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Topic: alp death (21 msgs / 1091 lines)
1) From: Gak
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My trusty alp has recently passed on, after several 
hundred roasts.  Unfortunately, I'm not willing to let it end there.  
Since It's now long out of warrantee, I decided to perform an autopsy and see if 
there was any hope of recovery.
 
I think I've isolated the problem to the 
transformer.  I'm getting 40 ohms across the input side, and about 1 across 
the output side, so I'm looking for a replacement.
 
I searched around a bit on the web but came up 
blank.  I've never shopped for electronic components on the web, so I'm not 
sure where to look.  Any alp experts have a suggestion?  
Thanks!
 
-Garrik

2) From: Jeffrey A. Bertoia
Gak wrote:
<Snip>
That doesn't necessarily sound wrong.
The primary should be much higher impedaance than the secondary.
What does or doesn't it do?
jeff

3) From: Rick Farris
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Message
What were you 
thinkiing would be the proper impedance?
 
-- 
Rick
 

4) From: Gak
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Something closer to 0.  As I remember it, each 
side of a transformer is just a coil of wire, so the resistance should be very 
small.
 
-Garrik
 
 
 
From: "Rick Farris" <rick>
What were you thinkiing would be the 
proper impedance?
-- Rick

5) From: Rick W
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Garrik

 

I hear you and if my Alp died, I wouldn’t give it up for dead either.  Here’s a couple of places that I’ve bookmarked and I expect the Electrical Engineers on this list would have some favorites.

 

 

">http://www.web-tronics.com/webtronics/index.html

 

 

">http://www.alliedelec.com/

 

Hope you get it fixed and please post what happens.

 

Rick Waits

 

 


6) From: Jeffrey A. Bertoia
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For a small xfrmr like that the resistance that you mearsured may not
be unreasonable..
I would expect that the primary would measure _much_ less than the
secondary.
Typical failure modes for transformers are windings open (infinite
resistance) or shorted turns
(lower resistance of  a winding) or primary to secondary short (a value
when measuring between
primary and secondary that is not infinite).
But if you want to replace it try www.digikey.com
jeff
Gak wrote:
  
  
  
  Something closer to 0.  As I
remember it, each side of a transformer is just a coil of wire, so the
resistance should be very small.
   
  -Garrik
   
   
   
  From: "Rick Farris" <rick>
  
  
What were you thinkiing would be the proper impedance?
  
-- Rick

7) From: john roberts
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In 
general if it's a step down transformer the primary will have more turns of 
smaller gauge wire so I would expect the primary to measure higher resistance 
than the secondary. That said I have no idea of what the actual resistance 
should be be, but since both factors are increasing resistance and 
multiplicative you may have a good transformer.
 
If you 
were to measure an open circuit on one winding that would be definitive that 
it's bad, the numbers you measure would be more useful if you knew what a good 
transformer for that model should measure. I would keep looking for an problem 
elsewhere.
 
JR 

8) From: Paul Goelz
At 09:35 AM 11/16/2003, you wrote:
<Snip>
Yes, the two likely failure modes for a transformer are an open winding or 
a shorted turn.  Unless you either smell something that doesn't smell like 
coffee or measure an open winding, the transformer is probably OK.
Maybe we should start at the beginning..... what are the symptoms?
Paul
Paul Goelz
Rochester Hills, MI
paul
pgoelzhttp://www.pgoelz.com

9) From: Rick Farris
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Message
Your 
measurements sound about right to me.  Especially for a stepdown 
transformer where there are many more windings (of smaller wire) on the input 
side than on the output side.
 
Failure modes 
of transformers are usually either a) a breakdown of the insulation, causing a 
short with the associated smoke, meltage and smells, or b) a physical 
interruption of the wire (usually at a terminal) causing an open 
circuit.
 
-- 
Rick
 

10) From: Gak
Well, the machine lost all power in the middle of a roast.  Fan stopped,
drum stopped, screen went blank.
After tearing things apart, I measured the incoming voltage at the input
side of the xfmr at 120V.  There was 0V on the output side.  That seemed
pretty clear to me.
-Garrik
At 09:35 AM 11/16/2003, you wrote:
<Snip>
Yes, the two likely failure modes for a transformer are an open winding or
a shorted turn.  Unless you either smell something that doesn't smell like
coffee or measure an open winding, the transformer is probably OK.
Maybe we should start at the beginning..... what are the symptoms?
Paul
Paul Goelz
Rochester Hills, MI
paul
pgoelzhttp://www.pgoelz.com

11) From: Jeffrey A. Bertoia
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Try it with the transformer unloaded and see if you still measure 0v
jeff
Gak wrote:
  Well, the machine lost all power in the middle of a roast.  Fan stopped,
drum stopped, screen went blank.
After tearing things apart, I measured the incoming voltage at the input
side of the xfmr at 120V.  There was 0V on the output side.  That seemed
pretty clear to me.
-Garrik
At 09:35 AM 11/16/2003, you wrote:
  
  
    In general if it's a step down transformer the primary will have more
turns of smaller gauge wire so I would expect the primary to measure
higher resistance than the secondary. That said I have no idea of what the
actual resistance should be be, but since both factors are increasing
resistance and multiplicative you may have a good transformer.
If you were to measure an open circuit on one winding that would be
definitive that it's bad, the numbers you measure would be more useful if
you knew what a good transformer for that model should measure. I would
keep looking for an problem elsewhere.
    
  
  
Yes, the two likely failure modes for a transformer are an open winding or
a shorted turn.  Unless you either smell something that doesn't smell like
coffee or measure an open winding, the transformer is probably OK.
Maybe we should start at the beginning..... what are the symptoms?
Paul
Paul Goelz
Rochester Hills, MI
paul
pgoelz
http://www.pgoelz.comhomeroast mailing list
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12) From: Rick Farris
<Snip>
Well, there is good news and bad news.  :-)  The good news is that I got
a great deal on my auto insurance!  (Just kidding.)
Transformers are a chunk of iron with some wire wrapped around them.
Generally speaking, they are one of the most reliable components in any
electronic assembly.  However, just on the other side of the transformer
(looking from the AC line side), are some diodes -- one, two or four.
Diodes, being semiconductors are among the more likely things to fail.
Especially the ones that are close enough to the input line to be
subjected to the slings and arrows of outrageous line perturbations.
There are two common failure modes of diodes: open and shorted.
Sometimes they start out shorted and within a few milliseconds become
open.
I suspect that one (or more) of the diodes in your rectifier assembly
has shorted, shorting out the secondary of your transformer.  That's the
bad news.
The good news is that, like I said above, transformers are sturdy
beasts, and if you remove the short from its output it will probably be
fine.
-- Rick

13) From: john roberts
I didn't realize that the measurements were made in situ.
Agreed, in my experience most diodes do fail as a short, unless they heat to
the point of removing themselves from the circuit. To measure a short across
the transformer secondary you would need at least two of them to short
(possible but less likely).
Also beside the possibility of the transformer winding failing open due to
over current it may have internal overload protection. If this internal
protection is a one shot the transformer is pretty much a throw away, if
it's resettable you will be OK once it cools.
It looks like you may need to remove a few components to make a better
diagnosis. If a shorted diode has caused the transformer to fail you will
need to replace it before replacing the transformer or the new transformer
will fail also.
JR
PS: I'm not disagreeing with Rick's advice just offering another POV. By
definition we can't disagree over the basic behaviors involved, without one
of us being wrong. I suspect we are drawing upon slightly different personal
experiences.

14) From: Rick Farris
<Snip>
Actually, what happens is that the first one shorts, which immediately
doubles the voltage seen by the other diode in the same side of the
bridge.  Guess what happens, then?
<Snip>
Err, yes, I'm drawing on five years of managing the test department in a
power supply factory.  :-)
-- Rick

15) From: John Abbott
Rick Farris wrote:
<Snip>
Hey a half wave rectifier!  National used to make those, we sorted them 
at the end of line - some were bridge rectifiers, some half wave :O))  
In all cases they were fuses of varying stand off current!

16) From: Gak
Ok - a little clarification:
The 0V measured was directly out of the transformer, with it disconnected
from the follow on circuitry.  I had already disconnected the xfmr from the
circuit board.  It has been a long time since I took that circuits class in
college, so the memory is a little thin on some stuff (why do mechanical
engineers have to take in anyway?  It gave me just enough knowledge to screw
things up good...)
I did manage to locate the diodes and they seem to be working as I'd expect.
Using the diode setting on the multi-meter, they all are O/L in one
direction, and not in the other.
So whats the best method for checking the transformer to see if it's ok?
It's out of the system (sitting here on my desk, actually).
And, if the transformer checks out ok, any alp experts have any other
thoughts on what failure could cause the whole thing to go dark?
Thanks so much for all the help - I certainly appreciate it!
-Garrik
From John:
<Snip>
<Snip>
to
<Snip>
across
<Snip>
<Snip>
<Snip>
<Snip>
<Snip>
personal
<Snip>

17) From: Paul Goelz
At 10:39 PM 11/16/2003, you wrote:
<Snip>
If the transformer is not hot AND if you are not smelling anything, then it 
is likely open.  Not sure about the resistance reading you got other than 
maybe you measured it in circuit and it was not in fact the winding you 
were reading.  If you are very adventuresome you might try to take the 
transformer apart.  There is probably a thermal cutoff inside that has 
opened.  If you know what you are doing and are willing to take a risk, you 
can find it and either replace it or bypass it.  Better to try to replace 
the transformer though.  Have you tried Swissmar?
Paul
Paul Goelz
Rochester Hills, MI
paul
pgoelzhttp://www.pgoelz.com

18) From: john roberts
If I understand you correctly, with the transformer secondary not connected,
but voltage applied to the primary you are measuring 0V. This suggests that
the transformer has indeed failed. Of course be sure your multimeter is set
to measure AC volts.
Transformers typically have internal thermal fuses and one of these may have
opened up. If you are getting 0Vac output with normal line voltage in it
looks like you need to replace the transformer. As your diodes check out as
not shorted, replacing the transformer should get you up and running.
Regarding how to best test a transformer, measuring winding impedance is
useful if you know what a good one measures, I can't speculate. Of course an
open winding (infinite resistance) is always evidence of failure. Testing in
circuit can be useful but be careful as line voltages can be dangerous.
JR

19) From: Rick Farris
You had 117VAC connected to the primary and got 0VAC from the secondary?
You've got an open transformer.
Are there any markings on the transformer?  If not (and maybe even if
so) you need another Alp owner to open their Alp up and measure the
output voltage of the secondary...
-- Rick

20) From: Gak
Ok - a little clarification:
The 0V measured was directly out of the transformer, with it
disconnected from the follow on circuitry.  I had already disconnected
the xfmr from the circuit board.  It has been a long time since I took
that circuits class in college, so the memory is a little thin on some
stuff (why do mechanical engineers have to take in anyway?  It gave me
just enough knowledge to screw things up good...)
I did manage to locate the diodes and they seem to be working as I'd
expect. Using the diode setting on the multi-meter, they all are O/L in
one direction, and not in the other.
So whats the best method for checking the transformer to see if it's ok?
It's out of the system (sitting here on my desk, actually).
And, if the transformer checks out ok, any alp experts have any other
thoughts on what failure could cause the whole thing to go dark?
Thanks so much for all the help - I certainly appreciate it!
-Garrik
From John:
<Snip>
<Snip>
heat to > the point of removing themselves from the circuit. To measure
a short across > the transformer secondary you would need at least two
of them to short > (possible but less likely).
<Snip>
due to > over current it may have internal overload protection. If this
internal > protection is a one shot the transformer is pretty much a
throw away, if > it's resettable you will be OK once it cools.
<Snip>
will > need to replace it before replacing the transformer or the new
transformer > will fail also.
<Snip>
<Snip>
By > definition we can't disagree over the basic behaviors involved,
without one > of us being wrong. I suspect we are drawing upon slightly
different personal > experiences.
----Original Message----
From: homeroast-admin
[mailto:homeroast-admin]On Behalf Of Rick Farris
Sent: Monday, November 17, 2003 9:57 AM
To: homeroast
Subject: RE: +alp death
<Snip>

21) From: Rick Farris
Apply 117VAC to the input and measure the (AC) output voltage.  If it's
zero, as you've indicated, it's bad.  Make sure your meter is on the ACV
scale.
-- Rick


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