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I'm new to this forum. I have a TA-1630 that is truly baffling me. I know that some of you have had experience of this amplifier or a similar model, and wondering if you can offer some experience/advice.

This amplifier has a volatile output section, in that it likes to blow the transistors to oblivion. The result if the transistor stays intact is the one of the output pair shorted sending around 29v out the speaker terminals. The other one goes OC.

I have pulled the 2 previous stages and diode tested all of them. All other transistors seem fine. I changed a couple of suspect resistors and the capacitors of the bad channel. I have changed the bias diodes to the Sony suggested rubber diode and that made little change - it worked with new output pairs of TIP41/2C as suggested in the ST transistor cross reference list. It worked until I turned up the volume. The same channel exploded again.

I checked everything I could find, traced PCB tracks for bridges or breaks. Checked the rubber diode, checked the bias pot, though I see the rubber diode overrides that. Rechecked the transistors. Reflowed solder joints. Changed the output ones again. Double double checked, power on - bang.

Checking component values from left to right, all seem to read the same.

With this PCB, it is almost impossible to work on it in situ. Testing is a nightmare, as the solder side of the PCB is inaccessible when you come to power on. Every time I have to change something, it's weakening the heatsink screws, the aged PCB tracks, the hardwired cables. I have a virtual scope (usb) and a multimeter. I cannot get the thing to be stable enough to test via the scope.

I have no way of running this on low power to reduce chance of blowing up yet more parts, no way of easily testing it minus the output Transistors. I do not want to apply power to it with the PCB sticking out of the chassis, nor undo perfectly good original wiring to the preamp. What I wanted to do is simply replace the dud parts and move happily on, how wrong I was.

Anyone had a similar issue? I realise many did with the SV04F, but mine is not that straightforward. I just cannot see what is causing this. Could the driver stage be faulty even though they read fine on a diode check?

I had another Sony that was "fun" to fix - TA-F30. That one though didn't explode transistors, it just had a tendency to overheat and fluctuate the bias. That was down to the diodes. Fixed that.

Update: I've had another tour round the PCB, referring to the service manual. I then noticed that in the parts list of the manual, the transistors differed from those on the Schematic! Also, someone had repaired this unit in the past and for some reason, replaced the drivers with smallish signal 0.75w transistors. The proper drivers were sitting where small signal transistors should be, constant current and pre-driver. In the schematic, the same pair 5w driver transistors are also listed in the position of constant current Q1/206 and pre-driver Q1/207. I also believed it must be correct until yesterday I noticed the parts list have different transistors specified for them. I cannot get the required transistors, and those subs for the drivers didn't spec close enough, so I swapped the drivers to the correct positions and subbed the missing small transistors with pairs of A733 and C945 which are close enough. I don't have the replacement power transistors, but running the amp on the 1 good channel with reduced power as suggested resulted in the good channel actually working better, louder as one would expect. It didn't blow up even with the incorrect drivers where as the other channel did, but let's see if my correction of the circuit would make a difference.

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  • \$\begingroup\$ Best guess is a capacitor you haven't changed yet. \$\endgroup\$ – evildemonic Jun 9 '20 at 15:14
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Suggest you replace the output stage emitter resistors with 100 Ohm or higher for the purposes of testing. This will at least allow you to power it up without the O/P failing before you can even make voltage measurements. Suspect the bias pot, in many circuits if the bias pot opens up or the wiper goes high resistance the bias into the O/P devices goes to the maximum the circuit can support. O/P device failure is near instantaneous if that's the case.

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  • \$\begingroup\$ Thanks for the info and tip! \$\endgroup\$ – Philip Tusar Jun 10 '20 at 15:30

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