Sansui G-7000 Driver Issue

[Zekeman]

Back on the bench is a problematic G-7000. The unit has been fully recapped but there was a previous problem with this unit's right channel (details unknown) because the outputs were mismatched and some work had been done to the driver board.

I am currently getting a bright DBT bulb upon power up and no relay click. IF I pull out the driver board completely and power it up - both relays engage with the proper timing (one instantaneously and the other after 4-5 seconds).

The rails are producing the proper + and - voltages.

I have replaced all transistors and diodes on the driver board.

New pots installed for bias. The bias will not adjust properly as well for either channel.

I installed all new output transistors...OnSemi.

All small ceramic caps were tested on the driver.

I'm concerned about further testing voltages on the driver when the amp is pulling current through the DBT.

Any advice on next steps would be appreciated.

 

[Hyperion]

Pull and test the driver transistors, especially any recent replacements - check them for type and orientation.

 

[Zekeman]

Yes I did that and confirmed the orientation but I will give it a second look.

 

[Zekeman]

All transistors are confirmed. So as I was checking the driver board I decided to clean off all the residual rosin flux with acetone. The conductor side the the board was a mess with all the rework. After I cleaned off all the flux I decided to touch up a few of the older-looking solder points that had not been touched.

I powered it up and no DBT brightness....relay kicked in and I have functionality!!

Thank you!

 

[Zekeman]

I still do, however, have an inability to adjust the bias in the left channel...

 

[Hyperion]

Have you changed the trimmers? make sure trimmer is actually adjusting.

Compare voltage readings L & R and check the transistor associated with the bias trimmer. Measure C-E voltage of that transistor as you adjust the trimmer.

 

[Zekeman]

Yes. New Bourns 200 ohm pots. Also, I replaced the 10 ohm resistors which are in parallel. There must be a bum resistor in there somewhere.

 

[deltalight]

G-7000 New Output Cap Board

I am currently restoring a G-7000. I did not want to "gut and stuff" the old Nichicon "Dual C" caps, so I designed a new cap board for four 15,000uF 63v caps. That part works perfect. One side is blown so I will be replacing both sides with OnSemi parts. There was some Bias discussion on another thread (Pioneer SA-9500???) on the difficulty of biasing new OnSemi outputs since they have a different Vbe and the Bias range didn't cover the necessary range. I forget what the fix was. I think the issue was you couldn't get the bias low enough because the STV-4 Bias diode was meant for the original Toshiba transistors, etc.

 

[deltalight]

Each electrolytic has a 4.7uF polypropylene film cap across it. [see photo]

 

[deltalight]

G-7000 Output Cap Board

Bottom view after mounting.

 

[Zekeman]

Thanks for the info. I had the same problem with the Toshibas. Those had been messed with by someone prior to my receiving the amp though and were mismatched....so I decided to install all new TO-3's.