Intermittent short!!!?

RuffzGuts

Super Member
Hey Yamafolk!...I've a little CR-450 receiver here that is driving me nuts...

I was hoping maybe someone could possibly shine a light on something I am overlooking.
The amplifier was given to me with a blown fuse...this amp runs 4x 3A glass fuses...the blown fuse is closest to side of unit.

I replaced fuse and plugged into Dim Bulb, it flashed up straight away so I turned the unit off and began troubleshooting....
I have been over every transistor in the amp now....checked as much as I could without de soldering including all caps while I was there..

I could not find a fault...so started checking all wires...nothing....then after switching on and off on and off through testing, suddenly it came good...

puzzled I thought ok...I shall leave it open and listen to it for a few days...adjusted bias, dc off set all good...sounded wonderful, first Yamaha amp I've had and I like....definitely has its own signature sound...

So, performed fine via dim bulb for a few days, I closed it back up...

I then decided to open back up to change a bulb out......turned her on again, bang short came back....disconnected new bulb back on dim bulb,,,flashed up...
at this point I went away to bang head against wall....

Came back went over it couldn't find anything...noticed some suspect (cracked)solder joints so I touched them up, all around power supply circuit...

switched on.....all good, No short....Hmmmm

went away few a few hours, plugged in again....and dim bulb flashes up!

I think its haunted...

any ideas?
 
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The dim bulb flashing on turn on may be from inrush current as the power supply caps charge up. The correct behavior for the bulb is it can flash bright on turn on but it should dim after 1 or 2 seconds. If it doesn't then there is a problem.
 
Yes....no its defenetely staying on.
.

That's bad. Now you need to find out where the current is going. Get some mini clip leads for your meter and measure the voltage across one of the large ceramic emitter resistors in the output stage. Measure one on each channel. Divide the voltage by the resistance to get the current. You want it to be about 20mA, not more than 40mA. I expect one channel to be really high, over 100mA.
 
Back at it this morning....retested with dim bulb presenting short once again...

So I flipped the amp on its side and started checking emitter resistor Voltages from underneath, turning power on and off quickly to get readings....

Took two measurements, Dim bulb flashed up both times....went to take a third measurement, got the probes on flicked on the Variac....Dim Bulb dims!!!!

now the amps back to life again, perfect both channels...

This is just no good...I need to know why....
 
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First, with the power off, measure the resistance of VR501 and VR502, the bias pots. If one of them is high or intermittent it would cause it to draw too much current. If those are ok and about the same, tighten all the screws that secure the boards to the chassis, and any that secure wires to the chassis. Bad grounds can cause over current situations.
 
You have a bad solder joint somewhere, go over your solder joints with a fine tooth comb- not just the PS and check what warren said with the grounds.
 
Both bias pots around 560 ohm, the short circuit is back....re appeared again after attempting to check emitter resistor Voltages again...I've given all boards a knock around with a chop stick...cant find anything......Its a nice clean unit, has been well looked after...as you have probably noticed the manual isn't very clear....
head scratch....
 
Dim bulb tester

Its ALIVE!,:yes: I got the magnifying glass over it and actually found a fair few bad joints....The next Yamaha I work on I think this will be the first thing I look for...the two worst were at TR507,508 where the trace had actually lifted away from the board and broken, I've had to run a jumper off one leg of one transistor....

Thanks for your help guys...and for anyone else watching, Right there is the value of the DBT! should be the first tool on anyone's bench(even if you do not intend on repairing equipment)....very easy to knock one up and saves a lot of heart ache...cheers:thmbsp:
:music:....touch wood the ghost is gone...
 

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