No Clue wrote:
"John Popelish" wrote in message
news
No Clue wrote:
"John Popelish" wrote in message
...
No Clue wrote:
Here's some pics of the inside. It's a Yamaha G100-112III guitar amp.
I've
circled the resistor in yellow, and red dots on back of circuit board
where
it connects.
Is it possible for you to read any part number on the small
black rectangular block a half inch from the resistor, that
has 9 pins in a row? The resistor seems to be connected to
it and the black 24 VDC relay block.
The numbers on the side of that block are a large (T), then TA7317P -
4E.
The 4E is a lot smaller than the rest of the numbers. I marked the pic
with
a red arrow to make sure we're talking of the same block.
You got it. That number (TA7371) allowed me to Google for
the data sheet.
Hit "download data sheet on:
http://www.datasheets.org.uk/search....317&sType=part
It looks like this resistor is approximately the same one
that connects pin 6 to the relay coil on page 3 of the data
sheet (500 ohm 3 watt). The value may be different, because
the supply voltage is different. It uses up all the extra
supply voltage that exceeds that needed to operate the 24
volt relay. The relay coil may have partially shorted out,
causing extra power to be dumped into this resistor.
The voltage between pin 6 and pin 4 should be either about
1.2 volts (when it is turning the relay is on during
operation) or the full supply voltage (when the protection
relay is off for a while at start up, to prevent a big
speaker thump). This is the output voltage speced on page 1
to be typically 1 volt and not more than 2.
Can you measure the voltage across the resistor and provide
its color code (looks like orange, something, black gold),
so we can figure out whether its (and thus, the relay coil)
current is reasonable?
Thank you for all your help with this. I don't know if I'm reading my meter
right or not since all I've ever used it for is continuity testing and
checking 9 volt batteries, but what I'm seeing is about 42 volts across the
resistor. Does that sound right? Across pin 4 and 6, I'm getting about 48
volts. As for the resistor, it's so faded out, my best guess would have to
be orange, yellow or possible orange again, black, gold.
The standard 5% (that is what the gold means) values that
begin with orange and end with black are orange orange black
(33 ohms) orange blue black (36 ohms) and orange white black
(39 ohms). That 24 volt relay probably should draw much
less than 100 mA, when 24 volts is connected across it.
That means that the resistor should have much less than 100
mA going through it and that current would drop less than 4
volts. If the black used to be brown, then those resistance
possibilities would be 330, 360 and 390 ohms, and the
voltage drop would be 10 times bigger, and these seem more
likely.
Something is wrong with your volt readings, since the
voltage between pins 4 and 6 and the voltage across the ends
of the resistor should be equal to or less than the total
power supply voltage (pin 4 to the other end of the relay
coil). I expect that total to be less than 0 volts.
There should be a diode connected across the relay coil
(black cylinder with a stripe around one end), and that may
have shorted, putting an extra 24 volts across the resistor.
If you replace it, make sure you put the new one in with
the stripe at the same end. Any 1N400X (with X being
1,2,3,4,5,6,7) would work. If your meter has a range
labeled with a diode symbol, you can remove the diode and
check it with that range. One way, it would read off scale
(blank) and the other way, 6 hundred something (the forward
voltage drop in millivolts). If it is bad, it will read
near zero and the same in either direction. This failure
would not only explain the overheated resistor, but the
relay not pulling in and connecting the speaker to the
amplifier.
If the diode is good, before you solder it back in, check
the relay coil resistance (between the two holes where you
removed the diode). It should probably measure around 600 ohms.