1975 Small Stone Phaser with Strange Rate Pot Response

pmkipp · Post by **pmkipp** » 15 Apr 2016, 16:07

Working on a non-functioning 1975 Small Stone. Switch was shot, so I replaced it. Now it works, but there is no response from the Rate knob until a couple of minutes before 3 o'clock where it begins with a medium slow, very nice sounding rate response, then the rate increases rapidly all the way through a couple of minutes past 3 o'clock before (like in less than 5 minutes of rotation) before going into that wacky self-oscillation. I thought it might be the pot, but when I hooked it up to my meter, it read 1 meg all the way counter-clockwise, and moved smoothly all the way to zero as it was turned clockwise. When it is in that sweet spot it sounds great in either regular or "color" mode. Before the sweet spot there is just some tonal effects, and after is oscillation. Where should I look next? Attached is a schematic.

Scruffie · Post by **Scruffie** » 15 Apr 2016, 16:17

I don't quite understand what you're saying regarding the 'minutes' but if you're referring to knob position rather than actual time it sounds like the pedal is using a B1M pot (probably because it's all EHX had on hand at that time) when it should be a C1M pot for an even range spread.

pmkipp · Post by **pmkipp** » 15 Apr 2016, 18:10

Yes - I was referring to positions in the turning of the control - 3 o'clock being pointing straight to the right. And you are right - it must be a linear pot. Turned all the way down it reads 1 meg, and as it is turned up (clockwise) it reduces resistance to zero. How could this have been around for 40 years and no-one notice this or try to fix it?? Very strange. It is in excellent shape and has the original box - maybe it just got put away in a closet and forgotten about. I hate to take off the old pot with the date code, but I will see what happens. Thanks!

The schematic just says 1m at the rate position - how do you tell it is reverse log?

pmkipp · Post by **pmkipp** » 15 Apr 2016, 18:26

Can I just reverse the 1 and 3 lugs on this pot and get the desired result? Finding something with this kind of D-shaft in a C1M value could be challenging, unless someone has a source...

Scruffie · Post by **Scruffie** » 15 Apr 2016, 18:44

No, all that'll happen then is the pot will work the same... only backwards.

You could always wire in a smooth/knurled shaft C1M pot with a replacement knob to fit the new shaft rather than trying to locate a D-Shaft which yes will probably be tricky.

Personally, i'd just leave it as is rather than tampering with a vintage EHX pedal (especially one as you say in good condition with the original box too) quite a few of them seemed to have this issue so it's not abnormal.

Possibly a tapering resistor might help though which is at least a less invasive mod...

pmkipp · Post by **pmkipp** » 16 Apr 2016, 00:53

Thanks. I may try it with a different pot just to see if it solves the problem. It is not terribly usable, except at that one setting.

pmkipp · Post by **pmkipp** » 08 May 2016, 14:25

Interesting - from what I read, these require a reverse log 1Meg pot. I got one from Small Bear, and it had almost the same effect - no effect until about 1 o'clock (on the knob rotation), then crazy oscillation after 3 o'clock. For some reason I tried a C500K pot, and it works MUCH better - effect starts at 9 o'clock, mild oscillation only at full turn. This is truly an interesting specimen. I may try a C250K pot to see what happens there.

Scruffie · Post by **Scruffie** » 16 May 2016, 21:17

Are you saying you get no sweep at all at lower rates? If so then perhaps the pot is not the issue, the 1uF cap going in to pin 1 of IC6 on the schematic or the 50uF cap to ground from the 10k coming from IC6 could be failing.

pmkipp · Post by **pmkipp** » 17 May 2016, 18:28

I will try those next. The C500k pot works quite well, but I would assume what you are thinking. And if one of the caps is going bad then it will eventually effect the pedal no matter what value pot I use. Thanks.

moonshiner · Post by **moonshiner** » 23 May 2016, 21:54

Ya it should be a reverse pot. YOu should have most of your resistance before noon on a reverse pot . Thats the only way too tell besides a C on the pot . If its a 500k you should have over 350k-400k resistance or so at noon.

pmkipp · Post by **pmkipp** » 05 Jun 2016, 14:24

Scruffie wrote:Are you saying you get no sweep at all at lower rates? If so then perhaps the pot is not the issue, the 1uF cap going in to pin 1 of IC6 on the schematic or the 50uF cap to ground from the 10k coming from IC6 could be failing.

"Interestingly, there is no 50uf capacitor anywhere on this pedal. There is a Tantalum 1uf, which I replaced. But no 50uf. Want a pic of the board?"

Scruffie · Post by **Scruffie** » 05 Jun 2016, 14:42

Ah no, that's normal, it was omitted.

Did replacing it help? You still haven't answered if you actually get no sweep at all at lower settings so it might not have been necessary.

pmkipp · Post by **pmkipp** » 05 Jun 2016, 14:54

pmkipp wrote:
Scruffie wrote:Are you saying you get no sweep at all at lower rates? If so then perhaps the pot is not the issue, the 1uF cap going in to pin 1 of IC6 on the schematic or the 50uF cap to ground from the 10k coming from IC6 could be failing.

"Interestingly, there is no 50uf capacitor anywhere on this pedal. There is a Tantalum 1uf, which I replaced. But no 50uf. Want a pic of the board?"

"So, after checking the archives at PedalArea, mine is the 2nd iteration of the Small Stone, V1. There is no 50uf capacitor after that 10k resistor. There seems to be an extra capacitor in the middle of the first version (with the metal IC's), and the large power capacitor is only 200uf and on the trace side in the first version. Maybe the schematic I have is some combo of the two? Anyway, I will try it now with the replaced 1uf cap and see what happens."

pmkipp · Post by **pmkipp** » 07 Jun 2016, 13:00

Replaced the Tantalum 10uf cap, and paired with the C500k rate potentiometer, this now works beautifully. Does not work correctly with a C1meg pot (which is what is on the schematic). Interesting. Thanks for all the suggestions.