| Electronics > Beginners |
| Help Fixing receiver |
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| JohnPen:
Q803 base should have +15.6v relative to the chassis, the voltage rating of the Zener diode. If it was a normal silicon diode or perhaps a Zener diode connected in reverse Q803 would have only +0.6v relative to earth. If the capacitor C803 was s/c that would also lower the voltage below +15.6v. Even though you originally measured only +24v from the bridge rectifier there should still be enough volts to provide the +15.6 voltage on D803/Q803 base. This does assume that R806 and R811 are both OK. John |
| tpowell1830:
--- Quote from: AngraMelo on June 12, 2018, 04:17:42 pm --- --- Quote from: JohnPen on June 12, 2018, 08:30:52 am ---Starting back at the beginning check that the transformer centre tap is actually connected to the chassis. Another thought check that D806 and D807 are still diodes. A simple check might be to lift one end of each diode to eliminate any effect they might have on the regulated supplies. The transistors Q802-5 are set up as simple voltage regulators using a Zener diode on the relevant transistor base to set their appropriate output voltage on their emitters. Check each of the voltage regulator outputs to see if they have the correct value. In each case it is the emitter of the transistor that has the regulated voltage output. Q803 should have +15V relative to the chassis. Q802 should have +6.6v but only if Q803 is providing +15v to it. Q805 should be -21v relative to the chassis and Q804 should be -15v relative to the chassis. Similarly the QR01 should be providing +28.6 v relative to the chassis. Hope this helps. John --- End quote --- Thank you very much for your help, what I did noticed from probing around is that Q803 is not receiving any voltage at its base. the Zener D803 is also conducting. Center tap of the transformer is connected to the chassis and given all of this, Im completely lost. I have no idea what might be causing this. Do you know why would D803 start conducting? is it supposed to be conducting all the time? --- End quote --- You stated earlier that there is 15.5 V at the D803, yet you have no voltage at the base of Q803. Is there a break in the board trace? |
| lordvader88:
As for a common zener diode rated 15V and 1W, that would probably be passing around 10mA from cathode to anode (conventional current). So yes if it's working it would be passing a fairly small current with a big voltage drop. Lower voltage zener's like a 3V usually pass a lot more current, like 80mA https://www.vishay.com/docs/85816/1n4728a.pdf |
| mzacharias:
I think either a short or near short is pulling down a voltage line, or a main filter cap is open circuit, so that any load lulls it down, or the ground reference is faulty, such as a burned foil run or resistor. I would check resistance to ground at any of the DC voltage regulated outputs, like +5, +28. +15, etc. One of these may have unusual low resistance. Does Q803 0r Q802 get hot very quickly at power on? Make (Sony I presume - Legato Linear is NOT the Brand) and model number PLEASE. |
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