I'm trying to figure out this jfet switching and I'm having some issues. I'm not sure where my issue is so I put my voltage measurements throughout the circuit. My main issue is that there is no flip flop. The voltages don't change regardless of pressing the switch. I have my transistors all socketed and when I pull Q4 I get the LED to come on, otherwise it's off. I do get a very faint flash from the LED when it press the switch but it's next to nothing. I can confirm that the SPST footswitch does work.
Does it matter what NPN transistors I use? I'm using a combination of 2N4123's for the input and output buffers and 2N5088's for the flip flop and LED.
Does anyone see any mistakes?
Where should I begin my troubleshooting?
Thanks!
(https://i.postimg.cc/WFVv48Tr/Schematic-JFET-Switching-Voltages-2021-8-31.png) (https://postimg.cc/WFVv48Tr)
I think you are missing a resistor on the base of Q2. The transistor driving the LED.
(BTW, the low 2.4V on the base of the input buffer could be due to the meter loading, *but* the low emitter voltages means it actually is low. Check you haven't got E and C swapped on the pinouts.)
Or Q2 shorted B-E?
Is this MicroCAP ?
I have just downloaded the software and could possibly assist with the simulation shortly
QuoteI have just downloaded the software and could possibly assist with the simulation shortly
I'm fairly sure the missing Q2 base resistor will cause problems from the start.
You can see the correct circuit here,
http://4.bp.blogspot.com/-W90Rsg7YA9Y/T75MYyT9szI/AAAAAAAABZE/SrJ-EsvSkNg/s1600/Ibanez+STL.gif
My very first circuit that I built when I was 14, was a similar flipflop. I made it with AC128 on vero. It had 2 little filament bulbs. I think I kept it on for about 6 months, always in amazement at what electronics could do !!
So this circuit brings back happy memories !
Yes it does appear that the missing part made the circuit into a flop,
instead of a flip-flop !!!
QuoteMy very first circuit that I built when I was 14, was a similar flipflop. I made it with AC128 on vero. It had 2 little filament bulbs. I think I kept it on for about 6 months, always in amazement at what electronics could do !!
I think the astable versions were some of my earliest projects. Flashing lights and high-voltage zappers (using a transformer). The self-oscillating zappers were less troublesome but you had to scrounge around for a suitable transformer.
I added a 1Meg resistor to the base of Q2 and the LED turned on, unfortunately now it won't turn off when I press the switch. I'm going through the circuit and I see one mistake, a 10k resistor was in with my 22k resistors and I placed that for R7. I'm going through the rest of the circuit now for any more mistakes.
Edit: I can confirm that that's the only stray component.
Do you have same value for R14 and R20 ?
The schematic that Rob posted has different values
Quote from: Vivek on September 01, 2021, 01:41:49 PM
Do you have same value for R14 and R20 ?
The schematic that Rob posted has different values
I do. I've looked at various schematics and I'm not sure if that value makes a difference. I suppose I could give it a shot.
I've always found the two transistor flipflop a little touchy to make work perfectly. There's always some little thing to be tinkered. I find that it helps me to think of it as two common emitter amplifiers in series, with a wire connecting the output of one of them back to the input of the previous one.
You may already have read it, but if not this might help: http://www.geofex.com/Article_Folders/bosstech.pdf (http://www.geofex.com/Article_Folders/bosstech.pdf)
Quote from: R.G. on September 01, 2021, 02:02:06 PM
I've always found the two transistor flipflop a little touchy to make work perfectly. There's always some little thing to be tinkered. I find that it helps me to think of it as two common emitter amplifiers in series, with a wire connecting the output of one of them back to the input of the previous one.
You may already have read it, but if not this might help: http://www.geofex.com/Article_Folders/bosstech.pdf (http://www.geofex.com/Article_Folders/bosstech.pdf)
I have read that article and found it informative. I've decided to go back to the breadboard and figure out the issues. It came to my attention that the pinout of the flip-flop transistors in the original schematic were ECB and not EBC which seems to account for some of the issues I was having. With that said, I breadboarded both and got the following results:
(https://i.postimg.cc/8s2hL3XQ/2021-9-1-Jfet-Switching-1.png) (https://postimg.cc/8s2hL3XQ)
(https://i.postimg.cc/N5RRD4Zy/2021-9-1-Jfet-Switching-2.png) (https://postimg.cc/N5RRD4Zy)
I believe the top one is correct, however, when I press the switch there doesn't appear to be any change in voltage when measuring at the outputs. I'm not sure why this is. I've yet to add the other NPN transistor for the LED or the jfets until I have this part figured out. Any idea on why I'm not seeing a change in voltage?
I got frustrated with staring at the circuit and went off to the circuits machine and dumped it into the simulator.
You're right - with values as shown, it doesn't work in simulation. Your results are correct.
I did mention that it was always some value or other that needed tinkering, right? The amount of base drive looked excessive, so I tinkered with that. Try changing the 47K base drive resistors to 220K and the base pull-down resistors to 1M. That got it working in my simulator. I used 2N3904 models for the transistors.
One comment. I don't know if you have the transistors hooked up wrong or mislabeled. Irrespective of where the pins are on the package of the transistor, the one that is the collector has to go to the 56K resistor, the one that is the base has to go to the was-47K-but-now-220K resistor. The labeling on the schemos seems wrong.
The too-low base resistors lead to a 4.6-ish collector voltage on the "off" transistor, as you measured. That rises to over 6V with the bigger base feed resistors.
So I've still been working on this and made some progress with the suggestions in this thread and from elsewhere. Here is what I have:
(https://i.postimg.cc/QBsqJQ4R/Schematic-Bistable-Multivibrator-2021-09-06.png) (https://postimg.cc/QBsqJQ4R)
It's working reasonably well on the breadboard however it's not perfect. I'm using a soft touch SPST footswitch in a 1590LB enclosure with a standard cable connected to the board. The switching works (as in the LED turns on and off) between 50% and 75% of the time. I'm aware that the switching for a soft touch SPST footswitch is not going to be 100% accurate, however, it is significantly less accurate then I would hope and I'm not quite sure why. I've played with a number of values, increasing and decreasing, but the accuracy remains where it is for the bistable multivibrator portion, and when I add the jfet switching. I should add that the switch is practically brand new and I have no reason to suspect that it's faulty. Does anyone have suggestions on where to proceed?
QuoteI got frustrated with staring at the circuit and went off to the circuits machine and dumped it into the simulator.
You're right - with values as shown, it doesn't work in simulation. Your results are correct.
Spice often biases these things up with equal collector voltages. There's no asymmetry for it to decide to go one way or the other (except perhaps the 1M output load). You need force an initial state where one of the transistors is on. After that button presses will toggle the flip-flop.
One way to start-up correctly is to set some initial conditions on one or more of the caps. Another is to create a start-up circuit which pulses some current into one of the bases of the transistors, for example a spice current source set to narrow pulse say 10us.
The fix will only show-up on a transient analysis, not a DC bias.
If you it to start-up on a DC bias you will have to make some of the resistors unequal to create some asymmetry. Off-hand, by a reasonable amount.
I might have posted a sim for the Boss unit in the past, it's probably got the current start-up pulse method.
http://4.bp.blogspot.com/-W90Rsg7YA9Y/T75MYyT9szI/AAAAAAAABZE/SrJ-EsvSkNg/s1600/Ibanez+STL.gif
R21 and R22 are different values here, possibly to pre-determine a start condition ?
Quote from: Rob Strand on September 06, 2021, 06:50:09 PM
Spice often biases these things up with equal collector voltages. The no asymmetry for it to decide to go one way or the other (except perhaps the 1M output load). You need force an initial state where one of the transistors is on. After that button presses will toggle the flip-flop.
Yeah, I'm familiar with the oddities of SPICE startup. The "nothing starts" problem was even more infuriating back when we had to submit text jobs and get results printed on watermelon paper an hour or so later. :icon_eek:
However, in my runs, the circuit did in fact start up with one collector down at about 0.2V, one at between 4 and 5 volts. It just would not switch until I raised the base resistor values.
I also messed with the relative values of the trigger and speed-up caps to see if there was some race condition that happened to be just perfectly balanced and prevent switching because the values were exactly the same. That didn't help either, although slight offsets in the specified component values is another way that can trick SPICE into an initial transient, as well as using close-but-not-quite transistor or diode models in digital simulations to stop the too-perfect condition. These dodges were published in the (archaic!) instructions for the older SPICE versions we used inside the corp.
Could it possibly have anything to do with the SPICE option "Skip initial Operating Point Solution" ?
Have you got this ticked or unticked ?
your last shown version is missing the cap to ground from the C8//C10 node. it might be switch bounce. on the breadboard anyway, does spice include bouncy switches?
LTSpice hardly has any physical switches built in.
One has to make models for all physical switches. It's cumbersome to make and use switches in LTSpice.
The way LTspice works, it seems impossible to "press a switch" in the middle of a simulation run.
Spice has Current controlled and Voltage controlled switches though
Microcap might be different, I have not gotten up to speed on it yet.
> The way LTspice works, it seems impossible to "press a switch" in the middle of a simulation run.
Look deeper. Even the antique version of SPICE that I use has a time-operated switch.
When you do this, check the parasitic values. Resistance can't go infinity to zero without blowing SPICE's mind, so they put some in. But the defaults may not be appropriate for your setup.
(https://i.postimg.cc/rKnDx98F/Sw-t-Close-42.gif) (https://postimg.cc/rKnDx98F)
Quote from: duck_arse on September 07, 2021, 12:07:46 PM
your last shown version is missing the cap to ground from the C8//C10 node. it might be switch bounce. on the breadboard anyway, does spice include bouncy switches?
I've tried adding a cap to ground there but it doesn't seem to make any difference other than slowing down the recovery of the voltage.
LTspice does not allow the user to interact with any component while the simulation is going on
So you cannot move your mouse over a switch and click it on/off in the middle of a simulation run
LTspice does not have any standard mechanical switches in its standard library. Other flavours of Spice might have switches
Some of the ways to emulate mechanical switches in LTspice are to use current controlled switches, voltage controlled switches, resistors whose value is set by step parameters, or behavorial resistors which accept a function with time as a parameter.
One way to emulated a bouncy switch is to use a piecewise function and encode the bounces we need.
Quote from: Vivek on September 07, 2021, 03:01:31 PMLTspice does not allow the user to interact with any component while the simulation is going on...
IMHO that would be weird (I know some do this).
" to simulate a simple switch that opens and closes at specific times "
https://www.analog.com/en/technical-articles/ltspiceiv-voltage-controlled-switches.html
Nota bene: The variant of SPICE I'm using now does have switches that can be flipped by key presses, and other switch models that can be voltage triggered. In my testing I used a 1/2Hz clock source to trigger a switch so I could watch it repeatedly in the oscilloscope view.
True, I paid way too much for this variant of SPICE and the stuff that goes with it, but I'm old and my eyesight's weak, and the sun is in my eyes and ... :icon_biggrin:
I'm continuing with the jfet switching schematic. I decided to start from scratch and trace the circuit of my Ibanez TS9, measuring both the values of components and the voltages along the way. What I've come up with is what I believe to be a highly accurate drawing of the Ibanez TS9's switching system, and it's very similar (although different in some ways) to a drawing by Steve Cerutti with help from R.G. Keen. Here is what I've come up with:
(https://i.postimg.cc/CZcbHkhS/Schematic-Ibanez-TS9-2021-09-12.png) (https://postimg.cc/CZcbHkhS)
Here is the drawing that I used for reference:
(https://i.postimg.cc/fS9XNPxR/Ibanez-TS9-Schematic-2.jpg) (https://postimg.cc/fS9XNPxR)
I didn't bother doing the effect side of the build because I'm not so much interested in that right now. The main differences between the two drawings are the resistor and capacitor in series going from the collector to the base. I measured mine at being 100k and 1nF, whereas the alternate is 47k and 100pF.
Also, they drew two 56k resistors from 9V to the collectors of the NPN's whereas I measured one 56k resistor and one 39k resistor.
I decided to breadboard this once again and I get accurate voltage measurements between my Ibanez TS9 and my breadboard version when the LED is on. There is one problem still remaining. For whatever reason, I cannot get this to flip flop. It doesn't matter what switch I use, for some unknown reason it won't flip flop on the breadboard but it will on the PCB of the TS9
Here's a picture of my breadboard, I decided to make it as neat as possible:
(https://i.postimg.cc/v1T4yjyw/IMG-20210909-230734472.jpg) (https://postimg.cc/v1T4yjyw)
I know it's been said that the circuit is finicky but I don't get why it would work with the PCB but not on the breadboard. Unless the breadboard itself is faulty but I do see anything to suggest that. Any thoughts on getting this to work? Do any of you see any errors?
QuoteI didn't bother doing the effect side of the build because I'm not so much interested in that right now. The main differences between the two drawings are the resistor and capacitor in series going from the collector to the base. I measured mine at being 100k and 1nF, whereas the alternate is 47k and 100pF.
Also, they drew two 56k resistors from 9V to the collectors of the NPN's whereas I measured one 56k resistor and one 39k resistor.
There's many variants of the JFET switching circuit, even if we stick to Ibanez.
There's two major variants of the Ibanez circuit. One with the R+Diode in the emitter and later versions without R+Diode.
Across all circuits the collector resistors are anything from 10k to 56k.
The RC network on the base is a tricky one. I've seen both 100k and 47k. Your voltages look like it should be 100k but I don't believe the voltages should be symmetrical like on your schematic. I'm not so confident about the cap value (1n might be correct). Measuring either R or C in-circuit is likely to measure incorrectly because of the parallel R. Even if you removed the RC network from circuit you would need an LCR meter. A generic cap meter is likely to measure incorrectly due to the presence of the parallel R.
The 10nF cap across the switch is often 1n. However 10nF is a much better value. In fact 47nF is better again. What happens is as the switch wears out it bounces and the 47nF fends off the bounce a lot better. (The 1nF is a well known problem on the old Ibanez potato bug 5-series TS5, CR5, PL5 etc. which have cheap switches.)
Also note that they're using different transistors than you are unless you ran down some 2SC1815s.
Good engineering practice is to design so that variations in the transistors don't matter (much, at least), so I would expect the design on the PCB to have been done that way. But it's not possible to design all the variation out across all transistor types, only minimize the effect.
Try just changing the base-to-ground 56k resistors to something higher, maybe 100K or more. These resistors do two things. First, the historical reason they're there is that they shunt collector-base-leakage out of the base, enabling an "off" transistor to be really off. They're a legacy of logic design with germanium devices, which really, really needed a leakage path for switching. And they are a "desensitizing" resistor, in that some of the current from the opposite transistor's collector has to drive them as well as the transistor base, so the lower their value, the less sensitive the base they're connected to - they eat some drive current.
As Rob notes, the sizes of the caps get into the act too. The cap at the momentary switch stores up a bucket of charge that gets dumped into the connecting caps to the bases when the switch is closed. The caps to the bases and the resistors on the bases determine how much voltage that chunk of charge causes at the bases and how long the bases are driven by the transferred charge. The collector-to-base resistors and the base-to-ground resistors eat some of the transferred charge too, in the instant before the "on" transistor starts turning off. The coupled charge from the switch has to last long enough so the "on" transistor's collector starting up can reinforce the coupled charge in turning the "off" transistor "on". That's a long winded way of saying that the parts values matter, as do the gains of the transistors.
That still doesn't completely answer why it works on the PCB but not in a breadboard. To focus on some possible differences, you might try soldering up a test circuit with just parts lying flat on the work bench. I do this sometimes because I'm horrible about abusing breadboards, and my breadboards often develop loose contacts and give me very odd results that change if I move the circuit to a different part of the breadboard - and hence different contact points. It might be your breadboard, maybe.
FWIW, I did a quick ltspice sim to look at the range of cap values that work.
Constrained to old Ibanez circuit with the resistor and diode in the emitter circuit of the flip-flop.
Transistors vary but are are usually 2SCxxxxGR, GR means gains of 200 to 400.
I checked a few cases of different gains, cap across the switch and supply voltage.
The breaker is operating at low battery.
I found with those CR networks at 100k in parallel with 1n the flip-flops switch
the circuit will switch reliably over a wide range of value and down to low supply voltages.
With 100k in parallel with 470pF the behaviour at low voltages is marginal and is unlikely
to be what is used.
I conclude the 1nF cap in the CR neworks is likely to be correct.
[Don't draw the same conclusion for the boss circuit as they use different caps between the flip-flop and the switch.]
Here's the test set-up.
(No start-up circuit was added as it was starting up OK after the first switch pulse. However, the flip-flop collector voltages were stuck on equal values and "incorrect" at start-up)
(https://i.postimg.cc/NLZKjPP6/Ibanez-JFET-switching-Old-R-D-Version.png) (https://postimg.cc/NLZKjPP6)
I did the same thing with the later version of the Ibanez switching circuit which doesn't have the resistor & diode in the emitters.
While the circuit is shown with a 1n cap on the switch 10n or 47n would be much better choices.
(https://i.postimg.cc/3kBBVbXT/Ibanez-JFET-switching-New-Non-R-D-Versions.png) (https://postimg.cc/3kBBVbXT)
This circuit uses three less parts.
The advantage is it can pull the JFET gates down lower which means the switching has more headroom to keep the JFETs off.
The collector-base caps can be a little bit smaller, however, the 100k + 1n CR network works fine and are likely to be the correct values. A few factory schematics have these values.
Rob
You never give up on a problem !
You dig as deep as necessary, till issue is understood
Deep respects !
QuoteYou never give up on a problem !
You dig as deep as necessary, till issue is understood
Deep respects !
You have a similar attitude. I suppose part of it is comes from the fact you can see a solution should exist.