Samson SX 1200 repair

[erict35]

Hi

I have the right channel OK et left channel blown.

I changed most transistors (2SC5200 and KSA1943 were short circuited) and diodes on the wrong channel but I still have this problem. On the D101 anode I have 2.5v between diode and ground without input signal and 65 v or more with signal.

I measure this value on the correct channel and it's always 1.1V on D101 anode whatever the input signal level.

I don't understand what is the function of the Q127 and Q137 transistors. I changed them but I replaced the Q127 with a 2SC3200GR, not a BL so the Hfe is different. Do you think it could be the problem ?

Regards

Eric

 

[llwhtt]

Q127/137 are the VI limiter transistors, you can remove them and the amp should run fine for troubleshooting purposes. There are many fusible resistors, FR, on that schematic, go thru the whole channel and make sure they're still good.

Craig

 

[erict35]

SX 1200 repair

I have found one bug. Both 131 and R132 were opened. I changged them with a "normal not fuse" 47 ohms/2W resistor and it's works but with some distorsion. Many thanks for your help Craig.

Best Regards
Eric

 

[erict35]

Hi Craig

I found that many fuse resistors were burned so I replaced them with "regular" 2w resistor. The amplifier works fine since 15 seconds then all the power transistors burned (short circuited). I bought these on Ebay so perhaps they wre fakes.

Do you think I can replace fuse resistors with regular resistors because fuse resistors are generally wirewounded and so they have a self inductance value that could limit VHF self oscillation ?

Regards
Eric

 

[llwhtt]

I usually use 1/4 or 1/2 Watt metal film in place of fusibles. All depends on what fits the board, they should also be raised off of the board when installed.

Craig

 

[MAAC0]

Have You replaced driver transistors ? Usually when the output ones burns, they also burn.

 

[erict35]

Yes I have
I finally repair the amplifier and it works fine during 15 seconds then all output transistors burned once more time



I suppose it's a hf parasitic oscillation. I verified the Zobel network components and all looks ok. When I received the amplifier le 10 ohms/3W (R127) was open so I suppose it's a parasitic oscillation too.


How can I do to suppress this oscillation ?

 

[kudakwashe]

Hi guys .need hybrid B2400 circuit diagram