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 Interesting little HK fault today ...

HK Actor, chassis only - the shop had removed it from the cabinet for me, as
they always do on these to save workshop space. Reported problem was "powers
up but no sound". At a first look, that did appear to be the case. However,
when the board with the PSU on it was moved ever so slightly, it came on,
but then stayed on until it was next turned off. Once on, no amount of
provocation would make it go off again. Likewise, when it was wrong, no
amount of tapping or hitting would make it work, but as soon as you turned
the chassis over, on it came. I then noticed that when it was wrong,
although the mains switch was lit, giving the impression that it was powered
up, the fan wasn't running. This is powered from its own personal supply
with a single diode and filter cap, straight from one side of the split
secondary winding of the power transformer, so if there is power to the
transformer's primary, then the fan pretty much has to run.

A bit more observation of what happened when it did come on, revealed that a
relay clicked. This is across the inrush thermistor, and again, its coil is
fed from a simple supply derived directly from the transformer secondary. It
then dawned on me what was happening, and a quick tug on the thermistor
proved it. One leg had sheared off inside the hole passing through the PCB,
making the thermistor effectively open circuit, so no supply to the
transformer primary. The switch of course lit up, as the neon is internal to
it. The gap in the broken leg was obviously big enough that the vibration
from tapping, was not enough to close it up, but when the board was flexed,
that was enough, and the leg momentarily made again. This was enough to get
the relay coil supply up, the relay contacts then bridging the thermistor,
so it didn't matter any more that it had gone open circuit again. And once
the relay contacts were shorting the thermistor, no amount of disturbance
would make it die again, until it was actually turned off.

What a pleasant change to get a nice positive fault with reasoning that
works .... :-)

Arfa

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Default Interesting little HK fault today ...

Arfa Daily wrote in message
...
HK Actor, chassis only - the shop had removed it from the cabinet for me,

as
they always do on these to save workshop space. Reported problem was

"powers
up but no sound". At a first look, that did appear to be the case.

However,
when the board with the PSU on it was moved ever so slightly, it came on,
but then stayed on until it was next turned off. Once on, no amount of
provocation would make it go off again. Likewise, when it was wrong, no
amount of tapping or hitting would make it work, but as soon as you turned
the chassis over, on it came. I then noticed that when it was wrong,
although the mains switch was lit, giving the impression that it was

powered
up, the fan wasn't running. This is powered from its own personal supply
with a single diode and filter cap, straight from one side of the split
secondary winding of the power transformer, so if there is power to the
transformer's primary, then the fan pretty much has to run.

A bit more observation of what happened when it did come on, revealed that

a
relay clicked. This is across the inrush thermistor, and again, its coil

is
fed from a simple supply derived directly from the transformer secondary.

It
then dawned on me what was happening, and a quick tug on the thermistor
proved it. One leg had sheared off inside the hole passing through the

PCB,
making the thermistor effectively open circuit, so no supply to the
transformer primary. The switch of course lit up, as the neon is internal

to
it. The gap in the broken leg was obviously big enough that the vibration
from tapping, was not enough to close it up, but when the board was

flexed,
that was enough, and the leg momentarily made again. This was enough to

get
the relay coil supply up, the relay contacts then bridging the thermistor,
so it didn't matter any more that it had gone open circuit again. And once
the relay contacts were shorting the thermistor, no amount of disturbance
would make it die again, until it was actually turned off.

What a pleasant change to get a nice positive fault with reasoning that
works .... :-)

Arfa



I've always expected to see problems with polyswitch "fuses", considering
the temp they run at, but I don't think I've seen a problem one. I expected
a "welded" lead to come loose or internal cracking of the material


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Default Interesting little HK fault today ...


"Nutcase Kook "


I've always expected to see problems with polyswitch "fuses", considering
the temp they run at,



** FFS - you steaming great ****WIT !!!!!!!!!

Have you got no ****ing idea what a "thermistor" is ???

FYI - you asinine, trolling POS !!!

Polyswitches are PTCs and thermistors are NTCs !!!!!!!!!!!!

Wot a ****ing retarded pile of sub human pommy ****.




..... Phil











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Default Interesting little HK fault today ...


"Arfa Daily" wrote in message
...
HK Actor, chassis only - the shop had removed it from the cabinet for me,
as they always do on these to save workshop space. Reported problem was
"powers up but no sound". At a first look, that did appear to be the case.
However, when the board with the PSU on it was moved ever so slightly, it
came on, but then stayed on until it was next turned off. Once on, no
amount of provocation would make it go off again. Likewise, when it was
wrong, no amount of tapping or hitting would make it work, but as soon as
you turned the chassis over, on it came. I then noticed that when it was
wrong, although the mains switch was lit, giving the impression that it
was powered up, the fan wasn't running. This is powered from its own
personal supply with a single diode and filter cap, straight from one side
of the split secondary winding of the power transformer, so if there is
power to the transformer's primary, then the fan pretty much has to run.

A bit more observation of what happened when it did come on, revealed that
a relay clicked. This is across the inrush thermistor, and again, its coil
is fed from a simple supply derived directly from the transformer
secondary. It then dawned on me what was happening, and a quick tug on the
thermistor proved it. One leg had sheared off inside the hole passing
through the PCB, making the thermistor effectively open circuit, so no
supply to the transformer primary. The switch of course lit up, as the
neon is internal to it. The gap in the broken leg was obviously big enough
that the vibration from tapping, was not enough to close it up, but when
the board was flexed, that was enough, and the leg momentarily made again.
This was enough to get the relay coil supply up, the relay contacts then
bridging the thermistor, so it didn't matter any more that it had gone
open circuit again. And once the relay contacts were shorting the
thermistor, no amount of disturbance would make it die again, until it was
actually turned off.

What a pleasant change to get a nice positive fault with reasoning that
works .... :-)

Arfa



Dry joints on these thermistors are surprisingly common. I think it may be
because the higher powered ones are kind of tall and weighty and have no
support other than their leads.
Powered speakers are a favourite for this, possibly because they get a lot
more mechanical shocks in transport than your average punter would give to,
say, his desk or FX rack, and also because they get a LOT of vibration in
use.

I always wiggle them whenever I see them, and if the PCB comes out I will
always resolder them (along with the usual suspects, e.g. zener diodes,
power resistors that get hot etc).


Cheers,


Gareth.


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Default Interesting little HK fault today ...

Wot a ****ing retarded pile of sub human pommy ****.

It's pome (or POME) -- prisoner of mother England.




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Default Interesting little HK fault today ...

On 06/01/2012 11:28, William Sommerwerck wrote:
Wot a ****ing retarded pile of sub human pommy ****.


It's pome (or POME) -- prisoner of mother England.


Rubbish!

R
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Default Interesting little HK fault today ...

On Jan 6, 1:23*am, "Phil Allison" wrote:
"Nutcase Kook "

I've always expected to see problems with polyswitch "fuses", considering
the temp they run at,


** FFS * - * you *steaming *great * ****WIT * !!!!!!!!!

*Have you got no ****ing idea what a "thermistor" is ???

*FYI *- *you asinine, *trolling POS *!!!

Polyswitches are PTCs and thermistors are NTCs *!!!!!!!!!!!!


One quibble: While their resistance did increase drastically when they
heated up, the polyswitches I worked with years ago seemed to just
have two states: a low resistance one while carrying lower currents,
and a high resistance one when excessive current passed through. Not a
traditional linear or logarithmic (etc.) relationship. The reason was
that heat made the polymer matrix flex, which tended to separate the
conductive particles embedded in the matrix. As the matrix cooled it
shrank once more, bringing the conductive particles into more intimate
contact, and so on.

We eventually decided not to use them for loudspeaker protection,
because they cycled so rapidly that we decided consumers would
complain. Even though these were the same consumers who didn't notice
that their speaker's voice coil former was bottoming out against the
motor structure.
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Default Interesting little HK fault today ...


"spamtrap1888"

We eventually decided not to use them for loudspeaker protection,
because they cycled so rapidly that we decided consumers would
complain.

** Polyswitches respond slowly and reset only after a long wait.

Even though these were the same consumers who didn't notice
that their speaker's voice coil former was bottoming out against the
motor structure.

** ********.



..... Phil


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