Guidance in reapir of Technics SA-410

M

mattr100

New Member
Hi,

I have gotten a Technics SA-410 for next to nothing and I am trying to revive it. Honestly it is a bit of a sentimental value as I used to have one many years ago during High school. That one has long been gone and now I have gotten my hands on a not so functioning one.

Now for what it does and does not do.

Does
1. Powers on Tunner appears to work
2. All lights works
3. Computer Drive Seems to works as it starts in Preheat and ends in Auto in the 15 seconds.

Does not:
1. Have any sound out put from the speakers.
2. Have any sound output from the headphones.
3. protection relays does not close.

What I have found so far is that the output of the power pack on both channels is -41VDC so the DC protect is operating properly and not letting the protection relay close. What I am wondering if the power pack (STK2048II) is bad of is the current stabilizing transistors (Q601, Q602) or the voltage Amp transistors (Q71, Q72) On all of those transistors I am getting 39V on the E,C,and B of the transistors. My guess is the power pack is bad, however I think I will just replace them all.

What I am wondering is if anyone has a good source for these parts that is not China on Ebay and has anyone seen this failure before and am I on the right track.

Thanks I look forward to the guidance.
 
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Usually the STK is bad, and there are no replacements available anywhere. Don't bother with the Ebay fakes, there are no originals in existence, other than in stil functioning untouched amplifiers.

Chuck it in the garbage. You cannot fix it and it has no value in scrap.
 
STK2068II sometimes surface and have not yet seen a counterfeit of that number (yet)

Check for collector-emitter shorts (power supply pin(s) to speaker pin) pin 1 to pin 2..... Pin 4 to pin 2

Pins 19 21 and 22 for other channel
 
Hi

I checked the for shorts on the STK. yep both channels are shorted to the negative side.

Thanks. I'll update this if I get it fixed. I did manage to find some parts. A 2048 and a 2068.
 
The 2068 will be the one to fit... More stronger than a 2048 - do check current stabiliser transistors for shorts too
 
Before fitting the new output, check supply voltages. I had experiences with blown STK taking some voltage regulator with them. Just in case double check you don't have DC at the STK input and output pins.
 
HI Thanks,

I have checked the voltage rails and other transistors everything seems fine and I did replace the STK. I did get a 2048 and a 2068. I am using the 2048 now just in case I blow the crap out of it while fixing it. I did get it to work by replacing the STK, however still trying to get it to fully work. I have also adjusted the IQC to the specified 15mV.

Now the left channel sounds great. The right channel has some distortion and crackling so to speak. I am trying to chase this down. I believe that it is Q72 but I am having difficulty as I am using a function generator at 150 hz. This is where the distortion is the worst and tracing through the circuit, the distortion disappears when I touch the probe (10x) of the oscilloscope to the base of Q72. It will then be fine until you power cycle the receiver. Any one have any thoughts on that? I am now suspecting Q72 or IC71.
 
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