Is the volume wheel value wrong on the GBA schematics?

Question

This GBA schematics says the GBA volume wheel (VR2 at the center bottom) is 30k ohms. Another schematics can be found here and it also says 30k.

However when you do a ebay.com search for GBA potentiometer you will only get B103 potentiometers which have 10k ohms. Same on aliexpress. Everyone seems to use them.

I asked a seller and he told me the schematics is wrong. Another seller told me the wheels are wrong.

I have a GBA here but it's not possible to measure with the wheel still connected to the board.

Anyone knows if the schematics is wrong?

I already asked a similar question but it assumes the schematics is right.

I asked a different question about the GBA hardware on SE electronics and they told me to ask here. So I hope this is the right site.

Here is a picture of a 10k ohms B103 spare part potentiometer. (Not the original.)

Edit: Also see my comment in the accepted answer.

You apparently bought a replacement pot. Presumably you removed the original, or you're going to remove it. Remove it! What did it measure at? Have written on it? — dave, Sep 14 '22 at 11:31
@another-dave I don't want to remove it before knowing the replacement is the right one. — zomega, Sep 14 '22 at 11:43
@another-dave When I measure in-board I get 17k ohms increasing 0.1k ohms per second. Does this mean it's not 10k ohms safe? — zomega, Sep 14 '22 at 12:16
Someone could remove a working one from a gameboy and measure it. — user253751, Sep 14 '22 at 15:27
It’s quite possible that it doesn’t matter; if the output of the potentiometer goes to a high-impedance input (which is quite likely) then it’s just acting as a voltage divider so changing the value by a factor of three will make little difference. 30k is an unusual value for a potentiometer. — Frog, Sep 14 '22 at 20:13
@somega - my fairly limited understanding is that, if the end-to-end resistance of the pot is 'R', nothing in the powered-off circuit (i.e.., effectively parallel to the pot) will make it read as more than 'R', only a lower value is possible. Since you're reading more than 10K end-to-end, the pot itself is more than 10K, but the exact value remains unknown. — dave, Sep 14 '22 at 22:36

Kaz · Accepted Answer · 2022-09-18T16:53:52.973

I opened up one of my Game Boy Advances (AGB-001) and took some measurements.

When measuring the potentiometer between pins 1 and 3 in-circuit, the initial resistance I measured was circa 7k Ohm, increasing rapidly to 10k+ and slowing, then tailing off around 18k. This is reasonable, given that there are three capacitors shown in parallel with the potentiometer (C50, C51, C59), which charge up at a non-linear rate when a multimeter is applied to the circuit. I'm mainly quoting these values as something that's easy for others to measure and compare against.

Having desoldered the contacts from the board, I measured the resistance between pins 1 and 3 as 26.5k. This would be slightly outside a 10% tolerance band for a 30k resistance (30k - 3k = 27k), but could be reasonable given its age.

I then proceeded to desolder one of the potentiometer's mounting points, and lever the package up to have a look underneath.

At first glance, all I saw was the mark 18C. There is, however, another mark, higher up, and fainter. It's slightly obscured by the scratches I made in levering the potentiometer off the board, but it appears to read 334. Confusingly, this would indicate 33 x 10^*4, or 330k, an order of magnitude higher than what I measured! (18C could be an EIA-96 code, but that would represent 15k.)

In short, neither of these possible values match the schematic, nor the value I measured on the aged component.

Package markings aside, this discrepancy in volume is unlikely to have much impact on the performance of the GBA's audio. Assuming the VOL pin on the AMP chip is a high-impedance input (drawing little current), applying circuit theory to the 30k potentiomenter turning from 0% to 100% would give a range of voltages from 0.401V to 2.959V, assuming the batteries were providing exactly 3V. Swapping this for a 26.5k pot gives a similar range of 0.445V to 2.955V. If a 10k part were used, it'd be 0.929V to 2.907V.

Based on these numbers, the exact resistance value of the potentiometer is not important in this circuit, and Nintendo may have chosen to use other convenient values in GBAs at other times during its production. Swapping in a 10k part results in a loss of ~0.58V range in voltages to the VOL input, or 22.7%. This is practically all at the bottom (quiet) end of the volume scale, so it'll have no practical effect on how loud the sound output can be.

In my personal experience, the bottom end of the volume range was too quiet to be audible on the built-in speaker, and I'd have to turn it up a good quarter of the way in order to start hearing anything. The loss of that part of the range would therefore be inconsequential.

I now got the info from a popular seller that it was always 30k. I also desoldered a potentiometer and also measured 30k. I think this is the right answer. When using a B103 potentiometer I was told to bridge PIN 3 and 4 (when looking at the board) so you have 10k fixed and 5k variable resistors. — zomega, Sep 18 '22 at 17:00

score 2 · Answer 2 · answered Sep 18 '22 at 10:55

It is unlikely that the schematics can be said simply to be wrong.

You may have the schematics for a different version or revision of the product which may or not match the product you have or someone else has.

So if there are reports that different resistance values have been used, it's likely true. The products may use completely different audio amplifier, and thus require a completely different pot as well.

You need to figure out which version of the product you have to figure out whict kind of pot your product has in order to know what is the correct replacement for it.

Is the volume wheel value wrong on the GBA schematics?

2 Answers2