TECHNICS SA-700 breakdown

N

NicoM13

New Member
Hello,

I discovered this good forum while searching for problems encountered with the TECHNICS SA-700.

My neighbor gave me an old TECHNICS SA-700 that was stored in his garage for a lot of years.

Yesterday I wanted to test it and I connected a CD player to the aux imputs.
I switched the amplifier on and the sound was really good and powerful but a few seconds later the amplifier started to emit a bad smell. I switched everything off.

In order to know where the smell was coming I removed the cover of the amplifier before switched it on again.
A few seconds after I saw a smoke coming from the PCB on the right of the power transformer (while facing the amplifier).
I waited for the smoke to disappear and I noticed a leak under C625 and C628 (100uF / 63V electrolytic capacitors) and there were still very hot.
Beside these capacitors R708 was a bit burnt but showed 650 ohm as it should.
I checked the other components and saw that R912 was also burnt (I don't understand why because it's far from C625 and C628) but still showed 1.8 kohm.
I took the enclosure apart to check the solder joints under the PCB and found out that the solder joint of the collector of TR701 (NEC D381 transistor) was cold so that the leg was not connected to the PCB anymore.

Do you think C625 and C628 blew because of this cold solder joint of TR701 so I just need to replace them (or replace all electrolytic capacitors if I want to be on the safe side), or could it be a another faulty component?


Thanks for your help.
 
i doubt a bad solder joint would cause a cap to fail, it most likely already failed and if it hasn't been power up in years, that just made it worse. need to look deeper into it. if that cap was shorted it would take out more parts
 
Thank you Bassblaster,

I removed C625 and C628 and I didn't see any apparent short.
I also removed C627 which sleeve started to cut.

As I dont't have the schematic I don't really know what other component to ckeck.
 
i am suspecting if this unit has a voltage selector it is set wrong for the voltage being supplied at the mains plug . failing that the rectifier is kaput .
 
So the main caps arent testing as shorted? If the unit has been sitting around for years unused caps, if they arent subject to a charge, can pose a safety risk when the unit is powered up again.

Suggest if the caps are OK, power up the unit again only thru a dim bulb tester and see what the bulb tells you. Then as petehall suggests test the basics in the powersupply, that the trafo has voltage at the secondary (that should be fine) and the bridge rectifier isnt shorted and is putting out DC.
 
i doubt the rectifier diodes would be harmed from high voltage. every diode bridge ive ever seen have been good for at least 200V
 
Here are the voltages I measured with the selector on 240V on the SUP 11911C PCB from pin 708 (black cable) to:
Fuse F2 (Brown): 5V (6.2V)
Pin 703 (Blue): 44V (41.8V)
Pin 705 (Blue): 44V (41.8V)
Pin 702 (Blue): 0V (N/C)
Pin 704 (Orange): 16V (7V)
In parenthesis the expected values reported on the schematics.

The voltage on pin 704 is too high.
 
I've checked other random voltages:
R702 / C703 (20.2V): 24.2V
TR701 Emitter (31.4V): 36.4V
TR701 Collector (56.1): 61.3V
TR702 Base (-31.4V): -31.3V
TR702 Collector (-57.6V): -61.5V
 
I see traces of glue on the big dissipator attached to the power amp transistors and transistors TR607 and TR608.
These transistors' purpose is thermal compensation so i asssume that they need to be stuck to the dissipator.

Please can anyone confirm?
Thank you.
 
Thank you Petehall347.

Now there are not touching the dissipator (maybe this amp was already serviced?).
I'm gonna replace all the electrolytic capacitors and every suspect resitor and glue TR607 and TR608 back to the dissipator.
 
After changing all the electrolityc caps I switched the receiver on. The safety LED lighted up but R640 and R642 started to fry then R627, R628 and R629 then C617.
All 5 resistors that fried were previously replaced as well.
I don't know what's wrong with it.
I think I'm gonna bring it to the rubbish dump.
 
trouble is in 2 channels by the looks of it ... c617 is maybe wrong way round or just failed .
other channel transistors maybe inserted wrongly according to pin outs . failing that or what might have first failed is the biasing circuit to run wild as in flat out and cooked those drivers etc .
 
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