• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Sony TA-4650 Restoration

39cross said:
OK, I have to ask what is probably a dumb question. I came across this thread as I've been reading up on the 5650 et al. - just picked one up off of CL. Works fine although the pots and switches need cleaning. However after reading all the things that an go wrong it's beginning to make me feel paranoid about turning it on without making the VD1212 update etc.

So my question is, I am assuming you ground the tied together diodes as marked below by the black line (photo from ahess!). But if I was 100% confident I wouldn't ask, but it's given these VFETS are like gold
.View attachment 764116
Click to expand...

Sounds like you are are confusing two separate diode issues. Posts #113 and #118 is to address a specific diode issue in the TA-4650 where a specific diode should be replaced with two diodes in series but grounded in the center as in the pic. EchoWar's diagram in post #127 shows how to make the same mod for the TA-5650.

The completely separate issue is the VD1212 diodes. You do NOT add a center ground with the two 1N4148 diodes that are wired in series to replace VD1212 "suicide" diodes.

I suggest you start a separate thread for your 5650 refurb. Don't be hesitant to ask a basic question. Everyone here started at square one at some point and everyone understands that if you make a bad assumption it could fry a nice Vfet amp, which everyone wants to prevent.
 
Last edited:
Sorry for any confusion, I thought my question was clear but it wasn't. I am referring to the issue addressed in posts #113 and #118, the two diodes in series but grounded in the center.

If I have any questions during my restore I'll start a new thread.
 
39cross said:
..... However after reading all the things that an go wrong it's beginning to make me feel paranoid about turning it on without making the VD1212 update etc.

....
Click to expand...
Well then I probably should have just said that your paranoia is not paranoia because it is justified and left it at that.
 
joshvi said:
Hi Andrew Hess,

Yes ,Now you can put vfet back and adjust the bias to 65-75 mV across R333 or R334 with no input signal after 1-2 miniute warmup . Please refer the service manual page no.4 and 5.

Regards
joshvi
Click to expand...

I realize, this an old thread but is this measurement taken with a dummy load or without any load? I am about to check my tan 5550's but live in fear at destroying something. Thanks
 
Hi all - I realise that this is an old thread - but it seems like the best compendium that I have found for this amp, and I want to ensure I do things correctly. Note that for background, I have built a coupe of VFET kit amplifiers, but never worked on a stock unit original unit containing VFETs.

I have an old 4650 and it has the old diodes installed. I want to replace these and check that all is running fine with this amp, voltage & bias-wise. Would the basic process be like this?:

- Check the bias running to the VFETs before doing any work on it - note that section 2.2 of the SM refers to setting bias - however this seems in actuality to be setting DC offset at the speaker terminals - its a bit ambigious but me reading of this is to ensure that mV readings here shoud be less than 75mV. Am I wrong here? If someone could confirm this basic point for me, that woud be great.

- Remove the VFETs (after confirming the above). Measure across S and G of the empty VFET terminals to ensure that I have between 8-18 volts.

- As per the SB, replace resistors R313, 314, 363, 364 from the current 1.5kohm to 3.3kohm.

- Replace the 3 x death diodes D302, 352 & 414 with 2 x 1N4148 in series each.

- Add another suitable rectifier diode in series with D403 and tie the centre of where these 2 diodes join (to amplifier ground).

- Replace trimmer pots R301 & 351 with new 2.2k modern types.

- Fire the amp up with VFETs still removed. Check that I have 20V at test point by adjusting RT401.

- With the VFETs still removed, turn the new R301 & R351 to maximum voltage (measure across S and G of the empty VFET terminals to ensure that I have between 8-18 volts).

- Reinsert the VFETs and set the bias to proper levels.

As you may be able to tell - there is some vagueness for me about setting the bias to required levels. Its a bit annoying that the SM is soo vague about this.

Any guidance on this aspect - or any aspect of my proposed project - would be welcomed.

Cheers
 
Last edited:
slimecity said:
Bump
Click to expand...
Probably to late, but still....
First step for me would be to pull the v-fets and test them as per Sony service bulletin no. 62, confirming they are ok....
After that all the rest that should be done....
 

Attachments

  • IMG_20200329_142237.jpg
    IMG_20200329_142237.jpg
    45.1 KB · Views: 44
Hello I just bought a Ta-4650 It works. but no mods yet lol.

1 Initially on a cold start up there’s no audio at all. And then a 15 seconds or so you can get some output. (the speaker rely might be going out )

2 The left channel only has a time interval popping or motor-boating only on low Hz notes (change the source change the popping / motor-boating ) sound like a discharge capacitor and stops after few mins and the audio is normal. (I don’t think it’s in the power supply side because it’s only one channel and because the interval is a few seconds between each pop I believe it’s maybe in the preamp section because I don’t believe it’s volume sensitive) Would a Full cap replacement fix it?

3 The speaker selector will not play audio through the A+B and B Position it will play on the A only. (i got nothing)

Any initial thoughts, the amp is in working order but definitely has a few gremlins and there.
 
I recently did a restoration on a ta-4650. Look for my thread. It has a list of all parts I used. Sounds fantastic.
 
I’m not a happy camper. I was very close to placing about a $200 order with Mouser and Digikey. No I’m gonna go try to get my money back (local record store).
 
You must log in or register to reply here.
Back
Top Bottom