replaced the bias pots with the new blue small replacements. Hooked up a 100w 8 ohm resistor to the + speaker output and used mcintosh mpi4 scope. got the idle current setting done just fine. cranked in power for current adj and went too high evidently on power and blew something. Have literally checked every transistor and diode and all good, and see no cracked resistors anywhere. I am absolutely lost on this, but I really blew it up. All output trannies are still good! It does have the 6013 amp boards. Help if you can please and thanks, Randy
sansui 5000a unable to bias. very high current at fuse holders
- Thread starter matisse
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- 5000 5000a 5000x sansui
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The Fuxtor
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Check the emitter resistors and drivers
The Fuxtor
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They are the bigger white resistors on the board where the output transistors are mounted usually? Say a value of 0.33 ohm.. possibly 0.47 ohm, I never looked it up...
thx, yes they are the 2watt units, 2 each of .33 and 1 ea of 4.7ohms. I'll replace the .33 units as they are off a little, but not much. Sadly all of the transistors on each board test good.
It will be worth a try, but??? thanks! The boards are loaded with 1/2 watt resistors, so will unsolder a leg and check each of them, but none are cracked
It will be worth a try, but??? thanks! The boards are loaded with 1/2 watt resistors, so will unsolder a leg and check each of them, but none are cracked
The Fuxtor
Addicted Member
They would be very far out of spec to cause problems...
How are you checking the transistors?
Btw.... When checking low value resistors you need to subtract the ohm value that the probes touching each other read. That is probably your little extra you read .. I typically would expect a reading of 0.4 or 0.5ohm when probing a 0 .33 ohm resistor
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dly66
Addicted Member
So you did both adjustments? If I remember this vintage Sansui the smaller trimmer for each channel was voltage and big one the bias current. Cap coupled receivers have single sided power supply so one output transistor in each channel gets about half the other at the collector (which is the case for these old TO-3 types). One will be getting the power supply voltage coming off the big filter cap which I believe here tends to run in the 67-69 Vdc range. You use the smaller trimmer to adjust the voltage feeding the other to about half that voltage. If like mine with the F-1040 boards I think I used two voltmeters seem to remember the two were not totally independent.
If it totally died on the channel did it blow a fuse? The so-called speaker protection fuses are actually between the power supply and each amp channel.
How are you checking the transistors? If using the simple diode check technique with a voltmeter it can miss bad ones. I have had some bad ones will test as two diodes tied at the base but if you pull one and put it on a transistor tester it will not test as a transistor but as a pair of diodes.
If it totally died on the channel did it blow a fuse? The so-called speaker protection fuses are actually between the power supply and each amp channel.
How are you checking the transistors? If using the simple diode check technique with a voltmeter it can miss bad ones. I have had some bad ones will test as two diodes tied at the base but if you pull one and put it on a transistor tester it will not test as a transistor but as a pair of diodes.
yes, the .33 ohm resistors come in at abut.5 to .6 so no issues there. I do value check transistors and not the diode check method. Nothing on the boards that I can find show any issues, and since I had new electrolytics, all are replaced with same value, some tho higher voltage. None of the pulled caps had questionable values. No fuses ever blew, and the excess voltages are at the speaker fuses. If an attempt to set bias, the analog meter instantly pegs
The Fuxtor
Addicted Member
I'm guessing you blew some output transistors.
Test them with the diode setting. Look up the 6 way diode test for transistors.
Test them with the diode setting. Look up the 6 way diode test for transistors.
Hello and thanks!
Yes, you are exactly right on the adjustments. The transistor checks are with an analog meter and ohms readings, not as diodes, so should be an accurate test for them. The idle current setting is currently impossible and is taken at the speaker fuse holders with fuses removed....Huge current there. Have checked about everything I can look at and put a meter on all over the unit with focus on power supply and diode stack but it all seems well. It will power up, but hesitant to let it run. I should get over it and just let it run with dim bulb and 40 watt bulb and start looking again. I think idle current should be about 40 watts.
Hmm, I can check again, but the ohms test was good. I have several sets of output transistors and they all test about the same. You are saying to diode check them?? I haven't tried that way, so will do it....
The Fuxtor
Addicted Member
40 watts?? More like 20mv
Outputs test just fine diode method. At this point going to clean up some prior wiring and put it back together for another attempt. I caused this by overpowering it I think looking for a clipping point on the scope. Thanks gentlemen, I will post again when it is re-assembled and can be powered up...
The unit was pulling about 38watts when running, and I did think it should have been much less. Couldn't find the idle watts specs posted anywhere
The Fuxtor
Addicted Member
Ah. Gotcha
If you have incandescent lights that sounds about right.
dly66
Addicted Member
Dim bulb and this power of receiver would usually be using a 100W in my DBT. Tried my 5000A in the living room at idle on AUX my kill-a-watt indicates about 15.9 watts and with FM went up to 18.6.
When checking did you check the bias trimmers. If they go open things usually go crazy. Restored a Pioneer SX-1500TD someones previous repair blew up and fried the replacement trimmer. Might also double check the cases of any of the output transistor are not grounded.
If you are driving speakers at lower volume 38 watts would be reasonable. When I am shaking out a repaired unit in my workroom setup which is not high in power requirements the receivers in the 60 wpc range often are shown to be drawing wattage usually in the 40-60 watt range.
Don't drive speakers on a dim-bulb, headphones maybe. As a unit draws more power having the filament of the light bulb ahead of it the voltage the receiver sees can plunge.
Did you put new amp board trimmers in it? Those old ones can get loose have run into some starting to come apart. One of the ones used just have two thin bent metal tabs holding them together. If loose or dirty/corroded they can get flaky and incredibly touchy and try to go open and up goes the bias current. Have had some units I could not get bias down to where it should with new trimmers they biased nicely. Historically I have mostly either the 3352 or 3386 series from Bourns. For the big bias trimmers Sansui liked to use on the cap coupled units have to solder on leg extensions to the outside legs of the trimmer and may have to tilt it back a bit for tool access.
When checking did you check the bias trimmers. If they go open things usually go crazy. Restored a Pioneer SX-1500TD someones previous repair blew up and fried the replacement trimmer. Might also double check the cases of any of the output transistor are not grounded.
If you are driving speakers at lower volume 38 watts would be reasonable. When I am shaking out a repaired unit in my workroom setup which is not high in power requirements the receivers in the 60 wpc range often are shown to be drawing wattage usually in the 40-60 watt range.
Don't drive speakers on a dim-bulb, headphones maybe. As a unit draws more power having the filament of the light bulb ahead of it the voltage the receiver sees can plunge.
Did you put new amp board trimmers in it? Those old ones can get loose have run into some starting to come apart. One of the ones used just have two thin bent metal tabs holding them together. If loose or dirty/corroded they can get flaky and incredibly touchy and try to go open and up goes the bias current. Have had some units I could not get bias down to where it should with new trimmers they biased nicely. Historically I have mostly either the 3352 or 3386 series from Bourns. For the big bias trimmers Sansui liked to use on the cap coupled units have to solder on leg extensions to the outside legs of the trimmer and may have to tilt it back a bit for tool access.
Thank you so much, I did install the new style tiny blue trim pots, and got the idle current just fine and then went on to setting the other current pots and caused all the issues looking for a clipping point and shouldn't have. It was running just fine till I did this, and now it makes perfect sense to pull the new pots and change back to the old ones to see what happens. I see a very real possibilty here, as that was all i changed. This fits with back when cars were brought to me for repair. The first question was what has been done recently and what has been changed? Certainly makes sense here...Your wattage post sounds just right for this unit