Volume Sag with Attack? (germanium fuzz/MK1 content)

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Dr Tony Balls
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Post by Dr Tony Balls »

Hi. Looking at a fuzz for a friend and it's got a strange problem that i've not encountered before. It's a MK1 Tone Bender type (Maestro FZ-1, Zonk, etc) made by someone that's not me and it sounds pretty good but when you really dig into a chord, especially a low one it seems, there's a brief volume sag before returning to nominal volume. The resting bias on all transistors looks appropriate for this type of circuit but apparently something is briefly bringing it out of whack when its given a lot of input signal. On both Q1 and Q2 the voltages change slightly when you dig into a chord, but on Q3 the collector voltage drops quite a bit (from ~8V to 2 or so) when striking a chord. Has anyone experienced this before in a MK1 type build or elsewhere? My guess is that Q3 has gone south but not sure.

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stolen
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Post by stolen »

Hey!

We never built this circuit before, but we have a suspicion: This kind of sag probably occurs due to the asymmetric load on the coupling capacitors between Q1&Q2 and Q2&Q3 respectively: At large input levels the base diode clamps down hard on the caps and shifts their voltage drop outside of the headroom, thus resulting in a volume drop. If it were our pedal we would inspect if:

a) the coupling caps have indeed the correct value;
b) the electrolyte is not accidentially in the wrong orientation
c) if increasing the 100nF cap between Q2 and Q3 (by a lot; hook up a thicc electrolyte in parallel for testing) helps with the issue.

Since you speak of positive voltages; if this is a npn version or some other modified ground option b) is a likely failure case. You can just measure (without input signal) if the voltage during operation across the electrolyte(s) has the correct polarity to check for this error.

We find it unlikely that it's a transistor fault.

All the best :D!

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purelyviol
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Post by purelyviol »

bumping a dead thread. working on a mk1 and had the 'sag' issue as described. messing with bias resistors on q2 & q3 did little to help but messing with the caps after q1 and q2 FIXED the issue. Thank you.

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