repair akai am-49 help needed

[pavlos]

I have in which some resistors and transistors were burned.I replaced them and it seemed to work for a while but after incresing the sound resistors and one transistor were burned.While iwas testing it i noticed that when turning it on my fluorescent lamp made a sudden little blink.I read that there is probably a short.HOW CAN I FIND THE SHORT.
The amp has relay.Two pairs of 2sc3281/2sa1302 transtors.HOW CAN I FIND THE SHORT

[anatech]

Hi pavlos,
Do you have the schematic? A DVM? Anything else?

The reason it burned again was that it wasn't repaired properly the first time. Sorry. Now it's time to get into some detail on it. You may have an intermittent open, so try to consider other possibilities. Keep an open mind and the facts will lead you to the truth.

Also, please take a picture of it and post.

-Chris

Edit: Is there a small bias PCB on teh heat sink? Check all the connections, an open here will do what you describe.

[pavlos]

I have a DVM and some basic kwonledge in electronics but i don't have the schematic.I had made some measurements while trying to repair.At the moment i cant send any foto but i can describe a little the board. The amp has two pairs of high power amplifiers and between them there are are six smaller transistors which are in pairs too.I had change them all.There must be a defective part which i couldn't found.
The capacitors seemed to work.

[anatech]

Hi pavlos,
Did you use exact replacement parts? If not, please list the parts you changed and what you replaced them with.

-Chris

[pavlos]

I replaced 2SA970BL with 2SA970GR
2SA1248 S with 2SB649A
2sc3116 S with 2SD669A

power
transistors 2SA1303 with 2SA1302
2SC3284 with 2SC3281

bridge rectifier RBV602 with kbu602 or kbu604

The parts i replaced were given to me by the seller as replacements.

[ilimzn]

It is very likely that your 2SC3281/2SA1302 are fake...

[anatech]

Hi pavlos,
Your subs are mostly correct
I'll list a bit.

2SA970BL with 2SA970GR - Okay, just gain differences.

2SA1248 S with 2SB649A -Okay 160V 0.7 A compared to 160V 1.5A, the gain is the problem here(?). 2,000+ compared to 320 max for the B649A.

2SC3116 S with 2SD669A -same as above, at least they are compliments

2SA1303 with 2SA1302 -different case styles. At least the new ones are compatible and higher rated.

2SC3284 with 2SC3281
-same as above.

It appears that your driver transistors are completely wrong. I have attached the data sheet from Sanyo, it's not the "S" version and looks like a little higher gain, not a ton like my data book shows. It is entirely possible that your 2SC3281/2SA1302 are fake as ilimzn stated.

Try the On Semi parts from a proper distributor (MJL3281/MJL1302). They should be okay.

-Chris

[pavlos]

pic from my mobile phone

[anatech]

Hi pavlos,
Instantly I can see one source of potential troubles. A more clear picture would be needed to say much else.

You have not used the original "plate" style non-inductive resistors. Instead I see inductive, wire wound "bathtub" style resistors. These can easily cause the output stage to oscillate. You must use low inductance resistors or the original "plate" style.

-Chris

[pavlos]

If i replace with the corect parts,what posibility there is to burn or destroy something again.I dont understood what you wrote about th 2sa1248 and 2sb649a gain.If the capacitor or transormer is defective how can check.How can i check the amplifier before powering it up.