Sparkplug
Super Member
I am posting this hoping it’ll help someone down the road.
A while ago I acquired a C-70 in decent shape that needed cleaning more than anything. After cleaning came testing when right away there were problems with intermittent audio and also the Rch (or Lch, can’t remember) was completely missing. I traced the problem to the volume where incidentally noticed an ever so slight wobble in the volume shaft as well. Since this is a Japanese version of C-70, there are stark differences when compared to the US model. For example, the volume is a 24KΩ special taper Alps volume (RK21 series), as opposed to 20kΩ. On the volume label, it’s printed ‘special’ in Japanese.
I had already purchased an A-curve 20KΩ Alps volume (same series) for a different project and decided to replace the original C-70 volume. I made the change and ran some tests. The intermittent audio and missing channel problems were completely resolved but the volume was very low (anemic) as compared to the original pot. I now had to turn the volume to around 12 o’clock to get a decent level (not loud) out. It wasn’t the end of the world but still very irritating, not to mention the loss of overall audio quality. Next I took apart the original volume and ultrasonically cleaned it, oiled it up, re-assembled it and put it back in the C-70 hoping it would magically work and that I could continue using it, yeah right … take a hit …
The volume level was back alright, I’m talking about the difference of night and day here, but the Rch was one B-50 meter level higher than the Lch. This was confirmed by playing audio test tone at various frequencies and checking the meter. Even in Mono mode, the difference was readily visible.
So finally when last night going to bed and thinking what the heck to do, I suddenly had a vision, I’d put the original C-70 wafers into the new 20K Alps case where there was no wobble at all. So today I took everything apart again:
Top part is the disassembled new Alps 20k pot whereas the bottom part is the original, ultrasonically cleaned C-70 volume disassembled. Only the wafers from the original volume were ‘transplanted’ in the brand new case on top. I then reassembled everything and put the now upgraded original volume back in C-70. The audio imbalance was completely gone and full Yamaha volume was back. The new volume’s shaft is also 10mm shorter but if you fasten the volume knob a bit further back, no one, not even the Yamaha engineers can tell.
As for the difference in volume, it comes from the fact that the original C-70 Alps is a variation of steep logarithmic curve as opposed to the A-curve of the usual Alps volumes available commercially which are much softer curves. To see this clearly, consider the A15 and W (or special W) curves in top two graphs below:
The Alps RK21 series volumes sold at Mouser today seem to have only two taper types available (haven’t checked all models), A, and V given below:
The original C-70 curve is much closer to V-taper than A. With A-taper, you lose the ability to increase the volume effectively and in my experience a lot of audio quality. The difference is similar to playing music on two small speakers in your room and playing on two large professional speakers in a studio … go figure. As a last option, I was going to order a V-curve 50K (they don’t have 20k) Alps volume from Mouser and see what I could do with it. Thank god I don’t have to.
The moral of this story is that you can’t change the C-70 volume with just any A-curve volume, you’ll stand to lose a lot of volume and for me, audio quality.
Thanks for reading.
A while ago I acquired a C-70 in decent shape that needed cleaning more than anything. After cleaning came testing when right away there were problems with intermittent audio and also the Rch (or Lch, can’t remember) was completely missing. I traced the problem to the volume where incidentally noticed an ever so slight wobble in the volume shaft as well. Since this is a Japanese version of C-70, there are stark differences when compared to the US model. For example, the volume is a 24KΩ special taper Alps volume (RK21 series), as opposed to 20kΩ. On the volume label, it’s printed ‘special’ in Japanese.
I had already purchased an A-curve 20KΩ Alps volume (same series) for a different project and decided to replace the original C-70 volume. I made the change and ran some tests. The intermittent audio and missing channel problems were completely resolved but the volume was very low (anemic) as compared to the original pot. I now had to turn the volume to around 12 o’clock to get a decent level (not loud) out. It wasn’t the end of the world but still very irritating, not to mention the loss of overall audio quality. Next I took apart the original volume and ultrasonically cleaned it, oiled it up, re-assembled it and put it back in the C-70 hoping it would magically work and that I could continue using it, yeah right … take a hit …
The volume level was back alright, I’m talking about the difference of night and day here, but the Rch was one B-50 meter level higher than the Lch. This was confirmed by playing audio test tone at various frequencies and checking the meter. Even in Mono mode, the difference was readily visible.
So finally when last night going to bed and thinking what the heck to do, I suddenly had a vision, I’d put the original C-70 wafers into the new 20K Alps case where there was no wobble at all. So today I took everything apart again:
Top part is the disassembled new Alps 20k pot whereas the bottom part is the original, ultrasonically cleaned C-70 volume disassembled. Only the wafers from the original volume were ‘transplanted’ in the brand new case on top. I then reassembled everything and put the now upgraded original volume back in C-70. The audio imbalance was completely gone and full Yamaha volume was back. The new volume’s shaft is also 10mm shorter but if you fasten the volume knob a bit further back, no one, not even the Yamaha engineers can tell.
As for the difference in volume, it comes from the fact that the original C-70 Alps is a variation of steep logarithmic curve as opposed to the A-curve of the usual Alps volumes available commercially which are much softer curves. To see this clearly, consider the A15 and W (or special W) curves in top two graphs below:
The Alps RK21 series volumes sold at Mouser today seem to have only two taper types available (haven’t checked all models), A, and V given below:
The original C-70 curve is much closer to V-taper than A. With A-taper, you lose the ability to increase the volume effectively and in my experience a lot of audio quality. The difference is similar to playing music on two small speakers in your room and playing on two large professional speakers in a studio … go figure. As a last option, I was going to order a V-curve 50K (they don’t have 20k) Alps volume from Mouser and see what I could do with it. Thank god I don’t have to.
The moral of this story is that you can’t change the C-70 volume with just any A-curve volume, you’ll stand to lose a lot of volume and for me, audio quality.
Thanks for reading.
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