Help Fixing Technics SU-V4X

[Flibble]

Hello, I just got an old Technics SU-V4X in good condition which seemed to work ok (outputting undistored audio on both channels in a very brief test) so like a good boy I changed Q501-Q504 to A1124s/C2632s. However it now intermittently goes into protect mode together with causing Q501 to go short circuit between E and C and cooking R513. After some testing with Q501 removed I noticed that when it goes into protect mode I saw ~40v at the collector (I think it was the collector- I powered it off pretty sharpish to avoid damaging any of those unobtanium Technics chips...). It's -hard to say if the work changing those transistors caused the fault as I didn't test it extensively beforehand.

If anyone could offer any expert advice on what the fault might be I'd be really grateful - just worried about killing any of those old chips and I'm down to my last A1124 .

Thanks, Dan.

[Flibble]

Did some more testing and it turned out the shiny new 2SC2632 at Q503 was faulty (dead-short between collector and base) and swapping this out fixed the previous problem. But now the protection light flashes quickly after the initial slow flashing, and after about 30 seconds I see the full 52v rail voltage at IC502 pin 1 (input), taking Q501 out stops this happening (seeing the voltage at IC502 pin 1) but the protection light still flashes. Had to stop testing at this point but It seems to me that either IC502 (the SVI2004) or IC501 (AN7062) may now be dead...

[Harleyjon]

Not realy but I know a man in Weymouth Dorset that can fix it for you, if it is not too damaged. Welcome to my Web site

[Flibble]

Thanks for the reply Harleyjon. Turned out the shorted transistor took out one channel of the SVI2004A chip - gutted! Not sure I'll be ordering from Cricklewood again...

Would love to get the amp going if I could get another SVI2004 chip without spending a fortune

Cheers, Dan.

[stratus46]

Questions time. Did the unit work when you first got it? What was the expected improvement by changing the 4 transistors? Was there a problem with the original transistors before you removed them?

Where can I get datasheets on the old and new transistors?
G²

[Flibble]

Hi stratus46, It seemed to be working fine when I got it, Q501-Q504 (2SA1123/2SC2631) run hot in this amp (too hot to touch for more than a couple of seconds), a bit of searching showed that these are a weak point in the amplifier http://www.diyaudio.com/forums/parts/25436-svi-3206-a.html#post521797 and there are higher-power equivalents (2SA1124/2SC2632) and its a good idea to swap them out. Sadly one of the new transistors was DoA and killed the SVI2004A. Interesting, the SVI2004A contains two channels with symmetrical pinouts so by mounting the chip the 'wrong way round' I could see the fault move to the other channel and the initially faulty channel test out ok.

Data sheets 2sa1123
2sc2631
2sa1124 2sc2632

Thanks, Dan.

[Flibble]

I should add the the fault is DC at rail voltage on the the output pin (pin 3) triggering the protection circuitry and DC at the input pin (pin 1) of the SVI2004A. Dan.

[stratus46]

Originally Posted by Flibble

Hi stratus46, It seemed to be working fine when I got it, Q501-Q504 (2SA1123/2SC2631) run hot in this amp (too hot to touch for more than a couple of seconds), a bit of searching showed that these are a weak point in the amplifier
<snip>
Thanks, Dan.

The transistors you put in are the same electrically but have higher thermal dissipation without heatsinks. If that was mine I would look for clip on heatsinks (assuming there is room) rather than changing the transistors because in reality, if that 33% dissipation increase makes the difference, it's running way too close to the 'edge' for me and needs extra thermal help. IMO semiconductors (at the case) should run no more than the temperature of hot shower.

Thanks for the datasheet links. That helped a lot. As far as your faulty transistors on arrival, I _might_ have 'Ohmed em out' before installing but I have the fortune of working where the replacement parts are virtually certain to be good every time and have never had the misfortune of bad parts 'out of the box'. After your problems I will check unknown parts before installing.

It looks like those SVI2004 chips are out there but but not much fun to acquire.

G²

[Flibble]

Thanks for the all the useful info I got it working fine by swapping out all the faulty new transistors but soon after it developed distortion on the right channel, the collector of Q502 should be at +1.8v but it sitting at -2.5v. I cant seem to find anything obviously faulty between Q502/q504/SVI chip, I swapped around q501/q502 and q503/q504 but that made no difference - is tempting to think the svi2004 has gone bad but think I need some expert advice on what else could cause this fault

Cheers, Dan

[Flibble]

I got another SVI2004A chip from china off ebay and the amp is now back up and running! When I took the old chip apart it was actually a SVI2004C board with a SVI2004A cover on it (you can see the difference here - last pic). Pretty impressed with the sound so far .

Cheers, Dan.