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-   -   Sony TA-F6B PSU repair (http://www.diyaudio.com/forums/solid-state/181121-sony-ta-f6b-psu-repair.html)

ilimzn 22nd January 2011 10:04 AM

Mooly, there would be no sense in putting a linear regulator in front, you missed the connection of winding 7 from the chopper transformer, and the purpose of the inductors around the pre-regulator.
This setup is similar to all Sony PLPS amps, in that it has a buck pre-regulator in front of a power oscillator. So, it rectifies the input, pre-regulates it to a stable voltage, and the second stage self-oscillating chopper provides sonversion to voltages as needed by the amp as well as galvanic isolation. All the regulation is done on the primary side by the pre-regulator, after that the whole thing is unregulated to the output. Winding 7 is used to synch the pre-regulator and the chopper so that you don't get beat frequency problems with a separately oscillating pre-regulator. The pre-regulator starts in a pseudo-linear mode so the chopper gets power to start oscillating, at which point the pre-reg goes into buck regulator mode. The weak point in this setup is the possibility the chopper does not oscillate properly, at which point not only does the chopper dissipate much more than designed for, but also the pre-regulator might fail to exit linear mode, resulting in overheating of it's pass transistors and failure.

Having a higher regulated primary side voltage requires more windings on the chopper as the output voltage is directly determined by the winding ratio - the chopper transformer is really just a HF transformer.

Regarding two different models, it seems illogical but is down to rules and regulations. Certification is (was?) quite different for US only and international models. Seemingly inconsequent these days when everything seems to have IEC plugs and sockets, the power wire and connection tot he amp was different, for instance the german model had an IEC cable, while this required completely different certificates for the US where such devices were not supposed to have a ground line. The UK again used some different rules, japan had it's own, etc.

Mooly 22nd January 2011 11:32 AM

Hi ilimzn, thanks for the explanation... I hadn't fully appreciated that aspect of the design actually, that it slipped from linear to switching in the pre stages, it just didn't register if I'm being honest. Thanks.

So that brings another "critical" component into play, the flywheel diode D601. I'm not familiar with the type number listed but know from experience that the old rectangular "Semikron" brand SKE2G type and similar used to fail by intermitently going open circuit.

Must be losing it this morning lol ;)

johnm 22nd January 2011 01:21 PM

Wow - all that's way over my head unfortunately :bawling:. But does any of it have any bearing on whether to stick to the factory installed 100K for R607 (which should be for the US/Canadian units only), or to change that to the 56K which is mentioned 3 times in the manual for the UK/AEP E versions of the amp? (Mines the 'AEP, E' version). The only difference I can see between the UK and the 'AEP, E' version is the UK consumes 550W, and the AEP, E model consumes 450W...

Jaycee is of the opinion it makes little difference at the end of the day, but I really would like to fit the correct part if possible.

Just checked diode D601 and it tests fine thank goodness!

My spare 2SC1810s arrived today - exactly the same as the originals and still in their 70s style Sony packaging - niiice! :up:

Ta,

- John

Mooly 22nd January 2011 05:03 PM

Hi John...
I'd stick with the 100K.
Why... because it's what was fitted originally for one thing.

Second is that it is the value that makes sense, but that goes against the parts in the manual. My reasoning is this,

The resistor has 6.3 volts on one end. That's an absolute as it's set by the zener diode and the B-E volt drop of Q605. The other end goes to the unregulated side of things with around 340 volts DC on it. So the resistor passes around 3.3ma, and that is the current available for the error amp to work with. All seems reasonable.

If the resistor is dropped to 56 k there is nearly twice as much current available. Not a problem in itself, and it will not alter the operation of the circuit as such with regard to voltages. It would allow for "cleaner" switching as more current would be available to charge and discharge stray capacitance and switch the transistors cleanly. At the frequencies involved here that's not really such an issue. It will increase power dissipation though.

Thinking aloud now... if you fitted 56K then that would give a dissipation of W=V2/R which is 2 watts. And the manual says 1 watt rating...hmmm.
The 100K dissipates 1.1 watts... which is still over it's rating.

Thoughts anyone ? Is this where commercial ratings of parts come in. A 1 watt metal oxide type probably would be OK in practice and considered acceptable.

Getting back on track, the 56K makes sense for the lower unregulated supply of the US model as that keeps the current available similar to the 100K for UK voltages.

So keep it same as fitted :)

johnm 22nd January 2011 05:31 PM

Quote:

Originally Posted by Mooly (http://www.diyaudio.com/forums/solid-state/181121-sony-ta-f6b-psu-repair-post2442483.html#post2442483)
Getting back on track, the 56K makes sense for the lower unregulated supply of the US model as that keeps the current available similar to the 100K for UK voltages.

And yet according to the manual the US/Canadian model uses 100K hahaha! I'm thinking this must be a mistake in the manual, and the resistor values have evidently been transposed between the UK/US models by mistake. As you say the 100K is fitted to my 'AEP, E' model. The US *should* be the 56K. I'll correct that (or add a note) in the service manual as I'll PDF it at some point incase others need it.

I'll forget about it now - I'm getting way too OCD about this :eek:

As always, mucho thanks for all the help & advice :up: Hope everyone's having a good weekend!

- John

Mooly 22nd January 2011 06:07 PM

Quote:

Originally Posted by johnm (http://www.diyaudio.com/forums/solid-state/181121-sony-ta-f6b-psu-repair-post2442509.html#post2442509)
I'll forget about it now - I'm getting way too OCD about this :eek:

As always, mucho thanks for all the help & advice :up: Hope everyone's having a good weekend!
- John

You're welcome. It's quite an interesting PSU actually. I think 100K would work in all variants tbh particularly if the transistors in the error amp are high gain, particularly Q604.

The power consumption will come down to the efficiency of the PSU at different line voltages I suspect.

jaycee 22nd January 2011 07:51 PM

It makes me wonder why this technique was abandoned, especially nowadays when we have much more efficient SMPS technology, IC based controllers, low RDSon mosfets etc. Probably an emissions certification thing.

johnm 22nd January 2011 09:53 PM

That insulating gasket is on a 2 week back order with RS - arghhhhh! I guess that'll force me to continue to take it slowly with this project however. Very slowly - there's not much I can do with the board now until it arrives except solder in the new caps. The transistors will have to wait until the die-cast heatsink is back in place. I always seem to choose the out of stock items - it's already happened with 4 of the items I ordered from Farnell for this project :bawling:

Oh well - the amps nice to look at anyways :cool:

Mooly 23rd January 2011 06:27 AM

That's the insulation for the four switching transistors ?

We need to test as we go really. Are Q601 and 613 fitted ? as this section is the first to power up and test.

Bit hard to visualise exactly what it all looks like and how it all fits together. I can imagine but it's the details that count :)

johnm 23rd January 2011 12:49 PM

Hi Mooly.

Yes it is, but the insulation is sandwiched between the die-cast aluminium part (which is fitted to the top of the PCB), and the PCB itself. Until that is all back in place none of the BUT11A transistors can be soldered into place as all 6 use that die-cast part as the heatsink.

I'll aim to get some pictures up later today.


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