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'[OT] Need advice on part replacement'
2000\01\10@131123 by Eric Oliver

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Hi guys,

Long time no see.  I have a bit of an emergency that I need some advice on.
Our company has an older phone system  that has been discontinued.  Today,
the power supply went out in it.  I can find only one part that is suspect,
it's a Mitsubishi CR6CM Power Mosfet/Thyristor rated at 400V 6A 10mA gate
trigger current.  I went to Radio Shack and they gave me an SCR rated at
400V 6A 25mA gate trigger current.

Will it work ?  Should I even try it ?  Can someone recommend a replacement
that I can overnight from DigiKey or Jameco ? Obviously, time is of the
essence since our company is currently operating with one analog phone <g>.

Thanks for any advice you can offer,

Eric Oliver

2000\01\11@153557 by Brent Brown

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Eric,

Well I meant to reply, really, surely it's the thought that counts...

I replaced a device like this once, at least the part number was very
similar, and found that it needed to be a "sensitive gate" type SCR,
and a standard SCR wouldn't suffice. See if you can find some
more data on it and compare it to a C106D 400V 4A 200uA gate
trigger SCR.



Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile: 025 334 069
eMail:  spam_OUTbrent.brownTakeThisOuTspamclear.net.nz

2000\01\11@224100 by Eric Oliver

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Thanks !!  Since the phone system is older and obsolete ( and I still
haven't tried that SCR <g> ), they bought a new phone system.  However, they
have asked me to try to fix the system so they can sell it surplus.  This
brings up another point :

If I can't fix the system, I am willing to pay someone ( on this list ) to
fix it.  However, please keep in mind that it wouldn't pay too much since
I'd be surprised if the whole system was worth mre than $500. On the other
hand, I would expect someone on this list could probably find the problem
right away.

Paul pointed out in another post that I didn't give enough information so
here's some more <g> :

The faulty circuit is in the power supply. The power supply is a 24V
switching supply. However, that's about all I know. To be honest, I don't
know if the blown part is a cause or a symtom.

I will take Brent's (and James') advice. In the meantime, if you live in the
US ( preferably somewhere in the vicinity of Texas ), and your interested in
taking a stab at this, drop me a private email. I'll pay shipping to and
from.

Oh yeah, thanks to everyone who replied on and off list. Some of you seemed
slightly irritated regarding my "Are my posts getting through".  I promise.
I was not complaining. It was a pressing situation and I was watching my
inbox for replies.  Over time, I began to doubt. I even checked to make sure
I had actually sent a message <g>. I have changed my list settings.

Eric


{Original Message removed}

2000\01\11@230217 by Eric Oliver

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Brent,

Thanks !!  Since the phone system is older and obsolete ( and I still
haven't tried that SCR <g> ), they bought a new phone system.  However, they
have asked me to try to fix the system so they can sell it surplus.  This
brings up another point :

If I can't fix the system, I am willing to pay someone ( on this list ) to
fix it.  However, please keep in mind that it wouldn't pay too much since
I'd be surprised if the whole system was worth mre than $500. On the other
hand, I would expect someone on this list could probably find the problem
right away.

Paul pointed out in another post that I didn't give enough information so
here's some more <g> :

The faulty circuit is in the power supply. The power supply is a 24V
switching supply. However, that's about all I know. To be honest, I don't
know if the blown part is a cause or a symtom.

I will take Brent's (and James') advice. In the meantime, if you live in the
US ( preferably somewhere in the vicinity of Texas ), and your interested in
taking a stab at this, drop me a private email. I'll pay shipping to and
from.

Oh yeah, thanks to everyone who replied on and off list. Some of you seemed
slightly irritated regarding my "Are my posts getting through".  I promise.
I was not complaining. It was a pressing situation and I was watching my
inbox for replies.  Over time, I began to doubt. I even checked to make sure
I had actually sent a message <g>. I have changed my list settings.

Eric


{Original Message removed}

2000\01\12@043709 by William Chops Westfield

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   The faulty circuit is in the power supply. The power supply is a 24V
   switching supply. However, that's about all I know. To be honest, I don't
   know if the blown part is a cause or a symtom.

It may be easier and faster to replace the entire power supply.  Lots of
switching power supplies out there in the surplus market (many more than
SCRs.)  An SCR would likely be in the crowbar protection circuit of a power
supply, so a bad SCR woul be sign of some other problem.

For a (short) while, there was a market replacing the linear (30% efficient)
power supplies of a certain rather obsolete mainframe computer with modern
switchers (80%+ efficient.)  The energy saving in $$$ worked out to be more
than the (surplus) price of the mainframe (ie they were going for ~$5k, and
cost $5k/month in power to run and cool...)

BillW

2000\01\12@093149 by Eric Oliver

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You hit the nail on the head.  The problem is a replacement power supply for
this system costs $550.  Not because it's worth that, but because the guy
who has them is the only guy in town that has them.  When the replacement
cost is more than the phone system is worth .....

Your reference to the fact that the blown SCR may be a symtom is what I am
afraid of.

Thanks for your advice,
Eric

{Original Message removed}

2000\01\12@114515 by Eric Oliver

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Jim,

I apologize to everyone.  I am currently installing our new phone system and
my replies lack some needed information.  The current power supply is
enclosed in a black  box that mounts to the bottom of the phone system.
While I wouldn't care if a new power supply didn't fit, trying to sell it
with an obvious 'patch' might be little hard.  But that's not the real
problem. The real problem is that there are two connectors that connect the
power supply to the phone system. Each connector contains 10 wires for a
total of twenty wires. All wires are black.  I don't have schematics on the
the power supply or phone system. And even if I did, I'm not sure it would
do _me_ any good.

Eric

{Original Message removed}

2000\01\12@125701 by James Paul

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Eric,

I live in the Houston area, so if you need any help, I'll do
what I can.  Won't be able to travel far though.

                                 Regards,

                                  Jim



On Wed, 12 January 2000, Eric Oliver wrote:

{Quote hidden}

> {Original Message removed}

2000\01\12@161454 by paulb

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Eric Oliver wrote:

> But that's not the real problem. The real problem is that there are
> two connectors that connect the power supply to the phone system.
> Each connector contains 10 wires for a total of twenty wires.  All
> wires are black.  I don't have schematics on the the power supply or
> phone system. And even if I did, I'm not sure it would do _me_ any
> good.

 I sympathise with your problem.  That's a lovely description,
especially the "All wires are black" part!  (Now, where have I heard
something like that before - perhaps the Model T saying?)

 That said, even with 20 wires, it should really only have two outputs,
24V and 5V (though the 24V should be negative) to operate the "loop"
supply and logic respectively.  So many if not most of the wires should,
like the mass emanating from a PC supply, derive from three or four PCB
pads only.

 In any case, most of us *loathe* repairing switchmode supplies due to
the "domino" effect and would indeed, rather replace them.

 My exchange cost me $500 second hand, nine years ago (with phones).
This was a Philips SBX512, made in New Zealand (Baaa.aa!), i.e., five
incoming lines, twelve extensions.  Although I'm very suss about the
ni-cads still in there (hate to think of what PCB cancer is in there)
and its programming regarding divert-on-no-answer is faulty causing
problems with the fax machine,  it has been good value.

 I'm sure it chews far too much power with its *linear* regulated
supply but to me, that was part of a major advantage - the use of bog-
standard parts which could be replaced if they failed, rather than
impossible-to-obtain proprietary parts as in the smaller, leaner, more
modern digital units.  Nevertheless, I don't think resale value of such
equipment is more than a couple of hundred dollars, *if that*.

 Of course resale value is a matter of the whole second-hand scene - it
always depends *entirely* on how desperate the sucker is!  We've been
through this discussion recently regarding how much a Pentium 90 system
is worth - very close comparison.
--
 Cheers,
       Paul B.

2000\01\12@222701 by Marcelo Yamamoto

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>An SCR would likely be in the crowbar protection circuit of a power
>supply, so a bad SCR woul be sign of some other problem.


Or may be it could be part of a soft start circuit. SMPS uses it a lot.

Marcelo

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