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Author Topic: Yaesu FT 726R help for VFO trouble please!!!  (Read 4369 times)
CX6AV
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Posts: 4




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« on: October 31, 2012, 05:33:21 AM »

Hello, i just purchased an old Yaesu FT 726R.This radio are beautiful ,it´s works fine in all bands, 2, 6 and 7cm.
Unfortunatelly has a little trouble, when turn it On ,takes near 3 minutes to unlock de VFO change of frecuencies by the Knob (i can do right  with up/down mike butttons and by scan).
When the radio warmed 3 minutes the knob of vfo come to work fine.
I was cleaned with cleaner contacts (not lub) all the control unit, the encoder, the pins, but no effects, i believe that the encoder works good, due when cold trouble locked the radio move 2 decimals up and down, but only this.
Anyone can help me?, Sorry my poor english, Regards, Victor ,CX6AV from Uruguay
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W9GB
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Posts: 2600




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« Reply #1 on: October 31, 2012, 05:51:54 AM »

Victor --

This 2004 eHam.net Q&A about K1MCN issue should assist your FT-726R troubleshooting and repairs.
http://www.eham.net/ehamforum/smf/index.php?topic=55560.0
Quote from: K1MCN
I have a Yaesu FT726R that the VFO dial dosn't work when the unit is cold.  After it warms up for a while, the tuning works fine.  I have isolated the problem to the control board.  Is it possible that a capacitor or resistor could be cold sensitive? I have never run into this problem before.

A copy of the Yaesu FT-726R Service Manual, parts list by module and schematics are mandatory for proper disassembly and repairs.

Pete, K1ZJH offered this comment in 2004:
Quote from: K1ZJH
Erratic VFO tuning for the FT-726R really requires the use of a dual trace scope to look at the phase relationship and levels coming from the shaft encoder.
I've gone through this problem on two different FT-726R transceivers, and the most common cause is the 100pF disc ceramics used to filter the encoder noise become leaky. This is easily seen when scoping the encoder signal--the bad signal will be much lower in amplitude.
The debouncing circuit uses high megohm resistors and a small amount of leakage in the 100pF caps will kill the encoder output.
Unfortunately the capacitors are extremely tiny, and that area of the control board is densely packed! But, it can be done. I suggest using SMD replacements, right on the foil pads.

K5DOG offered this solution in 2004, which solved his issue.
Quote from: K5DOG
For those searching for answers to this issue, I want to say that I just fixed this problem by replacing the 100pf capacitors (4) on the Control Unit Board.  
They are C15, C20, C21, and C22.  This replacement fixed the issue.

Remove FT-726R top and bottom covers, then remove two side screws, and the front panel swivels down out of the way.  The Control Unit board is behind it.  Carefully loosen the connectors and the Control Unit board slides up and out of the radio.  The 4 capacitors are on the end, near the battery.

w9gb
« Last Edit: October 31, 2012, 06:16:11 AM by W9GB » Logged
CX6AV
Member

Posts: 4




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« Reply #2 on: October 31, 2012, 11:34:27 AM »

 Smiley Thanks for your complete help. i try the reparation and comment here, regards and many thanks from Uruguay!
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K9YLI
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Posts: 853




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« Reply #3 on: November 01, 2012, 08:15:14 AM »

 Since its a  heat problem  ..  try  heatng  parts of the radio  when its cold  to speed up the
time when it works correctly..    hair dryer  to start for   large section..

then  either  reduce  hair dryier out put to    soda straw size..   
low power and   paper cone  or  use  soldering iron..
slowly  heat components n the smaller area..  this may take  many trys,,or even days for it to cool off between cycles.

or  also get some  canned air  from  computer store.
turn can upside down.  lit will spray  very cold..
ONce the radio works  correctly..   spray  parts  until it fails.. 
 then replace  what might be bad.    of course using the schematic
to verify  you are likely  in the correct area.

also  first check the  power supply voltages  both  cold and when working..this could be a power supply  problem

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CX6AV
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Posts: 4




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« Reply #4 on: November 02, 2012, 07:59:26 PM »

Many many thanks,for the help of 2 friends!!! W9GB, and K9YLI , i was changed the capacitors, but the problem persists, ...we work many hours investigating the cause of trouble, i was connected my old oscilloscope and view that the square wave from encoder output was o.k., but in the pin nr.5 output of C.I. Q16, the MC14069UB isn´t square wave, fortunatelly i was go to one old electronic store in near Las Piedras city, and purchased for only 1 dollar this C.I.!!!...
Changed this C.I. the radio works fine as new!!!!!!!!!!!!!!!!!!!!
I´m VERY HAPPY...!!! thank for our help, due your atachments was explained for me how the encoder an c.i. works !!!!
I hope this post help for another HAMs. to fix this trouble.....
Regards from the route of fine wines in Uruguay!!!,and  both have a new friend here!!,best wishes for you and your family!!. Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Vic
« Last Edit: November 02, 2012, 08:03:00 PM by CX6AV » Logged
W9GB
Member

Posts: 2600




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« Reply #5 on: November 03, 2012, 12:13:12 PM »

Vic -

GREAT.  The MC14069UB is a Hex Inverter, still mfg. by ON Semiconductor.
http://www.onsemi.com/pub_link/Collateral/MC14069UB-D.PDF

I would rather see these radios properly repaired and operated,
than stripped for parts -- like automotive CHOP Shops.
« Last Edit: November 03, 2012, 12:15:42 PM by W9GB » Logged
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