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Old 24th November 2013, 05:24 PM   #1
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Default basic electronics repair question - Sansui G5500

I purchased a Sansui G 5500 which was not working. I bought it for the face plate and cabinet for another working unit. I would like to get the new unit functional.

On the Power Amplifier Circuit Board, there is a completely burnt resistor ( R46) which has also surface burned the two neighboring resistors. My understanding of resistors is that they rarely fail, something causes them to fail.

Looking at the schematic, Transistor TR20 is right next to it. The middle leg of the transistor is directly connected to the burned resistor. Transistor failure seems to be far more common. Should I be looking for something else that cause the bad transistor or just replace it?

My question is - should I just replace the transistor while replacing the resistor? Should I replace the neighboring resistors that look bad but may not have internal damage?

The transistor part number is 2sa992 and they seem pretty cheap on ebay. I am hoping my local electronic shop has them.
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Old 24th November 2013, 11:50 PM   #2
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TR20 appears to monitor current through the NPN output transistor, TR602. R46 restricts TR20 current, being in the path from its collector to ground. It suggests to me that if that resistor and transistor have both failed, the output stage has failed or at least been severely overloaded.

Measure the DC potential across the output terminals (it should be <50 mV) and measure the other voltages highlighted on the schematic to see if any problems become clearer. As usual, a bulb tester should be used to limit current to the whole receiver and prevent further damage to the amplifier. Also, deselect the tuner, remove all inputs connections and turn volume to min. when testing DC levels.

If the 2SA992 you find are genuine, they are a good find for low noise parts, such as the input stage. 'Can't see that are necessary here, other than for high gain. They are obsolete though, the replacements being Fairchild KSC992. A similar but also obsolete type is 2SA970, which might still be easier to find.
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Old 25th November 2013, 01:25 PM   #3
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would it be a good idea to pull TR20 and TR602 and test them prior to installing new resistor and giving the unit power?
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Old 26th November 2013, 12:36 AM   #4
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Yes, replace them if they measure shorted or different to similar types but there will likely be other casualties there too. You could also test for C-E shorts of both the output transistors and the drivers, TR16,18 before powering up. Measure similarly in the other channel since it can be useful to compare powered DC and also unpowered resistance measurements. This may help overcome any confusion in trying to measure transistors in-circuit.

Have a look at page 2 of the manual for the detail description of how the protection chip, HA12002 is supposed to work. It is controlled by TR19, 20 and is therefore involved in the problem. However, since the other channel is presumed working OK, the protection circuit may also be OK and the relay should turn on normally after a delay at power-up. If not, it gets difficult in more ways than one. Be certain to use a current limiting device like the bulb tester if powering up, it may be essential to prevent a repeat failure.

Perhaps others who worked on similar designs would like to comment as I don't recall having worked on this type more than once or twice. There's a typo in my reply too - the Fairchild type should obviously be KSA992.
Download the Sansui G-5500 service manual for free - Hifi Manuals
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Last edited by Ian Finch; 26th November 2013 at 12:49 AM.
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