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Antenna Switchboard Not Switching

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dmedic View Drop Down
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    Posted: 29 April 2010 at 8:23am
Hi,
 
I'm having a strange problem. It's probably something simple. The switching only happens on the 7473 on pin 13. the switching on pin 12 stays on. Measuring the voltage out on the pins of the 7473 I'm getting a difference of .6 vdc on pin 13, but only .3vdc on pin 12. The actual voltages are 3.8 to 4.4 on pin 13 but only 3.8 to 4.1 on pin 12. Looking at the schematic the only difference I can see is that I tied both protection diodes on the collector side with the 330 ohm resistors. Any thoughts?
 
Thank you
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amal Quote  Post ReplyReply Direct Link To This Post Posted: 02 May 2010 at 12:39pm
Sometimes, when made by different manufacturers, the 7473 can have different pinouts. Can you post the manufacturer info and maybe a scan/pic of the pinout information that came with the flip flop, if any?


Edited by amal - 02 May 2010 at 12:51pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dmedic Quote  Post ReplyReply Direct Link To This Post Posted: 03 May 2010 at 7:28am
Hi Amal,
 
The 7473 is a NTE. I built a test circuit and found the the flip flop works with just LEDs on pins 12 and 13. In the antenna circuit pin 13 switches but pin 12 stays high. The voltage variation that I was seeing on pin 12 was only a drop in voltage when pin 13 switches on. I used zener diodes for the protection diodes, do you think that they could be the problem?
 
The voltage at the collector of the two transistors is also different, the tansistor connected to pin 13 goes to zero when switched off but the other on pin 12 never changes it stays high. I also tried changing transistors but there was no difference.
 
I also tried a 74LS73 and strangely the problem reversed, pin 12 switches and pin 13 stays high. Very frustrating. I used sockets for the ICs so I could change them if necessary.
 
Thank you for your help,
 
Dan
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amal Quote  Post ReplyReply Direct Link To This Post Posted: 03 May 2010 at 10:19am
Hmm, I'm not sure if the Zener diodes are the problem, but I do know they shouldn't be used. They allow current to flow both ways, which is not something one expects of a "standard" diode. Even if they turn out to not be the cause of the problem, I'd replace them anyway.
 
I'll dig up a copy of the switching circuit later tonight and see if Zener diodes could cause that problem. In the mean time, replace those diodes for standard switching diodes and keep testing different configurations.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dmedic Quote  Post ReplyReply Direct Link To This Post Posted: 03 May 2010 at 12:01pm
Hi Amal,
 
I just added the transistors to the test circuit and I have the same problem. When I drive the LEDs off pins 12 and 13 it works, but when I add the transistors it no longer switches. I'm using Radio Shack PNP Transistors part #276-2023 which are listed as General purpose medium power switching and small signal amplification.
 
Thank you,
 
Dan
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amal Quote  Post ReplyReply Direct Link To This Post Posted: 03 May 2010 at 1:02pm
Have you replaced the zener diodes with switching diodes as well?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dmedic Quote  Post ReplyReply Direct Link To This Post Posted: 03 May 2010 at 1:06pm
Hi Amal,t
 
I have not used any diodes in the test circuit. I'm going to Radio Shack and pick up some as well as some other transistors.
 
Thanks,
 
Dan
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amal Quote  Post ReplyReply Direct Link To This Post Posted: 03 May 2010 at 1:22pm
ok. just out of curiosity, what did you have connected to your switching transistors? what were they switching?
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dmedic Quote  Post ReplyReply Direct Link To This Post Posted: 03 May 2010 at 1:32pm
Hi Amal,
 
I connected the LEDs to the collectors of the transistors. The bases are connected to pins 12 and 13 and the emmiters are connected to +5vdc. The LEDs have the annodes connected to the collectors of the transistors and the cathodes connected to 330 ohm resistors to ground. The LEDs light but do not switch.
 
Thank you,
 
Dan
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dmedic Quote  Post ReplyReply Direct Link To This Post Posted: 04 May 2010 at 11:44am
Hi Amal,
 
I finally have it working. It took some "pull down" resistors to make it work. I placed some 1K resistors between pin 12 and the base of the transistor, and pin 13 and the base of the transistor. The circuit is now switching. Clap
 
Thank you for all your help,
 
Dan
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