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Author Topic: Good resistor-bad resistor?  (Read 1420 times)
N5CCQ
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Posts: 15




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« on: May 09, 2008, 12:40:59 PM »

Hi, I purchased a Yaesu FV700DM digital VFO that I have discovered has a intermitent problem. When it manifest itself, which is usually shortly after being turned on, the LED that indicates the VFO is working either goes out or flickers and the VFO is inoperative. I have traced the loss of voltage from the LED back through the control board to the PLL board.  Here is what I have found: 5 volts DC comes from a IC to a 510 ohm resistor, on the other side of the resistor the DC voltage goes to a jack that supplies voltage to the LED and also to the collector of a transistor. There is always voltage on one side of the resistor (as long as the rig is turned on), however when the VFO stops working there is no voltage on the other side of the resistor, it quits passing the voltage, sometimes it will read less than a volt and the LED will flicker and the display will be eratic, but when I measure resistance it always shows the 510 ohms!  I have reflowed the solder on both sides of the resistor but still the same. Can the resistor be bad but still show the correct ohm value? Also the only resistors I can find are 560 ohm 1/2watt which physically is about double the size of the one on the board but should fit, will it be a o.k. replacement? Don't want to do any unnecessary soldering on this old board.

Thanks
Bill
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W5JI
Member

Posts: 146




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« Reply #1 on: May 09, 2008, 12:50:31 PM »

I don't have a schematic for your VFO, but based upon what you said about the 510-ohm resistor being on the PLL board......I'd bet the transistor that is hooked to the resistor is the "PLL Locked" versus "PLL Unlocked" switch and that the transistor is pulling the LED voltage to ground because the PLL is not properly locking.

The LED indicates the VFO is operating properly, but if the PLL unlocks then the LED will be turned off by the transistor turning on and grounding the LED end of the 510-ohm resistor. This is to prevent a bad VFO signal from being applied to your transceiver.

You need to find a schematic and/or service guide for the VFO. It will tell you how to adjust the PLL circuitry so that it locks properly.

73.....Jim  W5JI


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N5CCQ
Member

Posts: 15




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« Reply #2 on: May 09, 2008, 01:37:25 PM »

Hi Jim, the LED and the transistor are on the same side of the resistor, when the DC voltage isn't getting through the resistor neither the LED or Transistor is getting the voltage.  I have the schematic but have not been able to find a maintenace manual.

73
Bill
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N5CCQ
Member

Posts: 15




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« Reply #3 on: May 09, 2008, 03:46:54 PM »

To maybe clarify this for those that understand circuits (not me I just check voltage, resistance and look for burned componets). The DC voltage that should go through this resistor also goes to the collector of a 2SK19GR FET  labeled on the schematic as "UL", it leaves the PLL board (wired connection) and goes to the Control Board as input (Labeled "UL in") on the base of a 2SC1815Y AF voltage amp, also connects to pin7 of a MC14519B which is a and/or selector, or Quad 2 channel data selector, or a Quad Exclusive NOR gate. The emitter of the 2SC1815Y is the "UL" out (wired connection) that goes to the LED referenced in the first message.
Would like to know what "UL" is in reference to........

73
Bill
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W5JI
Member

Posts: 146




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« Reply #4 on: May 10, 2008, 08:22:43 AM »

Bill,

As I indicated before, if the transistor is turned on then the voltage you are measuring will be zero volts or ground. When you say there is no voltage it is because the PLL has failed to lock properly and the transistor is turned on and pulling the LED side of the resistor to ground....thus you don't measure any voltage.

The UL signal pin you described is the "PLL Unlocked" signal. This is the signal which is causing the transistor to turn on and pull the LED end of the resistor to ground.

If the PLL fails to lock, the designers of the VFO do not want a signal to be output from the VFO since it most likely would be off frequency or maybe even out of band.

73.....Jim  W5JI
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W5JI
Member

Posts: 146




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« Reply #5 on: May 10, 2008, 08:33:45 AM »

Bill,

Another possibility is that some other output from the PLL board is pulling the end of the resistor to ground rather than the transistor you originally described. From your last description, that is a definite possibility.

Either way, the problem is being caused by the PLL not properly locking, not by a bad 510-ohm resistor. Debugging a PLL circuit is difficult since an unlocked condition generally causes the PLL to halt. This makes it very hard to find what is actually causing the PLL to fail.

A PLL normally operates by locking to a reference signal such as a crystal oscillator. Often, the crystal oscillator freq will have drifted out of tolerance due to age and/or component heating. This will make the PLL fail to lock. You will need a good freq counter and a multi-trace oscilloscope to be able to properly debug your PLL. If you have no PLL experience, I suggest you try to find an Elmer with such experience.

73.....Jim  W5JI
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N5CCQ
Member

Posts: 15




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« Reply #6 on: May 10, 2008, 08:49:04 AM »

OK Jim, I appreciate the information, have read everything I can find on the web and ARRL handbook and they don't give a lot of info on troubleshooting the PLL. Guess what I wasnt understanding from your first reply was that when not working I wasn't able to measure a ground connection on the resistor or the transistor collector.  I think the reference oscillator is 30 MHZ so will see if I can pick up a signal there just out of curiosity.  Was going to join the local (County) ham club today in hopes of getting some hands on help but with a lot to do for mothers day tomorrow could not make it to the meeting, will go next month.  Again I appreciate your input and from the reading I have done have a better understanding of what you were saying. I didn't pay a lot for this piece of equipment and the seller offered to take it back but figure I can get my money back by the education I am getting trying to get it to work.

73
Bill
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