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Sansui BA-3000 runs super hot when bias set correctly

richardpp said:
Do I concur with Hyperion? Absolutely :bowdown:

2 amp fuse rating is good for testing the amp at idle

Correct, continuity between the board edge connections and the transistor cases is testing for any interruption in the supply voltage to the transistor collectors.
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Good to know. He certainly seems like he knows what he is doing and he has helped me too.
 
richardpp said:
I wouldn't rush into dismantling the left heatsink. My first inclination would be to isolate it from the power supply by removing the in-line fuses. Then I would retest your supply voltages at the main filter capacitor terminals.

Have you carried out the power supply check #61
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Forgot that twice now. Thanks.
 
Tina Louise said:
Forgot that twice now. Thanks.
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Does it only work for one power on/off/wait 10 minutes cycle?

The + side is still crawling up from 3.8kohms but it took me a few tries to get the clips to stay on. Sort of moving slowly now.

- side is going past that now. Goes pretty slowly after 3.8k.
 
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richardpp said:
I wouldn't rush into dismantling the left heatsink. My first inclination would be to isolate it from the power supply by removing the in-line fuses. Then I would retest your supply voltages at the main filter capacitor terminals
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+/- 67.2V. Fuses removed from back brick.
 
richardpp said:
Yes, I would agree that there is clearly something wrong within the rear heatsink assembly.

But besides the problems with the bias readings when connected to left channel heatsink, overall your readings appear lower than expected on the right channel. Adding to this, rail voltage is lower than expected. The question is whether this can be all tied to a single fault.

that 11.4mV and 10mV was with VR02 only slightly more than half way. I'm sure it will go to 20mV with room to spare on the front/right brick which is spec.
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Tina Louise said:
+/- 67.2V. Fuses removed from back brick.
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That's better, can you make the same test but this time with with terminals 29/30 disconnected from F-2511 as well as with both left and right channel fuses removed.

Also take an AC voltage reading across terminals 5 and 6 on F-2511 for good measure. Expect around 46VAC as per schematic.
 
Tina Louise said:
that 11.4mV and 10mV was with VR02 only slightly more than half way. I'm sure it will go to 20mV with room to spare on the front/right brick which is spec.
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... I would expect it to be higher with the pre-driver transistors you're using, but that's okay. It usually requires a solid 30-45 minutes of warm up time during which the bias reading will continue to rise.
 
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richardpp said:
That's better, can you make the same test but this time with with terminals 29/30 disconnected from F-2511 as well as with both left and right channel fuses removed.

Also take an AC voltage reading across terminals 5 and 6 on F-2511 for good measure. Expect around 46VAC as per schematic.
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+/- 67.6.

102.1V. That's weird I'm sure. I'm on a variac with a digital readout. Variac output is spot on.

those were the right pins. so red lead of MM on 5 and blue lead of MM on 6? I swear I tested the power supply output months ago and all the leads were fine. Should it be blue(5)/gray and blue(6)/gray? those are 51V.
 

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So, just to be sure everything apart from the left channel heatsink assembly is working as it should be, put the right channel driver board in right channel slot connected to the right hand heat sink. Leave out the fuses in the left channel, don't forget to reconnect 29/30 on F-2511. Let the amp warm-up for 5 -10 mins, check dc offset and set the bias to 15-20mV. Monitor 30 min or so adjusting bias as necessary until it settles.

If all good then we'll look closer into the left heatsink assembly
 
richardpp said:
So, just to be sure everything apart from the left channel heatsink assembly is working as it should be, put the right channel driver board in right channel slot connected to the right hand heat sink. Leave out the fuses in the left channel, don't forget to reconnect 29/30 on F-2511. Let the amp warm-up for 5 -10 mins, check dc offset and set the bias to 15-20mV. Monitor 30 min or so adjusting bias as necessary until it settles.

If all good then we'll look closer into the left heatsink assembly
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Bias is rock solid at 18.5mV thru the test. Card is warm to the touch but not bad at all.

The only weird thing is that the usable range of VR02 is super small. Really small. I bet one of those resistors on F-2507 can increase that range.

I just realized I forgot to test the voltages on right F-2507. Will do that.
 
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Tina Louise said:
Bias is rock solid at 18.5mV thru the test. Card is warm to the touch but not bad at all.

The only weird thing is that the usable range of VR02 is super small. Really small. I bet one of those resistors on F-2507 can increase that range.

I just realized I forgot to test the voltages on right F-2507. Will do that.
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The main heat sink should be just warm as well. Where are you running out of range?
 
richardpp said:
The main heat sink should be just warm as well. Where are you running out of range?
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Not running out of range, the opposite. the range from 0mV to 20mV is bunched up right at slightly more than half way on Vr02. One slip and it would easily go to 50.

The main transistors were not warm. I wasn't running audio through nor did I even have speakers hooked up. I can redo the test easily.

I can hook up the left card to the front brick and see what that biasing looks like too.
 
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Remove that board and check the trimmer's resistance through its range. I would expect the main heatsinks to be showing some sign of warming up with 20mA bias current under test conditions. Will wait to hear if the other board behaves any differently.
 
That weird behaviour you're getting with the trimmer could be caused by a bad pass through connection on output board. I would probably stop there, in case you get that sudden jump and blow the outputs. I think you have a problem with both of the F-2497 boards.
 
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