Sansui 7070 Blown Fuses

[buerwi01]

Hi everyone,

I’m a long time lurker, first time poster, so thank you in advance for any assistance you can offer!

I have an issue where the left channel on my Sansui 7070 stopped working, and was momentarily fixed by cycling the switches. However, it then stopped working again, and when I took a look at the driver board I could see that the main fuses (F02 an F03) were blown. I replaced them with a few 7a 250v fuses (I see they are marked for 125v, but was of the impression 250v should work fine since that’s the maximum voltage), but they also blew out as soon as I turned it on.

I checked the fuse resistors on F-2624 that seem to cause people problems (R33 & 34, and R41-44), but they all seemed to be working fine. Furthermore, I tried removing F-2624 entirely and replacing the blown fuses with new ones, only to have them blow out as well.

I’ve been combing through posts on this thread and see that the likely culprit is now the output transistors. Can anyone confirm whether this is what I should be checking next, and if so how I can determine whether they are functioning properly or need to be replaced? I would prefer to fix it myself, as I have been enjoying learning how it works. And I’m still very new to this and understand if I’m now in over my head.

 

[kwayde]

You'll want to build a dim bulb tester so you can quit blowing parts. Don't fire it up again until you have a dim bulb tester hooked up.
https://audiokarma.org/forums/index.php?threads/dim-bulb-tester-build-and-how-to.808399/

You'll get more help if you ask the mods to move this to the Sansui section. Just click on the report link in the bottom left of your post then ask to have it moved.

 

[Sandman1221]

The resistors usually have colored bands on them which indicate the resistance, you could start checking the suspect resistors with the unit off to see if they have the correct values.

 

[mbz]

the likely culprit is now the output transistors

Yeah F02, F03 feed the unregulated supply which powers the driver/output stage so these transistors should be checked. See diode test below. +1 on moving to Sansui subforum to get expert help, +1 on building a DBT, yes, it's a PITA but will save you $$'s and is needed to troubleshoot/identify suspect components.

Nice graphic from markthefixer on diode testing, clear concise... red/black indicate mm probe orientation, 2 results expected, 640mV or abouts and OL open loop/circuit/out of range.