Sony TA4650 Vfet -+75v problem - diyAudio
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Old 14th July 2013, 12:49 PM   #1
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Default Sony TA4650 Vfet -+75v problem

Children at home did some bad thing with my amplifier, a book was on it, so overheated and all Vfets out.
I found news Vfets bur put them back.
I read all the threads, change caps and diodes, and get again +- 43v, 20v, 90v but the -+75v are bad, 85/90v and are changing continuously.
An idea ?
I was thinking that I must get +-75 rails and -+ 54/59 on vfet cutoff command without the Vfets ?

Instead I got a floatind value of about 60to70v (not stable)for cut off command , when I turn the bias potenitometers.

I put 2a fast fuse on the 43v rail in case of...

Thank you for your help.

Kind regards from France.
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Old 14th July 2013, 04:42 PM   #2
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I found a bad cap, so now I have +74.5 and -76v stable but an idee to have -75 and +75 ?

Gilles.
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Old 14th July 2013, 06:15 PM   #3
ilimzn is offline ilimzn  Croatia
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Location: Zagreb
The amp has protection resistors at the output of the +-75V power supply. The supplu is not regulated, only the +90V is, so it will be slightly higher than +76V on a 230V mains voltage.

When VFETs fail short (not often - mostly the gate line will go open preventing control of the VFET), the charge pumps used to get +-75V will polarize the 47uF capc in reverse resulting in their destruction. Change ALL of them. Also, change the series resistors, you will probably notice they will be slightly burned, but often when there was failure on both channels, they will go open or intermittent without burning.
Before putting in new VFETs it's possible to check the bias voltages without supplying a signal to the amp. The bias should be set to minimum current (maximum negative bias voltage) before installing new VFETs anyway. MEasuring the voltage Vgs will then show usually at least -20V, which is OK. Less than that means you have a problem with the driver circuit on the main amplifier board. More than about -30 or close to -40V also means a problem.
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Old 14th July 2013, 06:26 PM   #4
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Default Vfet

Thank you this helps.

Gilles:-)
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