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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 20th Feb 2018, 4:33 pm   #1
Diabolical Artificer
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Default Metal film resistors going high.

I've had several (5) Vishay 10 ohm 2w 1% metal film resistors go high - up to 200k. They're not being run hard either, part # is CPF210R000FKE14. also one go low 4 ohms.

I'm using them as sense resistors in the the cathodes of EL34's. Maximum voltage is 1v at 100mA or 100mW. But this is hardly ever, only for testing. A safety circuit cuts power at 1v.

So am I using a too low a wattage? I would have thought 2w an adequate safety margin. they are diddy though in comparison to your average 2w R, IE 8mm by 3.5mm.

To replace them with 5w resistors will be problematic, 5w 1% R's being uncommon. Of course I could use 5% R's and match them.

Lastly I've found lower value R's tend go go drastically high as opposed to your higher values.

Andy.
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Old 20th Feb 2018, 6:32 pm   #2
G8HQP Dave
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Default Re: Metal film resistors going high.

Are they genuine Vishay or fakes? Anything which some people are prepared to pay good money for means someone else is willing to fake them.
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Old 20th Feb 2018, 6:42 pm   #3
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Default Re: Metal film resistors going high.

Data sheet is here: https://www.vishay.com/docs/31021/cpf.pdf

Sound dodgy to me. Vishay / Dale resistors have been absolutely bomb proof for me, across all lines. Where did you buy them?

One point with such resistors though; they have pretty terrible pulse handling capability so if there are any spikes it might kill them dead.
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Old 20th Feb 2018, 7:45 pm   #4
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Default Re: Metal film resistors going high.

I have had several (maybe three or four ?) usually US-made amps in for service where the fault has been failure of one or more shiny brown Vishay-Dale resistors. I'm afraid I'm suspicious of them now.

Last time I needed 10ohm cathode current sensor resistors I bought 5% ones, soldered ten of them in series and passed just under 200mA down the chain, then measured the voltage across each one on the 2V range of my DMM with the aim of trying to find four within 1%. I needn't have bothered. I think they were all within 1%.

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Old 21st Feb 2018, 7:11 am   #5
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Default Re: Metal film resistors going high.

Got them from Farnell, so they should be kosher Dave.

Not sure if they've seen spikes on the cathode of a valve but there has been a few de-bugging issues where the amp oscillated but only briefly.

They are the brown shineyish jobbies G. Vishay made stuff though is usually rock solid, wonder if it's worth contacting Vishay, though I don't want to look a plonker.

Will get some 5% ones in the meantime and give them a whirl.

Andy.
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Old 21st Feb 2018, 10:31 am   #6
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Default Re: Metal film resistors going high.

Would Vishay even talk to you? Before Vishay was Vishay it was Rodestein!!

I have NEVER had a failure!! and I bought up almost all that the Aus government sold to surplus 20 years ago!! Still were brown, still NEVER had a failure. In fact I have individual resistors worth 1/2 a cent packed in inert gas in individual packages with the packaging worth $3!!!

Go figure.

Joe
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Old 21st Feb 2018, 12:36 pm   #7
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Default Re: Metal film resistors going high.

The little brown RN55 / RN60 ones are the usual milspec ones. They are heavily derated commercial CN series. I have a whole bunch of 0.1% tol ones for calibration purposes. I think the CPF are a little looser in quality.
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Old 21st Feb 2018, 12:43 pm   #8
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Default Re: Metal film resistors going high.

These days even reputable distributors sometimes find that they have been sent fakes. You should alert Farnell; they may wish to look into it. Fakes or a batch fault are two explanations.
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Old 21st Feb 2018, 1:05 pm   #9
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Default Re: Metal film resistors going high.

I'm afraid none of mine came from Farnell or, as far as I know, from a common batch. They arrived in different pieces of commercial equipment which was originally built in the US and then exported here, either new or second-hand. I see a lot of these resistors - indeed I returned a Rogue Audio Magnum 88 amplifier to its owner just last Sunday at Audiojumble. It was full of them and they were fine. But in three or four cases they haven't been.

The curious thing is that in 6+ years of equipment repair I can think of only one other case of a resistor failing when it hadn't been over-stressed by a fault elsewhere in the circuit. So even 'just' three or four Vishay-Dale failures out of many hundreds is a disproportionately large number.

Cheers,

GJ
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Old 21st Feb 2018, 1:33 pm   #10
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Default Re: Metal film resistors going high.

Use wirewound resistors? W21 are not huge and are pretty bombproof!
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Old 21st Feb 2018, 2:33 pm   #11
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Default Re: Metal film resistors going high.

Looked online and have ordered some larger Vishay jobbies from RS and some bigger 3W MF Trueohm from Rapid. Looked for W21's Chris, I have none and there are none to be had on ebay. Also looked for some Russian mil spec types, no go in 100 ohm.

BTW, went to Farnell and found most of their 10R resistors incur the dreaded special £15.95 delivery surcharge. This is one aspect of Farnell that really gets my goat. That's dearer postage from France than ordering from Digikey in the States - madness.

I've scoped and monitored the amp whilst turning on/off, cranked it up to full whack and observed no adverse voltages or currents. I did find the reason one was low though, there was an intermittent short to ground on that valve's protection circuit. That still leaves four definite duds.

I'll contact Farnell and see what they say.

Oh, nearly forgot, the R's in question haven't been stressed physically.

A.
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Old 21st Feb 2018, 3:05 pm   #12
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Default Re: Metal film resistors going high.

The ones you've ordered will probably be fine Andy, but if you want them for the future the W21s are here http://cpc.farnell.com/welwyn/w21-10...10r/dp/RE04667.

Cheers,

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Old 21st Feb 2018, 3:06 pm   #13
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Default Re: Metal film resistors going high.

Andy, I will see what I have later today and get back to you.
Rob
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Old 21st Feb 2018, 4:48 pm   #14
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Default Re: Metal film resistors going high.

Sorry Andy. The bag I had in mind are just a whole load of 47R W21.
I also have 10R 5% carbon film. Enough to hopefully match a few within 1%.
Unknown make but about physical size of Piher 1W, but I think you ordered some.
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Old 22nd Feb 2018, 6:47 am   #15
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Default Re: Metal film resistors going high.

Thanks G, will see how I get on.

Cheers Robin, thanks for having a look.

A.
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Old 24th Feb 2018, 7:07 am   #16
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Default Re: Metal film resistors going high.

Andy,
If they are the old carbon types- brown shiny Bakelite body, Onmite, TRW, etc, I have tossed almost all of mine. They seem to absorb moisture over time, from what I have been told, and generally go way high.
I use the new flame retardant, "ceramicky" coated ones. A few more cents, but much more reliable.
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