Must be read for v-fet 5650 5550 4650 owners

I recently bought his last nos vfets from a old sony service technician, he sent this interesting bulletin i attached, and also mailed that:

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Sony recommended to solder directly wires to CN 60,61 and 76 on power amplifier board(for TA 5650 but I think it's the same for TA 4650)
CN stands for connector;you have to cut wires from connectors and sold directly each wire at respective place on the board. This countermeasure prevents intermittent problem with protection relay and poor contacts affecting bias voltage on this board. Correct value for DC bias is 40 to 50 mv.
Also,be careful with Q311 or Q361 (excess leakage current beetween Emitter and collector )if relay doesn't switch on correctly after 4 or 5 seconds.
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really interesting

mesut

Thank you. Good info.

Take good care of those NOS VFETs for future generations.

As I recall, there were quite a number of Service Bulletins from Sony on the old VFET amps in their first and second generations as they worked out the complexity of these amps. I believe they discovered that many of the then-standard manufacturing techniques didn't apply to amps with these devices in them.

Simiarly, local warranty and service companies really didn't understand them - and Sony really didn't provide much info about them - until it was clear that many amps were not being dealt with properly.

The good news is that not many of these were actually sold, I suspect, since they were quite expensive in their day. There wasn't that much difference in price from a McIntosh at the time.

Perhaps there's a reason that Sony abandoned the VFET......

Cheers,

David

I have just looked inside my 5650 to see its state and got even more confused.

According to modification list:
-r421 power supply board resistor is correct
-c305,c355 current regulator circuitry caps are correct
-c501 spk protection circuitry cap too is correct, but r503 resistor just here in addition to all other resistors mentioned at other places are incorrect.
-At the mentioned poweramp board sockets, the big main cn60 is a socketed one thus being incorrect, cn61, cn76 are neither soldered nor socketed but handwired to socket pins.


If these modifications were a must, Sony may be might require the units -which are known serial numbered- to service center for free modification. Really few units? And why? I really dont understand why to decrease bias voltage to 45-50mV. To make them run suitably getting rid of heat and sonics? What about new units and new service manuals for bias?(my manual that i have bought is one with wrong values covering aep 500,001 and later / uk 600,001 and later). My 5650 has serial number of 600436 with its state I mentioned above. What is the first number to be correct unit? Are they running with 45mV bias, how about sonics?

The things; vd1221s, socket contacts, bias pots, recapping, input transistor optimisation, spk terminal, input terminals, yes above modifications may be logical but what about changing protection circuitry resistors and bias point?

Are these resistor changes for making vfets always run cold by this new suitable bias condition? Running on a sonically worse but cold point? What do you think of doing this modification?

mesut

Hello Mesut,

thanks for the information !

Can you remember, I posted, that there were different sized heatsinks for the TA-5650 on the market ? Perhaps they found, that the smaller sized heatsinks were not sufficient enough to dissipate the higher idle power !?

And perhaps they didn´t want to confuse the service people with a sentence like "If the amplifier has the smaller heatsink, then please adjust the bias to 45 mV !"

David:

Not every V-Fet amp from SONY had such reliability problems !
The TOTL TA-8650, TAN-8550, TA-N7 (at 8 Ohms) and especially the TA-F7 are known to be very mature, reliable products (typical SONY).

As an V-Fet enthusiast it doesn´t look like good news for me that there were not very many sold.

Hi Unda Maris

Instead of solving the heatsink problem by a modification of replacing it, doing another problematic modification by replacing 12 resistors calculating another bias point for just few units? Seems not so logical, I really wonder about modificated new generaton 5650 units running with 50mV bias. Cold units. This makes them really cold, many people tried to find why their units are cold while everyone says they must run hot. If vfets must run coldly why to produce a vfet running naturally hot.

Echowars, you did many things, what is the bias you adjusted for them, what were resistors? What values of resistors and bias value does your service manual have? What to do looking to what?

Change of sink, replacement of bad 1221s, bad sockets, unworking bias trimpots, change of many things at speaker protection, change of protection circuitry, change of bias, bad phono with sonically worse later-units, what is true at the unit when main source is vinyl at 1978? Is there any service bulletin asking removal of main fuse for reliability of the units? I dont know where to start to replace a thing in the unit. I think this very helpfull person working at sony service center answered my question of why they didnot continue on vfets by silently sending this modification bulletin. Finally a modification of removal of units completely? Are the sonically best units these problematic ones?

I wonder if my speaker will again blown in smoke up when vfets shorts..

Bulletin says "below are listing several components and adjustment values that have been modified from early production". Does this mean newer units are produced with these modified values and please change also ones at early production units too? If newer units still have older values with bigger heatsink then I can find a bigger heatsink rather than changing values of amp unlike Sony did not.

A survey, "Do units with bigger heatsinks have older values"??
Calling Axel for a possible survey...

the nevest vfet amp is 5650 at sony TA line and 5550 too (may be too quick to be sold before well achieved tests on protos). And uses 8 vfets which is a problematic number to divide by half for cost unlike 4650 and unlike 8550 or 8550. 4650 uses enogh chimney system with a secondariliy important well enough heatsink. Why bigger amps with more vfets dont have heat problem to me is due to their 2 x chinmey systems that used for each channels. 1 chimney system is not able to make heat remove from all 8 vfets at 5550 and 5650. So bigger or not, no sink is helpful unless a more effective chimney system is used. So the only thing to do with units are decrasing the bias... My thoght only.. I dont know

Fan under the amp works? anyone tring? or air conditioned room makes the amp cool? little local cheap secod hand conditioner? worths this if solves.

TA-5650 Heatsink

Hello David

I am new here and I own TA-5650 too, can you tell me where can get the heatsink?
Regarding TA-F7, there are one sale in store nearby my home, it is nice with two clock like Yamaha B2 in front, I think I can buy it for just under 400 USD but regrettfully I am in Vietnam

Warm Regards,

Unda Maris said:

Hello Mesut,

thanks for the information !

Can you remember, I posted, that there were different sized heatsinks for the TA-5650 on the market ? Perhaps they found, that the smaller sized heatsinks were not sufficient enough to dissipate the higher idle power !?

And perhaps they didn´t want to confuse the service people with a sentence like "If the amplifier has the smaller heatsink, then please adjust the bias to 45 mV !"

David:

Not every V-Fet amp from SONY had such reliability problems !
The TOTL TA-8650, TAN-8550, TA-N7 (at 8 Ohms) and especially the TA-F7 are known to be very mature, reliable products (typical SONY).

As an V-Fet enthusiast it doesn´t look like good news for me that there were not very many sold.

Click to expand...

Sony TA-5650 and TA-8650 VFet amps.

Hi,
This is a very interesting and useful thread. To add to the confusion here's some data.
I have two of these units, a TA-5650 from 1976 ,a European model bought in Sweden with a serial # 504824 and
a TA-8650 recently bought on the ebay in fairly rough shape, a very early example probably from 1974, with a bunch of zeros in the serial number.
I have the original Service Manual for the TA-5650 ,this states 90mV for the bias. That would cook the amp , so the new number is very welcome.
The TA-5650 would not run cold of that, just nice warm.
I also got the manual in pdf form for the TA-8656. That one is saying in the power amp board schematic 0.1V and on page 12 ,fig. 3.1 125mV.
Obviously both numbers are wrong. I tried them and ended up with 85mV ,but is still to hot. So now I will go down to 50mV.
Thanks for the Service Bulletin and the info.
Miklos

TA5650 Bias & Distortion Facts, Recap

rev10-10-2009/29-3-2011/12-7-2011: small changes

Hi Vfet-lovers,

Still interested in some info and facts (TA5650 Bias & Distortion, Recap) ?? See below!

Also I would like to know your experience with:
a) Recap, audio results? What did you use for recap of
feedback cap C305, 355 (100uF/35V) in power amp?
b) Recap with film capacitors instead of electrolytes in
the signal way of pre and power amp.
c) Other mods?

I did recap and some mods in poweramp & supply board for safety reasons and recap in pre amp for music improvement. The music quality is very much improved! Now exceeding my old tube amp with superb sound quality. No need to correct + 2dB for bass & treble anymore! Especially new feedback caps make remarkable difference.

I also did some measurements concerning required bias versus distortion and overall quality with an oscilloscope as well listening tests with Kef Reference 104ab speakers and Sennheiser headphones. Of course test can be better with expensive equipment, but tests performed does give more knowledge!

Test equipment: Cossor oscilloscope 2 beam 35 MHz, square wave generator/calibrator Cossor 25/250 mV 1 kHz, audio test CD with sinus waves 20Hz up to 20kHz.
Scope channel-1 direct input from source e.g. CD-player; channel-2 via TA5650 (pre and power amp)
By projecting Ch-2 over Ch-1 even small diffences are visible on the scope, can be blown-up 10x extra, and have been regarded as TA5650 distortion because with bypassing TA5650 there are no differences between Ch-1 and 2.

BIAS 0 mV --> Notorious ZERO cross-over distortion is NOT EXISTING at all, probably due to the use of Vfets, only slight distortion (~0.25 % of wave area) at 2/3 amplitude rise with 1, 10, 20 kHz sinus waves. Sound quality reasonable with low & high volumes.
BIAS 20 mV --> no visible distortion with sinus waves, however still slight overshoot needle (~ 4% amplitude; <0.06 % of wave area) with square wave 1kHz. Sound quality good with low & high volumes.

Note: Square wave test is not normal audio but ideal in testing to indicate quality of amp such as step response; is a very severe test; in a lot of amps overshoot or damped step response can be noticed.

BIAS 28 mV --> no overshoot with square wave 1kHz, not any trace of distortion and superb audible sound quality. Dynamics improved.
BIAS 50mV or 90mV --> idem 28 mV.

By establishing the minimum required bias there is in addition to cross-over distorsion a 2nd criterium to be considered! Vfet V/A measurements prove that in the low region (Id <30mA) the Vfet V/A-characteristic is not linear and not equal between different Vfets with the same rank numbers! So bias should be at least 60 mA (equivalent 20mV in R0.33) for the 2 parallel Vfets in the TA5650.

A bias of 30 mV satisfies both above criteria and the TA-5650 would run just nice warm (Thermocouple measurement: Temp-room 24C, Ffets ~55C, heatsink 47C). With 1 hour high power 2x 50w continuous sinus wave output: Ffets ~ 80C, heatsink ~65C.
Note: Max allowable Vfet temp acc. specs: 120C !

So, I made a choice for a bias adjustment of 30 mV (~100 mA in R 0.33 = equivalent of 50mA every Vfet), since higher bias does not improve oscilloscope picture or sound quality. Why Sony originally stated 90 mV (later modified to 50 mV) I don't known, nowhere in the literature I found requirements for operating with very hot Vfets.

Pre Amp distortion?
Situation before recap not known, after recap with film-capacitors in the signal path: No visible attenuation or distortion on scope!!
Not with:
a) tone control out,
b) tone control "in" and high and low attenuation at "0", with as well low and high signal levels!!
This contrary to an earlier tested Denon integrated amp: With tone control “on” no common zero positions to be found for treble and bass without attenuation! Probably therefore they emphasize “direct path”
For reference:
Literature show the human ear can detect differences in treble/bass as low as ~2 dB. This raises the question if the measurement equipment used is sensitive enough for above tests? The answer is YES: + or - 1 step treble or bass attenuation with TA5650, which is according to the TA5650 spec and scope readings the equivalent of +/- 2 db gives a big change in scope picture! Based on this scope picture change, by using the scope scale graduation and 10x magnifier, differences as low as of 0.2 db could be easily noticed with the test arrangement.

Feedback Cap Power amp
Please, any advice is welcome! Originally 100uF electrolyte. Although not in the signal path, suggestions for a better type with small dimensions? Even better, how to get rid of it??

regards,

Realworld

Hello,

I am trying to repair my Sony TA-4650 VFET amp. The right channel bias measures 75mV as per service manual. The left channel bias measures only 10mV. The bias adjusting pot has no effect. The bias pots and diodes have been replaced with the recommended equivalents. At R307 I am only getting very close to zero volts. The schematic says 0.8 volts. At R357 in the right channel I get around 1 volt which seems OK. The 38V voltage rails measure OK. Any ideas what could be wrong? Should I replace the caps etc? The VFETS appear to be working as audio is output from both channels.

My other Sony TA-5650 (new in 1979) works perfectly.

Any help would be appreciated.

Contact EchoWars.

normsp23 said:

Hello,

I am trying to repair my Sony TA-4650 VFET amp. The right channel bias measures 75mV as per service manual. The left channel bias measures only 10mV. The bias adjusting pot has no effect. The bias pots and diodes have been replaced with the recommended equivalents. At R307 I am only getting very close to zero volts. The schematic says 0.8 volts. At R357 in the right channel I get around 1 volt which seems OK. The 38V voltage rails measure OK. Any ideas what could be wrong? Should I replace the caps etc? The VFETS appear to be working as audio is output from both channels.

My other Sony TA-5650 (new in 1979) works perfectly.

Any help would be appreciated.

Click to expand...

Start a new thread please, and describe in detail what you have done (for instance, you say you replaced the 'diodes' [what diodes?] with recommended equivalents...what might those be?).

Old thread, but for correctness sake here's the right idle data for the TA-8650:
http://www.page.sannet.ne.jp/kokada/newpage28.html
25.9mV, it still makes the amp hot in the summer, with an ambient temp ca. 28C.