Vintage Sony TA-4650 VFET Amp Restoration and Diode Mods (Part 2)

[@noagospodin8037]

Hi, to reduce the DC offset on the speaker outputs, you need to match the diferential pair of transistors on each channel (q302/303, and q352/353). Match them closely, by HFE. You can use BC556B (beware of the different pinout!). Also, it is a good idea to thermaly couple the pair together, to have a steady DC offset. Please, remove the V-Fets when testing the new pairs (you can also measure the DC offset without the Vfets in), and once everything is verified to be fine, reset the bias, and set it to 50mV. Sony issued a service bulletin that describes this, and some changes to the resistor/capacitor values, for all Vfet models. You can find it by searching online. Hope it helps!

[@jonlaws4493]

Set the bias to 45mV on the TA-4650, 50mV is for the TA-5650.

[@roerdomp16e]

In one of the later service bulletins produced by Sony, they have changed the prescribed bias voltage to 45 mV. To get a better result in the DC offset you can also use 2 matched CT212L or 2SA1015 which have the same pinout as the original 2SA705. And I agree with Noa Gospodin... check the service bulletins for prescribed changes in the caps and resistors on the power amp board. And check resistors R365/15 and R373/23. They are normally toast. I change them with 1/2 watt resistors with the same resistance.

[@jonlaws4493]

You can use brass stand-off to extend the original post supporting the PSU board. 3mm thread kit with a few different heights will get you there.

[@vishwanathmirgal8374]

Thank You

[@adaboy4z]

I must have an older 4650 because the speaker relay switch is vertical and where you have plugs I have wire wraps? Also you have a capacitor your power supply board that is absent on mine. In its place is a transistor.

[@2008alde]

I just have a simple question if that exists. I have a Sony TA-4650 which until recently performed nearly flawlessly. The other night I wanted to play an LP and within 30, the speakers cut out. the unit recycled and then I played something through the AUX input and it was fine. Went back again to the phono selcetion and BANG.. speakers went out again. If I have anything connected to the phono input but dont feed it an active signal, like playing an lp, I can hear from one channel that fluttering sound. Could I have damaged the phono stage and is that something that could be repaired?

[@erbalumkan369]

When you do the voltage measurements on the pins of the vfets (vfets not yet inserted), what do you use as reference? Chassis ground or speaker ground?

[@75drdoug]

Tested the dc offset before starting, channel that was original had +4mvdc and channel that had been serviced, -36mvdc. My guess is some parts were electrically overstressed when the channel blew. Thanks for the info on matching the HFE of the transistors and checking resistors, I will certainly do that. Does anyone know what parts may have been damaged do to the electrical overstress? I have had this same issue on other amplifiers where one channel was blown, higher dc offset after repair.

[@bradelias9292]

Could you share your parts list?

[@ismaelnavarro9589]

Saludos, me puede guiar donde comprar el par 2sj18/2sk60?