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Is this a fake STK pack, or something else?

QSilver

QSilver

Super Member
I got this Technics SU-C03 with a blown STK1050. I bought one from a company called LittleDiode. I fitted the one they gave me and I could set the DC offset and everything seemed good until I hooked it up to a scope.

There's a lot of noise on a 1kHz sine wave on the channel with the "new" stk pack. I guess its just that the STK is a fake but I did wonder if it might be something on the circuit somewhere failing seen as half the STK had railed and not the whole thing.

I passed a square wave through too and had a large spike on the beginning of the peak.
 

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It could just be a real but bad one out the factory.
I bought an amp brand new in the late 1970s.... one channel made squawking and gurgling noises right from the first day.
The manufacturers claimed several times there was nothing wrong.
I discovered that putting a washer under the STK fixed it (bending the chip when the screws were tight), and then I replaced the IC - problem vanished.

BUT it could be another component acting up - and the reason the original chip failed in the first place.
 
Reminiscent of ringing artifacting, but I think that's only in square waves. Looks more like frequency modulation,

I notice that the ripple goes away on the decent from the peak and ascent from the trough, I wonder if that means anything?
 
is the preamp output clean ?
almost said look at input to stk but feedback circuit will throw that out somewhat .
might be worth looking at scope without any signal input to amp to see if its still looking like sawtooth .
and measure dc offset .
 
With no signal the scope shows a flat line. But, when powering it up, you can see the zigzags as it stabilises... I have adjusted the DC offset to as near to 0mV as I can on both channels. The amplifier has a "slot in" type circuit board that has two trimmers for the offset on the STK packs. I could try swapping the STKs and I'll check along the signal path for any problems.

Gazdatronik said:
Reminiscent of ringing artifacting, but I think that's only in square waves. Looks more like frequency modulation,

I notice that the ripple goes away on the decent from the peak and ascent from the trough, I wonder if that means anything?
Click to expand...

I noticed that too... there is a pattern there but I'm not sure what it points to. I did test it with a square wave and I get an overshoot on the rise but normal on the fall.
 
QSilver said:
With no signal the scope shows a flat line. But, when powering it up, you can see the zigzags as it stabilises... I have adjusted the DC offset to as near to 0mV as I can on both channels. The amplifier has a "slot in" type circuit board that has two trimmers for the offset on the STK packs. I could try swapping the STKs and I'll check along the signal path for any problems.



I noticed that too... there is a pattern there but I'm not sure what it points to. I did test it with a square wave and I get an overshoot on the rise but normal on the fall.
Click to expand...
BKtheking might know how to interpret that scope reading. He's pretty smart.
 
Post pic of suspect STK. It's very easy to tell if it's fake or not just by the exterior.

Littlediode does sell fake STKs (not exclusively), but they appear to have shills around here which will aggressively claim otherwise.
 
Just swap first. The voltage amp might be the problem, also. Anyway a lot of schematics using this STK 1050 amp employ small caps between pin 6 and 9 , as well as 2 and 5.
I have no idea how much this would interfere with the voltage amp in this one (those voltage amps are failure-prone also)

Fake? Probably not-fake does not exist anymore.
You simply need a good fake.
 
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I had thought it might be something to do with oscillation. I read a post by EW when he was doing a rebuild on an SX-1980 (I think?) and the amp showed a similar overshoot on the square wave which he solved with a small capacitor. Maybe that would go between 6+9 and 2+5? I'll post a pic of the STK pack and try swapping them. The voltage amp is a IC too... I'll do some testing and get some photographs and come back. Thanks for the help so far everyone - very much appreciated! :)
 
yes the caps are in between base of first transistor and collector of second transistor of the darlingtons inside. Just g??gle specs of the STK, and if you just select pictures searching this STK you see all kinds of schematics. 180 pf seems a common value used.
The scope picture shows ringing of not that high a frequency....

Anyway after swapping the STK you will know if the new STK is bad or prone to oscillations in this very circuit, and otherwise you might have found a reason the old one failed.....
 
i agree ..swap the packs and see what happens .
or remove suspect and have lots of fun measuring voltages that will be wrong anyway .
 
I checked Pins 0 and 1 on the bad STK and had that small sinewave with no signal present. I pulled the "new" STK and put the good one from the other channel in and it produced a perfect sinewave on testing and no ringing or oscillation on pins 0 and 1. So I guess that means the STK is bad?
 
That's a shame... but at least the rest of the circuit checks out good. Is there any trusted sellers of these or even a way of making one? I've seen some threads about making them...

EDIT forgot to add pictures. The one with the green "0" is the original and the other is the "new" one. An obvious fake?
 

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