Sony VFET TA-4650 Restauration, Part 2

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.

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:

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).

 

 

Sony VFET TA-4650 Restoration

Just obtained this nice vintage amp from an ebay auction.  It’s has rare VFET Transistors which were produced only in the 70ies and have triode like sound and properties. I am going to restore it and report on the progress.

These old Sony amps had some design flaws, which often caused destruction of the VFET transistors. The transistors were only produced during a very short time in the 70ies and are ‘unobtainium’ nowadays. After I got my amp on ebay, the first  thing I did, was to test, whether the precious 2sk60 and 2sj18 VFETs are still functional.

On Sony VFET models TA-4650, TA-5650 and similar the VFETs are easily accessible on the bottom of the amp:

I tested all VFETs according to the Sony Service Bulletin No, 62, and all were good. Lucky me !

To test yours, if you have such an amp, dusty sitting in a corner, or just got off from ebay, here are the testing instructions with a Multimeter:

Curve tracer measurements

I measured the 2sk60 and 2sj18 on a curve tracer to see how well they are matched. The previous owner reported that the right channel of the amp sounds a bit ‘soft’.

The 2sk60 are very well matched, but the 2sj18 show about 1 Volt difference in Vgs for the same current.  I am not sure, but this could be one reason why the two channels may sound slightly different.

New Capacitors

Received the caps I ordered.  I will replace all electrolytic caps in the amp with new ones. While some of the smaller caps in the power supply still measured good (despite being 40 years old), it’s better to replace them.  The large screw terminal Nichicons in the power supply are not available anymore.  Capacitors with same values which are close in diameter, height and screw-terminal distance are the Vishay BC 4700uF 63V (No. 222211418472).

The BC caps are 5mm less in diameter, but I wrapped some foam around them, and they stick quite nicely in the 40mm cap holder.  Also the screw terminal is slightly narrower than on the original (unobtainable) Nichicons. I enlarged the holes on the PCB slighly with a 6mm drill. That was enough to fit the new BC caps.  Other caps were replaced as well.

Potentiometers are also often a source of failure. The original pot’s were replaced by 2.2K Ohm multiturn pots.

I have disconnected (unsoldered) the terminals to the other boards, since  I wanted to test the power supply first standalone…

All voltages were ok, but higher than the values in the service manual:

+/- 43 Volt was  +/- 47 Volt
+/- 75 Volt was +/- 95 Volt
78V (to phono) was 95 Volt
20 Volt, was ok at 20 Volt (adjustable)

The higher voltage is due to two factors: In Germany mains electricity is 230V AC nowadays, and the amp is set to 220V AC.   Secondly the power supply is running without load. When adding a load the output voltage of the transformer will drop a bit.

I’ll ‘jumper’ the Sony to use 240Volts input before the next steps…

Cleaning contacts

Started cleaning contacts, here the headphone and speaker selection board. Contacts on he Alps rotary encoder are black after 40 years. The board:

I cleaned contacts with a small glass-fiber stick and isopropanol. Time-consuming, but worth it. After cleaning, tested all contacts with a multi-meter.

On the same board, two resistors that feed the headphone jack were burned.  I replaced them with 470R 2Watt types…

Also the relais was replaced with a new one with good contacts. Replacement type is Omron MY4-02 24V DC

Also the headphone jac was disassembled, contacts cleaned, re-assembled.

New relais and contacts after cleaning :