9090db smokes R34 without fuse 04

[Pilotlips]

I'm so sorry for this accident. I wanted to ask about bias current readings on Wednesday but I've decided against stating the obvious.

Yes always when something looks easy It is actually very difficult nevertheless I'm not giving up and knew the risk of not using the dbt.
My previous winter job was a Pioneer SA9800 and that's still working and sounding like a champ.

 

[Pilotlips]

For the time being, you don't have to connect the power transistors to trouble shoot your issues to save them from further damages. The driver board is the voltage gain stage and the power transistors are power gain stage.

Wow what an info I will get to work on it and come back with the results when I have it much appreciated. I noticed also the 25,5v in stead of -25,5 seeing its connected to pin 18 with -61v that would pose a problem, have to say slightly disappointed with the accuracy of the Sansui manual and layout of for example the dolby board. Nevertheless when assembled it's a thing of beauty.

 

[petehall347]

For the time being, you don't have to connect the power transistors to trouble shoot your issues to save them from further damages. The driver board is the voltage gain stage and the power transistors are power gain stage.

There could be many possible causes of the issues as many components have been changed.

The service manual is not very helpful as there are only very few voltage marked on the circuit nodes. But rough estimate is still possible with a calculator and tracking the circuit.

Here are few things I would start with:

1. Measure voltage between R18 and R38 to see if it's -25.5V (the circuit shows 25.5V, I think it's a typo missing the minus sign as the base of TR04 should be around zero volt and if it's +25.5V, the differential pair will be highly reversed bias).
2. Measure voltage between emitter of TR12 and 14 to see if it's around 1.2 V.
3. Adjust VR04 to see if the voltage in step 2 can be changed. If it can be changed, adjust to minimize the voltage.
4. Measure voltage across the base of TR12 and 14 to see if it's around 2.4V. It should be if step 3 is good.
5. Measure the voltage across R32 to see if it's about 0.6V
6. Measure the voltage across R22. Is it about 0.6V?

Note that I got the component number from one of the power amp. Do the same for the other channel.

The above steps should give us some clues on the root cause of the problem.

Good luck.

would it not be a good idea to use 1k resistors from base to emitter positions with outputs removed ?

 

[audio3000]

Wouldn't hurt but not necessary as there are already 150 ohm (R42 and R44) as the load for both drivers, TR12 and 14.

I like your attitude about learning but as other said already you need to access your risk...

 

[audio3000]

I just bought some bourns 25 turn 100ohm and will hunt for modern not fake transistors replacements.
Output I got MJ21193 and MJ21194 ON and bought some extras so I could gain match a pair. When I got the unit right channel 4x original Toshiba left 3x TIC rated 100w and 1 fake Toshiba.
I currently live in Berlin Germany there might be a tech nearby but for now I want to continue learning and figure it out for me half the fun is the repair/rebuild.

Using 200 ohm instead of 100 ohm should not cause the issues you reported. BUT if the trimpot was installed incorrectly pin-wise, it could cause high bias. The safest way to ensure is to, after the pot installed, turn the trimpot to make sure it's at the HIGHEST resistance between TR08 Collector and TR10 Base. This will turn TR10 on as much as possible meaning lowest CE voltage then lowest bias. At this point, bias is not necessary as cross-over distortion should be dealt with when the amp is up and running properly. With lowest bias condition, the chance of thermal run-away is minimized.

Cheers.

 

[audio3000]

Wouldn't hurt but not necessary as there are already 150 ohm (R42 and R44) as the load for both drivers, TR12 and 14.

I like your attitude about learning but as other said already you need to access your risk...

I actually meant "assess".. sorry for the typo...

 

[Skywatcher]

I just bought some bourns 25 turn 100ohm and will hunt for modern not fake transistors replacements.
Output I got MJ21193 and MJ21194 ON and bought some extras so I could gain match a pair. When I got the unit right channel 4x original Toshiba left 3x TIC rated 100w and 1 fake Toshiba.
I currently live in Berlin Germany there might be a tech nearby but for now I want to continue learning and figure it out for me half the fun is the repair/rebuild.

You don't need to gain match output transistors. Or any complimentary NPN/PNP pair for that matter. You should match the input pairs (NPN) on the driver board TR01-04, but it's not really necessary since you can adjust the DC offset. Make sure you get *authentic* ON Semi transistors or Fairchild. There are a lot of fakes out there. Being you are in Germany, I am sure someone near you can help out.