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Sansui 907 DC at speaker terminals

AmpRa

AmpRa

New Member
Hello all,

I have a Sansui AU-a907 audio amp. It sounds really really good!! A little problem is that there's about 250mv DC output at speaker terminals. I adjust it close to 0mv few times but when I turn it on the next day, it's out of wack. Both sides
This amp has Integrated and Power amp direct option. When I adjust the DC offset at Integrated mode and move the dial to Power amp direct, then the DC offset goes to around 200mv. Both sides
Anyone has any suggestions on where I should look at?
You can look at a close enough diagram here
https://elektrotanya.com/sansui_au-alpha907_au-707_au-607mr_mrx_sm.pdf/download.html
 
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Suggest you ask the experts in the sansui sub forum, they have deep knowledge of all thing sansui. Hit the "Report" link at end of post and ask that the moderator kindly move it. I've been lusting after an alpha for a while, congradulations on your pickup...
 
AmpRa said:
Anyone has any suggestions on where I should look at?
Click to expand...
If you adjust the DC offset and it keeps coming back then it has a fault which probably means some level of restoration is in order. In my opinion it is unlikely there is a single component causing the issue more like an accumulation of tired components requiring replacement. Note that it is normal to have some DC offset drift during warm up, but this should be well below 250mV, and be 'predictable' - in other words you should be able to adjust it to 0V when warm, and it should reliably return to that figure when warmed up again.

Please state the full model name - there are several different Sansui 'AU-α907' variants - the full name will be on the front panel of the unit.
e.g.
AU-α907MR
AU-α907XR
AU-α907 Limited - this model has MOSFET output transistors.

Whichever variant it is, you have indicated enough of the model name to indicate that restoration of this unit is almost certainly a job for someone with significant electronics knowledge.
 
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AmpRa said:
This amp has Integrated and Power amp direct option. When I adjust the DC offset at Integrated mode and move the dial to Power amp direct, then the DC offset goes to around 200mv. Both sides
Click to expand...
It is important to first set the DC-offset in power amp direct, normal mode. Then you set the DC-offset in integrated mode.
Could work better that way, although Hyperion's advice for a restoration still stands. Look for his own restoration from his AU alpha 907 MR (XR,wrong model). It's a lot less invasive then for a '70 model but still beneficial.
 
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Hyperion said:
:thumbsup:
It's actually an 'MR' - and there is no restoration thread by me, for this model. ;)
Click to expand...
Sorry for getting the model wrong ;), so many . . . with little difference
I understand it's not a restoration ( but you are doing the components necessary). Wanted to make the point that this more recent models mostly don't need a full restoration.
 
Very true, if it is a 'Limited' model, heat from the OP transistors collects at the top of the Driver/Output module and literally roasts the capacitors for the input stage. The one I did 'just' ( :) ) required trimmers and a replacement of those capacitors - (changed for 'films') and attention paid to the bias tracking transistors and their contact with the main heatsink, plus a full clean off and remount of the OP transistors. ;)
 
Pls have a look at the pic. This is one of the first generation of alpha series.
AU-α907MR
 

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AmpRa said:
Pls have a look at the pic. This is one of the first generation of alpha series.
AU-α907MR
Click to expand...
This IS the first alpha named AU 907. This means it is not a MR. Still the same rules apply. You first set the Poweramp dc-offset, both hot-cold and hot(cold) - ground. After this you set the pre amp dc-offsets ( Equaliser and flat amp).
If you don't do it in this order you will always get dc-offset when you switch to power amp direct.
 
Hyperion said:
Maybe the DC originates from the pre-amp? - but more likely that out of spec components on the power amp board are the cause.
Click to expand...
Thanks Hyperion,
You mean it's fitted with non-factory components?
I'm 99% sure I was the only one who opened this amp after they tightened the screws at the factory. It was full of dust and lint. No rust, no scratches and boards are untouched. Looks as if it was stored in a time capsule. Feel really lucky to get it.
 
GrisPato said:
This IS the first alpha named AU 907. This means it is not a MR. Still the same rules apply. You first set the Poweramp dc-offset, both hot-cold and hot(cold) - ground. After this you set the pre amp dc-offsets ( Equaliser and flat amp).
If you don't do it in this order you will always get dc-offset when you switch to power amp direct.
Click to expand...

Thanks GrisPato,

I totally agree with you. The problem is though.... this doesn't have any trimpots on Eq and flat amp board. I'm talking about JVR5/6(R and L). It has the usual KVR 1-4. I'll try to upload some pics.
 
AmpRa said:
You mean it's fitted with non-factory components?
Click to expand...
Some parts don't age well, like some resistors, and trimmers can get 'oxidised', electrolytic capacitors can develop high leakage, high ESR, and value drift, transistors can lose gain, all of these factors can affect the correct operation and stability of vintage audio. The above is a range of issues that may, or may not apply to your, or any vintage amplifier.

I've had a look around for the Service Manual for this model - and not having any luck partly due to so many references to the wrong model variant (i.e. the models I thought you had one of).

The look of the front panel suggests it might be similar to an AU-X901, but this doesn't appear to be the case. I think you may have a difficult time sorting this unit out if you can't get your hands on the SM. Sometimes there is a model with the same schematic, but with a different model name, but without more information on your unit it's going to be difficult. :(
 
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AmpRa said:
Thanks GrisPato,

I totally agree with you. The problem is though.... this doesn't have any trimpots on Eq and flat amp board. I'm talking about JVR5/6(R and L). It has the usual KVR 1-4. I'll try to upload some pics.
Click to expand...
Ok, that is interesting!
Because I don't have a schematic of this model I used the schematic of the X701/X901 assuming that this are the AU alpha 607/707.
The X901 has trimpots on the control board. But I see now that at least according the schematic the X701 doesn't!
But the X701 doesn't have power amp direct inputs. So doesn't need it.
Still, if you are using the amplifier only in power amp direct mode and you can not set the DC-offset to a stable value, this would implicate the amplifier needs some kind of restoration. Not wrong components but old components like the trimpots, capacitors/resistors that are damaged by glue, etc. . .
 
It has to have pre amp dc trim pots.
They might be hidden in the front board behind the fascia, like 911dg.
Look at the manual.
 
Myself and others have been asking and looking for a service manual for the first a907. It has not been located yet. It may not exsist.
 
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