KV-27EXR25 & STRS6301

S

Steve Kraus

Guest
Twelve year old Sony KV-27EXR25 was dead except for relay clicking. Based
on excellent information here and elsewhere online I replaced the STRS6301
on the power converter board and now it seems to work just fine. Measured
a bit over 160 degrees F. at the heat sink which I think is typical on
these older units (I did add a bit of extra heat sink on top of the chip).

One thing though...I do not hear the degauss coil buzz on power-on. I
suppose I can do without it but it has me wondering what is wrong. I did
plug everything back in the way it was. Is firing the degausser for a
second a function of the STRS6301 or something else on the board?
 
I have serviced many of these, but this was years ago. When changing the
drive IC, it is a good practice to do an ESR test on all the capacitors on
the power supply board. We found that a high ESR decoupling cap can easily
cause this IC to fail, due to non supression of noise. Other caps being off
value can cause faults that may also be a cause of failure.

Another cause of failure is the mounting screw that is holding the IC down
to the heatsink may loosen up with time. This is due to the thermo
contraction and expansion that happens when the IC cools down, and warms up
during use. I found a number of them with their mounting screws to be loose
when changing them.

When mounting this IC on to the heatsink take care that you cleaned the
surface of the heatsink, and use the proper silicon compound for maintaining
proper heat transfer.

This device does run very hot. it is normal that it may run at about 60 to
70 degrees Cels. We only used the origional Sony part, because they are
running this device very close to its limits. We found generic replacement
parts to be less reliable.

It is normal that the degausing coil may make a buzz sound when working. The
degause cycle is about 100 ms. Being exposed to the coil's AC type magnetic
field, the metal shroud that is around the tube may resonate a bit. The coil
itself may also be a bit noisy when it is working, by the way it is sitting
in the shroud. With age, some of the internal windings in the coil may
become a bit loose, and they may also contribute to the noise you are
hearing.

I found some set's degaussing coil to be less or more noisy than others,
when compared to others of the same model. I don't think there is a factory
spec for the degausing coil noise. If the set is switched off for about 30
to 40 minutes, the degaussing coil should activate when the set is powered
up again.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
=========================================


"Steve Kraus" <screen@SPAMBLOCKfilmteknik.com> wrote in message
news:jiEjc.12529$eZ5.3147@newsread1.news.pas.earthlink.net...
Twelve year old Sony KV-27EXR25 was dead except for relay clicking. Based
on excellent information here and elsewhere online I replaced the STRS6301
on the power converter board and now it seems to work just fine. Measured
a bit over 160 degrees F. at the heat sink which I think is typical on
these older units (I did add a bit of extra heat sink on top of the chip).

One thing though...I do not hear the degauss coil buzz on power-on. I
suppose I can do without it but it has me wondering what is wrong. I did
plug everything back in the way it was. Is firing the degausser for a
second a function of the STRS6301 or something else on the board?
 
I just powered it up after having been off all night and I did hear the
buzz of the degaussing coil so I guess all is well. Hope it goes the
distance.

FWIW the generic part (bought online) looks exactly like the OEM part I
removed except for a different SK number written below the STR6301 but the
fonts and print quality look the same so either they come from the same
source or they went to great lengths to duplicate the look. I bought a
spare and have it and the original on the keyboard and they are
indistinguishable save the solder and thermal goop on the OEM.

PS: Could not find my solder sucker and faced with endless searching or
buying another I used a little imagination and rigged something with a
vacuum cleaner, a bit of rubber hose, and part of a ball-point pen with a
metal tip. This worked so well I don't think I want to go back to a proper
solder sucker. Next project is a dead power board from a KV-8AD11 with
likely STRD1206 culprit.
 
The only thing that goes wrong with the degausing circuit on these Sony's is
bad solder connections on the 3 leg thermistor, or a bad thermistor itself. In
case you are not familiar, the 3 leg thermistor is the aprox 3/4"x3/4" black
square shaped part close to where your degausing wires plug into the board. If
you have a good quality AC meter you can measure the voltage across the 2
degauser wires at set turn on. You should see a momentary 50-70vac.
The generic STRS6301 looks just like the Sony part and generally will work OK,
but as mentioned the chip runs over 160 degrees F. As with any part, you have
to weigh the difference in cost of original vs generic. The Sony chip is $30,
generic $6.
The capacitors in the power supply rarely are bad, but I always check them and
also resolder any visible bad connections in the area.
Ron

Twelve year old Sony KV-27EXR25 was dead except for relay clicking. Based
on excellent information here and elsewhere online I replaced the STRS6301
on the power converter board and now it seems to work just fine. Measured
a bit over 160 degrees F. at the heat sink which I think is typical on
these older units (I did add a bit of extra heat sink on top of the chip).

One thing though...I do not hear the degauss coil buzz on power-on. I
suppose I can do without it but it has me wondering what is wrong. I did
plug everything back in the way it was. Is firing the degausser for a
second a function of the STRS6301 or something else on the board
Click to expand...
 
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