crossfire cfa-602, car audio amp problem

hi

i actuall posted this in the wrong section, so ill have another go :)

i have a crossfire cfa602 amp that is making a weird high piched noise about 10 sec after its turned off! it slowly drops in pitch for about 4 or so seconds then dissapears, almost like a cap or something discarging.
thats with the rca's dissconected, mostly in the right channel and a little in the left.

the amp still works fine its just annoying :D

i know this amp is getting on a bit(10-12years old!) but its 1 of 6 i am using at the moment, this one powers a set of usd HLCD's
if i recall correctly the amp at one stage of its life accidently had its 2 channels wired together, it shorted and blew the 30a fuse in one channel.

was that the likely cause?

anyone with any idears on what it is, i dont have a huge knowege of how they work but i can usually fix most things.

thanks
 
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ok, hears some pics,

i have been told to check the output protection circut, any idear when that may be located?
 

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Monitor the DC voltage on each end of R313. Place the meter probe on the resistor and power up the amp (apply remote voltage). The voltage should change significantly after a few seconds. Switch the amp off. The voltage should go immediately back to the voltage that's on the resistor initially (just after remote voltage is applied).

Place the black meter probe on the amplifier's ground terminal. Place the red meter probe on the point where you need to measure the voltage.

Post what you find.
 
ok heres what i got

measurements on either side of R313 were the same

with power connected but remote lead off, measured nothing
connected remote and took a second for the amp to turn on and power light to glow
but as soon as it went on the voltage went from nothing straight up to 40v

disconected the remote lead and it drops to 23v or so then continues to drop to 2-3v in about 4min and i assume is going to drop to nothing like it did initially

what does that mean?
 
That would seem to indicate that the problem is in the 100 series channel. That channel may have a leaky transistor. With no power applied, you may be able to find the defective component by measuring the resistance between the legs of the individual transistors and comparing them to the readings in the other channel. The components with similar names (Q103, Q203) should be the same in each channel.
 
i did some checking and it seams(according to my measuments anyway!) that theres 3 or 4 of the various trans are measuring differen't, i might just replace the whole lot on that side theres only 10 or so.

i might have to put another post up cause i carnt seem to find the numbers on them in any documents i have,

on another note, i had it setup for testing and i noticed that when the amo is at idle with no input and its just outputting what sounds like hiss or pink noise that channel(right) the one making the noise, isnt as loud a hiss as the left channel, but when playing music theres no difference.
 
Initially, the complaint was that it made a noise after switching it off. Is the hissing a new problem?

Before you begin replacing large numbers of components, there are a few other things you should confirm.

Is pin 12 on the power supply IC immediately going to 0v after you remove remote voltage?

Does the noise/hiss with/without RCAs plugged in? You stated it hissed with no input but that could be with the RCAs plugged in with the signal source volume set to 0.
 
the hiss has always been there, to hear the initial problem better i used a 1" titanium horn motor and flare that i had lying around, similar to what i have in my car, because these are quite efficient( 109db) they tend to show up amp hiss and anything else :)

all my checking was done with no rca's plugged in, i dont have an audio scorce where i was checking the amp, only power, and pulling it in and out of my car is a pain.

i was just switching the driver between l-R channels and it was a noticible difference in horn hiss with the left being quieter, i probably should compare it to a good one, i also bridged it across the 2 channels out of interest and it still made the noise as well.
 
If it's drops instantly to less than 1v, that's good enough.


The his is louder on one channel in some amps because additional active components are required to invert one channel (so it can be bridged).

What voltage do you read on the third leg of transistors Q101 and Q201 as remote is applied, after 5 seconds and immediately after the remote voltage is removed?