Project Amp

[thefragger]

Hey guys,

What would you suggest as a good candidate for a fix up project? I (think) I've recently lost an auction on a non-functioning Sony TA-F6B amplifier that I thought would be fun to fix up and get running again, but it wasn't meant to be.

I'm looking for something with the sharp styling similar to that Sony but also an amp what can push 100WPC. Meters are a plus!

Thoughts? Suggestions?

[thefragger]

Apparently I called it too soon, I won it! My brother chipped in $20 for my birthday!

http://cgi.ebay.ca/Sony-TA-F6B-Integ...QQcmdZViewItem


I'm waiting for my Paypal account to be verified then I can pay for it!


Okay, so it's in non-working condition. It's not the fuses (apparently). What else could it be? Does anyone have the service manual for the F6B?

[avionic]

Pretty vague ...Could be anything.

[thefragger]

Originally Posted by avionic

Pretty vague ...Could be anything.

Keeping it to the Pulse Locked Power Supply, what problems do they commonly experience?

[avionic]

Someone else will have to chime in.I'm not familar with that design.

[thefragger]

Originally Posted by avionic

Someone else will have to chime in.I'm not familar with that design.

Could you entertain me? To what extent could the problem be?

[dr*audio]

It is a switching power supply. These are very tricky to work on and if you haven't ever worked on a switching supply I don't suggest you start with this one. Here's basically what it is:
Instead of using a large transformer to step down the line voltage, then a bridge rectifier and large capacitors, a switching supply runs right off the line voltage and rectifies it, and filters it. There is a high frequency oscillator that usually runs at 50 - 100KHz. This oscillator switches on or more transistors on and off. These transistors complete a circuit on the primary side of a small transformer. The rectified line voltage goes to one leg of the primary, and the other leg goes to the switching transistor collector, and the switching transistor emitter goes to ground. When the switching transistor is on, current flows through the primary of the transformer, generating a voltage in the secondary. The secondary side gets rectified and filtered to generate the supply voltages for the output stage of the amp.
To regulate the voltage in the supply, the pulse width of the waveform on the base of the switching transistor is modulated. If the output voltage sags, the transistor is turned on longer. If the voltage is too high, the transistor is on for a shorter time.
There are variations on this theme, sometimes instead of regulating the output voltage from the supply, the audio signal is used to modulate the supply, and then the filtered output of the supply is the output of the amplifier.
Now if this is over your head, cease and desist. This is a nice amp and you can get someone who knows what he is doing to fix it.
To work on it, you need an oscilloscope and an isolation transformer. If you attempt to connect your scope to the supply without an isolation transformer you will get lots of nasty arcs and sparks and possibly damage the supply and your scope. There are high voltages inside the supply, 400V or so DC. It is LETHAL! 'Nuff said.

[thefragger]

Originally Posted by dr*audio

It is a switching power supply. These are very tricky to work on and if you haven't ever worked on a switching supply I don't suggest you start with this one. Here's basically what it is:...

Now if this is over your head, cease and desist. This is a nice amp and you can get someone who knows what he is doing to fix it.
To work on it, you need an oscilloscope and an isolation transformer. If you attempt to connect your scope to the supply without an isolation transformer you will get lots of nasty arcs and sparks and possibly damage the supply and your scope. There are high voltages inside the supply, 400V or so DC. It is LETHAL! 'Nuff said.

I've got a nice HP 1725A on my desk, but haven't gotten an isolation transformer for it yet because I've dealt mostly with signals and low-voltage applications. I've mucked around with PWM before, it's fun.

What currents should I expect inside the supply?


400V sound like fun. I'll DEFINITELY discharge the caps before working on it. At all.

I'd like to try my hand at it, but first thing I need is a service manual. I won't touch the HV circuitry without that.

[dr*audio]

Currents within the supply should be less than 1 amp, with no load. Sounds like you may know enough to repair this bugger. The main failures to expect are shorted switching transistors. It is mandatory that you recap the power supply, since the electrolytics are also a source of failure. Replace all electrolytics in the supply with 105ºC caps, preferably low ESR. The isolation transformer is for the amp, not the scope; to isolate the power supply ground in the primary circuit from the power line.

[thefragger]

Originally Posted by dr*audio

Currents within the supply should be less than 1 amp, with no load. Sounds like you may know enough to repair this bugger. The main failures to expect are shorted switching transistors. It is mandatory that you recap the power supply, since the electrolytics are also a source of failure. Replace all electrolytics in the supply with 105ºC caps, preferably low ESR. The isolation transformer is for the amp, not the scope; to isolate the power supply ground in the primary circuit from the power line.

Thanks, electronics has been a hobby for me (no professional training) for roughly 11 years (I'm 21) and it actually got me a job at a research hospital in R&D.

Naturally I'll do a recap of at least the power supply. Low ESR vs. Ultra-low ESR? Does it matter? Vishay VS Panasonic VS United Chemi-Con? I know what kind of parts are acceptable generally, but with an audio amp power supply, how high should the standards be held?

Also, do you know of a good high-frequency transistor? OnSemi's pretty good for beefy power-supply-quality components that are pretty quick, too. I like their stuff, and I've seen some mention of them around here.

Furthermore, I did residential wiring in high school and have since been the family electrician, but I can't see why am I isolating the power supply ground. I've heard of employing the usage of an isolation transformer with scopes in extreme cases; apologies for the confusion.



Quite the bombardment of questions; you seem to know a lot more than me, amps are a new beast, and I like to prepare myself for what to expect. The best way to learn as I see it is to jump right in and let it go over your head.


Cheers mate.

[thefragger]

Does anyone here have this amp?

[dr*audio]

I'll answer the most important question first. Since the primary circuit to the transformer is connected directly to the power line, imagine what the circuit looks like. You have a wire coming from your wall socket into the circuit. It goes through a diode to rectify it, and then to a filter cap. The other side of the filter cap goes to the other wire going to the wall socket. This side is the ground for the primary circuit of the transformer. Now if you connect your scope ground lead to the "ground" of the primary circuit (which you have to do in order to be able to look at the PWM), you are connecting the ground of the scope to the AC outlet. "Flash!" "Ka-Blamm!" That's why you need to plug the amp into an isolation transformer.
I use Panasonic and Nichicon caps. I avoid United Chemicon, I see them in cheap equipment, which makes me dubious of their quality.
For the switching transistors, I would need to know what is in there. If the original part is available, I would go with that, otherwise I can find a substitute. You will need the service manual.

[thefragger]

Originally Posted by dr*audio

... This side is the ground for the primary circuit of the transformer. Now if you connect your scope ground lead to the "ground" of the primary circuit (which you have to do in order to be able to look at the PWM), you are connecting the ground of the scope to the AC outlet. "Flash!" "Ka-Blamm!" That's why you need to plug the amp into an isolation transformer. ...

Spot on. So the magic smoke from both the Amp and my Scope are released.

You will need the service manual.

I agree. Do you know of a good source?

[jimbofish]

http://www.hifiengine.com/manuals_ty...ony&mod=TA-F6B

[thefragger]

Originally Posted by jimbofish

http://www.hifiengine.com/manuals_ty...ony&mod=TA-F6B

Thanks for the link, but I was hoping for a better version to surface.


Philip.