Special Resistors. - UK Vintage Radio Repair and Restoration Discussion Forum

Clay1905 · 15th Jan 2021, 10:32 pm

Hi Folks,

Recently I have been trying to resuscitate an ancient Philips oscilloscope. So far things are going better than expected.

It turned out that there were a whole bunch of dead and dying resistors. Two types, composition, and carbon film. Each has it's own failure mode, and all in the horizontal circuitry have been replaced.

On the strength that there were so many open or hugely out of tolerance resistors, I've decided to rebuild the vertical circuits too. However, The inputs for this venerable instrument are different to more modern ones. There are a selection of sockets, and each one has it's own attenuator built in.

The attenuation is achieved by using a very small value capacitor in parallel with some resistor. It's these resistors I'm interested in. These are physically different to all the others. They are a rounded tapered shape. The colours are indistinct, as there is some sort of very glossy coating that has yellowed a bit over time. I would guess about 1/4 watt on the basis of size.

What I'd like to know is can anybody recognise these as being somehow special? Are they low noise types? What about induction? Why did Philips only put them around the input sockets, but use regular cylindrical types everywhere else then?

Below is a picture with four of the little items in it, as attached to an input socket.

Your knowledgeable input would be appreciated.

Thanks,
Clay.

Electronpusher0 · 15th Jan 2021, 10:51 pm

Have you measured them? they may be capacitors.

Peter

Clay1905 · 15th Jan 2021, 11:09 pm

Quote:

Originally Posted by Electronpusher0

Have you measured them? they may be capacitors.

Peter

Thanks Peter, but no, they're definitely resistors. The schematic and the meter say so.

Clay.

Skywave · 15th Jan 2021, 11:51 pm

I suspect that their shape is simply what was commonly available (for the required wattage rating) many, many years ago when that 'scope was designed and built.

Al.

Refugee · 16th Jan 2021, 1:42 am

Just test that the the sensitivity steps are on cal.
They are likely to be inductance free resistors in that position and are best left alone if possible.
They are usually reliable unless the input has been madly overloaded.

Herald1360 · 16th Jan 2021, 1:59 am

Two of them look like 10K 5% and 1M 5%.

Radio Wrangler · 16th Jan 2021, 2:35 am

5% seems disappointing setting the sensitivity of a scope. Tolerances build up and at 5% a pop you soon get bad accuracy.

They look like carbon film, and that tolerance would fit. They seem to have a protective over-glaze lacking from normal carbon film types. Maybe it goes with a high voltage rating?

David

GrimJosef · 16th Jan 2021, 10:59 am

The glaze overcoat looks like it might have been applied after assembly. The way the droplets are concentrated at the sharp bends in the wires of the upper resistor, for example, suggests the bends were there before the glaze was. I have to say it looks a bit like the 'conformal coating' that's sometimes sprayed over circuitry which is going to be used in a dirty or damp environment.

As for the 5%, I wonder if that was their as-manufactured tolerance but individual resistors were then measured and the particularly accurate ones selected out for use here ?

Cheers,

GJ

Clay1905 · 16th Jan 2021, 9:50 pm

Hi All,

Yes these resistors are a little different.

I can tell you that they've been here since new. I know this because Philips supplied a layout diagram, and these resistors are drawn as little tapered pods, whereas the rest are plain cylinders.

The gloss coating does indeed continue out to the first leg bends. And it looks to me that these resistors were glazed in place too. I would have doubted anything to do with moisture ingress. Not for a bench instrument I don't think.
I'm wondering if it's some extra insurance against flash over. The input sockets, and their attendant resistors are very close to the steel case.

In any case these test well enough, all are within that rather wide tolerance. As a consequence, I've decided to leave well enough alone in this case. I'm not one for fixing what doesn't need it. And change for it's own sake isn't always a good idea.

So, until I can know what quality these resistors have to make them desirable in this location (if there be one at all) I think I'll leave them as they are.

Thanks,
Clay.

15th Jan 2021, 10:32 pm	#1
Clay1905 Tetrode Join Date: Sep 2020 Location: Blue Mountains City, New South Wales, Australia. Posts: 51	Special Resistors. Hi Folks, Recently I have been trying to resuscitate an ancient Philips oscilloscope. So far things are going better than expected. It turned out that there were a whole bunch of dead and dying resistors. Two types, composition, and carbon film. Each has it's own failure mode, and all in the horizontal circuitry have been replaced. On the strength that there were so many open or hugely out of tolerance resistors, I've decided to rebuild the vertical circuits too. However, The inputs for this venerable instrument are different to more modern ones. There are a selection of sockets, and each one has it's own attenuator built in. The attenuation is achieved by using a very small value capacitor in parallel with some resistor. It's these resistors I'm interested in. These are physically different to all the others. They are a rounded tapered shape. The colours are indistinct, as there is some sort of very glossy coating that has yellowed a bit over time. I would guess about 1/4 watt on the basis of size. What I'd like to know is can anybody recognise these as being somehow special? Are they low noise types? What about induction? Why did Philips only put them around the input sockets, but use regular cylindrical types everywhere else then? Below is a picture with four of the little items in it, as attached to an input socket. Your knowledgeable input would be appreciated. Thanks, Clay. Attached Thumbnails __________________ The higher it goes, the fewer.

15th Jan 2021, 10:51 pm	#2
Electronpusher0 Nonode Join Date: Jun 2016 Location: Bognor Regis, West Sussex, UK. Posts: 2,296	Re: Special Resistors. Have you measured them? they may be capacitors. Peter

15th Jan 2021, 11:51 pm	#4
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Special Resistors. I suspect that their shape is simply what was commonly available (for the required wattage rating) many, many years ago when that 'scope was designed and built. Al.

16th Jan 2021, 1:42 am	#5
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: Special Resistors. Just test that the the sensitivity steps are on cal. They are likely to be inductance free resistors in that position and are best left alone if possible. They are usually reliable unless the input has been madly overloaded.

16th Jan 2021, 1:59 am	#6
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Special Resistors. Two of them look like 10K 5% and 1M 5%. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

16th Jan 2021, 2:35 am	#7
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,874	Re: Special Resistors. 5% seems disappointing setting the sensitivity of a scope. Tolerances build up and at 5% a pop you soon get bad accuracy. They look like carbon film, and that tolerance would fit. They seem to have a protective over-glaze lacking from normal carbon film types. Maybe it goes with a high voltage rating? David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

16th Jan 2021, 10:59 am	#8
GrimJosef Dekatron Join Date: Sep 2007 Location: Oxfordshire, UK. Posts: 4,311	Re: Special Resistors. The glaze overcoat looks like it might have been applied after assembly. The way the droplets are concentrated at the sharp bends in the wires of the upper resistor, for example, suggests the bends were there before the glaze was. I have to say it looks a bit like the 'conformal coating' that's sometimes sprayed over circuitry which is going to be used in a dirty or damp environment. As for the 5%, I wonder if that was their as-manufactured tolerance but individual resistors were then measured and the particularly accurate ones selected out for use here ? Cheers, GJ __________________ http://www.ampregen.com

16th Jan 2021, 9:50 pm	#9
Clay1905 Tetrode Join Date: Sep 2020 Location: Blue Mountains City, New South Wales, Australia. Posts: 51	Re: Special Resistors. Hi All, Yes these resistors are a little different. I can tell you that they've been here since new. I know this because Philips supplied a layout diagram, and these resistors are drawn as little tapered pods, whereas the rest are plain cylinders. The gloss coating does indeed continue out to the first leg bends. And it looks to me that these resistors were glazed in place too. I would have doubted anything to do with moisture ingress. Not for a bench instrument I don't think. I'm wondering if it's some extra insurance against flash over. The input sockets, and their attendant resistors are very close to the steel case. In any case these test well enough, all are within that rather wide tolerance. As a consequence, I've decided to leave well enough alone in this case. I'm not one for fixing what doesn't need it. And change for it's own sake isn't always a good idea. So, until I can know what quality these resistors have to make them desirable in this location (if there be one at all) I think I'll leave them as they are. Thanks, Clay. __________________ The higher it goes, the fewer. Last edited by Clay1905; 16th Jan 2021 at 10:14 pm.