technics SU Z45

b0ssmanracing · 8th March 2010, 09:24 PM

Can someone please help. i have inherited a technics stereo system from my grandparents. it all works well except the amplifier, (SU-Z45). when you adjust the volume, the levels jump up and down, and sometimes only one speaker at a time. if i fiddle with the control i can get it right, and it sounds amazing. is this the lack of use? is it a dust issue? please offer some advice.
it would be appreciated.

nigelwright7557 · 8th March 2010, 10:28 PM

I would preplace the pots.

Some will suggest contact cleaner but I would recommend replacement.

jaycee · 9th March 2010, 11:03 AM

Try contact cleaner in the volume pot first. It might simply be dust rather than wear.
A small warning - don't drive this thing too hard. It uses an STK2038 output module, and these have a habit of going pop. My own Technics SU-Z55 (similar internal circuit) did the same thing.

wahab · 9th March 2010, 11:16 AM

I did find a Technics SUZ25 (among others amps) recently at the local scrapyard,
and it doesn t work as well..
Does someone have a schematic of a nearby model?..
It use a high voltage IC containing the differential.
There s external components for the Vas and the output stage
devices.
Anyway, a bizarre design...

jaycee · 9th March 2010, 11:24 AM

It is probably AN7062 connected to STK2038... I think most of the SU-Z's had this arrangement.

I've got an extremely bad copy of the SU-Z55 schematic. Check for small signal transistors whose bases are connected to pins 8 and 11 of the AN7062 - they can die, unfortunately that usually kills the STK2038. They will probably be 2SA992.

Pin 10 should have about -15v on it, the power supply for this in the Z55 is dreadful - just a resistor and zener diode.

I didn't bother repairing mine - it cost me nothing anyway. Instead I used the parts and built an LM3886 based amp into the case. Later I scrapped it

wahab · 9th March 2010, 12:05 PM

Thank you jaycee.
Yes, the circuit is surely the same, apart from the lower voltage
component in the ZUS25..
Ic is a AN7060 and power output device is a STK2028.

Bad thing is that this kind of amp has the tone control network
inserted in the feedback loop of the power amp, a bad habit
to spare some costs, so using a gainclone, you ll have to build
a passive baxandall , as the pots are antilogarithmic ones...
That s a poor design globally, and as you mention it, it doesn t
deserve to be repaired.

Funny thing is that these two last years, i did canibalize far more amps
than i did repair them !!...
By there, they are massively got rid off at the scrapeyard..

8th March 2010, 09:24 PM	#1
b0ssmanracing diyAudio Member Join Date: Mar 2010	technics SU Z45 Can someone please help. i have inherited a technics stereo system from my grandparents. it all works well except the amplifier, (SU-Z45). when you adjust the volume, the levels jump up and down, and sometimes only one speaker at a time. if i fiddle with the control i can get it right, and it sounds amazing. is this the lack of use? is it a dust issue? please offer some advice. it would be appreciated.

8th March 2010, 10:28 PM	#2
nigelwright7557 diyAudio Member Join Date: Apr 2008 Location: Carlisle, England	I would preplace the pots. Some will suggest contact cleaner but I would recommend replacement. __________________ http://www.murtonpikesystems.co.uk PCBCAD40 pcb design software.

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9th March 2010, 11:03 AM	#3
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	Try contact cleaner in the volume pot first. It might simply be dust rather than wear. A small warning - don't drive this thing too hard. It uses an STK2038 output module, and these have a habit of going pop. My own Technics SU-Z55 (similar internal circuit) did the same thing.

9th March 2010, 11:16 AM	#4
wahab diyAudio Member Join Date: Nov 2009 Location: algeria/france	I did find a Technics SUZ25 (among others amps) recently at the local scrapyard, and it doesn t work as well.. Does someone have a schematic of a nearby model?.. It use a high voltage IC containing the differential. There s external components for the Vas and the output stage devices. Anyway, a bizarre design...

9th March 2010, 12:05 PM	#6
wahab diyAudio Member Join Date: Nov 2009 Location: algeria/france	Thank you jaycee. Yes, the circuit is surely the same, apart from the lower voltage component in the ZUS25.. Ic is a AN7060 and power output device is a STK2028. Bad thing is that this kind of amp has the tone control network inserted in the feedback loop of the power amp, a bad habit to spare some costs, so using a gainclone, you ll have to build a passive baxandall , as the pots are antilogarithmic ones... That s a poor design globally, and as you mention it, it doesn t deserve to be repaired. Funny thing is that these two last years, i did canibalize far more amps than i did repair them !!... By there, they are massively got rid off at the scrapeyard..

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