Broken rectifier/filter, blows fuse, schematics included

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Let me say that I'm a complete newbie when it comes to repairing  
electronics, but I have a problem with an old, and very simple appliance,  
and I think that it should be very easy to fix. The appliance in question  
is an expansion module to an home computer from the 80's. Essentially, it  
only consists of a rectifier and accompanying filter: the complete  
schematics can be seen here:

http://home.student.uu.se/jowi4905/mix/sv601rectifier.jpg

The produced currents are then used to power expansion cards. However, my  
problem is that the 5A fuse breaks. I have access to several copies of  
this appliance, and this symptom is common to most of them, and the fuse  
is fried regardless of which expansion cards are plugged in, even those  
that work fine in the healthy copy. So, I'm 99% sure that the fault can be  
found in some part shown in the schematics.

I guess I could manage to repair it by replacing all of the components  
(not an impossible task), but I would rather save the money and labour,  
and try to pinpoint the faulty component first. Any idea how to do that? I  
have no idea where to start. (At least I have meassured the 12V, and it  
shows exactly that, so it would seem the upper half of the schematics is  
okay.)

Re: Broken rectifier/filter, blows fuse, schematics included

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The most likely bet is either one of the 4 diodes in the bridge, or C4 being
short circuit. The fuse should probably be a " T " rated type. An " F " type
will almost certainly blow at power up, even with no fault present, due to
the high inrush current that a cap with a value like C4's, will have. As a
first move, with the supply disconnected from power, you should check with
an ohm meter across each of the four diodes in turn. You should read
somewhere in the region of 600 to 700 ohms, depending on the meter, in one
direction, and infinity or thereabouts, in the other. A reading of zero, or
just a few ohms either way across any diode, would indicate that it is
faulty. Across the cap, you should read initially, a low figure, which will
climb as the cap charges from the meter. A steady low reading would indicate
a faulty cap. After that, you would be into disconnecting various potential
current paths, and troubleshooting them individually.

Arfa



Re: Broken rectifier/filter, blows fuse, schematics included

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Quite right. The blown fuse is marked "T 5A 250V".

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I had made some measurements on the bridge, and the results were
somewhat... irregular, but I was a bit unsure about how to understand the
results, since I was thinking that the capacitor may be screwing up the
results. To be entirely sure, I disconnected one side of each of the
diods, and indeed, I found that two of them transmitted current in either
direction!

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Yep, that's what I see, so no problem there it seems.

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I'll replace the diods on Monday, and we'll see if that solves it. I'll
post my findings.

-- Johan Winge

Re: Broken rectifier/filter, blows fuse, schematics included

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What I found was that my local electronics shop had shut down, so I will  
order some parts from the web instead. This also means I have some more  
questions for you... ;-)

I couldn't find any diodes called only R250, but the parts list for the  
unit states that it should be specced for 50V, 6A. Based on this, I found  
one called 6A10, which was the closest I came at one particular vendor.  
Data-sheet can be found at http://www.elfa.se/pdf/70/07003916.pdf . Is it  
possible that this could be a valid replacement for "RTFR R250 50V 6A", as  
it is stated in the parts list? Which characteristics are relevant and  
important to take note of?

Similarly, for another unit, I will have to replace some of the 1N5400.  
However, most vendors seem to only carry 1N5401, which is specced for 100V  
instead of 50V ("Maximum recurrent peak reverse voltage" and "Maximum DC  
blocking voltage"). However, while this being over-kill in this case, I  
presume it won't have any adverse consequences?

-- Johan Winge

Re: Broken rectifier/filter, blows fuse, schematics included
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Diodes are very easily substituted and this application is low frequency
so not critical at all. You need something with equal or greater
amperage PIV (DC blocking, reverse voltage, etc) so the ones you found
will work fine.

Re: Broken rectifier/filter, blows fuse, schematics included

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I would also recommend that all four diodes in the bridge are replaced with
similar types, irrespective of how many are actually blown, and that you add
1000pf disc ceramic caps across them.

Arfa



Re: Broken rectifier/filter, blows fuse, schematics included
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Being a "new apprentice" or novice - you need to have a basic tool - a VOM
or DVM.  With this you can test for shorts (continuity); diodes and bipolar
transistors.

You are blowing a fuse - due to unacceptable current draw -- which is likely
due to a short caused by a wire or actual physical short or a failed
component.

gb



Re: Broken rectifier/filter, blows fuse, schematics included
In data Sat, 24 Jun 2006 13:59:38 +0200, Johan Winge  

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Where's that 5a fuse?



--
Inty.Evolution
snipped-for-privacy@email.it -> per email normali e con immagini Jpeg

Re: Broken rectifier/filter, blows fuse, schematics included

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looks like it is on the primary side of 8v xformer

Re: Broken rectifier/filter, blows fuse, schematics included

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I was assuming that these were two windings on a transformer with a ( not
drawn ) common primary, and that the fuses were wrongly drawn as the
elements that look a bit like switches, but with only one little circle ??
Perhaps the OP could confirm this, as it potentially makes a difference to
the diagnosis.

Arfa



Re: Broken rectifier/filter, blows fuse, schematics included

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Ah, yes, there's a common primary. I realised it is shown on the original  
schematic, but I didn't have room for it when I did the scanning... Sorry  
for that.

I believe though that the things that look like switches, are indeed  
switches. The inner workings of the transformer are not shown, probably  
since it was made by another manufacturer. I believe the various retailers  
provided their own transformer, or something like that.

-- Johan Winge

Re: Broken rectifier/filter, blows fuse, schematics included

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It's inside the transformer (which is external to the unit, and thus not  
included in the schematic). The blue text was added by me in a try to show  
that.

-- Johan Winge

Re: Broken rectifier/filter, blows fuse, schematics included
In data Sat, 24 Jun 2006 17:25:26 +0200, Johan Winge  

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try to remove everything after the 78xx regolator output, and see if fuse  
blowns yet.
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--
Inty.Evolution
snipped-for-privacy@email.it -> per email normali e con immagini Jpeg

Re: Broken rectifier/filter, blows fuse, schematics included

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Yep.  The power on surge current charging the 4700uf and 10,000uf
capacitors might blow the fuse.  These capacitors are a nice short
circuits while charging on initial power on.  A resistor between
either the xformer and diodes, or between the diodes and the
capacitors, should limit the surge current.  My guess is about 1-2
ohms should help.  You can also use slow-blow fuses.

If the unit is really 25 years old, you might want to check the
capacitors for leakage.  If there is any evidence of bulging or case
expansion, they're blown.  Big electrolytics deteriorate.  If they're
defective or dying, that might also be the cause.  A crude way to
check this is to substitute an ammeter for the 5A fuse and see if the
power on current approaches 5A.

As others have suggested, it might be a blown diode.  These are easily
checked with a volt-ohms guesser.

Incidentally, the design of the circuit is truely an abomination.  It
vaguely resembles something I vaguely recall from Atari game products.
The 7805 and 7812 regulator can only output about 1amp without
shutting down.  However, the game required more than 1A.  So, the
designers added an unregulated emitter follower in parallel with the
78xx regulators to boost the output current.  I'm not sure, but I
think you've drawn the schematic incorrectly.  The regulator input
line and the MJE2955 PNP xistors don't look quite right.

--
Jeff Liebermann     snipped-for-privacy@comix.santa-cruz.ca.us
150 Felker St #D    http://www.LearnByDestroying.com
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Re: Broken rectifier/filter, blows fuse, schematics included
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The circuit diagram looks perfectly drawn to me.  The current paths to the
regulators' inputs are taken through the 56 ohm (5R6 would probably be more
appropriate) resistors.  The transistors aren't in emitter follower
arrangements; they are common emitter circuits.
Taking the +5V circuit as an example; the transistor is turned on in response to
the current through R1, which is the current being passed through the regulator.
Until the voltage across R1 increases to the point that the transistor turns on,
the regulator passes the full load current.  When the voltage exceeds the
turn-on potential of the transistor, the transistor turns on, passing more
current to the load.  The more current demanded by the load, the more the
transistor turns on, thereby keeping the regulator from exceeding its current
rating. Full voltage regulation is maintained.

The circuit isn't an abomination; in fact, this arrangement is recommended in
the data sheets as a method to increase current capacity of the regulator.
--
Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in the
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Re: Broken rectifier/filter, blows fuse, schematics included

http://home.student.uu.se/jowi4905/mix/sv601rectifier.jpg

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The typical circuit on the LM340/78xx data sheet shows:
  R = 0.9 / I(in)
where I(in) is the input current to the regulator.  
0.9???.  I would expect 0.6v.
See page 13 of the LM340 data sheet at:
  http://www.national.com/ds/LM/LM340.pdf

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True.  The ones in the Atari power supply were emitter followers.
Sorry if I didn't make that clear.  The circuits are similar, but not
identical.

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to
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regulator.
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on,
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Thanks for the explanation.  I didn't see how it worked.  The 56 ohm
resistor had me confused as 1A through 56 ohms is 56 watts which isn't
going to happen.  Sorry.

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I stand corrected and the circuit will work with the correct resitor
value.  

However, 14VAC from the power xformer is barely enough to produce the
minimum voltage required to supply the input of the 7812.  The full
wave bridge produces 14VDC.  The regulator requires 12V plus at least
1.6V input-output differential.  Add 0.6V for the MJE2955 EB junction
for a total of 14.2VDC required.  The 14VAC xformer just isn't going
to supply that.

--
Jeff Liebermann     snipped-for-privacy@comix.santa-cruz.ca.us
150 Felker St #D    http://www.LearnByDestroying.com
We've slightly trimmed the long signature. Click to see the full one.
Re: Broken rectifier/filter, blows fuse, schematics included

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Off load, the 14v AC winding will provide around 19.5v DC at the output of
the bridge. Whilst I accept that this will start to dip from this peak
value, when the bridge comes under load, I would have expected a decent
quality 4700uF cap to have been able to hold the rail up to a level enough
for the dips to still be higher than the dropout voltage of the regulator ?

I have a little project on the go at the moment, where a small 9v output
torroid that " came to hand " out of the junk box, is supplying a 7810
that's being thrashed quite hard. I also have non critical circuitry ( some
relays and a fan ), being fed from the input side. The resevoir cap that
also came to hand, is a mere 1000uF. Agreed, the voltage across the cap is
quite ripple-y, but it never drops below the point where any disturbance or
lack of regulation is seen on the output of the 7810.

Arfa



Re: Broken rectifier/filter, blows fuse, schematics included
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The 14V secondary will produce about 19.75 V at the peak.  Without doing a lot
of math, the rule of thumb for the filter capacitor is about 2000 uFd per amp
for 2V P-P ripple.  That means that the trough of the ripple waveform is at
about 17.75 V.  That gives the 12V regulator over 5V of headroom, more than
adequate for regulation.  Even after subtracting the .65V drop from the B-E
junction of the transistor, that still leaves approx. 17V; again, more than
adequate.
One thing that does appear to be marginal is the transformer's output at low
line voltage.  With a 10% drop in mains voltage, the DC input to the regulator
drops to about 15VDC.  Getting close to the regulator's dropout point, but still
within ratings.
If operated within its specs, this power supply should perform well.
Cheers!!!!
--
Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in the
We've slightly trimmed the long signature. Click to see the full one.

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