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Technics SU-V2 - questions

J

Janochytron

Active Member
Hi,

I got this amp not working. The ICs are blown on both channels. I will replace them with a circuit from this thread.
https://www.diyaudio.com/forums/solid-state/319154-build-stk8050-equivalent-11.html

I have the ICs out now and my question is:

Should I get relay click and 0 volts on pins 2 and 3 (where the pins would be if the ic was in) ?

I get 2 volts on both channels. It starts low, relay clicks but then it rises and relay switches back.

As it is identical on both channels I just want to make sure it is not because the ics are out.

Because it's on both channels I suspect something they have in common, but supply voltages look good positives and negatives.
Without the ICs only connection from pin 2 and 3 is to protection circuit and to op amp (tone controls are after main amp).
the tone op amp I changed to a different one, no difference. I have high voltages even without the opamp in.

There are bad voltages on Q315 and Q317 (other channel too). The collectors that would go to pins 16 and 14 and should have 1.9 and -1.4 V have -35V and -39V. The collectors are connected through 2 resistors R361,367 and the trim pot R363 when the IC is out.

This causes other bad voltages on Q301 - Q314. Again, supply voltages are good, resistors are good. Transistors and diodes as well. Is it just that the IC is not there?

Schematics here:

https://www.hifiengine.com/hfe_downloads/index.php?technics/technics_su-v2_schematic.pdf

Thanks

Petr
 
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After looking at it more:

I removed the items in yellow (numbers on picture are for the other channel as this has voltages in the picture but I am looking at odd numbers)- R333 and R335 because I won't need that part fo the synchro circuit anymore.
C311 and R 367 I removed so that collectors of Q315 and Q317 are not connected through anything. Then I get -39V on C Q317 which I would expect, but 0V on C Q315. I would expect it to be similar to Q317 in positive. What puzzles me is the incoming positive voltages:

VOltages should be as follows:

E315 - 41.0
B315 = E313 = E311 = 40.4

But I have all 4 basically the same - E315 is higher but by only 0.1V

E315 - 41.5
B315 = E313 = E311 = 41.4

The resistors are all good. I have swapped the transistors on this odd number channel for substitutes (just in case both channels would actually have same bad transistors) but no change.

On the npicture you can see where the good voltages are - blue, orange are the one bit out as per above and red are bad. It's quite a simple circuit but I can't figure out what's causing the bad voltages.

I have checked connections to ground as well. There must be something silly I am missing

upload_2019-4-6_21-52-26.png
 
Hello Janochytron. Have you succeeded? I got the same amp, but a stk8040 was taken out but yet functional the other channel (now I'm looking for a STK), curiously when the bass knob is not centered the relay pulls off, not because the tone control IC, seems like it is detecting some false overload, today i plugged in an headphone and the bass did not triggered the protection (did not figured out yet), maybe needs some load?
Again, I am interested if you succeeded and how different sounds with these module?
Regards
 
Hi. Yes! When I put in the replacement pack on one channel it worked. It looks like it needs the output stage for dc to be fine and relay to click.
But my own personal story does not end here:
I then connected the other channel to my version of pcb with the same circuit as the bought one. And now I am getting the dim light bulb light up. But it's not the second channel but the first one that worked fine previously. Driving me mad. But it's been a while since I looked at that because I've been away. Hopefully it's just something easy as it was working before. Once I have it tested and confirm it all works fine I can send you the pcb design.
 
Hi,had it parked for ages but just today I am working on finishing this. I have made the two pcbs one was tested already, the other one not yet. I will post pictures once it's all in and working
 
witekkino said:
Hi i managed to complete this mod on my Technics SU-V4A which is very similar to yours, and it work like a charm. Original project is from RCS16 user of Audiokarma you can find all info in this thread: https://audiokarma.org/forums/index.php?threads/stk-0050-replacement-for-sx-780-and-others.721181/
Click to expand...
Awesome!, I'll check it out!
BTW managed to get a stk8040, an Audiokarma user had couple modules, one is dead but the other one okay. Already installed a few days ago, the new module has a bit less power output, but i can live with that, but I preffer your mod for long term servicing!
 
Hi,

it is in and working. Only thing I can't set the bias. Getting 0 V when idle (no load, no signal). I think when I did it the first time with the bought PCB I was able to set it to the 7mV accros one emiter resistor as suggested, but I am not sure I was able to do it.The circuit is identical (and it works) so I don't know:-)
Anyway, it works and sounds nice, will do more testing and listening and update if I find anything else.

IMG_20190721_191018.jpg
 
Yes, in my case i had to turn R7 completely clockwise for 0mV on emiter resistor and then rotate ccw till 7,2mV (i use slightly higher bias current) i use BOM from the first post of the thread that i mentioned above. I posted some pictures on page 37.
 
witekkino said:
Yes, in my case i had to turn R7 completely clockwise for 0mV on emiter resistor and then rotate ccw till 7,2mV (i use slightly higher bias current) i use BOM from the first post of the thread that i mentioned above. I posted some pictures on page 37.
Click to expand...
Hi, thanks. If you have a chance could you measure what resistance you have now set betwen pin 1 and R6? (accros the variable resistor. Thanks
 
Now the amplifier is playing Pink Floyd :music:Netx time i get my hands on it i will measure it. BTW Looking at your photo it looks like you are using 0,11ohm for emitter resistor, it should be 0,22ohm...
 
witekkino said:
Now the amplifier is playing Pink Floyd :music:Netx time i get my hands on it i will measure it. BTW Looking at your photo it looks like you are using 0,11ohm for emitter resistor, it should be 0,22ohm...
Click to expand...
Don't stop your Floyd. I got it. The resistor has so many turns I was just away from the 'zone where the bias was measurable. I dialed it down to around 200 ohms (which took a lot of turns:-) and got some reading and was able to adjust from there then. The emitter resistors are 0.22 - red red silver.
Thanks!
 
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