Sony TA-F6B PSU repair

[Mooly]

Hi johnm

I'll try and have a look at the PSU in more detail later...
SMPS can be drastic in their failure modes, and it might help to see if the PSU would run off load and with a "bulb" limiter in the incoming mains supply. It's worth removing (if needed) semiconductors and checking accurately all the resistors as any that see a high voltage can be prone to change value and go high.

[Mooly]

I think I can stitch the PSU together from the hifi engine link... it's the only way I can visualise what's going to have it complete on paper

I'll have a gander over the weekend hopefully... fixing stuff like this used to be the day job lol

[johnm]

Oh that's very good of you Mooly - thank you Sir!

And I know what you mean about having it out in front of you to make sense of it - I already have pages of photos and scans all covered in notes haha!

Cheers again!

- John

[jaycee]

Looking from the circuit, Q601 and Q613 are just pass transistors - so as long as you've got a transistor with a high enough Vceo and Ic here, you'll be fine.

The other transistors are the primary side switching transistors in the SMPS. These are probably a bit more critical. I would say the BUT11A transistor that Mooly mentions would be suitable.

One advantage of old circuits like these - parts such as the 2SC2023 would have been top spec at the time, but now SMPS's are so common, so are equivalent (and evenb superior) specced transistors.

[johnm]

Originally Posted by jaycee

Looking from the circuit, Q601 and Q613 are just pass transistors - so as long as you've got a transistor with a high enough Vceo and Ic here, you'll be fine.

The other transistors are the primary side switching transistors in the SMPS. These are probably a bit more critical. I would say the BUT11A transistor that Mooly mentions would be suitable.

One advantage of old circuits like these - parts such as the 2SC2023 would have been top spec at the time, but now SMPS's are so common, so are equivalent (and evenb superior) specced transistors.

Yeah it's pretty old - the Sony TA-F6B amp came out 1977, and was manufactured until 1981 (Sony TA-F6B on TVK) so I'm hoping it will be fairly straight-forward to sub in new/better parts and get her working again.

Here's a cheeky link to another thread I started as 'ST70fan' over at Audio Karma yesterday (there's a few TA-F6B owners there):

http://mail.audiokarma.org/forums/sh...38#post4297238

Once I have a consensus on here which parts are best to sub for the transistors I'll get to work on it - afraid I totally lack knowledge about transistor subs alas. Really grateful for all the help here

[jaycee]

The main reason they specify the same hFE on Q609-Q612 is because they form a self oscillating circuit. If the transistors were wildly different on gain, the circuit would oscillate unevenly - or may not at all. Gain should not be a problem on modern transistors

[Mooly]

I've printed the PSU out... I'll see what I can come up with as regards transistors and a means of testing.

You need to 100% sure all the resistors are OK, particularly the 0.22 ohm (R622 is it... bit blurry). Check them all.

I think a small 60 watt or filament bulb would be OK as a light (ha ) load across the secondaries with the feed to the amp disconnected (looks like plug/socket... if not then by lifting L607 and 608).

[johnm]

Originally Posted by jaycee

The main reason they specify the same hFE on Q609-Q612 is because they form a self oscillating circuit. If the transistors were wildly different on gain, the circuit would oscillate unevenly - or may not at all. Gain should not be a problem on modern transistors

I think that explains (along with the missing clamp/heatsink) why this PSU blew as the parts seem to have been placed with no heed given to the service manual at all. Q609-Q612 are currently: 2SC2023-0p, 2SC2023-R, and two NEC C2335s which are from different batches. I guess it probably was oscillating, and over-heating too :s

First thing first - will desolder all those transistors over the weekend, and really give the board a really good clean up, and repair any damaged/missing solder pads. Will then be in a better position to install new parts.

EDIT: Q602 (the one to the right of the two red ones in the pictures) looks a bit dodgy too so will replace that also - it's originally a 2SC1775F for the UK model.

[Mooly]

That's it for tonight

Check all those passives john... hopefully this can be a success story.

Do we assume the amp proper is Ok... have you checked the audio outputs just for interest to make sure they are not short.

[johnm]

Originally Posted by Mooly

I've printed the PSU out... I'll see what I can come up with as regards transistors and a means of testing.

You need to 100% sure all the resistors are OK, particularly the 0.22 ohm (R622 is it... bit blurry). Check them all.

I think a small 60 watt or filament bulb would be OK as a light (ha ) load across the secondaries with the feed to the amp disconnected (looks like plug/socket... if not then by lifting L607 and 608).

Wow speedy gonzales haha! Thanks Mooly - so you mean (once all fixed up again) I'd apply power to the PLPS and then see if the bulb lights up or not collected to the secondaries?

Is it straightforward to test the resistors when in circuit? I thought some of them might have different values as they're all interacting?

p.s. the photo back in post.8 is clearer than the scan in that manual.