Technics SU-V2X - faulty stk2038-4 ?

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woodyp

Member
Joined 2020
#1
technics suv2x repair


Hello
I am trying to fix a technics su-v2x. The issue is that the amp when switched on, has the 'safety operation' light blinking. It blinks slowly on switch on (as it seems it should) then progresses to rapid blinking (which i gather is an error condition)

service manual:
Technics SU-V2X - Manual - Stereo Integrated Amplifier - HiFi Engine


Examining the service manual, the flashing is directly driven by the IC601 MN1404STE chip. I can see a a LOW (0.112v) signal on pin 8 causing the pin 11 to output a square wave driving the LED.

So i suspected the STK2038-4 (IV) stereo power amp chip. I am not able to source any datasheets for this chip unfortunately.
I decided to check for obvious shorts, but I couldnt find any except some legs that are tied to GND or the -ve voltage.
I only noticed one anomaly, on leg 14.

this a table of measurements : BOLD is where is see some noteworthy deviation


Pin, Service manual, Measured (vs GND)
1 1.8 1.95

2 43 45
3 0 0
4 -43.5 -45
5 -1.8 -1.85
6 0 0
7 0 0
8 0 0
9 0 0
10 0 0
11 0 0
12 -1.8 +0.1
13 -43.5 -45
14 0 +43

15 43.5 45

16 1.8 +0.13

Pin 14 looks the oddest, and I am not completely sure what that is. I am guessing it is the Right output channel?

I traced it onward to Q506 and i checked this transistor out of ckt and its fine. D504 is also fine.
So at this point I am not sure if the STK is the culprit, or something else. Any pointers on how to test the STK2038-4 and to be sure its at fault?

Many thanks
 
Last edited:
#2
I realised there was a jumper from Pin 14 onwards, which i desoldered leaving one channel output disconnected.
Success! well atleast a partial one. The safety warning doesn't come any more, and the amp plays through only one channel (the left).

So now the question is, is the power STK the problem for sure? and how can i source a replacement?
 
#4
thank you for checking
I am wary of aliexpress/ebay because I am not sure those advertised are genuine. Would gamble on one, except that something i ordered in March hasn't arrived yet.

But How can one be fully sure teh STK module is infact the offending party here?
 
#5
Update --- not very positive.

if you recall the left channel was working fine after the right output was disconnected. I actaully was impressed by the sound.

I was still puzzling over whether the issue was upstream of the STK, so i was scoping out some input/output waveforms. I did notice that the output for the right channel from the AN7062 has a greater DC offset than the left channel.
But in the course of this investigation, I accidentally shorted pins 3 and 4 on the STK. i immediately powered off. But powering back on, i noticed one difference that pin 5 now had +7v whereas it was (correectly) 0 before.
The sound still played, but it sounded very ragged, and almost felt like the sound was cutting out intermittently. I checked if it was a speaker issue - it wasnt.

After a while, the right output (pin 3) also now shows +5v and so the dc protection kicks in and im back at square one with the dc protection.

SO now i am waiting for a replacement stk2038 to arrive, and i still am not sure if that was the original issue.
 
#6
Woodyp, did you ever get a replacement stk2038? Did it work?

I just got myself a dead one of these amps with a dead short between pins 2 & 3, pin 4 burnt off just below the module with a dead short between pin 2 & 4 (from plus rail to negative rail).

I have ordered a new module but will it be a real one?

The rest of the amp seems okay and just got to wait for the STK2038-4 to turn up (from the USA).
 
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