Hello everyone
I need a help to repair Technics SUV90D amplifier.
I'm using a translator, so I apologize in advance if anything is unclear.
The problem is that the amplifier suddenly stopped working. After removing the cover, I noticed a blown fuse. I thought that was the only problem, but after replacing it and turning the amplifier on, resistor R558 started burning (after measurements, it turned out to be good).
After a closer inspection, I noticed that IC 502 was damaged and transistor Q552 had two burnt-out pins.
If I measured correctly, two large power transistors in one of the channels were also damaged.
I'm a beginner in electronics and would like to save this amplifier, so I need help. Please advise which components might still be damaged and help me find replacements for the original components.
The motherboard is currently desoldered.
If anyone can help, I would be grateful for any suggestions.
A few photos and a schematic are attached.
Best regards, Kamil.

 

Hi,
Good pictures.
I'm sure someone will be able to help you.
What is the number on the transistor/FET that you have your DVM attached to?
Good luck.........

 

Cześć,
Dobre zdjęcia.
Jestem pewien, że ktoś będzie mógł ci pomóc.
Jaki jest numer tranzystora/FET, do którego podłączyłeś DVM?
Powodzenia.........
[/CYTAT]
oba tranzystory dają podobne wyniki po pomiarze

 

Start with changing both the 2SA1302 and 2SC3281 on the bad side. Also, with R558 being hot, I would also change goth Q556 (2SA1535)and Q554(2SA3944), one of which is likely shorted as well. I'm a bit surprised that this unit it not in protect mode so I would also replace IC601 (AN7073) which should have detected the DC on the output line and shut down the amp. All of these are common to replace when an output transistor fails.

Start there and if possible, do not connect the center pin of the output transistors when testing. In this state, there shouldn't be any current draw and you can check for proper voltages without fear of another failure. Also check any feed resistors to Q554 and Q556. Perhaps there is a poor solder connection at the voltage regulator(s) that feed these.

-Dan

 

Thanks for your reply and suggestions. Can you recommend good replacements for these components?

Start with changing both the 2SA1302 and 2SC3281 on the bad side. Also, with R558 being hot, I would also change goth Q556 (2SA1535)and Q554(2SA3944), one of which is likely shorted as well. I'm a bit surprised that this unit it not in protect mode so I would also replace IC601 (AN7073) which should have detected the DC on the output line and shut down the amp. All of these are common to replace when an output transistor fails.

Start there and if possible, do not connect the center pin of the output transistors when testing. In this state, there shouldn't be any current draw and you can check for proper voltages without fear of another failure. Also check any feed resistors to Q554 and Q556. Perhaps there is a poor solder connection at the voltage regulator(s) that feed these.

-Dan

 

Repairing power amplifiers is for the brave only.



The first thing to do is to stop blowing anymore components.

Disconnect all loudspeakers.
If you already have discovered blown fuses and other blow components, then you need to check the power output stages, both left and right channels.

Unplug AC power and do all resistance tests with no power.
With a DDM set to measure continuity or ohms, check for shorted power transistors Q553 to Q560.
If you discover any shorted transistor, remove all four transistors in that channel. Make sure that you label each transistor that you remove.

Don't replace any component until it is safe to do so. And I will tell you when it is safe.

 

With a DDM set to measure continuity or ohms, check for shorted power transistors Q553 to Q560.

Hello MrChips, the continuity test does not show a short circuit between the transistors.

 

Hi,
Your transistors are Toshiba, avoid cheap replacements. Quality counts.
Good luck...........

 

There are replacements available for the listed transistors. I have "old stock" of the 2SA1302/2SC3281 and also have 2SB649 as a substitute for the others listed.

-Dan

 

If you find no short, then you can try a limited power ON test.
Do not connect loudspeakers.
Remove the suspected bad IC502 and Q552.
Instead of what would have been Q552, install two 1kΩ resistors, one from Q552 collector to GND and the other from Q552 emitter to GND.

Before doing a power ON test, build this AC test circuit.




Plug your amplifier into the AC outlet. The incandescent light bulb will limit the AC current in the event that there is a fault in you circuit. The light bulb should be 60-100 W. If the lamp lights, then you have an over current situation.

 

There are replacements available for the listed transistors. I have "old stock" of the 2SA1302/2SC3281 and also have 2SB649 as a substitute for the others listed.

-Dan

please send me a private message with your offer

 

Today I desoldered several damaged components. Please confirm that I'm correct.
What's the next step or what else should I check?

Currently desoldered components:
R560
R558
Q556
Q554
Q504
Q502
Q552
IC502
2SA1302
2SC3281

@MrChips, @abrsvc Can you help me choose replacements for previously damaged components?

 

I have marked in red the components you have removed.



Without replacing the components, perform the power ON test with no loudspeaker connected.

1. Check that nothing else smokes or blows up.
2. Measure the DC voltage at the LEFT channel output.
3. On the LEFT channel amplifier, measure the voltages at base, collector, emitter of Q557 and Q559.
4. On the RIGHT channel amplifier, measure the voltages at base, collector, emitter of Q558 and Q560.

 

Without replacing the components, perform the power ON test with no loudspeaker connected.

Should the connection and measurements be made via a test circuit with a light bulb?

 

Should the connection and measurements be made via a test circuit with a light bulb?

Do the measurements with the light bulb first and let's see the results.
We may choose to remove the light bulb set up at a later date.

(It may prove to be useful at some stage to replace the light bulb with a screw-in style fuse, or even a 5-10 A circuit breaker, if you can find one locally.)

 

Hello.

I had some time today and measured the current on the transistors. Nothing smokes or explodes.

The current values are not constant; after several attempts, the measurement gives different results.

On the first attempt, the bulb lights up briefly and then goes out. After a few seconds, it starts glowing slightly. Transistor Q557 heats up very quickly.

On the next attempt, the bulb lights up briefly and then goes out, but then doesn't light up again. Transistor Q557 doesn't heat up. Measured values in parentheses.