PA Amplifier, Repaired, then Very Broken...

I was gifted a mixer and integrated PA Amp.
This one is on-going as I have had to ordered parts.

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OK, I was given a 12 Channel Mixer with an integrated 300W amplifier.
The mixing desk works perfectly, and aside the power supply is a totally standalone unit.
The amplifier was distorted on the right channel, and gave nothing on the left. I also suspected the Bias on the output transistors was way off as the left channel got seriously warm.

The photo of the horrible wires all over the bottom of the board is from the left channel amplifier board and was taken when I got it. The first thing I did was remove all that and put it back to as near stock as possible (some of the tracks had gone, and this is a dual sided board, so there were some interesting areas to solder).

Once that was done there was still a very quiet output from the left channel. I started troubleshooting with no schematic, armed with a meter, I found no fault components (all the transistors were fine). Next step was to grab the tone generator (android apps are excellent for this). Shoved my old valve oscilloscope in circuit at various points and worked out where the signal was going on each channel, and where the left side was getting lost. 

I feel I should mention 2 things at this point. One, the left and right amplifiers on this unit are pretty much mirrors of each other, this is the only thing that made troubleshooting possible. Two, there are scary voltages and currents available inside these things (this led to my undoing later), if you are unsure I wouldn't poke about with power amplifiers capable of this level of output (my oscilloscope occasionally touched down in the wrong place and took 140V DC, it's hardcore and survived).

Anyway 2 hours later tracing and checking, I found the break in the pre-amp to power amp circuit (the signal carrying wire went to the wrong terminal) this is one of the few places the boards weren't symmetrical. Spot of soldering and we were in business. 

At this point I was very pleased with the progress. Nearly there I thought. Just need to adjust the Bias to stop the overheating. A little research revealed around 20mV across the output resistors would give an acceptable current flow through the power transistors (enough to keep the junction open, but not get them too hot). After adjusting the Bias pots, and re-checking each time, I was struggling to get the voltage to an acceptable point. 

On metering for the fifth or sixth time I slipped and one of my meter probes (which was just tinned wire for accuracy *see life lesson at the end) crossed between the input and output track of one of the transistors (between DC +-70V) unfortunately as soon as it made contact, it welded itself to the board (as per the scorch mark image) put the power amplifiers out of balance, and promptly (about 1/2 a second) caused one of the power output resistors to explode. Further putting the circuit out of balance. 

It's hard to convey at this point, the scale of time and moment of panic that ensued. As I realised the device in my lap was infact on fire, I pulled the plug, unfortunately I was waaay too late to save it. After the smoke cleared and I had a cup of tea, I checked the damage. All the power transistors, several resistors, several of the control transistors and preamps on the left channel had been damaged. I was not best pleased. 

*Life lesson, always use proper meter leads. 

I had a decision to make at this point, repair or replace. In light of no schematic for the existing amplifier circuit, the potential of downstream damage to the rest of the preamps and resistors etc. I opted to replace the board. I am currently awaiting the arrival of a preamp, protection board, and a pair of amplifier boards (see the parts list). I intend to reuse the case (providing it fits), stabilized power supply and anything else I can salvage. 

Watch this space... 

  • Update : The amplifiers (and other bits) have landed...

    ric86604/23/2014 at 15:33 0 comments

    OK. I took delivery of the new amplifier boards, pre-amp, and speaker protection board a little while ago. Unfortunately they are slightly too big (or more the wrong shape) to fit in the original amp board locations.

    So after a bit of jiggery-pokery, some serious hole cutting and hack saw action for the air vents, and a bit of re drilling the inputs and screw holes, things are looking like they will line up and fit. Hopefully this is the last of the physical modification of the unit I need to do. Next comes the re-wiring phase of the project, which could get quite interesting.

    The newly uploaded picture shows everything mocked up, trouble is I have lost my heat sink paste and my toothpaste started to taste funny. I just hope the two aren't connected. Anyway production will have to stop until a new tube arrives. 

    Oh and just for reference, those smoothing capacitors on the power supply board, are still charged after sitting for almost a month. Fortunately the case took the power out of them (and not the back of my hand). 

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