Project Amp

Discussion in 'Solid State' started by thefragger, Feb 11, 2008.

  1. thefragger

    thefragger Certified Crazy. Subscriber

    Project Amp Ideas?

    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?
     
    Last edited: Mar 20, 2008
  2. thefragger

    thefragger Certified Crazy. Subscriber

  3. avionic

    avionic Aim High !!! Subscriber

    Pretty vague ...Could be anything.
     
  4. thefragger

    thefragger Certified Crazy. Subscriber

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

    avionic Aim High !!! Subscriber

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

    thefragger Certified Crazy. Subscriber

    Could you entertain me? To what extent could the problem be?
     
  7. dr*audio

    dr*audio Fish fingers and custard! Subscriber

    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.
     
  8. thefragger

    thefragger Certified Crazy. Subscriber

    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.
     
  9. dr*audio

    dr*audio Fish fingers and custard! Subscriber

    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.
     
  10. thefragger

    thefragger Certified Crazy. Subscriber

    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. :thmbsp:

    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.
     
  11. thefragger

    thefragger Certified Crazy. Subscriber

    Does anyone here have this amp?
     
  12. dr*audio

    dr*audio Fish fingers and custard! Subscriber

    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.
     
  13. thefragger

    thefragger Certified Crazy. Subscriber

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

    I agree. Do you know of a good source?
     
  14. jimbofish

    jimbofish for TRULY GREAT MUSIC

  15. thefragger

    thefragger Certified Crazy. Subscriber

  16. dr*audio

    dr*audio Fish fingers and custard! Subscriber

  17. thefragger

    thefragger Certified Crazy. Subscriber

  18. dr*audio

    dr*audio Fish fingers and custard! Subscriber

    Vintage Audio Manuals provides an extremely high quality hard copy, while service manuals.net provides a very good to excellent scan (depending on the condition of the source material) and high quality hard copies. Twice service manuals.net sent me the original! You can't go wrong with either, but if you want an excellent reproduction, as good as the original, to hold in your hands, use Vintage.
     
  19. thefragger

    thefragger Certified Crazy. Subscriber

    The service manual arrived about two weeks ago from Vintage Audio Manuals and it's pretty good, aside from the colour being off (or absent) on some pages.

    I received the amp yesterday; it's in a lot better shape than I expected, and the packing was brilliant-- very sturdy. Big plus in my books. The cosmetics are alright; the face is good and shiny but there's some surface rust on the top cover-- nothing major. I couldn't tear it down yesterday because it was late and I had an early class today, so I tore into it as soon as I got home :D

    I plugged it into a properly grounded outlet with a surge projection bar and stood back, using a broom to turn it on. As described, it does not turn on. With manual in hand I started testing. Wires are continuous, some resistors has drifted in value, but not open, fuses are good, and then I came across a thermal fuse.

    It was open. It's a 10A 109°C fuse.

    Does this mean that the fuse opens up at 109°C and can handle up to 10A?

    I'm not sure that I want to replace it just yet as I'm seeing in the amplifier section that there are some capacitors undressing themselves (heat shrink label is all but off).



    P.S. Big mistake on my part; I thought that thermal fuses were standard kit with electronics dealers, I guess I was wrong. Anyone know where I can source this part?
     
    Last edited: Mar 20, 2008
  20. dr*audio

    dr*audio Fish fingers and custard! Subscriber

    Get the part number out of the service manual and try
    http://www.partstore.com/default.aspx?s=google&sw=nonspecifc
    If it looks like a normal size fuse you can try replacing it with a fast blow 10A. That should be ok, it will probably blow before the thermal one would. If it looks unusual, order the Sony part.
    The caps with the shrunken wrap will definitely need to be replaced. In any switching supply, when you service it, it's standard procedure to replace all the caps, using 105C caps. That may or may not be the problem. Usually what happens is one of the switching transistors shorts, and the thing is crowbarring the non - switching supply to ground through the transformer and shorted transistor. Check all the transistors on the heatsink before you do anything else. A quick way to get an idea of the health of the supply is to ohm across the big cap that is near the input of the supply, after the rectifier. If it reads a short, you probably have shorted transistors.
     

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