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From: Arfa Daily on 29 Jun 2010 13:03
I have a JVC hifi on my bench. Brand new, purchased by a British soldier
from a base somewhere else - possibly an American one, I seem to recall
being told. Oddly, for modern equipment, it has a voltage selector switch,
which was of course set to 120v, when he applied 240v UK power to it ...

The unit has a switch mode power supply (why though?) which is one of the
old style designs where the front end has two main filter caps, and the
input voltage is configured by how you jigger the connections of the reccy
and the caps. That is where the back panel voltage selector switch is
connected. Even more why though ? I would have thought that if you were
going to bother putting a switcher in a junky piece of hifi that was for
worldwide use, you would have used a design with an efficient PFC front end,
that makes it universal in terms of input voltage. Heaven knows, there's
enough cheap and cheerful examples out there, used by the million in LCD TV
sets.

Anyway, that aside, the bridge was twatted, as were two diodes?? , one
across each of the filter caps. These devices *look* like diodes, and the
symbol on the board is for a diode, and they're called D34 and D35, but I
can't recall having seen diodes in this position before. Voltage sharing
resistors, maybe. The type number on them is R4KL, and that's where the
problems start. All of the usual suspects - Datasheetarchive, Allicmall etc
deny knowing anything about this device. I can't seem to find any references
to the it on the net, other than lots of people on groups and forums, also
trying to find data. An admittedly brief trawl of some of those postings,
didn't seem to turn up any definitive answers. Some seem to be calling the
device a "transient suppressor" so maybe it's some kind of avalanche diode ?
Appears to be used in some monitors as well, so I'm surprised that it is so
difficult to turn up info.

Anyway, I've replaced the bridge, removed the short circuit
whatever-they-ares, and fired the supply up in isolation. It runs quite
happily without the "diodes" as you might imagine it would, and the output
voltages seem reasonable.

Apparently, the owner is being posted to Afghanistan or some such at the
weekend coming, and wants to take the hifi with him, so my back's against
the wall a bit on this one. In case of the remote possibility that anyone
has a service manual for the item, it's a JVC MX - KC68 or CA-MXKC68
depending on whether you have it listed by system or model number.

Any help then, please lads. TIA

Arfa

From: Vale on 29 Jun 2010 13:50

"Arfa Daily" <arfa.daily(a)ntlworld.com> ha scritto nel messaggio
news:mHpWn.93263$tH4.24979(a)hurricane...
>I have a JVC hifi on my bench. Brand new, purchased by a British soldier
>from a base somewhere else - possibly an American one, I seem to recall
>being told. Oddly, for modern equipment, it has a voltage selector switch,
>which was of course set to 120v, when he applied 240v UK power to it ...

http://www.clubdediagramas.com/archivo/audio-equipos-de-musica-amplificadores-reproductores-etc-a7/jvc-m148/jvc-ca-mxkc68-manual-de-serviciopdf-f86697.html

Just register and download.
Spanish is not a problem, isn't it?


From: Phil Allison on 29 Jun 2010 22:45

"Arfa Daily"

> The unit has a switch mode power supply (why though?)

** Lower cost and much lower weight - that PSU has to deliver over 500
watts.


> I would have thought that if you were going to bother putting a switcher
> in a junky piece of hifi that was for worldwide use, you would have used a
> design with an efficient PFC front end, that makes it universal in terms
> of input voltage.

** That only increases the expense.

The PSU in that JVC is likely to be of the unregulated kind - ie a simple
square wave inverter.

These are far preferable for audio amplifiers and cheaper to make too.


> Heaven knows, there's enough cheap and cheerful examples out there, used
> by the million in LCD TV sets.


** Irrelevant what LCD TVs use.

> Anyway, that aside, the bridge was twatted, as were two diodes?? , one
> across each of the filter caps. These devices *look* like diodes, and the
> symbol on the board is for a diode, and they're called D34 and D35, but I
> can't recall having seen diodes in this position before. Voltage sharing
> resistors, maybe. The type number on them is R4KL,


** That type is an "avalanche " diode that is specified to zener breakdown
at a particular voltage - like 200 volts.

Does two jobs:

1. It sacrifices itself and goes short to protect the electo when some
idiot does what your customer did.

2. It prevents reverse polarity current from flowing into the electro ( via
the load) during the switch on surge period - this is only an issue for 120
volt use when the circuit is acting in voltage doubler mode. Any diode with
adequate voltage an current ratings is OK for this.



..... Phil


From: Arfa Daily on 30 Jun 2010 05:34


"Phil Allison" <phil_a(a)tpg.com.au> wrote in message
news:88vpecFfloU1(a)mid.individual.net...
>
> "Arfa Daily"
>
>> The unit has a switch mode power supply (why though?)
>
> ** Lower cost and much lower weight - that PSU has to deliver over 500
> watts.
>
>
>> I would have thought that if you were going to bother putting a switcher
>> in a junky piece of hifi that was for worldwide use, you would have used
>> a design with an efficient PFC front end, that makes it universal in
>> terms of input voltage.
>
> ** That only increases the expense.

Well, if it does at all, not by very much. Many switchers now have a
'universal' PFC front end, based on a single chip, one cheap FET, and a
small piece of magnetics. Having this would save on the cost of the rear
panel voltage selection switch, having to fit that switch, the wire that
connects out to it, the plug and socket on the board that connects to it,
two whatever-they-are 'diodes' and one high voltage smoothing cap. Also, the
cost of the warning label about using it on the wrong voltage, and the
warning sheet in the box. Apart from this, it would slash the standby power
consumption, which is why many manufacturers have adopted this scheme in the
first place.


>
> The PSU in that JVC is likely to be of the unregulated kind - ie a
> simple square wave inverter.
>
> These are far preferable for audio amplifiers and cheaper to make too.

It's actually not. It's a fairly sophisticated design, apart from the
stupidity of not being universal on its input voltage. As to them being
better for audio work, just last month, Elektor magazine did some lab tests
to assess whether off-the-shelf switchers could be used to replace linears
in some of their well respected and time-tested designs. In every case, the
THD figures were worse.


>
>
>> Heaven knows, there's enough cheap and cheerful examples out there, used
>> by the million in LCD TV sets.
>
>
> ** Irrelevant what LCD TVs use.

Well, it isn't irrelevant, because these are now being built by the million,
and they have driven the development of cheap, sophisticated, and above all
efficient power supplies more than any other consumer or industrial product
has, in the civilized world. LCD TV manufacturers - including JVC
themselves - have developed the techniques of front end PFC usage to the
point where it is absolutely standard in their products, so why do they
bother to go back to an old technique of an inefficient voltage doubler
front end, for one of their audio products ?

>
>> Anyway, that aside, the bridge was twatted, as were two diodes?? , one
>> across each of the filter caps. These devices *look* like diodes, and the
>> symbol on the board is for a diode, and they're called D34 and D35, but I
>> can't recall having seen diodes in this position before. Voltage sharing
>> resistors, maybe. The type number on them is R4KL,
>
>
> ** That type is an "avalanche " diode that is specified to zener breakdown
> at a particular voltage - like 200 volts.

I too felt that it probably was an avalanche diode. Do you have any data on
the device to support that contention ?


>
> Does two jobs:
>
> 1. It sacrifices itself and goes short to protect the electo when some
> idiot does what your customer did.
>
> 2. It prevents reverse polarity current from flowing into the electro (
> via the load) during the switch on surge period - this is only an issue
> for 120 volt use when the circuit is acting in voltage doubler mode. Any
> diode with adequate voltage an current ratings is OK for this.
>
>

Agreed on the purposes if the device is, as suspected, an avalanche diode.
The unit is never going to be used on 120v, so I am going to leave the
switch disconnected anyway, which places the unit firmly in the 240v
bracket, meaning that in theory, the two diodes are not required. The supply
works absolutely fine without them, and I have seen plenty of examples over
the years, where there are just voltage sharing resistors in that position.

Arfa
>
> .... Phil
>
From: Phil Allison on 1 Jul 2010 06:33

"Arfa Daily is off with the Goblins & Pixies"


>> Do you still think that Marshall amps were designed by Jim Marshall ??
>>
>
> What's that got to do with this thread ?


** Proves my point.



.... Phil






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