Sansui 8900ZDB Keeps Blowing Left Output Transistors

[schoenradt]

I am having a terrible time trying to keep the left output transistors from blowing. I have replaced the output transistors and upgraded all the 1/2 watt resistors to 1 watt resistors. Unit states it supports a 4 ohm load. I have checked the bias and all the transistors ahead of the final outputs. Unit ran fine for several days using 8 ohm load at about 25 watts per channel. When performing a load test using 4 ohm dummy loads volume was set to level 4 and unit was consuming about 2-1/2 amps on the AC line. Test ran about 15 minutes and poof. I also noticed the left output sounded weak in low frequency response, but no clipping under a level 5 setting.
Any suggestions on getting the left output to work correctly? I have never had an issue repairing a receiver until this nightmare came along. Some of the history was this unit has been into Muse Audio several times for servicing. The work looks good and they did replace most of the electrolytic capacitors in the power supply, main output amplifier, and preamp/tone boards.

Just an update. I believe the problem is self explanatory.Since this repair has always blown the left channel output transistors and the right has remained operational it can only mean one factor. The replacement Sanken output transistors are counterfeit and will not comply to the OEM specifications. I am going to use substitute the BJT's and modify the heat sink so they will mount the same as the originals. I have found some OnSemi and SK transistors that should work.

So it looks as I found some substitute transistors for the output. They are as follows NPN FJL4315OTU and PNP 2SA1943OTU. I will have to revise the heat sink and center the new package outline for a TO264-3. The replacements are rated at 17 Amp IC, and 250 VCE. So they should work.

 

[schoenradt]

Initial test went well with substitute transistors. NPN FJL4315OTU and PNP 2SA1943OTU work. I only ran the unit at about 5 watts for a short test because I haven't mounted the substitute transistors on the heat sink, but it does sound good. I will remove all the hardware from the heat sink and drill and tap holes for new location of substitute transistors. Just waiting on some thermal pads for TO264 type transistor package. I will update on 4 ohm load test after completion.

 

[schoenradt]

Just another update I matched up the outline of the TO264 packages on the heat sink drilled holes all the way through. I used a #39 drill and a 3mm tap. My silicone pads came in for the transistors and I was curious how well they would conduct heat. The answer is quite well. I have had the test circuit running for 4 hours at about 25 watts and the heat sink is barely warm. I also added a small DC fan to run air across the heat sink fins like the 9900ZDB has.The fan really helps cool the transistors quickly. The real test will be a full 4 ohm load at medium to high volume with a dummy load. I only have the left channel rails connected for testing, but even at 25 Watts my variac shows only about 500mA draw on the 120 VAC line.

 

[schoenradt]

Just another update I matched up the outline of the TO264 packages on the heat sink drilled holes all the way through. I used a #39 drill and a 3mm tap. My silicone pads came in for the transistors and I was curious how well they would conduct heat. The answer is quite well. I have had the test circuit running for 4 hours at about 25 watts and the heat sink is barely warm. I also added a small DC fan to run air across the heat sink fins like the 9900ZDB has.The fan really helps cool the transistors quickly. The real test will be a full 4 ohm load at medium to high volume with a dummy load. I only have the left channel rails connected for testing, but even at 25 Watts my variac shows only about 500mA draw on the 120 VAC line.
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Just another update testing the left output with the new OnSemi transistors under a moderate load through 4 ohm dummy load. Getting about 50 V P-P at level 3 setting. Ran test for about 1 hour. No discoloration on resistors, or high heat. AC line drawing about 1 Amp of current. Looking like it will actually be better than the original output.

 

[Robisme]

Very nice job.

Rob

 

[Sansuiman]

That's a nicely done repair with the substitute output devices. These receivers (Z series) don't get a lot of respect, but they are no mere playtoys. I have a 9900Z here that is an excellent amplifier, and with a very good tuner (though I make little use of that anymore).

They are a bit flashy, as was the stylish thing to do in 1980 when the digital age set in, and I think a lot of people don't take them seriously for that reason alone. In any case, what's under the hood is very nearly as good as the preceding G-x700 line of receivers, as goes the output stage.

 

[schoenradt]

That's a nicely done repair with the substitute output devices. These receivers (Z series) don't get a lot of respect, but they are no mere playtoys. I have a 9900Z here that is an excellent amplifier, and with a very good tuner (though I make little use of that anymore).

They are a bit flashy, as was the stylish thing to do in 1980 when the digital age set in, and I think a lot of people don't take them seriously for that reason alone. In any case, what's under the hood is very nearly as good as the preceding G-x700 line of receivers, as goes the output stage.

Just a note, After modifying the output transistors and preforming a 4 ohm dummy load test. I noticed that unit didn't draw as much current and no longer dimmed the display and back lights. I did however, discover the preamp begins to clip with the bass at maximum and loudness on at about level 5 setting. Still quite impressive even at 50 V p-p the output sounded strong and punchy. Ran it hard for over 8 hours and not a problem. I added a fan like the 9900ZDB and it helps cool better than just convection. Just a note to anyone going through one of these units stay away from cheap Chinese transistor replacements from ebay. They are not the Sanken originals they are garbage.