Little gem mkII problems please help!

[fredy]

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.

[petemoore]

  hard to say, or have an idea as presented.
  see the debugging sticky thread.

[fredy]

ah yes, sorry. Here is how i have wired it:


here is the schematic i am using:



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?

[Zben3129]

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

[mdh]

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.

[mdh]

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.

[Zben3129]

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.

[mdh]

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.

[fredy]

thank you guys! i will try that as soon as i get back from work. Thank you!

[fredy]

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

[petemoore]

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..

[fredy]

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?

[fredy]

i just tried to check my voltages and then it started to make that tone again. grrrr!

[mdh]

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.

[fredy]

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.

[fredy]

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.

[mdh]

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.

[fredy]

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?

[Zben3129]

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

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?

[mdh]

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.