Thanks Frank.
By "shutting down" I mean I switch the receiver on, and before any lights or
display comes on, a relay opens, disconnecting the input 120VAC from the
power xformers. I'm not real experienced with repair (although I have
managed to get a few things fixed), but I am aware of the practice of using
a relay to protect the speakers from DC damage at the audio output. This
one, however, has the relay in the power supply section. The PS goes:
120v in = small xformer = relay = big xformer = rectifier stages.
I need to look around more in there (I don't have a lot of time to work on
it) to see exactly why there are 2 xformers in this arrangement. The small
xformer has a CT output with 15v on either side and feeds a 7806, and the
big xformer has 3 taps - one that must be +/-15v (because I can see the 78
and 7915s), another that must be in the 40-60v range for the output IC, and
2 brown leads (?). There is some type of surface mount device on the
underside of the board, but I'm not sure if it is a uP.
As you may have guessed, I have no schematic. I also immediately suspected
the output IC, but now that it is completely disconnected from the circuit,
and the amp still shuts down, I began to suspect the PS circuitry.
"Franc Zabkar" wrote in message
news
On Sat, 18 Dec 2004 11:47:14 -0500, "tempus fugit"
put finger to keyboard and composed:
Ok I did some more poking around. There are no shorts on any of the pins
you
suggested on the IC. Also, I couldn't find any shorts to ground near the
filter caps/PS area - everything seems to check out OK there. There are 2
taps from the big xformer - I'm wondering if I should disconnect 1 of
them
and see if the unit will power up then, to save me checking every single
connection. Is it safe to do this (I can't see why it wouldn't be)?
IME amp problems of this kind have nearly always been due to blown
outputs, whether discrete transistors, or hybrid modules. The only
other thing I can think of would be leaky rectifier diodes which may
not test as shorts under static conditions, but which may cause the
filter capacitors to look like shorts under dynamic conditions.
Also, is
it possible that the unit is shutting down because the output IC is
disconnected (again, I don't know why this would be, so I'm wondering if
you've had experience with anything that did)?
I'm not familiar with your amp, so I don't quite understand what you
mean by "shutting down". Does the amp have some kind of uP or overload
sensing circuit?
Thanks
"Franc Zabkar" wrote in message
.. .
On Sun, 12 Dec 2004 17:30:57 -0500, "tempus fugit"
put finger to keyboard and composed:
Thanks Frank.
What is a fusible resistor?
It's a safety resistor that is designed to fuse under fault
conditions.
The IC is an STK 4161 which I can;t find a data sheet on.
Is this the one?
http://www.satel.hu/file/document/El...K/STK4161V.pdf
This was one of my
first suspects, but now that I see that it is desoldered, I assume the
problem must be somewhere else.
You should test for shorts between the load side of each of the
fusible resistors and ground. You will always measure a short on the
winding side. If after desoldering the STK4161 IC there are no longer
any shorts on the load side, then the IC must have been the culprit.
Just to be sure, test for shorts between pins 10 & 11, 10 & 9, 13 &
14, and 13 & 11.
It's kind of tricky to test something that
you can't power up. I think my next step is to desolder the xformer
and
see
if the low resistance is still there to see if the problem in the
xformer
(if the low resistance is a problem) or somewhere on the board (I'm
assuming
that the resistance from the DC + to ground shouldn't be quite that
low,
otherwise it'd need to source 50 - 75A, which seems unlikely).
Any other thoughts?
Thanks
The transformer is the last place I would look. In any case you will
always measure close to zero ohms on the secondary side.
"Franc Zabkar" wrote in message
.. .
On Sun, 12 Dec 2004 13:18:19 -0500, "tempus fugit"
put finger to keyboard and composed:
Sorry bout that - should've put that in m OP. It's a Sansui RZ-590.
There are a number of 1 ohm resistors (they are marked FR and have
the
symbol of a fuse in series with a resistor?).
These are fusible resistors.
I know how to measure
voltages, but as I said in my OP, the unit is dead - it shuts down
immediately after power up, so there are no voltages to measure.
There
are 3
secondaries - a pair of reds with a black, a pair of blues with a
white,
and
a pair of browns. The one in question is the reds and black, which
I
assume
is for the high(er) voltage supply to the output IC, as they are
connects
to
50v electros.
Also, on further inspection, I found that the output IC, although
mounted
well, doesn't have any of the pins actually soldered to the board.
I
don't
know if this could cause the fault, and I don't know if they have
been
desoldered for a reason.
This IC is probably a hybrid amp (STKxxxx ?). The previous tech
probably desoldered it to isolate the source of a short circuit. Be
prepared to replace it.
thanks
"Mark D. Zacharias" wrote in message
...
The low resistance you measure may be normal. Some Sansui
receivers
had
bad
1 ohm resistors in the power supply. There's lots of
possibilities.
Model
number would help.
Mark Z.
"tempus fugit" wrote in message
...
Hey all;
I'm working on a Sansui receiver that shuts down as soon as I
switch
it
on -
no lights or anything. I've taken it apart and inspected the
power
supply
section, and found that there is a very low resistance in a
couple
of
the
transformer's output wires. If I measure between either of the
reds
(there
are 2) and the black (which is connected close to the reds, so
I'm
assuming
that these taps go together) the meter reads about 0.6 or 0.7
ohms.
Is
it
possible that this is the correct reading for this, or should I
take
that
to
be a dead short? I get somewhat higher readings on the other
taps -
more
like 2.5 or 3 ohms.
Also, has anyone had experience with a reciever with similar
symptoms?
Any
tips?
Thanks
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