Kenwood tm-731a blowing line fuse

[KB3SVJ]

Hi  ALL:

First,  I am  not greatly  skilled in electronics but  if  I farm out the   task, I'll  never  learn.  And   while I  did have  some  books  on  basic  electronics,  they went  up  in  smoke along  with my  home the  evening after  Thanksgiving  so, I  don't have   those  resources  to  fall back on. 

I  bought a   used  731a from a   fellow   in my  county  who   had given up the  hobby.  Of  course  when he packed  away the   unit a  couple years  ago, it was in working order.  Well, it's  not  now.

I  attached the leads to the   battery after first checking the   inline  fuse to assure it  was   good;  it  was ok.  I pulled the   leads  from the   battery  post checked the fuse  again and  saw that  by merely making the  connection, the  fuse   blew.  I dismantled  the  solder/tape splice   connecting the   radio's  power  cord to the   zip line. 

I  placed one  meter probes  to the NEG lead  into the  radio and  the other  to the  chassis-with an  alligator  clip- and   had   complete   continuity.  I then   placed the  probe that  was on the  radio's  NEG   lead and   place it  on the onto the  POS lead.  I  have   continuity but not to the extent of  going from  NEG to  chassis.  I  obviously have a  short and  am  wondering how  to  go  about tracing  back from the  power   in  point in the  back of the  radio  to the   offending and  seemingly  shorted component therein.   I  understand I'll need   schematics  and am trying to track them  down on the net.

There is also   a   serial cable   spliced into  a   phone   line  cable   with  an  rj22 plug at the  end. I  assume  that   got   plugged into a TNC.   I  am  wondering if this  could  be a   cause for the  shorting.  I  don't  want to  unravel  that   part   until I  consult  with  somebody in the know.

If   it   helps,  I can  post   pics of  what  I am dealing with on my facebook page  of  put  up a   quick  video  on you-tube.

Thanks  in  advance for  your   consideration  and   help.

John  KB3SVJ

[K8AXW]

John:  Rather than repeat myself and what others have said about this kind of problem I suggest you slide down 5 on this forum to the subject, "Sick TM261."  He has the same problem and the possible solution should be the same.

I would add one thing else John.  The first thing I would do is replace that crapped up power cord with the proper color coded wire, inline fuse and eliminate the problem of what goes where.

[KA4POL]

Concerning the phone line cable there is an easy way to check it. Just pull it before you try to blow the next fuse 
I doubt that this is a cause.
A battery is not a good way to check the rig. I recommend using a power supply with a current limiting function.

[N4ATS]

Its quite simple , where the power cord goes into the back of the TM-733 , almost right where the positive wire solders onto the board , you will see a diode , which is there for this very reason. Replace it.

[KB3SVJ]

Thanks  all for the  advise.  I'll check  back  in   hopefully   b4 the  weekend to  let  you lnow  if  the problem is  corrected.

Also, I  found  a   link  to  731  service   manual  PDF format.

http://www.google.com/url?sa=t&rct=j&q=schematic%20for%20kenwood%20tm-731a&source=web&cd=3&cad=rja&ved=0CDoQFjAC&url=http%3A%2F%2Fhome.comcast.net%2F~ko6kd%2Fmanuals%2FTM731A_Service_Manual.pdf&ei=bNc9Ue-ELO250QGex4CYCQ&usg=AFQjCNHy8Rrz97si6Offgma6XiAXiyY7zg&bvm=bv.43287494,d.dmg

73
John

[K8AXW]

John:

It seems that this radio uses more than one diode for protection. 

Check Page 19 of the service manual.  Look in the chart for:

D101 for reverse current for the lithium battery
D102 for reverse current for the microcomputer
D103 for reverse current protection.

Page 21 & 23 also lists more diodes for reverse current protection

With a bit of patience and a good magnifying glass you should be able to see if these diodes are the problem. 

There could be more John..... but these will get you started.

[N4ATS]

Typically its one doide near the power wires where they solder on the main board.

[K8AXW]

ATS:  You're correct but I couldn't find such a diode on the schematic.  Normally it's ahead of the on/off switch which protects the switch from the short circuit surge.  No here though.

[KA5IPF]

D12 in the Final unit.

Clif