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Sansui Eight help - hum on one channel

You checked them with an ohmmeter?
Measure the voltage at the top of the zb-1-8 . Looks like it.should be 8 volts.
 
Checked with DMM in diode mode.

Just so I understand, I should measure the voltage across ZD801 on the voltage setting? Which side +, which side - ?
 
Yes with power on, neg lead to ground or chassis, pos dmm lead to top or kathode of zd801.
Should be about 8 volts.
You replaced the pots also?
 
bob777 said:
Hey John, feel free to get back in the game. This is an odd problem.
Click to expand...
Thanks Bob, a pair of new eyes on this problem are very welcome. ;) I'll follow along for the time being and see if I can notice anything revealed by your further investigations, I don't want to confuse Keith and get into a 'too many cooks.....' situation. ;)
 
bob777 said:
Yes with power on, neg lead to ground or chassis, pos dmm lead to top or kathode of zd801.
Should be about 8 volts.
You replaced the pots also?
Click to expand...
Yes, on that driver board all 4 VRs replaced. That is also the board where I checked a lot of the components at John's direction. If you page back in the thread, a lot of the values I tested are there.

Will do that measurement this afternoon.
 
OK, it appears that tr 804 is not conducting or working. Tr804 along with tr851 set up the voltages for tr807 andtr808.
But tr804 is setup by tr801 and something seems wrong there. Tr801 does not seem to be biased on enough to bias tr804.
I don’t think that the positive half of the amp is turned on hard, I think the negative half is not turned on.
R813, r815, Tr804 up thru tr851 thru r819, r811, and r812 do not seem to be right, controlled by tr801..
Did we ever check tr806 to be sure it is not shorted?
 
bob777 said:
OK, it appears that tr 804 is not conducting or working. Tr804 along with tr851 set up the voltages for tr807 andtr808.
But tr804 is setup by tr801 and something seems wrong there. Tr801 does not seem to be biased on enough to bias tr804.
I don’t think that the positive half of the amp is turned on hard, I think the negative half is not turned on.
R813, r815, Tr804 up thru tr851 thru r819, r811, and r812 do not seem to be right, controlled by tr801..
Did we ever check tr806 to be sure it is not shorted?
Click to expand...
I can pull TR804 and check that it is ok. Is there a recommended replacement?

Since you have a working Eight, can you tell me the voltage at pin H with the driver boards removed, pre-out/main in separated (jumpers pulled and speaker selector set to C? I'm wondering in that condition should there be zero volts at pin H on each channel, or is there some non-zero reading. I'm getting about 0.2 to 0.5 V at pin H under those conditions, and wondering if that is normal?
 
I guess it won’t hurt to check tr804 and tr803. While they are out, check r13,14,and 15.
The small voltage at pin h is probably just leakage, I don’t think I would worry about that right now.
I think you should verify the trim pots are the correct value and in the correct location and double check tr801 and 802 pin out. Seems like both channels went south after replacing these. If you don’t find anything there, I am at a loss and would start checking all the resistors on the one board we are working with.
Check vr804 is not set all the way to one end. Check tr806 while the board is out (mounted on heatsink on bottom).
 
Have you measured the B3 and B4 voltages with the board in? If not measure the voltages.
there is a 150 ohm resistor in both sides in the power supply (r11and r15) and you did say that r13 and r15 on the driver board smoked.
Worth Checking them first.
 
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bob777 said:
Check tr806 while the board is out (mounted on heatsink on bottom).
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I think you'll find TR806 and TR805 are both on the board - protection circuit transistors. You may be referring to TR851 & TR852 = the Bias transistors. ;)
 
Thanks for the correction. My mistake , meant tr851. Any more thoughts on what’s going on here.
 
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bob777 said:
Any more thoughts on what’s going on here.
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Not really, I am still stumped on this one. Some nice detective work and circuit understanding by you though, but like me you followed 'leads' (almost literally) and they all seem to lead to dead ends.

I think I am right in saying that TR806 & TR805 can be removed from the board and the amplifier will still work. :)
 
Yes I agree they can be removed without affecting the basic operation in this state.
I keep thinking something wrong with tr801 and or tr802 circuit. They do not seem to be biasing tr804 on?
 
Actually tr807 and tr808 could be removed also and get it down to the very basic voltage divider network or biasing network. I Think !
 
Yes, that sounds possible, you'd lose base drive to those transistors, but we don't care because there isn't any :)
 
That might be the next thing to try. Not worried about current drive just try to get the voltages right around tr804 and tr851. Remove tr805,806,807, and tr808. That's bare minimum
 
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