SX450 Signal breaks down under load

[V12A]

I replaced 2/4 broken high power amplifier transistiors
replaced all Elkos
Sound on headphone is fine
Signal on Speaker breaks down under load (after switching to Speakers)

(yellow=Input)
(red=Speaker Output)





Amplification without load is fine (different scale)

 

[Watthour]

What replacement devices were installed? I've had good success using MJE15030G or 32G for outputs.

Were the driver transistors, surrounding resistors, and speaker switch/connections checked? The 2SA720 and 2SC1318s should go away anyway, since the 720s are in the family of noise-prone devices. Reasonable replacements are KSA1220AY and KSC2690AY. The board holes may have to be slightly enlarged to accommodate them, but it is a worthwhile endeavor.

 

[Watthour]

Incidentally, be VERY careful about replacing the driver transistors. The factory schematic and board artwork can be wrong. This schematic is correct:

http://www.wwdsltd.com/files/SX-450Schematic.jpg

 

[V12A]

I have now repalced all the four power transistors with MJE243G.
I started replacing the 0,5R/2W resistors - they had about double the value

The Switch has been cleaned witch contact 60.

I wonder how the signal on the basis of the power transistors should look like.
Is it the full wave or just half of the wave on each transistor.

What I also did not find out which 2 belong to which channel.

I'll check for the change in the driver layout - what has been changed? Connection or polarity of the transistor
One of the drivers was faulty, another had bad soldering

what about the Tantals should I replace them?

 

[petehall347]

is the problem one or both channels ?

 

[Watthour]

If you view that schematic, the driver trannie in RED is changed.

 

[llwhtt]

How did you mount the MJE243s? The pin out is just the opposite of what came out of the unit. Original 2SD313 is BCE and the MJE243 is ECB. Those are a bit on the small side also.

Craig

 

[petehall347]

If you view that schematic, the driver trannie in RED is changed.

i call that a deliberate mistake to keep us on our toes ..schem i have is wrong for other channel

 

[Watthour]

That would make more sense if you were in Australia, but...

 

[Watthour]

How did you mount the MJE243s? The pin out is just the opposite of what came out of the unit. Original 2SD313 is BCE and the MJE243 is ECB. Those are a bit on the small side also.

Craig

Agreed. They are backward, and are rated for a peak of 15W. That's pretty skinny for a 15WPC amplifier. The MJE15030G is rated to 50W, and they reject heat better.

 

[V12A]

How did you mount the MJE243s? The pin out is just the opposite of what came out of the unit. Original 2SD313 is BCE and the MJE243 is ECB. Those are a bit on the small side also.

Craig

Oh thank you!

That's it - I'll cross B&E and see what happens, before I change to the MJE15030G.

 

[V12A]

is the problem one or both channels ?

On both, but with different signal form on the output.

 

[llwhtt]

Just pull them out and mount them with the lettering towards the heatsink. Much neater than crossing the leads. Make sure they still good, no telling what installing them backwards did to them.

Craig

 

[Watthour]

The problem with that is heat rejection. They are plastic cases and may not have the die exposed on the front/face.

 

[llwhtt]

I thought it was just temporary until he installs the MJE15030.

Craig