154 - adding a 2nd scale to the meter - and how accurate is the TF2331 THD meter?

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In part 1, I repaired this vintage Marconi TF2331 distortion meter. Unfortunately I had to replace the defect original analog meter with a generic one. This video is about adding the missing 2nd scale to the generic meter and trying different approaches to determine the accuracy of the THD readings.
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0:00 TF2331 intro
0:40 removing the scale
1:01 making a new scale
7:53 install new scale
8:28 THD average vs True RMS
14:14 too much noise?
14:46 simulation approach
17:31 rectangle signal
18:47 saw tooth signal
19:41 sine+1 harmonic
21:05 conclusion

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Опубликовано:

12 июл 2024

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Комментарии : 8

@davidv1289 Месяц назад

Thank you for another interesting video. Your solution for the meter scale replacement is excellent - I have filed it away for future reference! Regards, David

@t1d100 Месяц назад

Thanks, for another great video!!! Precision printing tips = I print my DIY PCB board layouts to 100% scale, in order to proof the footprints to the actual component, itself. In learning about doing that, I discovered that inkjet printers have a common problem... Even set to 1:1 scale, they do not print true. I checked mine and, yes, it prints slightly smaller than it should. Here is my work-around... I add a 100mm line to a layer that is not active on the actual board... In KiCad, I use the Users Comments layer. I print the board with the User Comments layer set to be included. After printing, I measure the line with a ruler that I know is true. My line measures 98mm, so I adjust my printing scale to 102% This works well enough. This procedure should work correctly for meter faces, as well. HTH.

@TheHWcave Месяц назад

That is a good tip, thanks for sharing. I remember similar issues when using an ink jet printer. These days I am using a Samsung laser printer and find that it is quite accurate

@ivolol Месяц назад

That square wave filtered graph would seem to be the most informative - something causing the lop-sided waveform. I wonder if it's more caps needing to be replaced, maybe even selected for some moderate matching.

@geirendre Месяц назад

14:15 I was able to do a quick test of this on my TF2331 this morning before heading out on a trip. With shorted inputs it read somewhere in the middle between 0 and 1 on the 0 to 10 scale of 0.1% So in the 0.006 - 0.007% area of distortion. And with a slight movement on the needle, sugesting that it's noise it's picking up. Your's looks to be at about 0.004 (not 0.04). Probably not that bad considering the old germanium transistors in this units... I tried measuring the voltage on the output, but it was all over the place. Starting at several voltages and slowly counting down into the milivolt range. Have to investigate this further when I get back to my shop later this week.

@TheHWcave Месяц назад

Many thanks. Please share what you find

@geirendre Месяц назад

@@TheHWcave I made a mistake and measured the DC volts at the Output, that explained the strange readings I got. Must have been way to early in the morning... Anyway, when I hook up my HP3400A True RMS meter to the output, things look much better. I put the HP meter on it's 100mV range, and then do the "Set reference level" on the TF2331. With a 700Hz 2V p-p input signal from a Uni-T sig gen, I was able to dial it down to around 0.0025% THD. The controls (frequency and phase) are extremely touchy at this stage, so I'm considering replacing them with newer ones. Changing to a 10KHz signal I was able to get the THD even lower, down to around 0,0015%. I had a scope connected in parallel with the HP meter to watch the signal, and it's basically only noise left at that point. At least that's what the scope is able to pick up...

@TheHWcave Месяц назад

@@geirendre Very interesting. So using an external meter works fine with your unit. This means I need to go on a hunt to find the source of the excess noise in mine.