All About Circuits Forum  

Go Back   All About Circuits Forum > Electronics Forums > The Projects Forum

Notices

The Projects Forum Working on an electronics project and would like some suggestions, help or critiques? If you would like to comment or assist others with their projects, this is the place to do it.

Reply   Post New Thread
 
Thread Tools Display Modes
  #1  
Old 03-26-2007, 07:13 PM
Andy S. Andy S. is offline
New Member
 
Join Date: Mar 2007
Location: Cincinnati, OH
Posts: 9
Default Latching circuit doesn't seem to work right

Folks,

Well, I came across this site by accident. I was looking for some guidance on a circuit I am trying to repair and found a similar issue here on this site. Actually, just a similar circuit. Hopefully I can get some good info here and relearn stuff I probably forgot from 20 years ago.

What I have is a guitar effects device that has a Momentary switch that "should" activate a latching circuit, turning on the distortion. Press the switch a second time and it should turn off the distortion. Situation is when the switch is pressed, nothing changes. But, I discovered that when I put a test lead on either the point of R40 or R43 where it meets with pin 2 of eihter Q16 or Q16 respectively, then the LED comes on and the circuit is active. However, it stays active ONLY if the switch is kept in a closed position.

Am I looking at a failed device in Q6 or Q4? And why doesn't the switch work to activate or deactivate the latching circuit.

Or, worse case, I am totally wrong in my thoughts on the latching circuit and there is something real obvious that I am missing.

Any guidance would be greatly appreciated.

Regards,

Andy S.


Schematic is at:

http://www.schematicheaven.com/effec...luesdriver.pdf
Reply With Quote
  #2  
Old 03-26-2007, 09:16 PM
Ron H's Avatar
Ron H Ron H is offline
E-book Developer
 
Join Date: Apr 2005
Location: Idaho, USA (GMT-7)
Posts: 7,050
Default

What is your test lead attached to?
Reply With Quote
  #3  
Old 03-26-2007, 09:29 PM
Andy S. Andy S. is offline
New Member
 
Join Date: Mar 2007
Location: Cincinnati, OH
Posts: 9
Default

Red test lead at point near resistor mentioned and black lead, from a VOM, attached to ground. Seems like since I'm touching what is around 7.9volts and ground, I'm dumping the voltage straight to ground. Does that sound right?

I already swapped out the two transistors Q16 and Q15 as I thought eihter or both of those were the culprit. Guess not.

thoughts?

Andy S.
Reply With Quote
  #4  
Old 03-26-2007, 09:45 PM
beenthere's Avatar
beenthere beenthere is offline
Senior Member
 
Join Date: Apr 2004
Location: Missouri, USA (GMT -6)
Posts: 15,815
Blog Entries: 10
Default

Q15 & 16 make a flip-flop. One should be on and the other off. The switch input should change the state. It's odd that your meter will do anything but measure the collector voltages. You might suspect a poor solder joint is causing the problem. It looks like a bad FET (Q4, 5, or 6) would not be able to mess with the flip-flop and allow an audio signal to pass.
Reply With Quote
  #5  
Old 03-26-2007, 09:47 PM
Ron H's Avatar
Ron H Ron H is offline
E-book Developer
 
Join Date: Apr 2005
Location: Idaho, USA (GMT-7)
Posts: 7,050
Default

I need to see if I understand you. Are you saying that, with the switch depressed, nothing happens until you connect your VOM to either Q15 or Q16 collector (pin 2), at which time the LED comes on? And then, if you remove the probe, the LED goes off? And the probe does nothing if you don't have the switch depressed?
I can believe that a multimeter could cause the flip-flop to toggle, due to the capacitance of the probe and VOM, or even the resistance of the VOM if you have a cheap one. But I don't understand why this would be a function of the switch position. And I don't see why the LED would go back off when you remove the probe, at least not consistently.
Reply With Quote
  #6  
Old 03-26-2007, 09:51 PM
hgmjr's Avatar
hgmjr hgmjr is offline
Super Moderator
 
Join Date: Jan 2005
Location: Tennessee, USA (GMT-6)
Posts: 9,030
Blog Entries: 11
Default

It almost sounds like the voltmeter is in current measure mode rather than voltage measure mode.

hgmjr
Reply With Quote
  #7  
Old 03-26-2007, 09:58 PM
Ron H's Avatar
Ron H Ron H is offline
E-book Developer
 
Join Date: Apr 2005
Location: Idaho, USA (GMT-7)
Posts: 7,050
Default

Quote:
Originally Posted by hgmjr View Post
It almost sounds like the voltmeter is in current measure mode rather than voltage measure mode.

hgmjr
Well, I thought about that too, but then he mentioned the voltage being 7.9V.
Reply With Quote
  #8  
Old 03-26-2007, 09:59 PM
hgmjr's Avatar
hgmjr hgmjr is offline
Super Moderator
 
Join Date: Jan 2005
Location: Tennessee, USA (GMT-6)
Posts: 9,030
Blog Entries: 11
Default

I was keying on the phrase it was "dumping 7.9V to ground" which made me think shorting.

hgmjr
Reply With Quote
  #9  
Old 03-26-2007, 10:07 PM
Ron H's Avatar
Ron H Ron H is offline
E-book Developer
 
Join Date: Apr 2005
Location: Idaho, USA (GMT-7)
Posts: 7,050
Default

Quote:
Originally Posted by hgmjr View Post
I was keying on the phrase it was "dumping 7.9V to ground" which made me think shorting.

hgmjr
Good point. However, as I implied above, it doesn't take much probe capacitance to cause the FF to toggle. What I don't understand is how the switch would have anything to do with that. If he wants to test the FF with his VOM, he needs to have it in Volts mode and attached to a collector, and then activate the switch. However, the LED is there as a monitor already.
Reply With Quote
  #10  
Old 03-26-2007, 10:15 PM
hgmjr's Avatar
hgmjr hgmjr is offline
Super Moderator
 
Join Date: Jan 2005
Location: Tennessee, USA (GMT-6)
Posts: 9,030
Blog Entries: 11
Default

What if C30 is open or faulty? Would that give the symptom that is being described? That way if Q16 is triggered manually, then the release of the switch would turn off Q16 but with a faulty C30 the switch would not reactivate Q16.

hgmjr
Reply With Quote
Reply   Post New Thread

Tags
, ,


Related Site Pages
Section Title
Worksheet Flip-flop circuits
Worksheet Latch circuits
Worksheet Electromechanical relay logic
Worksheet AC motor control circuits
Textbook The Silicon-Controlled Rectifier (SCR) : Thyristors
Video Lecture Representative System (Part 3) - Executing a Program - Microprocessors
Textbook The gated S-R latch : Multivibrators
Textbook The S-R latch : Multivibrators
Textbook Class B audio amplifier : Analog Integrated Circuits
Textbook Motor control circuits : Ladder Logic


Similar Threads
Thread Thread Starter Forum Replies Last Post
Help with latching circuit xile6 The Projects Forum 6 06-11-2011 11:55 PM
Best Momentary Latching Pushbutton Switch Circuit trader007 The Projects Forum 8 04-23-2011 12:20 AM
Will This Circuit Work? Chris15 The Projects Forum 7 11-10-2010 11:50 AM
How does this circuit work? asdff1818 General Electronics Chat 4 12-21-2009 01:30 AM
the term "common" shelly50 General Electronics Chat 27 01-23-2006 04:02 AM

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT. The time now is 07:30 PM.


User-posted content, unless source quoted, is licensed under a Creative Commons Public Domain License.
Powered by vBulletin
Copyright ©2000 - 2014, vBulletin Solutions, Inc.