#47 Joyo ZoMBie guitar amplifier teardown

My bandmate got his hands on this Joyo ZoMBie about a week or two back; it was sold as faulty. No idea how or why, but there's something dicky with the tone control - on some power-ups it works fine, but sometimes quits working (ie. sounds like the setting got stuck at some random point along the travel of the knob), no matter how much you twist it. Power cycling sometimes helps, but inconsistently, and even if it does, it just "gives up" again, sooner or later.

Lots of Allen-head bolts on the outside. Upon a cursory glance though, i was quite surprised to see some "chinglish" (aka "engrish") right in amongst the markings on the case. See if you can spot it.

Now, i'll freely admit i'm no rocket surgeon and my math is nowhere near stellar, but i'm pretty sure a plural involves at least two items, doesn't it? Or who knows, maybe it's just me...

At first i thought it might only be my imagination, but nope - the handle on the top really IS cocked over to one side (ie. the vertical parts at the end aren't quite the same length.

The power brick's just... well... creepy. No OEM markings whatsoever, just about as cheap and generic as you can get. Some might even call it a death-trap (as is the case with far too many no-name aftermarket phone and laptop chargers - cue horror stories of housefires and whatnot). Oh, and it weighs about the same as an older 2A Samsung Galaxy Note 10" charger, i kid you not. Which, incidentally, actually IS UL-listed and has been safety-rated and certified etc.

But first thing's first - we gotta get inside. 8 Allen bolts later, the top cover comes off, revealing the awesomeness(?) of the Chinese (albeit Joyo-branded) 12AX7, in all its "tubey"glory.

Four more bolts off the bottom, and the amp "chassis" can be slid out of the casing. Peeking under the skirts, we see...

... The two boards making up this thing. One on the bottom, and one upside down, on the top. I must admit, i like the bottom silkscreen on the former, showing all of the components on the other side, both SMD's and through-hole. Somewhat horrid soldering on some of the connectors though. I wonder what're the odds the through-hole parts were soldered in by hand... That would explain the state of the soldering, as well as how nowhere-near-straight the DC input jack is.

In order to gain access to the top board, the bottom one needs to come out first, by removing the nuts around the three rear jack sockets. I must say, a very nice touch - all the connectors between the bottom board and the rest of the circuitry are keyed, and the two 3-pin ones have their removable sides at different ends, so it's quite impossible to mix them up (that's if the cable lengths allowed it, in the first place).

As one might expect, no-name hardware (the jack sockets, anyway), but surprisingly enough, brand-name silicon - TI and JRC opamps, and a class-D power amp chip.

Three RC4558's handle the effects-loop, buffering before and after the volume control, and the Bluetooth audio. Speaking of which, a CSR8635-based module is in charge of that. That's powered from an LM1117, and a no-name 78M15 supplies all the analog circuitry. A lone NJM4560 drives the stereo headphone output.

The power amplifier is a PBTL-configured TPA3118. From earlier tests of other amplifiers based on this chip, it's capable of considerably more than what this is rated for, but it needs to be provided enough "juice" to be pushed that far, so...

[Later edit]

The amplifier, for whatever reason, seems to like to run pretty warm, if not even hot. Granted, i only noticed this with the thing powered on but without a speaker connected and the volume down to minimum, but still... The little blue boards i used in an earlier project, despite using the same chip, idle at a much lower temperature. Go figure...

[End of edit]

At the far end of this board is an XLsemi XL6007 boost converter. Looks like they're running the 12AX7 at an elevated voltage, which is quite nice, i suppose. Given the 63v output capacitor, i'd reckon it won't go higher than 50-55v or so, but we'll see about that later.

Moving on to the top board - there are four TI TL072's here, a pair of Fujitsu FTR-B3 relays (for the channel switching), three transistors marked "J6", which may or may not be S9014 NPN's, and a pair of diodes (the "T4" markings on the SOD-323 packages may well correspond with 1N4148WS). And, of course, the socket for the dual triode, on the other side. Slightly disappointing, though - right in the center of the socket, there's an LED - orange, i'm guessing, to give some extra "mojo glow". Or no, wait - extra [ahem] "tube warmth" (since the heater wasn't enough). Groan...

Just as on the bottom board, the silkscreen with all the component outlines is replicated on the unpopulated side of the PCB. Not many traces routed on the bottom, which bodes well for potential later reverse-engineering efforts...

Oddly enough, the tops of the two relays look quite dome-like to me. To the best of my knowledge, they both work just fine, but it's still very unusual, i hadn't encountered "bulged" relays before.

Nice thing - the potentiometers are held together with metal rivets. Not-so-nice thing - the rear cover of the tone pot looks less tightly sealed than the other two. Could this have suffered a firm knock on the tone knob? Since it's the one presenting issues, i'd call that plausible, to say the least. Slight bummer that the only(?) way to investigate that, short of outright replacing it, is a quite a one-way disassembly operation. Unless the two rivets can be removed non-traumatically and i can peen over two pieces of wire to replace them, or something...