Sansui Frequently asked, and more...

Anyone have the 2000x readily available? I don't wanna seem greedy, but I'd love to make sure everything's up to spec on mine.
 
5000 series owners with 1040 driver boards - HOW TO RETROFIT THEM!

If you have a sansui 5000, 5000a, or early 5000x that uses the F1040 or F-1040-1 boards, this thread will instruct you on how to upgrade the bias devices so that the machine can operate safely.

A big thanks to Johncan for sending me the 5000a for the subject of this thread.


You will need:

solder wick
solder (64/40 is good)
good 50 watt solder station
120 watt soldering gun
various sizes of needle nose pliers
surgical forceps are a plus
#2 and #1 Phillips screw drivers.
non-conductive epoxy
(4) NTE-605a or STV-2h diodes.
22ga wire
100pf capacitor
(2) 2sc2240 or 2sc631
shrink tubing
exacto knife
mixing stick (for the epoxy)
2 jumpers with alligator clips
150 ohm 2 watt resistor
A variac is nice too.


Let's begin!​

Start by taking it out of the case and removing the top cover and bottom covers. bleed down the power supply caps with the 150 ohm resistor to the chassis. Pull out the amp fuses so any residual energy does not get to the driver board.

Here is what the F-1040-1 boards look like in the 5000a
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The problem lies with the thermal feedback diodes. They are the big black square things. They will open, causing full forward bias to be put on the output transistors. This is about 800mv, which saturates the outputs and kills them. WHen that happens it will destroy most of the components on the driver board. If that happens, its best to find a new board. They never really can be successfully repaired.
 

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Dismantling

Don't try to work on the driver boards with them installed in the chassis. its a headache and will only cause you troubles. They MUST be removed from the chassis if your repair is to be a success.
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There are 8 leads to unsolder per board. Rail, output cap, 2 ground straps, 2 protector leads, and two input leads. Note their locations. You'll need a soldering gun to remove the ground straps.
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Now that you have the leads unsoldered, its time to take the amplifier out to work on it. The entire assembly can come out including the output transistors and heatsinks. This saves time unsoldering the output transistors.

Remove the back panel covering the heatsinks, then unscrew the heatsinks and the screws that hold the driver board in. It will all come out as one piece.
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Now that the board is out you can get a better look at what you're up against. The old devices will come out easily enough, but now you will need to adapt the new devices.

The devices we will use are the NTE605a or the STV-2h. They are dwarfs compared to the originals, but they can be adapted to work.

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With your new devices prepared, check to see if they will install. You may need to attach wire to the ends of the device for it to reach the 2sa485 pre-driver transistor if your leads aren't long enough.

I also place shrink tubing over the leads so that no electrical current can touch any other device.

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DO NOT SOLDER THEM IN UNTIL THE EPOXY HAS SET. This can take anywhere from 15 mins to a half hour depending on what you use. YOu will need to make sure the new devices contact the body while the epoxy is drying.
 

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More shots of the new devices mounted up and epoxied in palce:
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Once you've made sure the epoxy has set, you can solder in the new diodes. DOUBLE CHECK TO MAKE SURE YOUR POLARITY IS CORRECT.

You also want to solder them in as quickly as possible. I would take a 30 second break between leads. If you overheat the new devices they will fail and your work will be a waste.
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Also compensate the 2SC485 with a 100pf 100v capacitor between the collector and base. This gets rid of a small oscillation that will occur at higher frequencies, and is absolutely critical if you have to replace the pre-driver transistors. On some boards there is actually a provision for this labeled c810.
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While you're in here, look at the initial audio amp transistor near the input leads of the board. if its a 2sc458 REPLACE IT. These transistors are notorious for becoming noisy and opening. This will cause all sorts of pink noise and crackling. Replace it with a 2sc631 or 2sc2240. I prefer the later for its low noise and low leakage characteristics.
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And there you have it!​

That is what is required to update the F-1040 series boards so that they operate safely. Again, if the amp has blown out, just find another board. The success rate at which the board can successfully be repaired and reliably work again is very low.

Hopefully this will be of use to the folks who have the skill to perform this task. This is NOT for the novice. Try it out on a 5000 series you don't care about, or have a parts unit standing by in case of a failure. If you install the diodes backwards or overheat them, the amp will self destruct when you power up.

After the installation, the old bias procedure is no longer valid. measure between the emitter (blue lead towards the bottom at the back of the driver board) on the final output device and ground. Adjust VR-02 for 12mv adjust again after 15 mins. I will use a variac to slowly power up and monitor bias one channel at a time.
 

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Holy smokes that is a lot of work. Makes you really appreciate modular designs like the AU-9500 even more!

Good work. Thanks for taking the time. Nothing like a good picture-walkthrough thread!
 
Holy cow!!!!! I had no idea it involved so much. That is an absolutely amazing pictorial.

You are the man!

Thanks!
 
I knew there was a reason I decided to not buy that 5000a that I saw at the flea market a while back. :D
 
Thanks, Jordan, for taking the time to document this procedure for us, it was very informative, and easy to read and understand. :thmbsp:
 
5000x

Nice work Jordan, and I agree, it should become a sticky.
Did this mod apply to the 5000X model?
I always thought the issue was solved by Sansui with the 5000X.:scratch2:
Casey
 
No, actually some early 5000x models have the 1040-1 boards believe it or not. There are very few of them, but they exist.

Sansui lost a TON of money when it came to repairing these. Anyone who was a sansui authorized service center at the time probably had a ton of work on their hands.
 
So by "retrofit" do you mean the earlier models of the 5000 series did not have the problem and it was the later ones that did?
 
The 5000, 5000A and very early production 5000X models were originally built with the F1040 boards. These boards all had the inherent problem Jordan described and were prone to failure. The permenent countermeasure Sansui developed was a redesgned board, the F-6013. The later production 5000X were built with the F6013 boards.

Many, Many 5000s and 5000As had the 6013 boards installed for free under warranty. Sansui continued to offer this free upgrade to 5000 owners up thruough the early 1980s, 10 to 12 years after these units were originally produced. How's that for a warranty?

So the only way to know what boards are in your 5000, 5000A or 5000X is to open them up and look. If they have the 1040 board, consider doing Jordan's modification. If they have the 6013 board, Rock On.

- Pete
 
no, the 5000 and 5000a have the problem almost guaranteed unless they were sent back to sansui for the 6013 boards.

Early production lines of the 5000x have the 1040 board too, however by mid 72' they were all F-6013 boards

The real way to find out if you have the problem is to pop the top on your machine. If it has a 1040 board, look for the black square things sitting atop o f the T0-39 pre drivers --you can't miss them.

There are moron techs out there who simply put 1n4004 diodes on the board just to make it work again. If you see a 1040 board without anything sitting on the pre driver transistors FIX IT PRONTO!

In lamen's terms The whole purpose of the setup is to regulate bias current with the varying heat conditions of the pre driver stage. As the amp heats up from playing it loud, the bias will creep up. Without any regulation the bias would keep going up even after you turn the volume down. Eventually the transistors would go into a thermal runaway state and fry.
The device is a temperature compensating dual-forwarding diode. Standing alone, it reads as two diodes would be in series. However as it heats up, the voltage drop across the diode goes down. When in circuit, this effectively lowers the bias automatically as the amp heats up.

You can still overheat the amp if you play it at crazy volumes on a 4 ohm load, but when you cut the music, the heatsink will cool because it backs off the bias instead of remaining hot or getting hotter!

I hope the explanation is helpful. I'm terrible at trying to explain these things, I understand it completely in my head, but have a hard time trying to convey what's in my head to others.
 
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