 Jan 2019 16:33:41 -0600, tubeguy wrote:

I was always under the impression that film resistors are identical
(electrically), and their only difference is appearance. Now I learn
that is not true.....

WHY?????

Really need to see a schematic. AFAIK the only functional difference
between film and carbon is that latter are noisier.





A bulk carbon resistor has less self-inductance.


I kind of thought that might be the case....

I prefer using the old carbon resistors in vintage tube gear, but they
are getting hard to find and expensive. They are all NOS now. i dont
think there is any place making them anymore.


Rapidonline does carbon comps. I'm sure it's far from the only supplier. There's no upside to them other than pulse power ability & for some cases looks.


NT


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On 2019/01/20 7:16 p.m., John-Del wrote:
On Sunday, January 20, 2019 at 9:35:52 PM UTC-5, John Robertson wrote:
On 2019/01/20 5:23 p.m., Tim Schwartz wrote:
On 1/20/2019 6:24 PM, Phil Hobbs wrote:
On 1/20/19 5:50 PM, Cursitor Doom wrote:
On Sun, 20 Jan 2019 16:33:41 -0600, tubeguy wrote:

I was always under the impression that film resistors are identical
(electrically), and their only difference is appearance. Now I learn
that is not true.....

WHY?????

Really need to see a schematic. AFAIK the only functional difference
between film and carbon is that latter are noisier.

They also have a large negative voltage coefficient.

Cheers

Phil Hobbs

Also, Carbon composition resistors will take spikes and surges that will
render a film resistor open circuit, and are more tolerant of short term
overloads.

Regards,
Tim


If I am not mistaken that is why film resistors are used where you want
flame-proof behaviour.

Carbon comp let out lots of magic smoke...

John :-#)#



And when they don't, they change value in a big way when overheated.


On our jukebox tube amplifiers we test every resistor as many of them
have happily drifted off-spec more than their tolerance. Usually plate
of cathode resistors of course because they pass the most current.

John :-#)#

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On Sunday, January 20, 2019 at 6:25:07 PM UTC-5, Phil Hobbs wrote:
On 1/20/19 5:50 PM, Cursitor Doom wrote:
On Sun, 20 Jan 2019 16:33:41 -0600, tubeguy wrote:

I was always under the impression that film resistors are identical
(electrically), and their only difference is appearance. Now I learn
that is not true.....

WHY?????


Really need to see a schematic. AFAIK the only functional difference
between film and carbon is that latter are noisier.


They also have a large negative voltage coefficient.

I was going to say they also have a negative temperature coef.
But I'd be wrong. A 1.5 kOhm CC in hot air gun and resistance rises.
(I know we use to use these as low T (LN2/ liquid He) temp sensors
with a resistance that rises with lower temp.
(Kinda like it was a thermally activated semi-conductor.))

So I stuck the same 1.5 k resistor into LN2... R increased to 2.1 k!

George H.



Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com




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Jon Elson wrote:



Very high value film resistors are made with some composition sprayed on a
ceramic tube, and then an abrasive jet cuts a spiral groove in the film
element.



** In fact, film resistors of all values are made that way.


That can add inductance, and the higher the resistance value they
are aiming for, the more inductance.


** Absolute CRAP !!!!

Where does this ****e come from ??

ONLY with *low value* resistors can the TINY inducatance created by spiralling become significant and then only at RF frequencies.


In tube circuits, it is not unusual to
have megohm resistors, so I can see how this could be a problem.


** Huh ????

Jumps straight from the rediculous to the absurd.

Go away fool.


..... Phil

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Ralph Mowery wrote:






I am well aware that the older carbon resistors often go up in value as
they age even if new. I always measure the resistors before I use them.
I do the same with the electrolytic capacitors.



** If you find an electro well above its marked capacitance value - what do you conclude ??



.... Phil
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On 21-1-2019 22:51, Phil Allison wrote:

Ralph Mowery wrote:






I am well aware that the older carbon resistors often go up in value as
they age even if new. I always measure the resistors before I use them.
I do the same with the electrolytic capacitors.



** If you find an electro well above its marked capacitance value - what do you conclude ??



... Phil

That the oxide layer has degraded, and might be about to fail?
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Sjouke Burry wrote:







** If you find an electro well above its marked capacitance value
- what do you conclude ??



... Phil


That the oxide layer has degraded, and might be about to fail?


** How do you check for this?


...... Phil
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