Other Soundstream Class A 10.2 issue

[azvrt]

I have an other Soundstream Class A 10.2 with a different issue.

This one powers on and plays the music.
However, when looking at the analogue indicator of our power supply, during the 2 seconds of turn-on delay the needle goes crazily fast back and forth between 0 and 1 ampere.
After the 2 seconds it plays the music (though maybe slightly distorted) and the analogue needle goes crazily back and forth between 1 and 2 ampere, non-stop.

When we opened the amplifier we noticed R202 and R172, right next to the speaker terminals being blown/burnt. We replaced them but the current draw remained inconsistent.

Could it be the amplifier cannot choose between High-Power / High-Current mode ?

Thanks

[Perry Babin]

With no speakers or RCAs connected, measure the DC voltage across every emitter resistor. Does one or more have a pulsing voltage corresponding that the current drawn by the amp from the power supply?

Pin 7 of U9 would be pulsing if it was having trouble switching between HP and HC modes. (<< that's from a 10.0 schematic. I don't have the 10.2).

[azvrt]

So I need to connect power/ground/remote wires which will result in the powering-on of the amplifier and then measure the DC voltage (how much voltage should I expect (to what range should I set the DMM ?) across all of the resistors indicated in this picture:

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Am I correct ?

Perry, I think you should have the 10.2 schematic as I've sent you batches of schematics half a dozen of times, and some batches contained duplicates of schematics from previous batches so you must have that 10.2 schematic at least a couple of times.

But in case I am wrong, I have just sent you the schematic

Hmm, I tried to find U9 on the 10.0 schematic to see whether I could then find the right part on the 10.2 schematic, but only managed to find the large U10 and the smaller U8 and U9a and U9b. While U10 clearly has pins, U9a and b seem different. Can you help me out here using the 10.2 schematic. We'll definitely check for pulsing on the pin, then. How should we check pulsing on a pin ? DC voltage, positive probe on the pin, negative somewhere else ? Just guessing. Let me know and we'll definitely have a look.

[Perry Babin]

The voltage will be no more than about 1/2v but the exact voltage would depend on the bias setting.

I had it but not on the computer I was working on.

Looking at the 10.2 diagram, pin 8 of U105 is the output for the HP/HC control.

Is the HP/HC LED switching on/off?

[azvrt]

0.5 volt ? Okay.
Bias should be 25 mV for this particular model, contrary to my other 10.2 I have not yet checked/set the bias on this one.

U105, pin 8. All right. I should check for pulsing DC voltage there, right ? How much voltage to expect ? 40 volts max ?

Will have to check on that LED. Hopefully we will have time for this amp tuesday evening, as our priority is the 705s as it is for a friend of mine. Unfortunately we only work together on the amps one evening a week.

[azvrt]

Hi Perry,

The high power / high current led should be red or green depending on which mode.
It stays red all the time (high power).
Also, pin 8 on U105 is not pulsing.
However, we measured the rails, and voltage was varying very rapidly between 27V (high power) and 20V (high current).

The bias was around 20 mvdc for each channel, should be 20 / 25 mvdc on this particular amp so that's okay for now.
We started checking voltage across the emitter resistors. We checked 3 of them (all were approx 9 mvdc), by the time we reached the fourth one of our DMM's probe slided a millimeter and we shorted with a transistor. No harm done we believe, but we stopped performing the check as on this particular SS amp, the resistors/diodes/transistors solderings are very very close to one another.

We have no clue what is causing the problem, do you ?
Maybe I will replace the TL074 (U105) and SG3524 (U106) by 14 pin DIP sockets and put back in the chips, and replace one at a time to see whether there is something wrong with one of these chips. I don't know.

Fred

[Perry Babin]

What sort of pulse are you looking for on pin 8?

Are you sure that you're driving the amp hard enough to get it to switch?

Do you have another 10.2?

[azvrt]

Hi Perry,

If you read through your posts above you asked me to check for a pulse on pin 8 on U105, first after consulting the 10.0 schematics and later the 10.2'

I am not driving it at all as you asked me to perform the tests with no load and no RCA connected. It appears to be switching between High Power Mode and High Current mode even with no load connected. Also, before I knew it was malfunctioning, I once did have it play music in 4 ohms stereo to test it and that's when I saw the analogue indicator of our power supply going crazy instead of idling nicely around 2 ampere, this model of amplifier's standard.

According to owner's manual of the older 3.0 and 6.0 which had the manual switch, the amplifier should be in High Power Mode up to 1/2 ohms stereo / 1 ohms mono and must be set to High Current mode at a lower impedance, for instance 1/4 ohm stereo / 1/2 ohms mono.
In the 10.2 manual it does not say how the automatic switch exactly works. I always thought it would detect the impedance hooked up.

Yes I do have another 10.2, the one with the minutes-long turn-on delay of which we never found the problem but cured itself ?!
I was thinking of using it to do comparative measurements, maybe you could give me some pointers of where to perform these ?

Otherwise I might just replace the two chips in the power supply section and see what happens.

[Perry Babin]

The circuit monitors the current draw from the power supply and when it reaches a certain threshold, it decreases the rail voltage. If the current draw never reaches that point, it won't switch. Unless you have a reason to believe that it's defective, I'd leave it as it is.

[azvrt]

Hi Perry,

I can´t leave it like this as it´s clearly defective. Instead of nicely idling at 2 ampere, the current draw varies crazily fast between 1.7 and 2.3 ampere. The needle of the current indicator of our power supply goes back and forth real fast and non-stop.
At some point I tested the sound and I believe it was distorted, but even if it wasn´t, I cannot sell the amplifier in its current state.
The rail voltage goes back and forth between 20V and 27V (HC and HP mode) so there is definitely something not right.

I could use the other 10.2 to do comparative measurements to find the source of this problem, but I wouldn´t know where to start.

Which circuit monitors the current draw and decreases the voltage if necessary and where is it located, it would be a good place to start checking I guess.