Sansui B-2301L / Sansui C-2301 restoration

[Bumpalump]

Sometimes its possible, on the phono boards i didnt managed to repair capacitors leads and had to replace them with modern Wima

 

[JOMARK911]

Did you try to set the dip switches ?
What exactly do they change ?
Also when you finish restoration , can you please tell me what temperatures do you see at the back? Is the pre amp getting really warm after 2 hours ?

 

[Bumpalump]

They are for offset settings, about 50 degrees

 

[JOMARK911]

Did you adjust offset ? When you do are you measuring on the phono output?

 

[Bumpalump]

I measure direct on the output from board

 

[JOMARK911]

Ok thanks a lot.

 

[JOMARK911]

One more question.
Do you have a complete service manual of c2301 , that includes a page with adjustments ?
As far as dc offset for line and phono stages?

 

[Bumpalump]

Yes i have, remind me in monday, so Ill upload that page

 

[JOMARK911]

ok will do.

 

[JOMARK911]

Yes i have, remind me in monday, so Ill upload that page

So i thought to remind you about the adjustment page of c2301 .

 

[JOMARK911]

Once more.

 

[Bumpalump]

I did not found c2301 scheme, but the procedure is the same in c2302

 

[JOMARK911]

ok thanks for that.
I assume you set the switches in a manner so to have the min dc offset on the output of the board correct ?

 

[Bumpalump]

Exactly

 

[fojica]

@JOMARK911

Let me try to explain it in simple terms - there are basically two ways of adjusting the offset:
version ( A ) -> trimming the "equivalent source resistors" of the JFET's LTP
version ( B ) -> connecting the CCS coming from V- directly to JFET's LTP (no source resistors) and putting a trimmer on the drain side, on the V+. The other side of the LTP has a fixed resistor in the drain, like usual.

For some reason, in case of C-2301 and C-2302 Sansui opted for version (B). But, instead of putting a trimmer (variable resistor) they put 4 resistors in series, each of them being (or not) bypassed with the 4 switches from SW1 - depending on each switch position. This SW1 arrangement behaves similar to a stepped attenuator. Based on the switches position, you obtain the values from the above tables. ON = switch closed (R shorted), OFF = switch open (R in circuit). The simplified schematic is below.
Depending on the double JFET matching, you are basically trying to "compensate" that 75R from the right side of the LTP with some combination of resistors from the left side, so that LTP is better balanced and DC offset is minimized.
Hope this clears it up.

Regards