My son's yamaha cr800 rebuild thread

Oscar23

Active Member
I found this yamaha cr800 and my son who's 11 has shown an interest so I thought I let him have a go showed him some basics and let him go all parts are ordered by him and soldering. I offer advice and guidance as he goes but pretty much on his own. Here's pics of his power supply. Anything that he may have missed please point out he'd love the assistance
 

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Looks good.. I assume the two caps to the right have already been soldered and leads clipped .
 
Oh my no he didn't but I'll tell him.he's been reading over posts finding out what to change he's pulled all the 2sc458 and replaced them with ksc 1845. Cleaned all the pots and repaired the amp section. He's recapping the amp/protection board.Takes him some time to pull the caps
 
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OK he got the amp board done regretted the outputs.he set the bias to zero on both boards. And the idle current on the left however we can't get the right side lower than 57mv. The unit will not come out of proteprotection.
 

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57mV wont trip protection.

He's recapping the amp/protection board.
Probably a fault in the protection circuit.
Double check orientation of all replaced caps, would expect smoke...
Check for dry solder joints, google for pix.
Check for any solder bridges during cap replacements.
I'll try and get some test points for the protection circuit.
 
Check dc offset of both channels as per the sm.
Target is less than 10mV, need a "few hundred mV" to trip protection

Lousey sm scan, may be errors below(?)
C731(47/16) and R769(150K) perform the mute power on timing
Check C731 orientation and solder joints.

TR724 and 725 should then be turned on to activate the relay.
Wait 10 seconds after powering on then measure b/c/e voltages
to GND for TR724 and 725. Please do not probe transistor legs,
measure at adjoining component,

eg, TR725c, measure at D717 cathode
TR724b, measure at R769 or R765
etc...

I have not worked on a CR800, if the requested measurements are too difficult
then don't do them.
 
.he set the bias to zero on both boards. And the idle current on the left however we can't get the right side lower than 57mv.
Bias and idle current are the same thing. Same adjustment.
 
.he set the bias to zero on both boards.

Mid-point voltage and DC offset are the same.Mid point is adjusted to zero or as close as you can get to zero.

Bias and DC offset are two entirely different adjustments and are not to be confused.
 
In the first photo. Looking at the side that says C458 the pinout is, left to right, BCE.

KSC1845's are ECB. Just like the diagram on the right.
 
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The confusion is caused by outhouse items having their id etc printed on the larger back face.
The bevel/slanted face is the front. Strictly speaking, the 2SC458 are ECB however with the
printing on the back it's easier to explain as above.

So if the TR803,4 (2SC458) are incorrectly installed then power to the protection circuit is
probably down...
 
OK we have the transitors in right I power on the right side idle current is 27mv and the left is 430mv something is not right
 
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