• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Sansui Eight help - hum on one channel

OK, what happens to the Base voltage of TR804 when you adjust VR804 ?
 
Last edited:
Hyperion said:
OK, what happens to the Base voltage of TR804 when you adjust VR804 ?
Click to expand...
Initially the base voltage moved a bit as I turned VR 804 to the extremes, but then it just settled out to a constant value no matter the position of VR804.
 
KeithD said:
but then it just settled out to a constant value no matter the position of VR804
Click to expand...
I am assuming it didn't change - in other words that 'constant value' was around -45.5V the same as your voltage reading - is that correct?

What is VR802 doing? - turn it so it's slider is nearest to R810 - see if anything changes when you try the above adjustment with VR804.
 
Last edited:
Hyperion said:
I am assuming it didn't change - in other words that 'constant value' was around -45.5V the same as your voltage reading - is that correct?

What is VR802 doing? - turn it so it's slider is nearest to R810 - see if anything changes when you try the above adjustment with VR804.
Click to expand...
Yes, the voltage stayed constant at -45.5V

Moved VR802 all the way clockwise - toward R810. VR804 still in center. Reading is now -35. Moving VR804 to the two ends, voltages goes to -36 at the two ends, moving it around, I can get the voltage as low as -34.6 just counter clockwise of center (about 5 O'clock).
 
KeithD said:
Yes, the voltage stayed constant at -45.5V

Moved VR802 all the way clockwise - toward R810. VR804 still in center. Reading is now -35. Moving VR804 to the two ends, voltages goes to -36 at the two ends, moving it around, I can get the voltage as low as -34.6 just counter clockwise of center (about 5 O'clock).
Click to expand...
OK - thanks for that - I'm still no closer to knowing what the solution is. :angel:

You could try changing the 2SA486 (for a KSA1220) on F-1201 board for the negative B4 supply - see if we can get that under control. The transistor itself doesn't have a TR number on the schematic, but there is only one 2SA486 on the board. Same rules apply, check pinout when installing. ;)
 
Last edited:
Yes, this one seems to be turning into a mystery. I appreciate all your help on this. I am wondering if it is time to check PS transistors, caps, etc. Possibly just go ahead with a restoration of the PS? Not sure. Will take a look at the 2SA486 on the board. Is there a BOM for the Eight boards?

Other option is to send this out for a restoration.
 
Could not remember if the driver boards were in or out when I measured PS voltages. So, pulled both driver boards and measured. So, any anomoly in the voltages would not be caused by a problem on the driver boards. Here are the readings (on DBT).

B1: -6.9
B2: +7.885
B3: +48.9 (+46.5)
B4: -47.5 (-46.5)
B5: 13.4 (14)
B6/7: 23.0 (25)
B8: 40.4
B9: -40.3
B10: 41.2 (40)

B3 and 4 are high as is B10.

2SA486 is TR006 according to what I see on the board and matching to the SM board layout (SM has that listed incorrectly as 2SC486). Schematic has neither of those labeled as to which TRxxx each is. It makes sense to me to test TR005 and TR006. Could also test TR003 on B10 circuit.
 
As B3 and B4 are a bit high, I am thinking this may be an effect of being on DBT and those odd little bits of circuitry connected to the 3.5V windings on the transformer. B10 only feeds the Phono pre-amplifier so, no big deal if it's only a volt or so over.

I am not personally aware of a BOM for this model, but that's not to say one does not exist, its surprising what the hive-mind of AK can come up with if asked. ;) However, if you do eventually send it to someone for repair and reconditioning, they will probably like to receive it 'as-is' with no major work done to it.

The problem with this Eight doesn't seem to be PSU related, all the supply voltages are there, and don't seem to be 'out' by enough to cause problems. The odd thing is all the semiconductors test OK, but the amplifier is stubbornly unbalanced in a way that might suggest a physical problem, like an open/cracked trace, dry joint, faulty connection, a missing ground connection, or something open or shorted. Or an innocently replaced component that has the wrong value.

If anyone else has any ideas for things to check, we are are all ears (or rather 'eyes') :)
 
Last edited:
OK, I did have the same though on a cracked trace and so checked the all the traces on the left driver board for continuity. All tested OK. I can check some of the ground connections to see if they are OK.
 
Aha! Pulled F005 and F006 to isolate drivers from anything between 8H and the speaker/headphone outputs. Measured 8H at R837 and I'm getting 0V on L driver. Put F003 and F004 back in for right channel and did the same measurement. Also getting 0V.

F-1334 (speaker protector block) and F-1202 (can't see where that board is) are both between driver 8H and speaker terminals according to the schematic.

Have I hit the problem here?
 
Aha! - so just to summarize what you said, you are now getting 0V DC offset (from both channels?) ? - after removing F005 and F006 ? - but with F001, 2,3,4 in place?
 
Hyperion said:
Aha! - so just to summarize what you said, you are now getting 0V DC offset (from both channels?) ? - after removing F005 and F006 ? - but with F001, 2,3,4 in place?
Click to expand...
Yes, that is correct. I did not change anything else. Off set reading is around 0V on left, 10mV on right. I'm reading the values at end of R837 connected to 8H.
 
Oh my goodness! - so, something on either F-1334 or F-1202 is causing the offset issue!

Well done for finding this, I don't think I would have thought of that for quite a while...

F-1334 PCB seems to be a crowbar circuit - when the voltage, (derived from the power amp output), appearing at the bases of those two transistors gets to a certain level it triggers the Thyristors 'on' and that in turn causes the fuses F005 and/or F006 to blow - thus preventing damage to speakers or amp - in theory at least. :angel:

F-1202 PCB seems to be a 'Balance' circuit of some kind - connected at point 'P' to one of the meters via a switch, why? - I have no idea. :)

How do you fancy having a crack at setting the bias? - while we're on a roll. :)
 
Last edited:
Yes, I think that is the next step.

The only other option is to disconnect F-1334, put F005 and 6 back in and see if that really was the issue. Possible 1334 got damaged with the shorted OT?
 
KeithD said:
The only other option is to disconnect F-1334, put F005 and 6 back in and see if that really was the issue. Possible 1334 got damaged with the shorted OT?
Click to expand...
It is possible that one or both of the thyristors got damaged with the other activities that went on.

But yes, just to prove it, you could try one fuse at a time perhaps? - while checking the DC offset on that channel?
 
Last edited:
With F005 and F006 out, went to set bias. Removed F001 and F003 per SM. Connected +/- of DMM in ammeter mode to ends of F001 fuse holder. It was reading about 26mA with VR803 all the way down (all the way clockwise). Turning the VR made no difference. Put F001 fuse in and moved to R channel check. Same current reading and again no change with moving VR803.

Then went to check DC offset on L channel. DMM common to chassis ground, + to 8H on R837. DBT stayed on, so turned off and discovered I was still in ammeter mode. Switched probes around and set to voltage reading. Don't know if this caused a problem, but now back where I was before. Getting about 40V at R837 on both channels. :(
 
KeithD said:
Then went to check DC offset on L channel. DMM common to chassis ground, + to 8H on R837. DBT stayed on, so turned off and discovered I was still in ammeter mode. Switched probes around and set to voltage reading. Don't know if this caused a problem, but now back where I was before. Getting about 40V at R837 on both channels. :(
Click to expand...
Yep, you might have blown a fuse or two, as amplifiers don't like having their output shorted to ground, but very possibly you may have blown OP transistors or drivers or both. Not sure why both channels now have +40V on the output, if you only checked one channel DC offset on a current range, (fuses F005 & F006 are still removed right?) - Time to check fuses and measure your B8 and B9 voltages. :(

If that checks out OK, and you still have +40V on the output, remove and check TR857 (and TR856) the 'upper' two OP transistors which may be C-E shorted.
 
Last edited:
No fuses blown. F005 and F006 were removed.

B8: 40.7V, B9: -40.8V

Confused on the transistors you mentioned. Did you mean TR857 and 858? Will check those.
 
You must log in or register to reply here.
Back
Top Bottom