It's very typical for shutdown noises to come from the preamp. The relay is supposed to break faster than the preamp power supply can collapse.
Speaker pop on shut off Sanyo JCX2900
- Thread starter curtisw99
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No luck
Found a lot of out of spec caps; most of the Sanyo branded were 20-30% high. All the Elnas were fine. Replaced everything
Still pops.
Q411 is only showing 3.7V, manual says it should be 4.3V. Is this difference significant?
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I don't have the scat open, so I'm not sure what the significance is of that Q. However, -.6V is a little outside what is considered the "normal" +/- 10% tolerance. Something could be pulling it down. Or, it could be fine. Real helpful, I know.
ETA: Q411 and the diodes and resistors surrounding it make up kind of an analog POST (power on self test). Q411 should be off when the unit is operating normally. It's worth investigating the condition of all of those diodes. D419 should show a drop of about 4.7V across it.
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Thx
I will recheck the diodes.
Can diodes test well on a multimeter, but be faulty? Or is the multimeter test always valid?
My rule of thumb with component testers is thus: if the tester says a component is bad, it's very likely bad. If the component tester says it's good, it miiiiiight be good. Or, it might well be bad. Sometimes things will test OK under the conditions provided by the tester, but fail under real use conditions.
Everything except the tuner has been recapped
Remeasuring D419, I get a 2.7v drop (1.3v to 4v) across the zener diode.
Q411 still measures 3.7v
But rerunning test by playing an iPhone straight into the amp and there’s no pop. I think I’m going to call the power section good for now.
I think next test will be running the preamp into another receiver to see if it pops.
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We've already established that it's very common for a preamp to make noises as its power supply collapses. The problem here is that the relay is taking too long to open when the power is switched off. Why that is occurring is the tricky part to figure out. That a 4.7V zener is only showing 2.7V drop might be a clue.
I think I can rule out the actual relays. It makes the noise on all 3 relays.
I may just shotgun all 9 diodes around Q411
Just to see what happens.
Nothing changed after replacing all the DS442x diodes.
What would be a good sub for that yz047 zenner? I have BZX55B4V7-TR (4.7v 500mW). Having a hard time figuring out the wattage of the yz047. I’m guessing it’s 1/2w from the size. Am I safe to assume 1/2w?
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Unless the parts list/schematic say otherwise, 1/2 watt is a safe assumption.
It should be showing about +1.2V on the anode, and 2V on the cathode. During normal operation, it functions as a regular rectifier (visualize the cathode/anode connections reversed). I believe it's there to keep Q431 from being destroyed when the positive supply collapses at power off.
The zenner is D419
Weren’t you saying earlier that the voltage drop across D419 should be 4.7v? Am j mixing thing up?
I had to zoom out and look at the bigger picture. With the positive lead on the cathode, and the negative on the anode, I would expect to see about a .6 volt drop across D419. Unless I'm totally overlooking something, which occasionally does happen.
That would be powered on, not using the diode drop function of the meter.
That would be powered on, not using the diode drop function of the meter.
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Replaced the zener. Ended up with exactly 3v drop across D419. 1.3v to 4.3v anode to cathodeI had to zoom out and look at the bigger picture. With the positive lead on the cathode, and the negative on the anode, I would expect to see about a .6 volt drop across D419. Unless I'm totally overlooking something, which occasionally does happen.
That would be powered on, not using the diode drop function of the meter.
I checked the DC output again.I had to zoom out and look at the bigger picture. With the positive lead on the cathode, and the negative on the anode, I would expect to see about a .6 volt drop across D419. Unless I'm totally overlooking something, which occasionally does happen.
That would be powered on, not using the diode drop function of the meter.
During steady operation, channels are around 8mv DC.
At shutoff, DC spikes to about 100mv
Is this the source of the pop?
If it were sitting at 100mV+ quiescent, then that would be a concern. The relay is supposed to open before the output can spike. I'm sure if you pulled the preamp jumpers, you would have no pop. But that's not the problem. The problem is that the relay is opening too slowly.
I'm going to be super busy for the next week or two, so not much help during that time. There is typically a tab open on my computer, but I probably won't be particularly active here during that time. We will revisit this though.
Thought it was a 4.7V zener? Volt drop should be nominal 4.7V, 3V is notok
Digikey have a capacitor discharge calculator, time for 47uf from 4.7V to 1.0V (2xVbe) through a 220ohm is 0,016sec. Also R431 and R433 will speed this up a little. Time slightly better for a (measured) 4.3V.
edit YZ-047 looks like a 1W 4.7V item, maybe the 500mW item is burning up?
edit Ooops! error on my part. The 47uf discharge through the 220ohm is tied to the mute switch S14-1, S14-2 and not power off swich S15-1 S15-2. That would mean the muting is delayed because zener D419 is leaking volts keeping Q431, Q432 on longer than normal. When D419 cathode drops below 4.5V (about) the anode should be 0V
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After power up, allow 5 or so seconds for transients to settle then D419 anode should be +1.2Vdc (Q431, Q432 Vbe). D419 cathode should be about 4.7 + 1.2V = +5.9V (depending on the actual zener voltage(4.7, 4.6, 4.5,,,)
There will be slight drift (xxmV?) with mains but supply is regulated (Q902,3,4, D...), the volt drop across the zener should be roughly constant at about 4.7V.
Previous posts indicate abnormal zener behaviour.
With a 500mW item a 4.7V zener can handle 106mA, plenty for this circuit, R419 would cripple the cathode voltage for any slight increase in current.
Suggest focus on D419, if possible replace with another 500mW item.
If you have a dbt then dbt power up and if possible monitor D419 anode to GND and cathode to GND (2xmultimeter). Wanting to see stable cathode voltage at about 5.9V allow xxmV drift, anode voltage +1.2V expected.
If cathode voltage is less than 4.5V than anode should be near zero, maybe xxxmV leakage???
If cathode voltage is low and unstable then need to look elsewhere...