Restoring uncle's Sansui 9090DB

Juan9568

Juan9568

Member
Hi guys. I'm about to begin with the restoration of my uncle's old Sansui 9090DB. The receiver is in a pretty bad condition, it has been serviced by bad technicians and they replaced parts with cheaper or wrong ones.

Issues:

. Burnt resistors
. Blown fuses
. Blown outputs
. Scratchy pots

A few months ago I decided to start with the restoration myself as there are no good technicians where I live and trying to get quality parts it's almost impossible.

I decided to do the following:

. Rebuild from scratch F2624 Driver board
. Replace capacitors and resistors from F2656 Power supply
. New fuses
. New outputs
. Clean all pots

Because I'm not an expert in electronics I thought it would be easier for me to start with a new F2624 instead of troubleshooting the whole board. So I got the new F2624 PCB and most of the other parts from mouser and digikey.

Also built myself a DBT cause I don't wanna blow everything up on the first run but I don't know yet what type of bulb I need to use for the thing to protect the amp, I got a 70W but don't know if it is the right wattage. I'm on a 220v area. Any suggestions??

What most scares me is when time comes to adjust the DC and Bias current, I know it is a very fragile thing and needs to be done correctly.

I would really appreciate it if someone can give me any tips, tricks, suggestions so I can make this restoration right.

THanks!!!


Here are some pics of the amp before I begin. I will be posting updates on my progress.

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I'm about to work on a 990DB so I will be watching. I found this thread: http://audiokarma.org/forums/index....ted-here-is-a-recent-replacement-list.426901/
Here is some advice from that thread:
tnsilver said:
If I could start over:

  • I'd use a less powerful soldering iron than the 15watts iron I use. This board is very delicate. Rails lift like there is no tomorrow...
  • Turning the board side to side while recapping it does not help either. It may cause wires to break and shields to cut through the insulation and ground the signal.
  • Wires from the Dolby board to the Dolby volume control board are way too short. It's almost impossible to mount and dismount the volume control board from the chassis more than twice before breaking one or more of these connections. I'd install longer wires the first time it's out.
  • The same applies for the master volume and balance switch - too short wires...
  • The 4 Orange, Red, Yellow and Brown shielded wires leading from the tape monitor terminals at the back, to the 5 Push Switch board, across the unit, may break while you service the Dolby board, which is connected to the Push switch board with flexible plastic coated connector wires. I'd artificially de-solder them lose from the pins on the Push switch board at the beginning of the process, tape them to the back, and avoid lot's of trouble later...
  • I'd try to find a PC or Laptop 9 pin assembly (A PC technician friend of mine just informed me tonight - they are available!!!) and replace the 9 pin assembly on the equalizer board - avoiding improvisation and hours of eye tearing work trying to fix the connections to the crappy flexible PC style ribbon that comes with the 9090DB.
  • I'd be more careful with the Power Supply board. It's huge!!! It's held by screws mounted to the chassis at the corners but an entire area is just lose in mid air. Any pressure applied to it (like when installing fuses) might bend it too much in the middle and cause rails to break. Those rails, like the Dolby board rails, are overly fragile too...
Click to expand...
 
Juan9568 said:
What most scares me is when time comes to adjust the DC and Bias current, I know it is a very fragile thing and needs to be done correctly.
Click to expand...
You got the DC/Bias directional label correct as per the new PCB. That's good. You won't get confused.
  • I start the bias/DC offset procedures with a DBT.
  • Bias trimmers all the way down on both channels and DC offset trimmers at 2.6KΩ.
  • I turn the unit on and as soon as the relay clicks, I start a timer and monitor my DC miliamp meter for 4 minutes.
  • It's important to notice the meter right away and make sure it reads low (about 3 or 4 mA) so we know we turned the trimmers the right way. The DBT lamp won't like it if we mess this up!
  • I gradually increase the bias current with the trimmer until I get 28mA at the end of the 4 minutes run. I do this for both channels.
  • Two meters are nice to have, but when only one is available, I give the unit 20 minutes to cool down between biasing each channel.
  • Then I adjust the DC offset as close as possible to 0 mV. It's typically fluctuating +/- one or two mV and that's normal.
  • Then I bias again and after that it's time to move to mains voltage.
So why 28mA? Because when going to mains from DBT there's always an increase of about 2mA in bias current.
I set the final 30mA on mains.

Do not stick the probes down there at the fuse and hold them for 4 minutes. You can clip them, but better use a spent fuse tool.
 
Last edited:
tnsilver said:
You got the DC/Bias directional label correct as per the new PCB. That's good. You won't get confused.
  • I start the bias/DC offset procedures with a DBT.
  • Bias trimmers all the way down on both channels and DC offset trimmers at 2.6KΩ.
  • I turn the unit on and as soon as the relay clicks, I start a timer and monitor my DC miliamp meter for 4 minutes.
  • It's important to notice the meter right away and make sure it reads low (about 3 or 4 mA) so we know we turned the trimmers the right way. The DBT lamp won't like it if we mess this up!
  • I gradually increase the bias current with the trimmer until I get 28mA at the end of the 4 minutes run. I do this for both channels.
  • Two meters are nice to have, but when only one is available, I give the unit 20 minutes to cool down between biasing each channel.
  • Then I adjust the DC offset as close as possible to 0 mV. It's typically fluctuating +/- one or two mV and that's normal.
  • Then I bias again and after that it's time to move to mains voltage.
So why 28mA? Because when going to mains from DBT there's always an increase of about 2mA in bias current.
I set the final 30mA on mains.

Do not stick the probes down there at the fuse and hold them for 4 minutes. You can clip them, but better use a spent fuse tool.
Click to expand...

Thanks Tom. What wattage would be the right one to use for my DBT bulb? I'm on 220V
 
Following Tom's assembly guide:

1. Ceramic Capacitors

Ceramic Capacitors

Ceramic Capacitors


Ceramic Capacitors (zoomed)

Ceramic Capacitors (zoomed)


2. Diodes

Ceramic Capacitros + Diodes

Ceramic Capacitros + Diodes


Ceramic Capacitors + Diodes (zoomed)

Ceramic Capacitors + Diodes (zoomed)


3. Molex

Ceramic Capacitors + Diodes + Molex

Ceramic Capacitors + Diodes + Molex


4. All resistors

All Resistors

All Resistors


5. Low power transistors

Low power transistors

Low power transistors

Low Power Transistors

Low Power Transistors

 
Last edited:
Looks nice! I'd wrap some shrink around the STV-3HY leads or make sure the systoflex goes all the way up, so it doesn't short with the heat sinks by accident.
 
I'm trying to bias the amp but the green light doesn't come up...It stays on blinking red. The DBT bulb turns on but immediately starts dimming...Could it be that the 70w bulb DBT doesn't let the amp power up correctly??? without the green light I still can read 2mA on the bias


Unit has been off for years...Maybe old capacitors need time to charge on DBT??


Here a video:

 
Last edited:
Juan9568 said:
The DBT bulb turns on but immediately starts dimming...
Click to expand...
That's expected while the main filter caps are charging. If the protector circuit "protects", it prevents DC from getting to the speakers. Most likely you got too much DC. While the red led is flashing, you can measure it for each channel on the gray and blue wires (each against chassis) coming out the bottom of the PS board. This will indicate if you got a DC issue.
 
tnsilver said:
That's expected while the main filter caps are charging. If the protector circuit "protects", it prevents DC from getting to the speakers. Most likely you got too much DC. While the red led is flashing, you can measure it for each channel on the gray and blue wires (each against chassis) coming out the bottom of the PS board. This will indicate if you got a DC issue.
Click to expand...

Should I measure these blue and gray wires with F04 and F05 fuses in the circuit?
 
tnsilver said:
yes, good, keep the black probe on the chassis. you can get negative reading. How much?
Click to expand...

I got around -200mv on both channels...Took it down almost to 0mv...But still i can't get the green light...

Gonna try if it goes out of protection without the driver board connected...If it goes green I should assume that something is wrong in the F2624?
 
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