Rusty Box/Traynor TS50B - are these caps backwards? + any tips

[MrStab]

there's a lot of "shorthand" in my layouts - i was aware of that PNP's orientation and traces under horizontal components, but i forgot to put em  on the schematic - thanks, though, it made me check to be sure.

i've populated the board (i'd fixed a few things in the layout before starting), but not tested it, although i have just gotten the charge pump working. i only had 3 schottkys so the rest i subbed with a 1n4001 and 2 germaniums (forgot which) i had lying around, which gives me ~25-27V. i wonder if im missing something or if i've just reached the limitation of the diodes. maybe i could get away with 25? i'll give your link a read, cheers duck!

[MrStab]

*sigh* debug time. full supply voltage on opamp output, that can't be right. no obvious flow to that strip. at one point i got a faint distorted signal which was affected as expected by the pots, so that's a start i guess - but i can't figure out how to repeat it.

[MrStab]

recent clues: i get a really faint signal, all affected accordingly by pots, when im using my middle or neck pickup position, but nothing on the bridge, and the signal totally disappears when i enable distortion on my amp (which you'd expect to make things louder).

i put the 4u7 cap after the NPN the opposite way from the schematic, i just can't see how signal can pass through to the PNPs if the negative is pointing away from em. cant think of anything else. no clue about opamp voltage weirdness either. hmm... wonder how much underpowering the circuit has to do with it.

[psychedelicfish]

i put the 4u7 cap after the NPN the opposite way from the schematic, i just can't see how signal can pass through to the PNPs if the negative is pointing away from em.

Which 4u7? The one I labelled C12? I'm pretty sure I put it the right way round, measure the DC across it just to be sure. The orientation of the cap has nothing to do with whether it passes signal or not, but electrolytic capacitors need to have a DC voltage with the correct polarity across them, or they won't last very long, or, if you reverse DC them, explode.

As for the full V+ on the op amps output, make sure you cut the trace between pin 1 and pin 8 of the op amp. You should also double check you've got the chip the right way round (everyone makes that mistake at least once)

[MrStab]

C12 is the one. i'll reverse it back if needs be, although that's probably not the issue if it'll still pass signal

i've tried looking for obvious leakage such as from pin 8, but no dice - iirc there's a few mV difference between either pin. no explosions of note - though it was very tempting for the sadist in me to try a backwards tantalum right at the end of the charge pump. lol
tried 3 TL072s (all right orientation) just to rule that out

i just ate and took a break, maybe i'll notice something if i look again

[MrStab]

the plot thickens:

with opamp removed, the socket reads:

(V+ = 24.6)

1 = 2.2
2 = 4.1
3 = 9.9 (bearing in mind vbias is via. 270k resistor)
4 = 0
5,6,7 = 0
8 = 24.6

the negative side of the wrong-way cap is about 2V higher than the positive side, guess that's evidence enough i need to change it.

i've examined all trace cuts compared to a reversed image - they all check out. no apparent solder bridges. i'll switch that cap & see what happens.

also, V+ sometimes drops to about 15V, but this seems to happen after the charge pump & i haven't figured out the cause yet

[psychedelicfish]

Without the op amp pins 1 and 2 should have no DC on them, if you look at the schematic there is no connection to VB or V+ from a DC perspective. There is a rather large resistive path between pins 1 and 2, and because pin 2's voltage is higher than pin 1, i'm willing to bet pin 2 is where your problem (or at least part of it) lies. Check, double check and triple check for solder bridges etc. around pin 2. You should also check if the voltage on pin 2 changes with the gain control.

[psychedelicfish]

I'll also print off the schematic and breadboard it, partially to make sure the schematic is good, and partially to see how it sounds.

[MrStab]

fixed it! (i think) - i tracked down the signal with my "biological audio probe" - no, i meant my finger - and isolated the problem to the bunch of 47nF caps, which i'd actually used 22nFs in parallel for so there must've been one or two sketchy solder joints. the voltage is loaded way down now, at like 11v, but opamp voltages seem to be in order, signal passes, and everything inc. boost works.
it was from touching the treble pot lugs and hearing the "buzz of hope" that i tracked it down. i do actually have an audio probe, but somewhere down the line it got caught up in a headphone - 1/4" plug conversion.

thanks a lot for helping me narrow it down. maybe you should go ahead with the breadboarding & see if things can be improved upon. i still wanna fix the voltage sag so it's more like the original TS50, but i have to spend the day with my lady tomorrow instead of making cool stuff  lol

[MrStab]

observation: with gain at full, and for the last little bit of its rotation (ie. not just shorted to one end), and the boost wires connected together, there's a high-pitched whine. i wonder if this is related to the charge pump IC, despite "S" models of the 7660 having the "power to pin 1" feature to increase oscillation frequency (i know it's also called "boost", but that would've confused things. lol)

annoyingly, as i'm using DPDTs (i'm broke), i won't be able to both add in an extra resistor AND use an LED in boost mode. damn.

on guitar at least, it sounds nice & chunky but with a kinda "glassy" feel. i don't know if these descriptions hold any meaning outside my head. maybe i could record it in a coupla days, but my sound card sucks and i can't find my USB interface. right, it's almost 6:30am - i can finally go to sleep without that nagging sense of failure!

[psychedelicfish]

observation: with gain at full, and for the last little bit of its rotation (ie. not just shorted to one end), and the boost wires connected together, there's a high-pitched whine.

It sounds to me like your input and output wires are too close together. The capacitance between them makes a positive feedback loop and causes the circuit to oscillate. It only happens when the gain is maxed because that's when the circuit has enough voltage gain to start oscillating. If I'm right, then you might be able to hear the whine kind of mixed in with your guitar when the gain is turned up just below the point of oscillation.
It could be the charge pump though, check the AC voltage on your supply rail. Also, I don't see any filtering on VB in your layout, you might want to add some (10uF to ground should do the trick)

[MrStab]

observation: with gain at full, and for the last little bit of its rotation (ie. not just shorted to one end), and the boost wires connected together, there's a high-pitched whine.

It sounds to me like your input and output wires are too close together. The capacitance between them makes a positive feedback loop and causes the circuit to oscillate. It only happens when the gain is maxed because that's when the circuit has enough voltage gain to start oscillating. If I'm right, then you might be able to hear the whine kind of mixed in with your guitar when the gain is turned up just below the point of oscillation.
It could be the charge pump though, check the AC voltage on your supply rail. Also, I don't see any filtering on VB in your layout, you might want to add some (10uF to ground should do the trick)

they're not physically near on the board, but the jacks may have been close at the time - i'll need to check that out. RG's mention of whine was my initial thought, too - maybe it is somehow the 7660's (pin 1) boost-mode frequency bleeding through.

there's only a 47u from V+ to ground on the main board itself, but there's a 4u7 to ground on both the Vbias and V+ rails coming out of the opamp on the charge pump board. a bit on the low side, maybe worth increasing? i didn't anticipate this and ran out of +35V-rated 10uFs, hence the 4u7s.

right, so check jack position, check for AC, check filtering. got it. cheers! i'll get on it on monday. then to worry about draining the circuit more with 2 LEDs...
here's the final layout i used (forgot to change the pot labelling - iirc Treble and Bass are backwards):

[MrStab]

btw, i could've misinterpreted, but didn't you mention at one point that the lower voltage would result in more gain? maybe that has something to do with it? i feel as though that's a less-likely cause, but just throwing it out there.

[psychedelicfish]

Lower supply voltage=same gain, more distortion.

[psychedelicfish]

To elaborate on my above post:
If you feed a 1V peak to peak signal into an amplifier with a gain of 20, the output will be 20V peak to peak. If that amplifier is running on a 10V supply, the amplifier will run out of supply and the output will be clipped to 10V peak to peak. If that amplifier is running on 40V, however, the output will be able to reach 20V peak to peak without clipping. While in both scenarios the amplifier has a gain of 20, the one running on 10V has more distortion than the one running on 40V. The reason you see controls on amplifiers etc. that control the amount of distortion labelled gain is that they use a variabe resistance to control the gain of an amplifier (think tube screamer), which is overdriven itself, overdrives the next stage, or has its output clipped using diodes. So always remember: gain≠distortion

[MrStab]

thanks for the explanation. i know gain and distortion arent interchangeable, but that helped me visualise whats going on here. so my paranoia over preserving the voltage~headroom~tone is unfounded so long as i make  accept the increased gain?

[MrStab]

just wondering - how problematic in terms of current do you think 2 dim-as-humanely-possible red LEDs might be in this circuit, given that it's already dragging down the charge pump output considerably? the ammeter function on my DMM is screwed following a wrong-setting blunder the other day. can't beat the classic electrical burning smell. 

[duck_arse]

run the leds from the supply before the 766a-ma-call-it cajigger. then you can rack them up to inhumane brightness, without problems.

[MrStab]

run the leds from the supply before the 766a-ma-call-it cajigger.

i'd thought of that but my tendency towards illogical assumptions thought "that must somehow be bad!" lol. awesome, this way i don't have to worry about the voltage. just got 60 LEDs through in the post. ill get back to work in a few hours to reverse those pots and look into the gain issue. cheers!

[MrStab]

first time hearing without headphones and bzzzzzz.

if i use the highly-professional method of holding my stings and touching the 2k2 resistor on the far-right of my layout, it disappears albeit with some tone loss. i'll get a wire holding a resistor to ground and poke around.

update: um... i usually try not to jump to this conclusion, but what are some less-obvious causes of external interference? i ask because at first i noticed that waving my hands above the board affected the sound, and thought nothing of it, but then  tried tilting the board vertically while on and there was a gradual decrease in hum. doesnt seem to be any offboard components touching. i've tried turning off my wifi router, monitor, even computer, fluorescent bulb... putting a metal tin over it did nothing. weird. been over the board a few times, no luck. there was a 220nF cap going to ground instead of vbias, but that was one of the ones you said was optional, Edward.