Sansui G-7700 receiver blows fuse

[woaimi]

A colleague gave me this Sansui receiver. He mentioned it was still playing until recently (couple weeks ago) lost its power and he thinks it blew the fuse. I brought is home opened up and sure enough the 10A fuse is blown. I found a spare one at home put it in and instantly blew that one too. I'm hoping some experienced users here would direct me in the right direction to bring this nice piece of art back to life. Having read through some posts in the other forum regarding this receiver, I realized this is not an easy task but possessing a nice Sansui G-7700 and learning new skill that what got my commitment.

there are no burnt marks or burnt components spotted on the board. One suggestion is to check the output transistors, I haven't got to it yet but will do as soon as I got a chance to play with it.

I'm very much appreciate all of your help and suggestions.

[petehall347]

Re: Sansui G-7700 receiver blows fuse

yes check outputs and if shorted check drivers . if they are bad keep checking all other main amp transistors .
if no outputs or drivers shorted check the power supply for short circuits starting with the main rectifier .
this amp isnt the best by a long way to learn on . things can jump out and bite you .
i think there are also fuse resistors that need checking / replacing . find what is blowing the fuse first and dont plug directly into the mains until its fixed ... use a dbt for testing it and initial bias current setting up .
there are threads over on audiokarma you can read .

[woaimi]

Re: Sansui G-7700 receiver blows fuse

thanks. I'm following both forums; the more the merrier.

[woaimi]

Re: Sansui G-7700 receiver blows fuse

finally got sometime for my hobby. I tested all the output transistors. It used a pair of 2SA1170 & 2SC2774. The left channel transistors only had reading on 1 direction. The right channel got readings both directions

2SC2774
(+) on B: B-C=127mV, B-E=127mV
(-) on B: B-C=145mV, B-E=145mV
C-E shorted

2SA1170
(-) on B: B-C=194mV, B-E=194mV
(+) on B: B-C=196mV, B-E=196mV
C-E shorted

i'll go through the others as peterhall347 mentioned. will see . thanks

[redwire]

Re: Sansui G-7700 receiver blows fuse

If output transistors have shorted, the driver and pre-driver stages can also get damaged. The transistors might test good- but actually be weakened. It is always good to replace those as well, Sony has it in their service policies to always replace one stage ahead of the shorted parts.

Sometimes you can tell a transistor is damaged if the VBE and VCB readings are way different, they usually are close together, or if the readings are hi or low, such as 400mV or 850mV.

[CapLeaker]

Re: Sansui G-7700 receiver blows fuse

I second what redwire said. I fixed one of these a few years back. After the initial output driver fix and playing music, about 1 minute and 30 seconds the pre driver failed.

[woaimi]

Re: Sansui G-7700 receiver blows fuse

If output transistors have shorted, the driver and pre-driver stages can also get damaged. The transistors might test good- but actually be weakened. It is always good to replace those as well, Sony has it in their service policies to always replace one stage ahead of the shorted parts.

Sometimes you can tell a transistor is damaged if the VBE and VCB readings are way different, they usually are close together, or if the readings are hi or low, such as 400mV or 850mV.

thanks for the suggestion. In fact, TR17 of the same channel shorted as well. I'm going to remove all of them from the board and test again. There's 1 resistor R17 seems to be cracked but I can still read its value with an ohm meter; it probably safe to replace some or all of them in the same area.