Electro Harmonix Bad Stone Phaser (1975) - Motorboarding/Cap replace?

[JapanVintageFX]

Hello freestompbox members,

This is my first post here, having hovered for a while. There is an incredible wealth of information and knowledgeable folk here and I'm only just beginning to understand the ins and outs of electronics in the pedal world. I have assembled a few kits, tinkered and fixed but as yet not much further. I primarily collect older vintage units and on occasion pass on units to fund others. Please excuse my beginner ignorance at times if you can.

As per the title I have a small issue with a newly acquired vintage Bad Stone which I'd be most grateful if anyone could shed some light on.

Now before I lunch a full debug, measure voltages and start unsoldering, here's a few basics and questions.

The pedal functions as a phasor should, producing a mild phased tone up to ¾ on the rate/speed dial. Above the visual guide point of 3 o'clock the signal disappears into a thumping and then moaning drone note. Reading around this appears to be "motorboating", although I am not sure if such an occurrence has the same cause per different pedal type?

Various YouTube videos attest to the BS producing very fast rates, unusual tones and a very full strong phasing overall, something my unit is currently incapable of. The signal is phased but quite mild and "normal", no over the top-ness.

What may account for this motorboating noise in the Bad Stone? Filter caps gone bad? (I realise it is difficult to be specific but in terms of low hanging fruit/experience this may be a common problem with a common solution known to members).

The circuit board has what appears to be an additional (physically) large capacitor and resistors soldered in the center. For what purpose might that be?  Filtering of the power supply? Again I appreciate this is guesswork at this stage.

(Note, the pedal exhibits the same symptoms whether powered by battery or adaptor). Bypass is fine.

As this is a 40 odd year old pedal would it be worth replacing the electrolyte caps straight off?

I am also concerned with the very thin traces lifting off the board if I attempt to unsolder, could there be a way to alleviate that happening?

It's a wonderful looking pedal which I hope finds a permanent place in my collection and would be fantastic to resurrect to its former glory. Any help or hints would be much appreciated. I will of course put up some gut shots, consult the schematic (although my understanding of them is limited) and measure voltages if required.

Many thanks,

Mark
JapanVintageFX

[thermionix]

As this is a 40 odd year old pedal would it be worth replacing the electrolyte caps straight off?

That's exactly where I would start.  It definitely won't hurt and just might address all the issues you mention.

I am also concerned with the very thin traces lifting off the board if I attempt to unsolder, could there be a way to alleviate that happening?

If you're unsure of your soldering it might be best to get a pro to do it.  Otherwise I would suggest cutting the legs of the caps you want to replace pretty close to the board, and be as quick as you can with the heat.

[JapanVintageFX]

Thank you for you reply. I shall open her up and carefully change the four caps and report back