#49 Joyo ZoMBie, part two - analysis and repair

It took the best part of an evening and a couple hours the next day, but i managed to lift the schematic for the whole preamp board. I also recreated the board, but that was mostly for sanity-checking. I used all the same component designators, but didn't bother measuring all the dozens of small ceramic caps, because there's only SO far i'm willing to go...

Quite involved, as you can probably notice. After the input buffer, the clean channel goes through one half of the 12AX7, one half of the gain adjust (dual-gang) potentiometer, including a bit of tone-shaping, and a twin-T notch-filter. The "OD" channel is more complex, as one might expect - a mid-boost (achieved via using a transistor-based gyrator, an "electronic inductor", in the feedback of an opamp), an adjustable gain stage (1.2x to 46x or so), a pair of clipping diodes, followed another mid-boost stage (using two gyrators this time) and another twin-T notch-filter.

Apart from the input, the rest of the circuitry common to both channels is comprised of the second half of the 12AX7, then a buffer driving the tone control, and the output stage. The audio jumps over to the power board and out to the effects-loop, possibly "through" the headphone output (there's a switch built into the jack socket), back to the preamp board to the volume control, and then finally into the power amp. The Bluetooth audio gets mixed into the signal at some point in that area as well (likely pre-volume-knob).

Right then, on to the tone control troubleshooting. The potentiometer itself actually seems fine, at least on direct measurements. And given the schematic and how it's connected into the circuit, if the wiper connection was wonky, the sound would go off completely, not "only" change its tonal character. Plus the "it works on some power-ups, at least for a while" thing shouldn't (or rather, couldn't) have anything to do with this potentiometer, despite its ever so slightly doubtful appearance.

Routed some music to an analog output of my audio interface going into the input, and the effects send back into an analog input, and listened back only to the latter. A few minutes in, and the fault manifested itself again - tone control ineffective, but leaning on the muffled side. Also, slow but noticeable volume variations seem to be happening. Briefly i considered the tube being at fault, but looking at the tone control chunk of the schematic, i'm now starting to wonder whether C91 and/or C93 might have something to do with it.

Either way, probing-around time, DC voltages edition. Pin 1 of the last opamp in the chain (IC4 in my reverse-engineered schematic) shows around 7V, as one might expect (mid-supply), which then goes to one end of C91. Probing the other end of C91 shows the same 6.9-7V, but there's a noticeable little "pop" noise whenever my probe makes contact there.

Slight correction time - i was struggling to redraw the resistors and capacitors in the tone control area. In the process, i discovered that one end of R93 did not in fact connect to the groundplane, but to C93 right next to it, and one end of the tone potentiometer. After that, on the third version of the redrawn arrangement, it actually DID start to make sense - an old-school tone stack as you'd find on a Fender or Marshall, but with only the treble being adjustable. Here's the corrected and rearranged version then.

Well, at least now we've got something to work with, and even some kinda-sorta-ballpark values to expect. I just removed them from the board, and tested them "externally"; all showed believable values, within normal tolerances (C86 & C93 27nF, C91 470pF, all three resistors fine).

 This is how the frequency response of the tone stack itself should be, at both ends of the treble knob (the "Tone" control on this amp). Hmm... If i reinstall only the 470pF cap but none of the other parts, that should result in a "flat" response, regardless of the tone control. Actually, it should then only act as somewhat of a volume control, in conjunction with the R40 after it, as well as somewhat of a high-pass filter (280-340Hz, depending on the travel of the pot).

So far, so good - i get a liiiitle level variation between the two ends of the pot travel. Adding the resistors back in (specifically R81 and R93; R89 would have one terminal unconnected to anything) should turn the tone control into an even more noticeable volume control. And that results iiiiin... Nothing at all. No more level variation, regardless of where the pot wiper is.

I whipped up a tonestack in LTspice, just to run some numbers, and with the two 27nF caps removed, but with the 150k and 18k resistors installed, i should get about 8dB of difference between the pot's ends-of-travel. What if i replace that 150k resistor, though? Nope. With a 1kHz test signal, i'm getting a variation of about 1.5dB, which is quite odd, at least to my mind.

Right then, i've had it, out come the two 27nF caps in the tone stack, in go two pairs of brand new 10nF ones, it's time to eliminate those as variables as well. While they don't seem to measure wrong, they may well be behind the seemingly intermittent fault. And even if they're not, they're one less thing to worry about. While i'm at the "shotgun approach" stage, out come the big guns - some frequency sweeps.

I didn't consider this before the capacitor swap, but one out of two isn't too-too bad, right? Either way, this is a sweep on the clean channel, at each end of the tone control (as per the labels at the bottom of the window). Keeping in mind there's a notch-filter upstream of this, as well as quite likely some high-passing as well, i'd say the sweeps match the Tone Stack Calculator predictions above quite well (a couple dB difference in the lows, and about 10dB in the highs).

I'm putting this thing back together, and see when some real-world tests can be arranged / run. I'll update this if the problem rears its annoying head again.