hell oagain, been a while since i posted here. Anyhoo, i have built a little gem mkii, problem is, when i turn it on i get no sound, after about 10-20 seconds i get a tone. I get this tone even with out a guitar plugged in and it doesn't change in pitch or volume. What do you think it could be? any help would be greatly appreciated, thank you.
hard to say, or have an idea as presented.
see the debugging sticky thread.
ah yes, sorry. Here is how i have wired it:
(http://i67.photobucket.com/albums/h287/fredy001/IMG_3737.jpg)
here is the schematic i am using:
(http://i67.photobucket.com/albums/h287/fredy001/littlegemMK2.png)
I have since removed the dpdt switch and changed the cap which follows the transistor from 221 to 220.
the voltages from the battery is: 8.3
from ground to + terminal on the battery is 2.6v
for the transistor:
the centre pin connected to the tip = 3.4v
Pin to ground = 0v
pin to battery and amps = 3.6
For the opamps:
1=1.2v
2=0
3=0
4=0
5=1.5
6=3
7=1.4
8=1.2
Whilei was taking these voltages, the sounds coming from the speaker changed from a constant frequency saw wave, to putting and to a quiet hum.
Is there any thing else you need to know?
Sorry, I didn't have time to read the info post, I have to go eat,
but the only time delay thing I can think of would be a charging cap, but 20 seconds seems way too much, so I'm not sure about that theory. Ill look at the info you provided in 30 mins. or so.
Zach
If I'm interpreting your drawing correctly, you have the input going to the source of the JFET, not the gate. The data sheet I have in front of me has the MPF102 pinout as DSG, pins down, looking at the flat side. I came across one other thread where someone had a Ruby/Little Gem/something oscillating due to an incorrectly wired JFET. It's always tempting to assume that the gate is the middle pin, especially since most TO-92 BJTs have the base in the middle, but that's almost never the case with JFETs, IME.
Oh man, I just looked at your voltages... it looks like the battery must be getting pretty hot when the thing is running, so that oscillation is probably drawing a lot of current (the only way the supply voltage could be reading 3.6V on a 9V battery with 8.3V unloaded is if the remaining voltage is being dropped by the battery's internal resistance, which means a hot battery). I'm a little weak on the details, but I bet the misoriented JFET is the problem... and the way you described the voltages confirms for me that I have interpreted your drawing correctly. Looking at your drawing, you want the input to be connected to the top pin, the middle pin to the 10k resistor to ground, and the bottom pin to +9V. That's assuming that your MPF102 is pinned out per the Fairchild data sheet, but I'm almost certain that it should be.
looks like your circuit is pulling TONS of current, judging by the extremely low v+ on power rail at opamp. Check to make sure all socketed parts are correcect: electros correct polarity, opamps in right-side up, and transistors in the right way. With that low voltage I'd imagine it must be an active component giving you issues.
Zach
EDIT: Looks like mdh already saw this before me, and I also would be willing to bet that that transistor is your problem.
I might add that dumping that much current through the FET and the amps may have damaged them, so if you still have problems after fixing the FET orientation, you might want to try replacing the FET and both amps.
thank you guys! i will try that as soon as i get back from work. Thank you!
ok, i have just change the transistor and the legs are where mdh suggested but now i am getting no sound at all.
here are my new voltages:
transistor:
input = 0.16
ground = 0.13
9v = 1.02
amps:
1 = 0.5
2 = 0
3 = 0
4 = 0
5 = 0.5
6 = 1.0
7 = 0.5
8 = 0.5
input = 0.16
ground = 0.13
9v = 1.02
amps:
1 = 0.5
2 = 0
3 = 0
4 = 0
5 = 0.5
6 = 1.0
7 = 0.5
8 = 0.5
A better reference for voltages like these is 9V = battery alone, I'm forgeing ahead assuming battery measures about 9v.
Voltage measurements like these [or .5v on an opamp V+ pin] means good time to disconnect battery and see what is pulling the voltage down so far.
Fair assumption is that a huge amount of current is drawn, likely an indication of a direct short between V+ and V-, enough to heat and destroy a battery.
Finding the short is usually a:
1. A Logic / findings affair, and may involve disconnecting stuff to get the short to stop [and taking notes and readings along the way to know which lifting of the connect ended the shorting condition.
2. Simple close-in light w/magnifying glass inspection of all the board traces, sometimes a teeny trace - culprit can be found this way.
..generally ground wires are going in many places and all over the place, so...finding a ground short to V+...well it's handy and often enough located near PS input [DC jack / input jack / offboard wiring or component etc.] but you can't count on it.
Maybe after else tries, lift V+ at the board and repeat tests looking for short...on board / off board..
turns out, i wired the transistor wrong, again! i am now getting a signal but it is very low and the bass is not comeing through at all. Could this be a dodgy solder after the countless rewiring or is there something else i should be looking at?
i just tried to check my voltages and then it started to make that tone again. grrrr!
One thing I'd like to know is which parts are getting hot. If you still get voltages anything like what you had before (supply being pulled well below 9V), then at the very least the battery must be getting hot, but the FET, the LM386s and possibly a resistor or two may be getting really hot as well. Any of those components could be compromised by passing tons of current. What I would do if I were you would be to sit down with the schematic and the board, and trace out every node to make sure that things are connected as you intend them to be. It might even be worth your while to set aside the schematic and your layout sketch, and sketch a new layout from the board, i.e., draw it the way you actually have things wired. You might find that there's a discrepancy that you can't find when trying to compare the board and the layout directly. The only thing I see wrong with the original layout you drew is the FET orientation, but I could be missing something.
You should also check for shorts in the offboard wiring as Pete suggested, and also use your multimeter in continuity mode (usually the lowest resistance setting) to check for shorts (do this with power removed from the circuit). At the most basic level, this involves checking for a short across the terminals of the battery snap (just let me emphasize again, the battery should not be connected to the circuit when you test for continuity!), but it could easily be more subtle than that. This type of problem is one of the hardest to debug by remote control, because it's really a matter of finding what's causing the short circuit, and we can't see how the nodes on your particular piece of perfboard or whatever are actually connected.
ah ok thanks, so it sounds like a short then? will have another check later. Right now i have to revise for a physics exam. Thanks for all your help.
i have looked all over the board, at the input, speaker, battery and swithcbut can not find a short. Nothing is getting noticably hot yet the voltage is still only 5v from the 9v battery. Although its is now a 7v battery. I am not getting any continuity through both the resistor and capacitor which are parrallel to the speaker, i am not sure if this is normal or not. Also, the tone that i am getting can be provoked by touching the input cap. Perhaps i am acting as a ground?
I really am stumped.
Well, it doesn't have to be a short, per se, but it seems that either something must be pulling a lot of current, or the internal resistance of the battery has gone up as it has been abused and discharged. If the battery is showing 7V unloaded, it's probably a goner, and you should try with a new one, but only after thoroughly checking the circuit to make sure that everything is wired correctly. And when you do plug in a new battery, the first things you should do are to check the battery voltage and see if it's getting hot. I can't imagine that it wasn't getting hot before if you were measuring the voltages correctly.
When you say you're not getting continuity through the resistor and cap in parallel with the speaker, do you mean that you're measuring them separately, or you're effectively measuring across the speaker terminals? In either case you shouldn't get continuity exactly (I believe the cap should look like it has infinite resistance since caps block DC). But if you're measuring across the speaker terminals, you should see a resistance in the ballpark of the speaker's rated impedance. And since there's nothing in parallel with the 10 ohm resistor, you should measure about 10 ohms across it. In any case, as for continuity measurements, I was looking for something more like a short across the power rails, or between adjacent pins of a transistor or IC.
Really all I can add is that you need to make sure that you understand how the pins on the physical components you have correspond to the parts of each symbol in the original schematic. If you wire it up correctly, it will work (assuming none of the components has sustained any damage from previous abuse). If you wire it up incorrectly, it may oscillate, heat up, kill your battery, etc. As others have observed here, Mother Nature is a stone cold bitch, and you have to follow her rules.
i changed the input cap and now my voltages seem a bit more normal but the sound is still as bad as before. Kere are my new voltages:
+ terminal of battery: 8.1
transistor
input: 7.3
ground: 7.95
9v: 0
amps
1: 6.7
2: 8.0
3: 5.7
4: 7.9
5: 3.8
6: 0
7: 3.9
8: 6.7
are these voltages normal?
Input, as in input of circuit?
Ground, as in ground of circuit?
It so, then you still have voltage problems. You should definitely not be seeing DC voltage on the input, maybe a tiny bit of AC should be all you can measure. And 7 something VDC at ground is also not right, as it should be 0.
If you notice, the 7 volts you see at your ground is also what you are seeing on your ground pin of the opamp (4)
Zach
Quote from: fredy on March 05, 2008, 04:22:43 PM
i changed the input cap and now my voltages seem a bit more normal but the sound is still as bad as before. Kere are my new voltages:
+ terminal of battery: 8.1
transistor
input: 7.3
ground: 7.95
9v: 0
amps
1: 6.7
2: 8.0
3: 5.7
4: 7.9
5: 3.8
6: 0
7: 3.9
8: 6.7
are these voltages normal?
I think you may be confused about how to measure voltages. You should have the black probe on ground, and the red probe on the point where you're measuring the voltage. So it's impossible to measure 7V at ground if you're measuring correctly, because all voltages are referenced to ground, so it is 0V by definition. Since we don't know where your voltages are referenced to, it's hard to interpret them. I suppose that maybe you're taking your measurements by putting the red probe on the positive supply rail and touching the black probe to the measurement point. Give us voltages referenced to ground, and then we'll talk.
Another observation: you report one set of voltages for "amps." The two LM386s are in different parts of the circuit, with some common connections, but some differences. Please give us voltages for both chips.
i thought that was wierd getting volts to the ground. I have the black probe on the sleeve, where ground is connected. IT is also attached to the -ve terminal of the battery.
And the voltages on each amp are pretty much the same give or a take a point of a volt.
Nope sorry, you were right, for the last set of reading i di have a probe on the battery. Whoops :icon_redface: sorry. When i get back from work i will try again. Thanks for all of you help with this, i do aprreciate it.
i have some pics now.
(http://i67.photobucket.com/albums/h287/fredy001/IMGP0723.jpg)
(http://i67.photobucket.com/albums/h287/fredy001/IMGP0722.jpg)
(http://i67.photobucket.com/albums/h287/fredy001/IMGP0732.jpg)
Sorry,Made
Too much of Zinn...
I thing:Did it again.
Hopefully Thilo
hi claas, sorry, but i have no idea what you are on about.
Here are my new voltages after the cap change:
the battery is at 9.1v
at +ive termanal of battery 8.09
transistor
tip = 0.0
ground = o.08
9v = 8.08
+ amp
1 = 1.30
2 = 0.0
3 = 0.0
4 = 0.0
5 = 4.6
6 = 7.96
7 = 4.02
8 = 1.29
- amp
1 = 1.28
2 = 0/0
3 = 0.0
4 = 0.0
5 = 3.85
6 = 7.93
7 = 3.98
8 = 1.29
Is it getting better?
Well, one thing is clear from your pictures, and that is that I'm not surprised you're not hearing any bass with a 220pF (picofarad = 10^-12 farads) cap in the audio path. That little ceramic cap labeled "221" is in the position that should be occupied by a 220nF (nanofarad = 10^-9 farads). I suspect that the other ceramic cap I see is about 3 orders of magnitude too small as well, but that's a guess based on its physical size. A 220nF cap will be labeled "224" and a 50nF cap will be labeled "503" (or more likely you'll come across 47nF caps labeled "473").
I think the voltages are credible, but I'm not really sure what to expect on the output pins of the amps (pins 5) or the gain control pins (1 & 8 ), and I'm not certain what we should see on the source of the FET (that's the pin connected to ground through the 10k resistor). Another thing, though, is that it looks like the battery is going to continue to take a beating, because even if there's no short (and I don't think being pulled down by 1V under load is evidence of a short), the circuit probably still draws too much current for sustainable battery power (note that ROG recommends running it off of a 9V wall wart rather than a battery; I just didn't want to suggest that while it seemed that you were shorting the power supply, lest you start frying wall warts).
Honestly, if I were you, I'd start over with an original Little Gem or a Ruby. I'm a little dubious on the utility of bridging LM386s, especially to drive a 2 or 3 inch speaker. On top of that, I think the LM386-1 chips you have are probably least capable of running in a bridged configuartion without overheating. You can probably find a verified vero layout for the Ruby or something similar in the layouts gallery, and get an easier start for yourself in this addictive hobby.
Edited for uninentional smileys.
i changes the caps now and the volume is better but still not great. The input cap had an "m" after the 224, does that mean anything? Here is a sound clip of the amp.
link (http://www.youtube.com/watch?v=PaOdCDZFuTc&feature=PlayList&p=6C76021B477FDCFB&index=0&playnext=1)
The M most likely means 20% tolerance, which is to say that the capacitance is guaranteed to be within 20% of the nominal value. It's nothing to worry about, 20% is pretty typical for capacitors.
The link to the clip doesn't work... could you try again, and make sure the link doesn't have an ellipsis (...) in it? Or make a tinyurl link and post that?
sorry,, should work now.
Um, sounds fine to me. What's your complaint about the sound?
well, to start with, it is very quiet. Its no where near the volume of my marhsall 1w micro stack. Also, with pins 1 and 8 disconnected, it is inaudiblely quiet. secondly, the camera makes it sound good, its not quite like that in real life. I will try and get another, more accurate recording of it later. Thanks for all of your help by the way. I really do appreciate it!
I haven't played with a Marshall Micro Stack -- are you sure that the speakers are comparable? Speakers really make a big difference in volume with these little circuits. A Smokey amp (single 386) can sound huge through a proper speaker cabinet.
tbh, i just have a cheap generic 3 inch speaker. I will try and get hold of another jack socket so i can hook it up to my cab, but i doubt it will be able to push a 4x12.
Quote from: fredy on March 09, 2008, 05:33:08 PM
tbh, i just have a cheap generic 3 inch speaker. I will try and get hold of another jack socket so i can hook it up to my cab, but i doubt it will be able to push a 4x12.
Have faith in the little 386, running them at 12v lets them punch out nearly a full watt of transistorised grunt.
my LG MKII is so loud I'm going to add a tonestack to try and take some of the sting outta it
running off 9v the 386 doesnt develop quite as much output, 1/2 watt, but this is still enough for the original little gem/smokey to run a 4x12
for anything up to 4 hours off a 9v battery
i just tried this amp with a greenback, sounds much better. Not much gain though and still quieter than you would expect from a 1w amp, but things are better and the greenback is a quiet speaker as far as i know. After about an hour or 2 of playing through it the battery went dead (well, 6v, sound went all fuzzy and mushy). I swaped in a nice new fresh one to find the amp farted at me! Lots of puts, odd distortion that does not correspond to what is being played and finally, "burrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr rrrrrrrrrrrrrr"...
perhaps there 9v is enough to cause some arcing across the strips where my soldering went bad? although that wouldn't be constant. hmmmmm... i'm stuck again!
anyone have any ideas about the problem? what would make it do that?
could it be the 0.5w resistors? i wouldn't have thought it would matter with the ones going to earth, but how about the one parrallel to the speaker?
Actually, from your pics the 10 ohm and 10k resistors appear to be 1/4W, not 1/2W, while the 1.5M appears to be a 1/2W device. I don't really know how much power should be dissipated by each of the resistors, though I suspect that it's negligible for the 1k and 1.5M since the signals in that part of the circuit should be very small. The 10 ohm resistor is part of what's called a Zobel network, and if there's a fault somewhere in that part of the circuit, I think it's possible that it could be responsible for the oscillation. I really don't know, though, I'm getting a bit out of my depth here.
I want to reiterate my suggestion that you set this circuit aside for the moment at least, and build one of the single LM386 amps such as the Little Gem or Ruby from an established layout. From your board pics, it appears that you need some more practice soldering anyway. I would also suggest that in your next build you use sockets for your IC and any transistors. This will make it easier to correct problems with pinout (at least for transistors), and will make it easy to swap out a part if you think you might have damaged it. I don't really have any idea what's causing your oscillations, but again judging from your pics, there could be cold solders or solder bridges anywhere on that board. There's also a very real possibility that some of the components are just flat out damaged, because early on in the history of the build, *something* was passing a whole lot of current. I don't really know to what extent each of these factors may contribute to oscillations that appear to be dependent on the supply voltage.
i hear what you are saying, and i would love to start all over again, but i just don't have the money at the moment and i would really really love to fix this thing. Unfortunately, i have worn my iron out and need to buy a new one, i have been using a 40w and i have a 50w psu which doesn't get used, would 50w be too much? it will be used for all sorts from amps, guitars, fx, at home, everything.
Thanks for everything, i will continue work with this untill i have melted it to a puddle, while i am doing that i would love to learn more on the theory side of things. Do you know of any good resources on the web that could help?
I understand how that is when you get into something new and you don't have the money to fix your initial mistakes, but you're also really excited about what you're going to learn, or what new things you're going to be able to do, so it's hard to get your mind off of it. I'm just concerned that you could work yourself up to a level of frustration where you'll throw up your hands and give up on the whole thing.
Quote from: fredy on March 11, 2008, 05:16:17 PM
Unfortunately, i have worn my iron out and need to buy a new one, i have been using a 40w
What is your evidence that you have worn out your iron? Are we talking about a safety issue like a frayed or otherwise damaged cord? Is it not heating up? Is it possible that it is heating up, but you think that it isn't because it isn't melting solder very well due to a dirty tip? What brand of iron is it/where did you get it/etc.?
Quote
and i have a 50w psu which doesn't get used, would 50w be too much? it will be used for all sorts from amps, guitars, fx, at home, everything.
You have a 50W power supply, is that what you're saying? And you're wondering what you can use it for? If this is what you're asking, there's a lot that we would need to know about this power supply in order to advise you as to the circumstances in which you should use it. Power
per se isn't really the main issue. A power supply won't "pump" more power into a circuit than the circuit would pull on its own, but it could be unsuitable for all kinds of reasons. At the most basic level, what is its intended use? Does it provide DC or AC at its output? What voltage(s) does it supply? Is it regulated? Are the outputs protected against short circuit or overload? Do you depend on it to run any particular piece of equipment that you already own?
QuoteThanks for everything, i will continue work with this untill i have melted it to a puddle, while i am doing that i would love to learn more on the theory side of things. Do you know of any good resources on the web that could help?
No problem. I've obtained enough free advice around here myself, so it's nice to give back. Finding good *and* free resources on the web for learning about electronics is something of a challenge. One place to look is the wiki (http://www.diystompboxes.com/wiki/index.php?title=Main_Page), and in particular the Tech Tips (http://www.diystompboxes.com/wiki/index.php?title=TechTips) section of the wiki. Also, for particular components and circuit elements, you can often learn a fair amount by reading articles on wikipedia. Another resource I know of is Lessons in Electric Circuits (http://www.ibiblio.org/kuphaldt/electricCircuits/). I'm sure there are other references to be found in the FAQ and the wiki.
the tip of my iron has crumbled away. I have a power supply for my weller 50w FE iron, but i hear that 50w is a lot for what i will be using it for, hence i have not used it.
Thanks for the sites, will check them out.
you know what, i think i will start again, this time, on a little gem MK1. Are the cap values in nano farads or something else? also, if i do not want a master volume, do i just leave out the 25ohm pot to ground or should it be replaced with a suitable sized resistor? One last thing, what ype of caps should i use? ceramic, multi layer, tantalum bead?
Thanks.
can't seem to find any 1w resistors, anyone know of a good internet supplier? also, could i wire 2 half watt resistors in series (or should it be parrallel if at all?) to handle 1w?
If you wire 2 half watt resistors of the same value in parallel you'll get a 1 watt resistor. The value will be half of the original values.
Before you abandon your original build are you sure there are no shorts between the pins of LM386s. On the photo of the underside of the board there don't appear to be very good trace cuts between the pins and there's a lot of solder around there. Get something like a 4mm drill bit and completely remove the copper from the traces to make sure there's no shorts.
i am fairly sure that there are no shorts but there is no harm done in making sure. i will give it a go. Thanks!
Ah, OK, you were still talking about the soldering iron situation. I'm still unclear on exactly what you have... a complete 50W Weller soldering station? If it's temperature adjustable and has a small enough tip (or if you can change the tip), you should be able to use it for just about anything. If it's just a simple plug it in and go type of thing, then it may not be suitable, but it may become perfectly suitable at some point down the road, when you get a bit more skilled at soldering. By the way, you may be able to replace the tip of your old soldering iron. Have you looked into that?
Now, about the Little Gem.
All of the cap values in the Little Gem schematic are given in microfarads (often abbreviated uF). To convert from uF to nF, just move the decimal point three places to the right. So 0.01uF = 10nF, 0.047uF = 47nF, etc. You can use ceramic or polyester film for the smaller caps, and for the larger caps (the 100uF and 220uF) you'll use electrolytics. I would just use aluminum electrolytics. Be sure to pay attention to polarity with electrolytic caps.
If you want to leave out the master volume, just take the output from the negative end of the 220uF cap.
As for the 10 ohm resistor in the Zobel network, I actually don't know how much power to expect it to dissipate, as I said before, so I'm not sure if you really need a higher power resistor there. If you want to try to replace it with multiple resistors of a lower power rating, you need to put them in parallel. The reason for this is that if you have a series circuit of resistors, the current flowing through each of them is equal, and power dissipation in a resistive circuit is P = I^2 R. If you put them in parallel, then the voltages across them will be equal, but the current through each resistor will be proportional to the inverse of its resistance. This is just a way of saying that parallel resistors split current, and therefore, power dissipation. If you're having trouble following this, go read about Ohm's Law and the properties of series and parallel circuits of resistors. Then, try to figure out for yourself how much current it would take to make a 10 ohm resistor dissipate 1W, and maybe even what parallel arrangement of lower power resistors would "look like" a 10 ohm, 1W resistor.
ah i see, thank you very much. Just went to check the current through the resistor, and guess what, no voltage :icon_question: oh, and yes, its a dull 50w soldering station with no variable temp. I cant change the tip on my current iron, partly because the screw has crumbled away and also because the iron is cheaper than the tip.
EUREKA! i had the 10 and 10k resistors in each others places and now i need a new 10k resistor, sods law i find this out just as i get the components for the little gem MK1.
Regarding the MK1, where do i connet the other speaker cable to? i know one will conect to the pin 5 ut what about the other?
just found out that the other speaker cable goes to ground. I have made it all up and guess what.... it doesn't work again! will take photos and voltages again but i can not see the problem with this one at all even with the wealth of experience gained. :icon_wink:
tell a lie, it works! however, the switch doesn't dangnabit. Here are some photos of the new build.
(http://i67.photobucket.com/albums/h287/fredy001/IMGP0742.jpg)
(http://i67.photobucket.com/albums/h287/fredy001/IMGP0743.jpg)
(http://i67.photobucket.com/albums/h287/fredy001/IMGP0745.jpg)
Thanks you everyone who has helped, i really needed it cand i appreciate it muchly! thank you.