Broken EH Small Stone

[eric90]

Hi I have an OLD Electro HArmonix EH4800 small stone USA.

I have been using it for years with batteries and decided it was time to get a DC connection going for my pedalboard. I followed a guide on another site on how to make an adaptor as it has the old style dc input, including voltage and polarity. However as soon as I plugged it in it has stopped working and hasnt worked since with batteries.

I was told by someone that I may have just blown the protective diode, installed for this kind of accident and that I could easily replace it.

Has anyone else had an experience with anything like this or got any useful info.

Thanks

[Mark Hammer]

Consider looking at this thread: EHX.com | Help needed identifying a Small Stone (v2/v3 mix?) | Electro-Harmonix Forums

There IS no protective diode on the SS. However, there is supposed to be a current-limiting resistor from the adaptor input to the rest of the circuit. That might be burnt out. Various schematics will show it as 47 ohms to 100ohms. Try 100 ohms.

Unfortunately, if it doesn't work with batteries, there is also a chance that one or more key chips are blown, although that seems unlikely. You could blow all the chips and still hear the clean sound at all times. My gut says that the power is simply not reaching the rest of the circuit.

Do you have a meter to take some readings?

[52 Bill]

And also don't forget that there is a battery cut-off contact on the power jack that may not be closing as well.

[eric90]

Thank you for your replies. I have a multimeter to check this out so I will have a further look into it. The pedal makes no noise what so ever when engaged but still lets the signal through in bypass.

Also could you elaborate on the battery cut-off please. I am relatively new in the world of electronics and especially regarding pedals!

Thank you

[Mark Hammer]

52 Bill makes a good point. The little phone jack on the rear of the pedal lets you plug in a power adaptor to run it off external power, rather than battery. The way it is wired, the pedal normally runs off battery UNLESS you insert a plug into the power jack.

That arrangement requires:
a) that the battery leads be in place and properly connected,
b) that the spring contacts on the jack itself have the required tension and spacing such that the battery connection is restored when you pull the plug out,
c) that the contacts be conductive enough (i.e., no tarnish build up) that battery power is restored when the plug is pulled out.

Personally, I've found that battery leads in many vintage stompboxes are of poor quality, and fracture easily. So, verify the battery snap is properly connected, and that there is a red wire of some sort running from the power jack to the circuit board.

[eric90]

I took a good look at the dc socket and it seems to be fine so perhaps it is the resistor.

I have been unable to identify the correct schematic for my pedal so I was wondering if anyone knows what schematic I need for the model attached. I believe it a version 3 model but not entirely sure about that.

Any help would be greatly appreciated.

[uvacom]

Well, I don't think there's a readily available schematic for your version, but judging from the earlier issue J Small Stone schematic (and common practice/good sense), there is a buffer between the dry input signal and the output when the pedal is engaged, so if something with that buffer is broken you won't hear anything with the pedal on. If all of the passive components look okay, the component I'd look at would be the LM324N - it's a quad op-amp and I'd bet one of those op-amps is the input buffer. I'd replace that, either with another LM324N (which is basically 4 741s on one chip) or even something slightly better like a TL074, you might perhaps notice a little less noise and/or a little more "transparent" sound, although I'm not making any guarantees.

[eric90]

Thank you for your replies. I will see what I can do with it over the weekend and hopefully with some luck it may just work

[oc disorder]

It appears yours is a version 3.
from:- Pedalarea - Electro-Harmonix Small Stone
Electro Harmonix - Small Stone USA Version 3

At the beginning of the 80's Electro Harmonix released the third version of the Small Stone. The circuit was again slightly modified to use dual OTAs - two
LM13600AN and one LM324N were used. The sound of this unit seems somewhat stronger and more balanced than the previous versions.