Vintage Sony TA-4650 VFET Amp Restoration and Diode Mods (Part 1)

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Parts list: docs.google.com/spreadsheets/...
PSU diode mod explanation and schematic: audiokarma.org/forums/index.p...
Service manual: www.hifiengine.com/manual_lib...
Some interesting VFET/SIT information: www.firstwatt.com/pdf/art_sony...

[@Jaymz87]

Believe it or not I actually picked up one of these off the side of the road on a verge collection, the funny part was it was in decent condition with the original vets inside. I gave it a little clean up recapped majority of the unit set the bias according to the service bulletin and it's been my daily unit for a year or so now! Absolute gem!

[@djamellarbi9830]

im a new subscriber and i really like you video keep it going im sure you will hit 20k in no time

[@adaboy4z]

What size lamp is the power button?

[@kenkworkshop]

Very helpful, and enjoyed the video. I'm about to start work on a TAN-5550, which I believe is similar, though just the power amp. With the double diode, is my assumption right that the cathode remains the striped side of the pair when replacing the VD1221?

[@Roudaki677]

Can you please list what brand/model soldering iron you are using in this video? Thanks in advance

[@75drdoug]

I'm in the process of doing my 4650, but this one has the power supply and connector modifications already done. Dual diodes are original, so those are my first priority along with the diode modification mentioned here (two 1n4004's to replace D403 with the junction of those added diodes to ground) to prevent drifting of the +/- 75v supply. The fixed bias resistors of 1.5k have already been replaced with 3.3k units. Is there anything else that is essential that needs to be done? Thanks in advance.

[@jonlaws4493]

Check the offset, if it is over 20mv then change the input transistors with matched hfe pairs.

[@ruibarbosa7221]

You should replace 4 resistors (R313, R314, R363, R364) in the power amp board as recommended in the service manual. Replace it with 3.3kOhm and adjust bias to 45mv! The current value of 75mv is to high!!! Check HiFi Engine for the service manual, it's the one in dutch.

[@messingwithelectronics7462]

I saw that bulletin, but it seemed like the majority of people didn't bother and haven't had issues. It's probably better safe than sorry though, so I'll put the new resistors in with my parts order. Regarding the bias, I probably should lower it. I was concerned that going from 75mv to 45mv might impact the SQ, but it seems like people have tested it with no problems. Luckily, I'm using the amp in a relatively cold room, so my heatsink temps have been reasonable.

[@ruibarbosa7221]

I got the same amp, but mine is a later production version. It has all those modifications from factory. New resistors, new diodes, and there's another modification that I never saw mentioned anywhere. In the power supply, the wires going to the Main Amp board, they replace the connectors with wire wrapping connections on both end. All the other connections still use connectors. I'm guessing that Sony figure out that in case of a bad contact from one of the plugs, it could take out the vfets. Anyway I think you should do the resistor mod and lower the bias. Just to be safe.

[@messingwithelectronics7462]

@@ruibarbosa7221 That's great to know, I'll definitely be doing the mods if they were shipping like that. Connector change makes sense too, that's one of the reasons I started it up without the VFETs first -- wasn't sure all the connectors were making good contact.

[@erbalumkan369]

Be careful when you change the power line voltage setting with the multi-plug. Two female pins broke off internally when i tried this. The pins are very weak design. I managed to solder the broken parts onto the pins again.