Sansui 8010 (771) Blowing fuses, no sound

Hello, I just purchased an 8010 in non working condition. The panel lights work, the tuning meter swings, and the relay clicks. No speaker or headphone sound.

I'm a noob, so this is my first unit I've attempted to repair, and all I want right now is to get it working. Recapping and other exotics (for me) will be later.

I opened it up, and fuses F01, F02, F04 were blown. F03 is O.K. I replaced the three bad ones, powered it up, and they blew again. SM troubleshooting points to TR 1,2,3,4 on F-1500 power board, or six transistors on F-1499, driver board. Could I unplug and set aside 1499 (one edge connector), replace fuses, and attempt to verify power board is ok by powering it up?

I also own another 771 which works. Maybe swap F-1499 from that, if F-1500 comes up alright?

From the forum, it looks like common driver board problems could be those six transistors, or 4 diodes? I do want to fix this unit, and have my 771 work also.

 

Those fuses supply rail voltage to the driver board. If they are blowing, you have real trouble. Start by checking your output transistors. It's likely at least one in each channel is blown. This would also be a good time to make a dim bulb tester. Don't power it up again without one.

When you have a chance, please post the serial number for the database. Click on the link in my signature block.

- Pete

 

Thank You Pete, It was that 3 of 4 outputs were fried, so I just replaced them all. Is there anything "downstream" that will kill these, or should I be good to go on the DBT?

-Ted

 

you need to check the driver transistors and associated resistors etc . also check/replace outputs emitter resistors .

 

Yes, agreed but still don't power it up without the DBT.

- Pete

 

I powered it on DBT, was popping and DBT was flashing. After a few minutes, it smoothed out. Powered on without DBT, ran well 10 minutes, then R51 caught fire. Looks like TR12 next to it is reading differently from its twin on the other channel. I'm guessing that's what smoked R51, but I really don't know. Ordered new R51's, and some 2SA706's from Ebay China. I would like to fix it sooner than China shipping allows. Found a "replacement P/N" 2N6126, but I think the legs are different, and I don't know if it's right.

 

Most, if not all, vintage transistors bought through eBay, especially eBay China, will be counterfeits. Counterfeits are usually very poor quality and prone to failure. When you get them, throw them in the trash. Use modern replacements sourced from a reputable supplier like mouser, digikey or arrow. A BD-140 is a typical replacement for 2SA706. http://www.mouser.com/Search/Refine.aspx?Keyword=BD-140-16. Before using these replacements verify the pin out. The leads may be in a different order than the original.

Popping and flashing is never a good sign. You should have turned it off at that point. Replacing the parts you mentioned is likely to just bring it back to the condition you first described. What output transistors did you use and where did you get them?

- Pete

 

The DIM bulb tester is not just a "soft start" device, it is also an indicator of problems. Using a 60 watt filament bulb on your 771 will show you if there is a problem by staying bright, from a softer glow to very bright is bad. When you power it on, the bulb will be bright for a second or two, then it should go very dim if all is right. Hence the name "dim" bulb tester. If the bulb continues to glow, turn it off immediately! Otherwise you will fry something. And obviously it was bright because of the "popping" and flashing. The flashing was indicating the death of components. Since you are a noob, you should be doing some research before trying anything again. And get the service manual!!! Read and understand it as well as reading about transistor types and uses. Especially the pinout of the Base, Collector, and Emitter as replacements can be different. And know the difference between NPN and PNP.

 

I bought output transistors on Ebay with this item number

282544063833

I saw some replacements for the 2SA706 with a P/N 2N6126, (or BD590, or KT602G?) I guess the pins are different, which I thought would be nice to avoid. There's a local supplier that has them at the counter. Guess I have to twist the legs around to make them work.

 

I do have the manual, and did go through the "suspect" transistors and diodes before powering anything. Those readings looked OK, so I powered it. It ran a little, then failed. It seems to me that you can DBT and check with a meter all you want, but at the end of the day, if something is going to fail and fry, it's going to fail and fry. The only "safe" way to prevent it is to "wholesale" replace everything first before connecting it to power.

You can be cautious and DBT till you're satisfied it's safe, then run it on normal outlet and enjoy it. Nothing is to prevent it from failing tomorrow, next week, or next month, while your trusty DBT is safe in a box on a shelf somewhere.
Am I missing something?

As a noob, I wish I could pick up a book of theory, that says "this flows to this, then this, and a failure here will then destroy this and this". When something burns up, who is the perpetrator, and who is the victim?

 

Those ON Semi transistors are often counterfeited. There is no good way to tell if they are genuine from the picture. The only good way to be sure you get genuine parts is to get them from a reputable supplier like mouser or digiky. It's best not to buy transistors on ebay because you simply don't know what you are getting. Get 4 new ones from a reputable supplier.

On the driver board you need to check the driver transistors, Tr10/TR12 and their counterparts in the other channel, all the resistors in the driver circuit and all the emitter resistors.

- Pete