EXPIERENCED Guidance Saught For Sony TAN-8550 Rebuild

I am starting a rebuild of critical components of a Sony TAN-8550, and am looking for someone experienced to check in with, for some assumption checks.

This is hardly my first rebuild, but given the unobtanium nature of the VFETs, I want to assume nothing.

All the VD1221 diodes have been replaced.

Next will be all the electrolytics, including the double 10000uf cap, which right now two singles are on backorder from Mouser.

I am also replacing critical components for the bias supply side of both boards.
This includes bias Resistors R725/775 and R731/732 and R781/782.
Also the 220K trim pots RT 701and 751 will come out. I will set these to the same value as the pulls before I install them.

Also changing out Q713-716 and Q763-766, as the service manual states that failure of these transistors will cause V-FET failure.

I have managed to get authentic Sony transistors as replacements

The service manual also has a procedure whereby you disconnect power to the V-FETs and measure voltage across R737 and 787 to make sure it is in the correct range, before applying power to the V-FETs.
It says to disconnect the brown and violet wires ... assuming they want brown and ORANGE, as the amp has no violet wires heading to those two boards.

The voltage I should be looking for after all the components have been replaced, is 38.8 - 43.8 for V-FETs with a ranking of 57.

Do I have this right, and are there any other critical components I need to replace

Are there any hidden dangers not documented in the service manual along the way

Any help much appreciated.

Vinylcafe said:

Are there any hidden dangers not documented in the service manual along the way

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The undocumented hidden dangers are usually the ones created by accidental mistakes made along the way (experience talking here); you're changing lots of things, arguably some of which might not be entirely necessary ... go slow and be careful. :thmbsp:

I've done one 8550 which I removed the Vfets altogether before power up and tested it thoroughly (no load- just a cro to look at driver wave forms and voltage/current checks around the circuit). I've never been comfortable with removing gate drive to a fet or vfet and leaving it in the circuit. The SM says remove +/- power to the vfets IIRC, but leaves the gates connected?
Even with Jfet amps, I take them out before power up if I'm not 100% confident and not on a variac+dbt.

restorer-john said:

The SM says remove +/- power to the vfets IIRC, but leaves the gates connected?

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It does. It is a procedure they recommend after replacing any V-FET and before applying power.
This is the section where they also warn about 4 potential transistors in the bias power supply, that if they fail, the VFETs fail.
My motivation to put in new ones in.

Vint Age said:

you're changing lots of things, arguably some of which might not be entirely necessary ... go slow and be careful. :thmbsp:

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Agree that the resistors and trim pots are optional. However, after reading in the SM,that there are 4 critical transistors in each side of the bias power supply that can cause V-FET failure, decided to put new ones in, just as a precaution.
This amp will be 40 next year, so thought if I could find the parts, it would be prudent, as the board is pulled anyway.
I am checking and double checking everything.

Vinylcafe said:

there are 4 critical transistors in each side of the bias power supply that can cause V-FET failure

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As long as you're confident that your new replacements are at least as good as the ones you're replacing. With all the fake parts floating around these days I'd sure be neverous ...

Vint Age said:

As long as you're confident that your new replacements are at least as good as the ones you're replacing. With all the fake parts floating around these days I'd sure be neverous ...

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I hear you.
All bought from reputable folks (all be it on that auction site).
Three of the four transistors are in original Sony packaging.
So unless they are even counterfeiting the bubble packs, I think I am good. Have attached a pic.

Vinylcafe said:

So unless they are even counterfeiting the bubble packs, I think I am good. Have attached a pic.

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Where there's a dollar to be made there's a way. But those do look pretty good ...

Vinylcafe said:

...I am starting a rebuild of a Sony TAN-8550.

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I sure hope you post pictures of it once you are finished with the rebuild and all.

That series of Sony audio components are awesome to say the least. :thmbsp:

R-2-R said:

I sure hope you post pictures of it once you are finished with the rebuild and all.

That series of Sony audio components are awesome to say the least. :thmbsp:

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Happy to ...

This unit is a dream to work on, as everything is modular and easy to access.

Left channel board comes out with the removal of 4 screws.
Two for the heat chimney, and two for the heat sink/board assembly (they are attached).
No need to disturb the V-FETs in their sockets.
There are 5 plugs to pull, and viola the board is out.
No wire wrapped posts soldered in for good measure here.

Service manual is also top notch. Well laid out and well annotated.

Left channel board is done here.
Bias supply resistors and bias trim pot replaced, diodes were already done, electrolytic caps replaced, 4 Sony bias supply transistors replaced and new thermal compound applied.
Both sides of board cleaned with isopropanol.
Ready to go back in.

Vinylcafe said:

No need to disturb the V-FETs in their sockets.

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Sure there is...the stock heatsink compound is crap and needs to be replaced if it hasn't already been done. Or you can buy these: http://www.digikey.com/product-detail/en/SPK10-0.006-00-05/BER111-ND/201914 (the Bergquist K10 pads are the only sil-pads worth considering, as most are crap).

EchoWars said:

Sure there is...the stock heatsink compound is crap and needs to be replaced if it hasn't already been done. Or you can buy these: http://www.digikey.com/product-detail/en/SPK10-0.006-00-05/BER111-ND/201914 (the Bergquist K10 pads are the only sil-pads worth considering, as most are crap).

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EW.
I have been hoping you would weigh in.
I know you are THE ONE.
It looks like these have already been done, as the thermal paste is pretty messy, when I compare to other stock applications.
I will post a close up picture and would love your perspective.
If Sony was really that messy, I am happy to pull them and redo the paste.
Have good stuff that does not conduct.

Plain-old white silicone grease is all you want to use. Nothing fancy, like computer CPU grease.

If it were mine I'd also be replacing the drifty old carbo comp R742 and R743 with some films ...

EchoWars said:

Sure there is...the stock heatsink compound is crap and needs to be replaced if it hasn't already been done. Or you can buy these: http://www.digikey.com/product-detail/en/SPK10-0.006-00-05/BER111-ND/201914 (the Bergquist K10 pads are the only sil-pads worth considering, as most are crap).

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As promised, here is a close up of the V-VETs.
Someone has clearly been in there before.

That said, these thermal pads are really interesting.
I never knew this product existed.
I presume from the literature that the totally replace the thermal paste-mica washer-thermal paste sandwich??
Are these as effective as the aforementioned sandwich at transferring heat?
Table pasted from data sheet also would imply the tighter you bolt these things down, the more effective they become.
Am I reading this right?


THERMAL PERFORMANCE vs PRESSURE
Pressure (psi) 10 25 50 100 200
TO-220 Thermal Performance (°C/W) 2.35 2.19 2.01 1.87 1.76
Thermal Impedance (°C-in2/W) (1) 0.86 0.56 0.41 0.38 0.33

If so, I will order some up.
Gives me an opportunity to clean up the sockets as well.
Will use some Deoxit and my favourite little cleaning brush pictured below.
Made to get food out of your braces ...

Vint Age said:

If it were mine I'd also be replacing the drifty old carbo comp R742 and R743 with some films ...

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Thanks for the input .. consider them ordered.
I will make them part of the same order as those thermal pads that EchoWars kindly pointed out.

Good to see your making progress with your Sony and taking the time to do it right.

Thanks Totem.

With the help of the help of the good folks around here, they make sure you get it done right!!

That's what I love about this place.

My 2 cents worth, I hate thermal pads. :thumbsdn: Give me a nice thin coating of paste and mica insulator any day. There, I said it.