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Need help with Sansui 5000a F-1040-1 boards

Andrew Lane

Andrew Lane

New Member
Hello everyone. Just picked up a 5000a at an estate sale. Very clean unit but the left channel would blow the 4amp fuses. Opened the unit and discovered the f1040-1 boards. This is my first major tuner rebuild/repair but have repaired other electronics. Ordered a f-1040-1 rebuild kit from eBay with new caps and transistors and installed them and also replaced the 4 2000mf caps. (One had a rather high ESR so I just replaced all 4) Apon first power up without the 4 amp driver board fuses in on my 60w dim bulb the bulb lit up but did not go completely dark. It got a little brighter when I switched to the FM/AM input selections. I proceeded to set bias current on the right channel and was able to dial it in at .023 amps (forgot the exact current). When I attempted to set the left channel the current was .123 amps and would not change when trying to adjust. When I was rebuilding I tested all output transistors and all checked ok. I’ve checked my work several times on the schematic and instructions and made sure the transistors and caps where placed with the right polarity but still can not seem to find the issue. I can post pics of the boards later. Also not sure why the dim bulb does not go completely dark. Both channels produce clear sound with fuses but I only tested for a few seconds in fear of the bias not setting on the left channel. Anywhere else I need to go poking with my meter or scope?
 
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Those rebuild kits usually just come with the bare necessities like the bias diode replacements, and the 4 transistors. They don't usually come with new drivers, and other transistors used on the board. Blowing the 4a fuse means that there was a definite problem on the board, either outputs being fed are shorted, or drivers or other transistors or diods or resistors have problems causing your issue of not being able to adjust bias (also check the bias pot itself to make sure its not open internally)
Other things in the power supplies or on other boards can cause dim bulb to be lit such as leaky filter caps in the power supply or on other boards, but if that is the case you may be able to identify them by seeing if they are getting warm or hot. With the 4 amp fuses removed for both channels, the dim buld should be very dim, or not lit noticably at all
 
It is possible that the bias pot on the left channel isn't in good shape. It may or may not help to hose it a couple of times with Deoxit. That sufficed on one of the bias pots of a factory 6013 board I have but next time I go in I'm putting in a new pot. Short of replacing the entire board I would at least replace every transistor and electrolytic plus that bias pot provided the dim bulb shows no problems with the channel fuses removed. IIRC the driver transistors on those are relatively easy to pull for checking, might be a good next step.
 
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Tom B said:
Those rebuild kits usually just come with the bare necessities like the bias diode replacements, and the 4 transistors. They don't usually come with new drivers, and other transistors used on the board. Blowing the 4a fuse means that there was a definite problem on the board, either outputs being fed are shorted, or drivers or other transistors or diods or resistors have problems causing your issue of not being able to adjust bias (also check the bias pot itself to make sure its not open internally)
Other things in the power supplies or on other boards can cause dim bulb to be lit such as leaky filter caps in the power supply or on other boards, but if that is the case you may be able to identify them by seeing if they are getting warm or hot. With the 4 amp fuses removed for both channels, the dim buld should be very dim, or not lit noticably at all
Click to expand...
I will pull the meter back out and check the resistors but all look to be in good shape. The drive transistors tested good before I installed the kit as well as the other transistors on the board. I’ll report back after I get done testing.
 
Guido57 said:
It is possible that the bias pot on the left channel isn't in good shape. It may or may not help to hose it a couple of times with Deoxit. That sufficed on one of the bias pots of a factory 6013 board I have but next time I go in I'm putting in a new pot. Short of replacing the entire board I would at least replace every transistor and electrolytic plus that bias pot provided the dim bulb shows no problems with the channel fuses removed. IIRC the driver transistors on those are relatively easy to pull for checking, might be a good next step.
Click to expand...
I did try spraying deoxit on the resistor and the current changed +- .005 amps when turning to min and max. Haven’t checked the resistor with the meter but I will report back when I test.
 
Ok so after testing it was indeed a short in the bias pot. not sure why I didn't check that first. schematic says they are 500ohm I think? As far as the dim bulb, I powered it again and tested some DC voltages to all the boards besides driver boards bc I still have fuses removed and all were showing approx. 5 volts under their voltages on schematic. All voltages were clean with little to no ripple. tested the main voltage off the main fuse and was only getting 90VAC. I am using a 60W bulb in my dim bulb so I'm pretty sure the bulb is restricting my voltage too much causing it to glow slightly. Then I just said screw it and plugged it straight to the wall and the indicators where significantly brighter and all voltages were correct. So I think that was my issue with the dim bulb. Im guessing I should find a 100watt bulb (which has been impossible lately) to set the biases or will a 100w still be too little?
 
@Wadebbi, unfortunately I do not. The only documentation that I've seen that even mentions them is the 5000x service bulletin and that merely says to replace them. My best suggestion is to start a new thread in Exclusively Sansui asking for help with any particular problems you are having. If both your boards are working and you just need to keep them that way I think the "Skippy fix" is the same as for the F-1040. If you want to do a recap get the cap values off the caps on the board, they don't always match what is in the manual anyway. If one or both boards are shot my advice to replace with reproduction 6013 boards and not attempt a rebuild of the 1040-1.
It's too late now, but I realize I should have kept one of the 1040-1 boards as a reference.
 
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