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F2624 V2 Help.

Discussion in 'Exclusively Sansui' started by cconger, Feb 10, 2018.

  1. tnsilver

    tnsilver AK Subscriber Subscriber

    Messages:
    3,241
    Location:
    Boston
    I think what you are seeing, is voltage drop of capacitors discharging b/c you are testing in circuit. I'm not sure what type of test you run.
    "OL" is open loop and it means no conductivity. No conductivity of collector/emitter is not an indication on it's own for transistor's health and a
    PNP transistor behaves differently than an NPN one. "Fried" transistors conduct on a path where they shouldn't, like C-B and E-B on an NPN transistor
    or the opposite way, B-C and B-E on PNP.

    If you get conductivity where you shouldn't, or get OL where conductivity is expected, the transistor is shot. At this stage, voltage drop or HFE (gain) is of no interest. All we need
    to figure out right now is the extent of the damage, if any, on your PCB. Test transistors out of circuit to be sure. You can make use of the following info:

    TR01...TR04 (diff pair) are NPN
    TR05/TR06 are PNP
    TR07/TR08 are NPN
    TR11/TR12 (power trans.) are NPN
    TR13/TR14 (power trans.) are PNP
    TR15/TR16 are NPN
    TR17/TR18 are PNP

    Follow the schematics and also test any low value resistor (under 1KOhm) that you see on that board. If you're not getting the specified value within the specified tolerance,
    de-solder out one lead of the resistor and measure resistance out of circuit. The same for diodes, except use the diode test mode on the meter. The standard 1N4148 diodes on that
    board, like any regular diode, only conducts when the red probe is on the anode (+) and black probe on the cathode (-). You should be seeing about 0.6V - 0.65V forward
    voltage when you a test a 1N4148 diode in diode test mode on the meter. The Zeners should show about 0.74V.

    You can easily check the trimmers (VR01...VR04) variable resistance between wiper and one of the leads to see if they function right. Replace them only if they
    are faulty, which I doubt. Please use the correct measuring units and it would really help if you specify what your meter scale is set to, when you report results.
    Whatever it is you're seeing, it's not 300 volts.
     

     

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  2. cconger

    cconger New Member

    Messages:
    14
    I finally verifiably found the problem. The 5k Ohm trimmers do not change value when I adjust them. They stay on 4.3k Ohms. The other 200ohm trimmer let me adjust them. Im going to order two new ones and hope they are my problem.
     
  3. tnsilver

    tnsilver AK Subscriber Subscriber

    Messages:
    3,241
    Location:
    Boston
    Maybe... but those are the DC offset trimmers and it's unlikely they both failed. R33/R34 would probably be open before this would happen. Try to renew the trimmers solder joints and measure resistance on a different pin combination.
     
  4. Joel_l

    Joel_l AK Subscriber Subscriber

    Messages:
    102
    Location:
    Colorado
    This is ringing a bell to me, don't assume the pots are bad. After my rebuild I popped a few parts on this board because I put some wrong value pots in. Once that was resolved it took a bit to get this board back up and running. I kind of recall seeing the same thing you are and it was not the pots.
     
  5. cconger

    cconger New Member

    Messages:
    14
    Sorry for being so late. Waiting for replacement transistors for mouser. I learned how to test the volume pots and they work. Is it possible this issue could be on the power supply board?
     
  6. tnsilver

    tnsilver AK Subscriber Subscriber

    Messages:
    3,241
    Location:
    Boston
    I lost track, what is the issue? If you ordered replacement transistors I assume you have faulty transistors on the driver amp and there's your issue.

    Without the driver amp nothing is driving the output transistors, you get no DC voltage, the relay clicks, no protection, all looks good and this is normal.
    With the driver amp in protection mode it means too much DC. You can measure the DC voltage (in volts) on the grey and blue wires down the power
    supply PCB that go to the relay and report what you have. It's one probe on the chassis, the other on the wire. Each is a different channel. If it's close
    to rails voltage your outputs are wide open uncontrolled. You'll probably measure around 50VDC in this case. Then, it's either you have one or more
    blown output transistor or you have some faulty shorted transistors on the driver amp, or both.

    It is unlikely to have shorted transistors on the driver amp without some open resistors and very possibly diodes too. If you don't find any faulty component
    on the driver amp and your output transistors are intact, I cannot explain this. You may be able to guess your way through this by shot gun replacing components,
    but you'll never know what went wrong if you do.

    Anyhow, I have gone over this thread and had a look at your PCB again. I suggest you take a look at the layout bellow and where a blue layer joins a red layer (I've
    marked most locations, but not all), reflow the solder joint all the way through using flux, to get a good contact.

    [​IMG]
     

     

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  7. DougBrewster

    DougBrewster AK Subscriber Subscriber

    Messages:
    513
    Here's a little test you may find helpful; pull the bias supply fuses F04 and F05 and power up (I did this on a 100W DBT because that's how you will probably want to do it) the board will hopefully stabilise and you can then look at the bias voltages at the bases of TR11/13 and TR12/14 to check they are about +1.1/-1.1 as appropriate and adjustable from the bias pot. It's tight in there so you'll need to be very careful with your probes...
     

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