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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Old June 29th 20, 03:22 AM posted to sci.electronics.repair
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Default Bad chip resistors?

On 29/06/2020 12:04 pm, Phil Allison wrote:

Terry Schwartz wrote:
====================


Not unusual for high value resistors to fail open.


** Well they don't fail short very often - but high rates of such in new looking equipment is very unusual.


**I've never seen them fail S/C. O/C, yes.



This was also true for leaded resistors.


** Only those with lots of DC across them, not the case here.


**Nope. Seen lots fail over the years. Even those in preamp sections,
when Voltages are well below 40 Volts.



The same environmental conditions that cause the surface corrosion
contribute to the resistor failures.


** Yeah - I did figure that.

But chip resistors are unusually vulnerable cos of exposed silver.


**Certainly are. I've seen a few fail.


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Old June 29th 20, 03:24 AM posted to sci.electronics.repair
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Default Bad chip resistors?

Trevor Wilson wrote:

================


**If I have a weird fault, I always suspect high value (100k) or low
value (100 Ohms) resistors.


** That is why I went on a bad resistor hunt - after everything else proved OK.


That said, mostly I find such faults confined to cracked carbon resistors.



** Small carbon film resistors regularly fail (open) due to high voltage "tracking" between turns on the spiral cut. Plate resistors in triode stages for one.

IME - metal film and cermet types ( ie chip resistors) are less prone to do the same.


..... Phil
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Old June 29th 20, 03:43 AM posted to sci.electronics.repair
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Default Bad chip resistors?

On 29/06/2020 12:24 pm, Phil Allison wrote:
Trevor Wilson wrote:

================


**If I have a weird fault, I always suspect high value (100k) or low
value (100 Ohms) resistors.


** That is why I went on a bad resistor hunt - after everything else proved OK.


**The most painful ones I've seen were popular in Sansui products a few
decades back. They were typically in the range of 100 ohms ~ 47k and
would go high. Frequently to around double the value printed on them.
Fault finding was very difficult, since Voltages were almost what they
were supposed to be. Almost, but not quite.



That said, mostly I find such faults confined to cracked carbon resistors.



** Small carbon film resistors regularly fail (open) due to high voltage "tracking" between turns on the spiral cut. Plate resistors in triode stages for one.

IME - metal film and cermet types ( ie chip resistors) are less prone to do the same.


**I've seen plenty of 100k resistors go high, despite being subject o
Voltages of less than 40 Volts.


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Old June 29th 20, 04:25 AM posted to sci.electronics.repair
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Default Bad chip resistors?


Trevor Wilson wrote:

===================



Not unusual for high value resistors to fail open.


** Well they don't fail short very often - but high rates of such in new looking equipment is very unusual.


**I've never seen them fail S/C.


** ROTFL....



This was also true for leaded resistors.


** Only those with lots of DC across them, not the case here.


**Nope. Seen lots fail over the years.


** Yawnnnn......

TW claims all kinds of impossible things.


But chip resistors are unusually vulnerable cos of exposed silver.


**Certainly are. I've seen a few fail.


** The amp in question is a "DV Mark" combo with 3 x PCBs covered in SMD.

Every chip resistor ( 100+ of them ) is corroded after storage in its original packaging for 6 years or so.

It uses just 2 obsolete Sanken SAP 5-pin darlingtons, has no VI limiting or rail fuses but has an extension speaker jack just waiting to be shorted. Peak available output current is +/- 30 amps double the max Ic of the transistor.

SMPS with the markings ground off the control chip.

Will drive 200W @ 4ohms but has totally inadequate cooling for this condition and no thermal shut down.

The fitted speaker is 16 ohms with a *tin* neo magnet.

No schem is available.

Total PITA.


...... Phil

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Old June 29th 20, 07:59 PM posted to sci.electronics.repair
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Default Bad chip resistors?

I've run into a lot of 500K resistors open in SMPS "start" circuits - where an IC gets its start Vcc from the rectified mains and divided down to 2.5V with half a dozen high value resistors and one low value resistor at ground. The same arrangement was also used to monitor the output and use the divided voltage for feedback. I can't recall seeing a high value smd resistor open with low voltage across them.

Whenever I see any inop SMPS supply with no blown output devices, I look for these first.

Interestingly, I've also found these resistors can be bothered by the adhesive beneath them and read *lower* value than their printed values. When I run into a supply that drifts, I remove these resistors, clean the adhesive off the board beneath them, and reinstall them. They read on the dot after that.



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Old June 30th 20, 02:00 AM posted to sci.electronics.repair
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Default Bad chip resistors?

legg wrote:

============
Phil Allison



The item itself looked new, but was in fact 9 years old.
Eventually I found 3 open resistors, all with values like 1Mohm.

Googling tells me this is a known hazard not shared with through hole parts - has to do with the use of silver plating on top surface of the resistor being subject to sulphiding and similar air borne contaminants.


As formation of sulphides is a surface phenomenon, I doubt
it plays a role in properly reflow-soldered junctions.


** Never said it did.

As the silver layer is on TOP of the SMD chip, failure occurs at the silver to resistive element interface.

You would need a microscope to see evidence of it.

My concern is that many more of the same resistors will fail open in time.


...... Phil
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Old June 30th 20, 07:47 AM posted to sci.electronics.repair
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Default Bad chip resistors?

Well the best thing to do is probably keep it cool and don't bend it.

What else ? Change them all ?
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Old June 30th 20, 12:24 PM posted to sci.electronics.repair
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Default Bad chip resistors?

Jeff Urban wrote:

=======================

Well the best thing to do is probably keep it cool and don't bend it.


** FFS temp was never and issue nor flexing.


What else ? Change them all ?



** Soon as pigs can fly...




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Old June 30th 20, 01:08 PM posted to sci.electronics.repair
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Default Bad chip resistors?

On Mon, 29 Jun 2020 11:59:40 -0700 (PDT), John-Del
wrote:

I've run into a lot of 500K resistors open in SMPS "start" circuits - where an IC gets its start Vcc from the rectified mains and divided down to 2.5V with half a dozen high value resistors and one low value resistor at ground. The same arrangement was also used to monitor the output and use the divided voltage for feedback. I can't recall seeing a high value smd resistor open with low voltage across them.

Whenever I see any inop SMPS supply with no blown output devices, I look for these first.

Interestingly, I've also found these resistors can be bothered by the adhesive beneath them and read *lower* value than their printed values. When I run into a supply that drifts, I remove these resistors, clean the adhesive off the board beneath them, and reinstall them. They read on the dot after that.


Adhesive is used when SMDs are wave-soldered, or for inverted
or repeated reflow. All those processes require additional
skill and technical knowledge to succeed, with a reasonable
service life.

There are a lot of other factors influencing parasitic
terminal leakage in those cases. You'd have to measure
the part and the board, before and after cleaning, before
and after each stage of rework, to get any idea of the
leakage source.

Reduction of R in higher voltage applications is also associated
with tracking and bridging of the part's element and board
surfaces. All of those were affected by your simple before and
after comparison.

RL
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Old July 1st 20, 03:54 AM posted to sci.electronics.repair
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Default Bad chip resistors?

** FFS temp was never and issue nor flexing.

Sorry for not reading your mind.

** Soon as pigs can fly...


They put monkeys up in orbit, why not pigs ?

Anyway so that leaves us with what ? They failed because they were pieces of ****, substandard junk ? What else are we to believe here ?


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