Sony VFET TA-4650 Restauration, Part 2

IMG_0627

TA-4650 Power Supply Problems

After recapping the boards,  I started powering up the amp,  first without VFETs.
+20 and +78 Volt outputs, as well the +/- 43Volt rails for the VFETS.
However the +/- 73 Volts for the BIAS were not symmetrical.  Voltage shifted to one side…, e.g. it became +100 / -60.

I found also others reporting in forums, who had the same problem: unsymmetrical bias-voltage.

…and solutions

After long searching and testing in wrong places, I found following:
The voltage doubler in the Sony power supply is a charge pump. It is not regulated. Any non-symmetrical load on it’s +/- outputs results in the observed voltage shifts.

I suspected a fault first, but confirmed through circuit simulation in Spice: It happens also in the simulation – so it is ‘feature’ of the supply.

Two questions arose:

  • What in the pre-amp board causes the not-symmetrical load ?
  • Can I change the supply to a regulated one – which would be also safer for the survival of VFETs during potential next 30 years of life of this amp…

Connectors

I searched some time to find good and matching replacement connectors for the on-board, partially oxidized 1970ies connectors.  I found a good replacement: Hirose DF5, which is a drop-in replacement due to it’s 5mm spacing.

IMG_0699

TA-4650 non symmetrical load

Started to investigate why there is non symmetrical load on the +/- bias voltage supply, which causes the sensitive power supply to derail. My suspect where the bias transistors Q307-Q310 (channel 1) and Q357-Q360 (channel 2).

Removed, measured all of them:

transistors1

Channel 2 seems quite ok. If you look at the amps schematic: Q357 and Q358 take care of upper half, connected to +75V, while  Q359 and Q360 take care of lower half, connected to -75V.
Both current and Hfe do match very nice:
Upper half: Q357+Q358  = 5,18 + 6,91 = 12,15
Lower half: Q359+Q360 = -7,77 – 4,68 = -12,45
But in channel 1 there are more significant differences in Ic and Hfe.

At the moment I think that this could be the cause of load differences on positive and negative rail of the power supply. I will replace the original transistors with new, matched replacement types…let’s see how it goes….

Result

Tested my changed transistors today.  As a result the initial symmetry of the supplies +/-75Volt is much better:  It only changes much slower to unsymmetry as before…
which could mean it takes a few minutes longer now until the VFET’s get destroyed…..(if I had them in the amp….which I haven’t, yet).

 

 

4 thoughts on “Sony VFET TA-4650 Restauration, Part 2

  1. Patrick KOMAN

    Hello,
    Yes, the poor symmetry in the +/-75V is due to the difference in load off the positive and negative rail.
    I had exactly the same trouble on a TA 5650 few years ago. Poor matching in the transistors is one point but what must be also taken in account is the load due to the VETs. But put the VFETs back if something is wrong with the biasing voltage is not really a solution.
    So I finally tried this configuration :
    – on the left channel : put a 2SJ18 on his location and instead of the 2SK60 connect a resistor of circa 220 ohms / 20W between his source and drain. In this configuration there is no risk for the 2SJ18 because even if the 2S18 is not well polarized the current will be limited by the 220 ohms resistor to 0.4A.
    – on the right channel : put a 2SK60 on his location and instead of the 2SJ18 connect a resistor of circa 220 ohms / 20W between his source and drain.

    In this situation the load of the biasing power supply is symmetrical and the feedback loop of the amplifier is closed. Now the voltage and the symmetrie of the +/-75V power supply can be checked and also the proper fonctioning of the bias adjustement. An action on the bias potentiometre must change the Vgs voltage of the installed VFET.
    Also don’t forget to replace the diodes D303 & D362 (VD1221) by two 1N4148 in serie.
    Those diode are usually responsible of the VFET’s failure .

    Reply
    1. Andrew Hess

      I have only 2 and 3 watt 820 Ohm resistors on me. Will those work? I’m in this same situation with a TA-4650.

      Reply
    2. Andrew Hess

      I have only 2 and 3 watt 820 Ohm resistors on hand. Will those work? I am in the same situation working on my TA-4650.

      Reply

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