Sansui 4000 help

Jon D

Member
I have been using this sight to gain info for quite some time now, but this is my first post. I have been trying to fix this 4000 of my dads. My knowledge is limited to things I have read on this site and Web in general. I have done minor repairs on receivers of my own. Got about 6 now. Anyway, I was using the 4000 when it died lights out and all. Main fuse blown. New one blew immediately. I misdiagnosis early on had me focused on the protection board. The problem turned out to be the bridge rectifier. I replaced it with one from Mouser now the fuse does not blow, lights work, no sound though. When you first power it up there is sound for less than one second. This thing is driving me crazy, but failure is not an option. Been spinning the knobs on it for many years. Any ideas as to where I should focus my attention would be appreciated. My tools are a multimeter, and a cheap esr,lcr. Thanks. Btw, I bought and eight deluxe last week. Was pricey, but worth every penny. Awesome receiver.
 
If the bridge blew, and you get brief sound, the first thing I suspect is the filter caps.

Do you have correct voltage in the filter caps (main rail voltage)?

Also, I would check the voltages on the power supply.
 
Thank you I will check the voltage. Couldn't do it before, because I couldn't power it up. One of the main caps is bulged a little but they all checked out ok with my esr. Just walked over and checked volage at cap. 69.9 manual says 66
 
The protection lamp does not come on. I checked all the bulbs when it was blowing the fuse thinking one may be shorted. I will check the protection one again. Maybe also check for voltage there.
 
Replaced the 2sf656 thyristors with nte5402. one of em was reading bad on my esrlcr. When that didn't work I bought a used board off ebay. When I put that in and it didn't change things was when I realized I was focusing on the wrong thing. The reason I was focused there is because I thought when I pulled the power supply wire from that board the fuse didn't blow. Tried it again and that was not the case. Only way I could get it to power up was disconnecting the purple wires from the rectifier. I powered this thing up tonight and gave a couple taps on the protector bulb, it seems loose in its housing but filament looks ok. I noticed a burning smell, and started looking around, the 68ohm resistor on the ripple filter block was red hot. Don't know what that means. Maybe I am in over my head here. Thank you for taking the time to help me by the way.
 
I don't believe that was happening before, because I had this thing on for longer periods of time and never noticed that. Whenever you get around to that will be fine. I need to get away from this thing right now anyway. Thanks again.
 
Well, tough call. That board feeds a few circuits. Could be the protection board. The schematic shows voltages. I would start by checking them.

Rob
 
I'm not positive about this, but I don't believe the 4000 has a protection circuit. When I turn mine on, it is on right away with no pause or click.
 
looks like the protection maybe lowers the rails when too much current is detected from the output ..this could be a genuine fault or a fault of the protector circuit .
i would be checking the amplifier first ...
might turn out to be a simple emitter resistor ...
check idle bias and centre voltage first .
 
I don't doubt that it does have protection, but mine turns out without a pause or delay too. I'm new to the unit and don't know if that's normal or not. Many other receivers I own go on with a short delay or click.

I know the unit was manufactured in 1969 to 1971 and not sure what was standard back then. I have at least one receiver.... PIoneer SX 434 that turns around right away too. It was the first receiver I ever owned and I always thought that it was not protected.
 
These early units don't have protection relays so they will turn on without a startup pause.

If it's handy, please post the serial number for the database. Click on the link in my signature block below.

- Pete
 
The so called "protection circuit" on the early Sansui's is just an over-voltage detector which signals you to shut down the unit and get it fixed. It doesn't shut down anything, it just tells you that it's got an over voltage.
4000 protection circuit.jpg
4000 protection adjustment.jpg
 
The Early Sansui receivers had "fused" output's as protection along with the "PROTECTION LIGHT" which IIRC was only an indicator that an overvoltage or overcurrent had occurred or was occurring and you'd better shut it off NOW. Whether it cut voltage to the Rail I don't know, the more technical guys will have to settle that one. There is a protection adjustment procedure in the Service manual which to me looks like it adjusts the voltages to a set point. Anything over that and the lamp lights up. The 4000 is CAP COUPLED.

GO GIANTS: PIONEER also fused the output's on the x2x and x3x series in the lower 1/2 of those series (incl. SX434.) NO relay. They were direct coupled with no output caps. All of them will start up almost immediately and sometime thump on startup.

Most of these units were capacitor coupled on the outputs. By the mid 70's most all receivers except the botl units in some brands were protected by a RELAY to the output's which opened on an over voltage, protecting the outputs.
 
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I'm having a problem with my 4000 going to protection mode while using phono. A little pop....then volume drops dramatically. This just started and only using phono. FM and Aux seem fine...at least for now. Any ideas...something shorting out perhaps? I know this model is known to have a bad board, but wouldn't that affect all outputs?
 
Hi,

It is likely that you have a transistor that is failing (going noisy/popping) on the Phono board causing the protection circuit to operate

The protection circuit in the 4000 does two things - it illuminates the protection lamp when an over current occurs at either output, and it also turns off the power supply to TR701/TR702 on the tone control board, this reduces or eliminates the signal getting through to the output stages.

Cheers

John
 
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