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Service info for TECHNICS SU-Z1 (Parts List)

audio_exp

audio_exp

Member
Hello, I'm working on one of these little units and can't seem to find a "Real" service manual with a parts list.
The only info I can find is a basic schematic but it does not have Capacitor, Diode or Resistor values?
Thanks
 
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The schematic on hifiengine looks complete. Unfortunately there are a few holes. Take note that R313=3.9k, R403=470, R401=120k (below D403). As usual, the values for the right channel are the same as the ones for the left channel.
 
When I got the unit the STK2028 was dead, I replaced and now have audio but I have a strange issue with the unit.
When I turn the treble control up or volume up it distorts and draws a lot of current? Some kind of feedback issue on both channels.
Sounds somewhat OK at low volume.
Anything I should look for?
 
It is possible that the amplifier self oscillates when he get out of clipping. It is easy to see this if you feed the amp with a 1kHz triangular signal, drive the amp at clipping (no load at the output) and observe the output signal at pins 3 and 14 of the STK2028. If you see oscillation, you might increase C329 and C330 to something like 47pF or 56pF.
 
ecluser said:
It is possible that the amplifier self oscillates when he get out of clipping. It is easy to see this if you feed the amp with a 1kHz triangular signal, drive the amp at clipping (no load at the output) and observe the output signal at pins 3 and 14 of the STK2028. If you see oscillation, you might increase C329 and C330 to something like 47pF or 56pF.
Click to expand...

OK I don't have any pF caps on hand so I'll have to order some.

Here are the amplifier symptoms:

1. When you first power up with volume at 0 and no input the meters go full scale and after 2 seconds they fall to back to zero.

2. With 8 ohm load, No input, tone controls flat, loudness switch off when you raise the volume control to about 2-3 it starts to squeal, meters go full scale and draws lots of current.
Definitely some kind of feedback/oscillation type problem and it is more sensitive to using the treble control.

I do have it on a current limited supply and only have it on for a few seconds for testing purposes as I don’t want to fry the new STK2028!

Could it be any diodes or small transistors, anything else I can look for while I wait for the capacitors?
 
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