UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - SINAD Meter

sentinel040 · 21st Sep 2022, 6:08 pm

If I may, I would like to ask a question about this.

Having tried to get my paws on an original one, I started to build one of these a bit back when I first saw the post. I purchased the meter from the link provided and removed the internal series resistor (it helps). I used the 8V regulator from the original circuit (circa +/-4V). However, when I powered it up it would only give circa 25% of the fullscale deflection. When the test audio signals were applied, the pro-rata decrease in readings appeared to be about right, which was some encouragement.

I subsequently went through everything, and it all seems in order, the notch filter is nice and sharp and the input stage controls the gain nicely. The output from the rectifier hits the supply rail (more or less) in the positive direction and not quite in the other. So I was a little mystified. I checked the fsd of the meter and it is 1mA. Going into mild desperation mode increasing the supply to circa 15V (to give circa +/- 7.5V), then reducing the 6.8k resistors gave a greater scale reading, but not in a linear fashion. Now this is not nice, but in desperation they went dowm to 680 ohms and this gave just over fsd. I suspect the output stage of the op-amp is limiting the current, the datasheet states it is short circuit protected, hence daring to go so low with the resistors. Bodgery I know but I only want to try to get a feel.

I was wondering if the meters actually are supposed to be 1mA, as the one I received had a different scale.

Cheers

Ian.

21st Sep 2022, 6:08 pm	#24
sentinel040 Hexode Join Date: Feb 2007 Location: Near Lichfield, Staffordshire, UK, most of the time and Crystal Palace, S London, some of the time.. Posts: 330	Re: SINAD Meter - Home Made for £15. If I may, I would like to ask a question about this. Having tried to get my paws on an original one, I started to build one of these a bit back when I first saw the post. I purchased the meter from the link provided and removed the internal series resistor (it helps). I used the 8V regulator from the original circuit (circa +/-4V). However, when I powered it up it would only give circa 25% of the fullscale deflection. When the test audio signals were applied, the pro-rata decrease in readings appeared to be about right, which was some encouragement. I subsequently went through everything, and it all seems in order, the notch filter is nice and sharp and the input stage controls the gain nicely. The output from the rectifier hits the supply rail (more or less) in the positive direction and not quite in the other. So I was a little mystified. I checked the fsd of the meter and it is 1mA. Going into mild desperation mode increasing the supply to circa 15V (to give circa +/- 7.5V), then reducing the 6.8k resistors gave a greater scale reading, but not in a linear fashion. Now this is not nice, but in desperation they went dowm to 680 ohms and this gave just over fsd. I suspect the output stage of the op-amp is limiting the current, the datasheet states it is short circuit protected, hence daring to go so low with the resistors. Bodgery I know but I only want to try to get a feel. I was wondering if the meters actually are supposed to be 1mA, as the one I received had a different scale. Cheers Ian.