Sansui au-4900 Burnt Resistor

vavila

New Member
I have a Au-4900 that was cracking and poping so I put it away for about a year. I have some time on my hands and decided to see if I could fix it. So i took off the cover and found that R36 was burnt found the service manual and replaced it. when I turned the unit on the R36 burns out. Any idea where i]I should look for this issue. I did notice that r28 has some oxidation on one of the legs (will replace) would that be the problem?
 
If a resistor is doing that you have another problem.

Register here and download this service and post the section of the schematic to this forum that concerns R36 - most likely a failed transistor that is taking the resistor out with it:

https://www.hifiengine.com/manual_library/sansui/au-4900.shtml

Also, look on this site for instructions on building a Dim Bulb Tester (DBT) and only power your amplifier up using that in future to prevent any further damage.
 
OK - so R36 is at the output stage of your amp - your output transistors are 2SD188A and 2SB545A - cant say I have heard of these outputs before.

R36 is connected to the base of TR14 (some of these resistors may well be fuse resistors too).

Suggest that you get hold of a good digital meter and following stickies on this forum - pull and test all your output transistors - you have one or more blown ones most likely.

Try this first.
 
You have a great little amp. Well worth fixing. Please post the serial number for the database. Just click on the link in my signature block.

- Pete
 
You have a great little amp. Well worth fixing.

+1 on this, l had a near mint condition one a while back and I really liked it's tone. Great advice from slimecity on checking the outputs first, prob a good idea to also check the drivers, TR08 and TR10. As suggested check the fusible resistors, l think from memory there are only four on the amp board? Anyway they are the ones that are standing up off the board to allow for cooling.
 
Last edited:
I just wanted to give an update. I have ordered new transistors, as i'm in Houston i'm not sure when i will get them but i will keep you guys updated
 
I have received the new transistors and I have a question. there is a clear plastic with a white substance on the old ones that the new ones dont have. What is it and do i need to install it with the new ones?
 

Attachments

  • File Sep 05, 5 14 23 PM.jpeg
    File Sep 05, 5 14 23 PM.jpeg
    28.9 KB · Views: 11
you could check dc offset with outputs removed . play music into headphones if offset is good .. thats if it has headphone socket ..never try with speakers if doing this .
 
. there is a clear plastic with a white substance on the old ones that the new ones dont have.
Not plastic. You can use the old mica insulators. Just clean off the old compound and apply a thin coat of new compound .
 
You can use the old mica insulators
I lay the old greasy insulators on a flat surface and wipe the mess off with a couple paper towels. The mica is rather brittle and doesn´t bend very much before it snaps in two. So a gentle touch is recommended.
 
Back
Top Bottom