Soundstream Class A 10.2 turn-on delay

[azvrt]

Hi guys,

I have a Soundstream Rubicon Class A 10.2 with a turn-on delay issue.
The amp had not been powered on for several months and it took several minutes before it really powered on (after hooking up the remote wire). During that time the clipping indicators were lit and there was no bias voltage, which I later properly set to 25 mVolts after it did power on. During the time the clipping indicators were lit it drew around 1.5 ampere idle current. After it really powered on it drew a little over 2 ampere idle current, like it should, and it played the music.
After removing the remote wire and putting it back after 10 seconds it took the amp 5 seconds to power-on, again with the 1.5 ampere idle current and clipping indicators lit during the 5 seconds.
After half an hour of inactivity it took longer to power-on, couple of minutes.

After reading another thread with a similar issue on a Rubicon 1002 which was solved by replacing a 22 mF 25V cap I decided to replace all 22 mF caps on my amp, namely C145 and C152. I somehow overlooked C161 which is of the same value, stupid ! There are about a hundred screws to onbolt on this particular model to remove the board and have access to the cap's solderings, so I am kind of annoyed at myself for not seeing it in the first place.

Anyway, unfortunately replacing the two caps did not do it for me.

Thanks

[Perry Babin]

Does the amp turn on quickly (no matter how long it has been off) if you remove D155?

[azvrt]

I have found D155 on the schematic near U106, right ?
However, on this picture:

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I only find D55 near U106, while D55 does not exist on the schematic.
Please advise.

If it is indeed D55 we must remove from the board, can it be desoldered from this side of the board or do we first need to remove the board from the heatsink ? My more technically skilled partner will know the answer to that question but if I know the answer before tuesday evening I can start removing the board from the heatsink before that, I might even be able to desolder it myself.

What does this diode do ? Does it have something to do with the thermal stuff ?
I suppose it is safe to power on the amp without it otherwise you wouldn't instruct me to do so, right ?

[Perry Babin]

Look at the connections on the schematic and see if D55 is connected to those points.

It connects the IC to the turn-on delay/softstart and to the thermal shutdown circuit.

I don't think there's any danger but you have to realize that I can't test everything that I suggest. There is always risk but I think it's minimal here. If you have a current limiter inline and have all transistors tightly clamped to the heatsink, it would be difficult to do any damage.

[azvrt]

Perry,

We easily desoldered the diode as requested, it did not affect the turn-on delay unfortunately.
It still took the amp several minutes to power-on.

[Perry Babin]

Measure the DC voltage on pin 1, pin 2 and pin 9. Does the voltage remain relatively constant on these pins up until the time the amp comes on or does the voltage slowly ramp up. Describe what happens on each of these pins.

[azvrt]

All right thanks.
Yes, my father told me we would measure several pins of U106 (if I remember correctly) next time.
Problem is we only get one shot at it during the initial 4 minutes, then we need to wait at least half an hour before the turn-on delay is exceedingly long again.

One question: would you like us to resolder the diode or leave it desoldered for the moment ?

Thanks !

[Perry Babin]

Reinstall it.

[azvrt]

All right Perry we put the positive lead of the multimeter/oscilloscope on pin 1 and the negative on ground. We repeated this procedure for pins 2 and 9, and we also measured the location after the diode (the one we had desoldered/resoldered).

During the several minute turn-on delay we measured:
Pin 1 2.72 VDC constant.
Pin 2 2.67 VDC constant
Pin 9 1.04 VDC constant

As soon as the amplifier fully turns on (and starts to play the music) which is after a couple of minute:

Pin 1 and 2 remain the same
Pin 9 increases from 1.04 VDC to 1.60 VDC. Can't remember if the increase was gradual or instant, I think instant if I remember correctly.

We also measured right after the diode (D155 on the schematic / D55 on the board).
During the delay the voltage is about 1 or 1.5 VDC, can't remember for sure as we measured this one after the amplifier had powered on fully, and each time we power it off and on afterwards the delay is normal, 2 seconds, so we don't have much time to check.
DURING the turn-on delay the VDC rises gradually. At about 2 or 2.5 VDC the music starts playing. It continues to increase slowly and gradually, at some point we measured 5.53 VDC.

On the schematic it says diode D146 (D46 on our board) should measure 18 volts, we measured only 13.2 volts there, both during turn-on delay and when fully powered on.

[Perry Babin]

D146 is used to limit the drive voltage to 18v. With a low duty cycle (normal when there is sufficient battery voltage and very little current demand from the audio section), the voltage may be lower.

It sounds like the amplifier is producing supply voltage (rail and regulated voltage for the audio section) well before the audio starts to play through. Is that true?