Calling Quadraflex 650 FET owners

Discussion in 'Solid State' started by waggs098, Mar 29, 2015.

  1. waggs098

    waggs098 Super Member

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    Im working on this receiver and it has been quite abused. The picture is of the amp board and the resistor that is circled in red I need some help with. It looks to me to be color coded brown-black-brown.
    According to that it should be 100ohm. Both measure 1 kohm. is there anyone that could double check that for me. I don't know if there just that far off or if the color changed from heat or if the color code was wrong to begin with.

    The unit seems to work except for a very quiet hum from the speakers. But when I check bias voltage I actually read 100 VOLTS! I checked it a couple times and both channels are 100 volt not 100mv. Im going to try and rebuild this board but these resistors have me baffled as to whats actually supposed to be there.

    The huge white resistors I installed just to test because one of them was burned open. (put in 10 watt and original was 2 watt. this will be changed when I order parts). Im checking bias across the smaller .47 ohm resistors.

    Any help is greatly appreciated.
    Thanks
    Justin
     

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  2. Robisme

    Robisme Sansui Enthusiast Subscriber

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    Just pulled mine out and opened it up.

    Color code is brown, black, red. Looks like 1K ohm is correct.:yes:

    Rob
     
  3. waggs098

    waggs098 Super Member

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    Thanks for the very quick reply. I figured 1k was correct since both measured that way but I wanted to be sure. The red must have turned brown from heat. There are many dark spots on board from heat.

    If theres any way you could get a bias voltage measurement just for a reference point for me that would be great.

    Thanks again.
     
  4. Robisme

    Robisme Sansui Enthusiast Subscriber

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    Do you have the service manual with bias instructions?

    Rob
     
  5. waggs098

    waggs098 Super Member

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    Can't find any service manuals for quadraflex units. Pacific stereo didn't put them out.
     
  6. Robisme

    Robisme Sansui Enthusiast Subscriber

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    So, the question is, are the MOSFETs in this biased the same way as a BJT?

    You have 100 volts across one the .47 ohm resistors? Ohms law says that = fire.:D

    Are there any 100 volt caps on that board?

    I will have to pull mine out again at some point (it's a pain to get at) and see what I get across the .47s.

    I can't believe 100 volts on that board is normal.


    Rob
     

     

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  7. waggs098

    waggs098 Super Member

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    I can't belive it's normal either. Really I can't believe it works. But like I said I had to replace a few parts to test it. Highest electrolytic cap is 63 volt. One ceramic cap that's 100 volt.

    If you can check your receiver that would be great and I'm not in a huge hurry. Due to the heat stress on this board I'm going to replace all transistors (except for finals) and all caps and see what I have after that.

    How to properly check bias on these I don't know. Maybe someone with more knowledge about mosfets will chime in.

    Thinking a little more maybe I need to rebuild the power supply board also. Could be an issue there and sending out to high of voltage.
     
  8. mech986

    mech986 Text ↓ optional Posts:>18,000 Subscriber

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    You might want to contact member Pacific Stereo at Tapeheads.net, he was a service manager at PS back in the day and if anyone is going to have info on that Quadraflex unit, he will. He can also do service too. No Affiliation but sending you an option.
     
  9. blhagstrom

    blhagstrom Mad Scientist, fixer. Subscriber

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    They way mosfets work is totally different than bi-polars.
    My guess is that you don't measure emitter resister voltage drop.

    Now I'm curious too.
     
  10. waggs098

    waggs098 Super Member

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    You reminded me of something.

    I have a Kyocera R451 with mosfets in it and in the service manual for idling adjustment it says,

    "Connect volt meter across the source of Q513 (which is the finals) and + of speakers output terminal."

    I might have to try this and see if I can get it to adjust. :scratch2:
     
  11. waggs098

    waggs098 Super Member

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    Well I contacted Pacific Stereo and not getting any help that way. This was his reply:

    "There are no specifications for that. And I have not had a 650 cross the bench since I started doing this again. I would set it by watching current consumption and distortion and I would then write those values down for future reference. But since I've not seen a 650 to be able to do that, I have no data. I would leave well enough alone."

    Of course I can't just leave it alone since this unit needs rebuilt. Guess I will order some parts, install them and hope for the best. See if I can get bias set properly using my scope. He also didn't have info on where to hook voltmeter just to check bias.
     

     

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  12. mech986

    mech986 Text ↓ optional Posts:>18,000 Subscriber

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    Well, at least you have the general outline of what to do. Setting bias by distortion does work, but you'll have to decide how much bias to use and whether to stay in Class AB or move more into Class A operation and higher thermal stress. With a relatively rare unit, I hope the transistors are fairly common so repairs could be easier.
     
  13. waggs098

    waggs098 Super Member

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    Yeah I've done a little research and I should be able to find substitutes for everything. Probably won't find originals though.
     
  14. waggs098

    waggs098 Super Member

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    Well I have brought this unit back to the bench to see if I can get it working properly or not.

    So where I'm at is there is a hum to the speakers. Its there with any input I select and its there with nothing connected to the receiver and its not volume controlled. I thought maybe the filter caps but from some info on a different thread they seem like they might be ok.

    Tested both filter caps to ground to read ripple voltage. One reads 140mV and the other reads 130mV, not a big difference there. The hum is constant and not volume controlled with no inputs hooked to the receiver. The hum gets a little louder if I hook something to the input.

    So I need to figure out the cause of the hum and then try and figure out how to set bias on this unit since it has mosfets and no service manual. I looked at the Kyocera manual I have tried setting bias according to its instructions but cant seem to make any difference when I turn the trim pot. I have also tried methods from some other mosfet receivers but cant seem to make anything work. I am however able to adjust DC offset down to almost 0.
     
  15. waggs098

    waggs098 Super Member

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    Here is a top pic of the entire unit just for some reference. DSCN0889.JPG
     
  16. cademan

    cademan Addicted Member

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    I'm sorry but those two big filter capacitors are not the power supply filter capacitors. The power supply capacitors (and power supply) is the board that is mounted vertically. No solid state power supply uses capacitance that large. The large capacitors are for the outputs. They say the larger the capacitance and/or voltage rating increases the bass response.

    Look at the bottom side. Those big filters are connected right off of the main amps outputs.
     

     

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  17. waggs098

    waggs098 Super Member

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    They do connect directly to the outputs. What would be the technical name of the large caps. I always thought they were considered filter caps, which I assumed were part of the power supply.

    The main power supply board is the vertical one yes. So should I recap the power supply and see what happens or is there a way to narrow it down? Given there is no service manual to deal with it makes it difficult.
     
  18. cademan

    cademan Addicted Member

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    The large caps are also filter capacitors. They are just bigger because of the higher voltage and capacitance ratings.

    It wouldn't hurt to replace all those capacitors on the power supply board as they look like the originals and some capacitor (or capacitors) there is causing your hum. I would just replace them all. Probably $30.00 to $50.00 I would think.
     
  19. jdurbin1

    jdurbin1 Tandberg enthusiast Subscriber

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    Hold up. Those two large vertical capacitors seen in the top view are absolutely power supply filter caps. The transformer to the right, those caps, and the rectifier under that U-shaped bracket in front of the two caps are the actual AC-DC power supply for the receiver.

    The board to the left of the two caps is not the power supply. It's the power amp, or more specifically it's the driver board for the power amp. The next chunk back of that is an internal heat sink for the power amp and you can see the MOSFET outputs attached to one side of it.

    This amp is a MOSFET direct-coupled design. It does not have output filter caps.

    John
     
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  20. century tek

    century tek Super Member

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    Vertical board to the right is the power supply. Vertical caps are part of the output. I would like to see a schematic before I speculate any further. waggs already confirmed that they do indeed connect to the output MOSFET's.
     

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