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Kef Kube for 103/3 rebuild and psu
Kef Kube for 103/3 rebuild and psu
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Old 18th July 2019, 06:07 PM   #1
vishalk is offline vishalk  United Kingdom
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Default Kef Kube for 103/3 rebuild and psu

Hi All

So i received a pair of lovely 103.3 with the kef kube.

As you can imagine they need some servicing. Unfortunately the PSU has seen better days and would like to upgrade it, perhaps use a torodial or rcore transformer instead. The output voltage is 19.1vac at 230vac. This drops down to around 18.4vac when connected to the kube.

I would like to also upgrade the capacitors and opamps. Any suggestions for opamps?

Thanks



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Old 21st July 2019, 12:12 PM   #2
Mark Tillotson is offline Mark Tillotson
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The film caps may be PP already - should be able to check this from the markings.


TL072 opamp circuits are not necessarily going to work with other opamps, the TL072 has relaxed requirements for decoupling and layout, and is ultra-low bias current. Another opamp might require high-speed decoupling or have too large a bias current for the circuitry (would need to analyze the circuit to check this). So some caution needed or things could be worse.
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Old 21st July 2019, 12:32 PM   #3
vishalk is offline vishalk  United Kingdom
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I was just going to replace the electrolytics, going by other threads and forums people have chosen opa2134s as a direct replacements. Maybe this will be OK?
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Old 21st July 2019, 11:00 PM   #4
indianajo is offline indianajo  United States
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TL072 has a minimum slew rate of 5 mhz. OPA2134 has a minimum of 15. This indicates a oscillation risk.
To correct the oscillation risk one usually puts a power supply bypass disk .1 uf cap near the op amp, and bypasses the feedback resistor with ~33 pf disk cap. This appears to be doable on this board. The large planes of conductive copper near the op amps may be the +and - supplies. In that case you could drill a #48 hole in each near each op amp and solder in a bypass cap. Make sure it is rated higher than 50 v, say 100 v since supplies might be +-15. You could also tack the 33 pf on top of each leaded feedback resistor, after you find it. Goes from output to minus input.
I did this on a mixer that oscillated after I upgraded from NJM4558 to ST33078, and got rid of the problem. Also decreased output hiss by maybe 10 db or more.
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Old 22nd July 2019, 07:14 AM   #5
vishalk is offline vishalk  United Kingdom
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I was hoping it would just be a direct swap, unfortunately finding a circuit diagram for this is impossible.



Hopefully there is no oscillation but if there is I will do as you specify to correct this, I will defiantly need to get more help with the mods!
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