Hifonics Brutus BXi1100D mk2 no audio

[Mote]

Rail voltages (black probe on GND) red on center leg of rectifiers: 48V and 48,3V
Voltage on inductor (black probe on GND) 24,1V on one terminal 23,2V on other, second inductor 24,2V and 22,9V.

[Perry Babin]

There should be a resistor (750 ohms?) between the 330uf cap and pin 2 of the LM833. Do you have audio on one or both ends of that resistor?

[Mote]

Now I got some really strange results, earlier I stated that I saw audio on 330uF capacitor. Now I am not so sure, started to check for audio, fed in 100Hz sine and got the signal (0,1V/10ms) on attached photo on both ends of 750 ohms resistor. The resistor is located right next to the cap, I'll give you the board designation later on. I grounded scope to the pin which is supposed to be screwed to heatsink. OK I looked at the signal and thought that looks a little bit like a sine, but not really. Then I realised that I had not even hit play on the head unit, I removed RCA cable, still the same waveform on resistor, then I switched off the amp and still had the same waveform.

OK, then I connected scopes ground to ground terminal of amp and made sure I had 100Hz sine on RCA input connector, but there was no audio on 750 ohms resistor and no audio on 330uF cap.

[Perry Babin]

You'll have to follow the signal through the preamp circuit to see where it's being lost. I'll assume that you have operated all controls through their entire range and you didn't see any audio on that cap.

You will only have audio on the cap side of the 750 ohm resistor if the op-amp is working properly.

[Mote]

OK, this amp has some intermittent problem, I was able to see audio for a while on one side of the 330uF cap, but not on the other side. There was still no audio on 750 ohms resistor only 6,30 VDC.
Voltages on 833N which I posted earlier seem OK?

Other thing I noticed was that when probing or measuring voltage on pin 4 of 494, the amp makes hissing noise, never seen/heard something like this before.

[Perry Babin]

Is there a lot of high frequency noise on virtually every component in the amp? If so, the primary filter caps may have failed.

If you need to measure the voltage on pin 4, connect a 0.1uf cap (or anything close) from pin 4-7. That should allow you to measure the voltage on it.

[Mote]

I read from your tutorial about it and looked at the photos of the caps and mine look similar to those which have caused problems frequently, but not exactly the same.

I used 100 Hz sine to look for audio, with the amp turned off I get nice thin sine on RCA connector, as soon as I turn on the amp the signal goes off from screen for a second and then returns but then the sine is a fat line. I do not have a photo of the waveform but I think that is high frequency noise on the sine. Same on every opamp in preamp section. On pins where I do not find audio there is also no clean line, also fat line which I think is noise.

[Perry Babin]

If you have a large electrolytic capacitor, connect it across the B+ and ground to see if that reduces the noise significantly.

[Mote]

Largest I had was 2200uF, it did not remove the noise when it was connected between B+ and GND terminals.

I thought that maybe it was too small. So I removed the 4 2200uF caps, I drew a red box around them on the attached photo. These are primary filter caps, yes?
I replaced them with two 2200uF caps, one for each side. No difference in noise.

Then I thought that maybe I got it wrong and you meant bigger, 2200uF/80V caps. I drew blue box around them. I have no big caps which can handle 80V or more. I did notice that when checking for continuity with DMM between their legs one set goes from - to 1 (no continuity) very fast, while the other set takes its time. I do not know if it means anything or not.

[Perry Babin]

Email me.
babin_perry@yahoo.com