Aiwa BX-770 pre-amp outputs only ~120hz HUM

[waspie]

I had another thread for the amplifier component of a mini system that had some issues. those are resolved via replacement of the 4 power supply filter caps.

The pre-amplifier of this system also has a problem. It outputs a powerful ~120hz hum. The amplifier has a spectrum analyzer with a 125 hz position and that's where it lights up. On the pre-amp is an equalizer. If I move that slider or one on either side up the spectrum analuzer at 125hz will just about peg at the top and this is audible through the speakers.

This system uses a proprietary interconnect cable. Using a pair of headphones and some test leads the hum also outputs to the headphones, this noise is definitely originating in the pre-amp. No matter which input of the pre-amp is used only the HUM is output, there is no music that can be heard, only the hum.

the service manual is available here:
https://archive.org/details/manual_B...e/n11/mode/2up

That manual has both the amp (gx) and pre-amp (bx). I'm having trouble with the pre-amp.

The Q21 looks to have been replaced previously. It also looks to have left its mark on the top cover. Not sure if this has anything to do with the problem

Any help is appreciated... not sure where to start.

[CapLeaker]

Re: Aiwa BX-770 pre-amp outputs only ~120hz HUM

Start with the capacitors. Specifically those near a heat source and the main filter caps

[waspie]

Re: Aiwa BX-770 pre-amp outputs only ~120hz HUM

Start with the capacitors. Specifically those near a heat source and the main filter caps

potential silly question... can i test if they're shorted while in place or do they need removed?

[redwire]

Re: Aiwa BX-770 pre-amp outputs only ~120hz HUM

Make sure the +12VDC (Q21) and thus the -13.3V (Q23) rails are OK.
Input to the spectrum analyzer comes from few parts - selector switches (CMOS) and some buffers. It's in the early stages.
I wonder if mixing up the three special cables could have damaged it?
Sony also had that "hot" transistor and I just bolted it to the bottom of the case for it to run stone cold. You need some wire and heatsink insulator added.

[waspie]

Re: Aiwa BX-770 pre-amp outputs only ~120hz HUM

Make sure the +12VDC (Q21) and thus the -13.3V (Q23) rails are OK.
Input to the spectrum analyzer comes from few parts - selector switches (CMOS) and some buffers. It's in the early stages.
I wonder if mixing up the three special cables could have damaged it?
Sony also had that "hot" transistor and I just bolted it to the bottom of the case for it to run stone cold. You need some wire and heatsink insulator added.

Q23 is supposed to have -19.2 and -14.4 and -13.3. The 14.4 is showing -9.3
the -13.3 is -10.7


Would those voltages indicate a problem with Q23 or something else dragging it down (up?)?

Those caps right next to it look like they've lived a hard life. I figure I'll replace those. Also the resistor just to the side of Q21 looks partially blown apart. Surprisingly it ohms out at 5.7k but that doesn't seem to match up with its bands, not that I can tell what they're supposed to be. red red brown? black? gold?

[redwire]

Re: Aiwa BX-770 pre-amp outputs only ~120hz HUM

What's the +12VDC rail measuring? Because, it is the master rail, the -13.3V rail is derived from it. If the +12 is low, so will be the -13.3 rail.
Is Q21 or Q23 running really hot?

[waspie]

Re: Aiwa BX-770 pre-amp outputs only ~120hz HUM

What's the +12VDC rail measuring? Because, it is the master rail, the -13.3V rail is derived from it. If the +12 is low, so will be the -13.3 rail.
Is Q21 or Q23 running really hot?

Q21 is showing a touch over 12v. I'll check their temps later - probably tomorrow.

[CapLeaker]

Re: Aiwa BX-770 pre-amp outputs only ~120hz HUM

potential silly question... can i test if they're shorted while in place or do they need removed?

That depends in what circuit they are in. Best way is to take them out and check them with an ESR meter.

[alfatv]

Re: Aiwa BX-770 pre-amp outputs only ~120hz HUM

Are you sure it gets clean power?
You need a scope to troubleshoot it properly.

[waspie]

Re: Aiwa BX-770 pre-amp outputs only ~120hz HUM

What's the +12VDC rail measuring? Because, it is the master rail, the -13.3V rail is derived from it. If the +12 is low, so will be the -13.3 rail.
Is Q21 or Q23 running really hot?

They don't seem to be for the few minutes I have it on but evidence on the board would say they run hot. +12 rail measures at ~12.67v

That depends in what circuit they are in. Best way is to take them out and check them with an ESR meter.

They must have been. I've ordered some replacements but in the meantime I paralleled 2 220uf caps for each one and the hum seems to be gone. Also replaced the 8.2k resistor that was blown apart.

Are you sure it gets clean power?
You need a scope to troubleshoot it properly.

I don't know anyone with a scope and don't have much intention on buying one at the moment.



So, I replaced the two filter caps nearest Q21 and Q23. I didn't have exact replacements but soldering some doubled up 220uf caps for the time being. Replaced the bad resistor as well. The HUM is gone. It no longer shows up on the amplifier's spectrum analyzer and its no longer audible.

HOWEVER, still no audio. I tried using the cd and aux jacks but there's no output. I don't hear any "static" when I adjust any of the GEQ sliders which i might expect from an older piece of equipment but I may be off track thinking that would do anything.

Should I may be try to find output at the IC4 chip with some test leads and earphones? Looks as though that's the first stop for the jacks and that IC is responsible for switching the inputs?


***also***

If i plug the CD player output into the phono input I can lightly hear the music. It doesn't matter which input the pre-amp is on, audio seems to always be coming in via the phone jack.

[CapLeaker]

Re: Aiwa BX-770 pre-amp outputs only ~120hz HUM

It could be IC4, but it could be something else bad too on top of it… time to grab that ole oscilloscope, inject a tone and start probing around.

[waspie]

Re: Aiwa BX-770 pre-amp outputs only ~120hz HUM

This will kind of be my notepad so if it gets annoying just don't pay any attention. I don't have a scope so I'll just try and trace the audio as far as I can go. So far ic4 seems fine as does ic5 the next in line. I can't seem to pick it up at the GEQ IC. It's either lost at the connector to the front board or?? I need more time to double check I'm probing the right spots ( ic2 19 ground 15 signal). 3 small kids and a wife makes this a difficult process!

[waspie]

Re: Aiwa BX-770 pre-amp outputs only ~120hz HUM

Make sure the +12VDC (Q21) and thus the -13.3V (Q23) rails are OK.
Input to the spectrum analyzer comes from few parts - selector switches (CMOS) and some buffers. It's in the early stages.
I wonder if mixing up the three special cables could have damaged it?
Sony also had that "hot" transistor and I just bolted it to the bottom of the case for it to run stone cold. You need some wire and heatsink insulator added.

That depends in what circuit they are in. Best way is to take them out and check them with an ESR meter.

ITS ALIVE!!

At last I was probing with my headphones on IC5 the (double?) op-amp I noticed the sound wasn't solid on pin 2 ( the output) but figured since i was just standing over it with wires it was fine. moved onto the connector right next to it that runs the sound to the front board with the GEQ and there was no output there but then thought maybe im on the wrong pins. i also noticed some static poking around when i touched it.

Fast forward to just now I got back at it and touched the wires to the op-amp to confirm its output but no sound. i physically moved the op amp a little and had sound again. also there are some jumpers right next to it lablled L and R that are the op amp outputs so i probed there and output was dependent on me touching the op-amp. i bent it over a little more and output is solid so i checked out the connector to the GEQ and there was sound there now too. Figuring that might have been it I hooked up the amp and BAM! I got music!

Probably bad solder joints. When I get the replacement caps in I'll heat them up and add some fresh leaded solder.

I thank you and everyone else that chimed in with your input. I've never tried to diagnose something like this.

[CapLeaker]

Re: Aiwa BX-770 pre-amp outputs only ~120hz HUM

Great! I hear you on the kids and wifey and I know how that is exactly. Anyway… congrats on your repair! �� The rest should be peanuts to fix.