Soundstream Class A 10.2 turn-on delay

[azvrt]

All right thanks for the info (D146).

Where should we measure this voltage ?

[Perry Babin]

On the center legs of the output transistors. Don't let the probes slip while measuring the voltage.

[azvrt]

We finally got around to check this amp.
Ah I see you told us where to measure the rail voltage. I hadn't looked on this pag prior to testing the amp so we measured elsewhere, but should be good.

During the turn-on delay the amp draws a little under 1.5 ampere and produces no sound.
The rail voltage is +20.1 and -20.1 volts.
When it finally fully powers-on it draws a little under 2 ampere and still has 20 volts rail voltage. This time it did not take 4 minutes for the amp to fully poweron by the way. Still, it took approx 20 seconds.

Anyway, rail voltage in High Power Mode (which the amp should be in with a 4 ohm stereo load, the way we tested it) should be 27 volts not 20 volts. High Current Mode should be 20 volts but that should be only at 1/4 ohm stereo loads, not at 4 ohms stereo.
The LED indicates High Power Mode, though (lights up red, in High Current Mode it should light up green).

[Perry Babin]

Post the DC voltage on all pins of U105.

Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11:
Pin 12:
Pin 13:
Pin 14:

[azvrt]

That's a TL074.
Will check it.

[azvrt]

U105 (TL074):

Pin 1: -13.34 V
Pin 2: +41 mV
Pin 3: -3.2 V
Pin 4: +14.88 V
Pin 5: -3.56 V
Pin 6: +41 mV
Pin 7: -13.1 V
Pin 8: -12.8 V
Pin 9: +41 mV
Pin 10: -486 mV
Pin 11: -15.00 V
Pin 12: -13.36 V
Pin 13: -12.56 V
Pin 14: -12.56 V

I don't know whether it's worth mentioning but we noticed that the +15 V and -15 V on this amplifier are +14.88 V and -15.00 V.

Also, turn-on delay has been way shorter than before the last two times we powered it on. Today was really short, perfect actually.
But as mentioned in the repair tutorial, if you haven't fixed the problem it will re-appear sooner or later.

[Perry Babin]

There's not much that you can do if it's working properly. Are there any components out of the circuit as of now?

If so, reinstall them to see if they have any effect on the turn-on delay.

[azvrt]

We desoldered a diode as requested a while ago, but as instructed we put it back in so the amp is like it should be.

I'll power it on again this afternoon, if turn-on delay is normal (short) I'll put the amp in my car and test it properly. But my guess is the problem will arise sooner or later.

Nothing abnormal regarding the voltage on the pins of U105 ?

[Perry Babin]

I didn't look at the voltages because the amp is working properly.

[azvrt]

Well Perry, I put it in my car and it has been powered-on several times since. Turn-on delay is normal. Very strange...

Merry Christmas !