[Kenwood] TS940SAT - troubleshooting questions

[rsakabu at comcast.net]

Hello Rob,
  Not to be insulting, but you are sure the timer button isn't pushed in?
That one bit me once or twice before.

73
Roy
AD6ZU


> You guys helped me fix this radio once before, so I hope you can do it again.
> 
> After operating my TS 940SAT a fair amount over the weekend without 
> problems, I was surprised to push the power button on Monday and get no 
> response -- no lights, no sound.  It wasn't the fuse or the power cord, so 
> I started pulling screws.  After a little bit of troubleshooting, I think 
> I'm on the trail, but before really taking the radio apart, I need some help.
> 
> The main supply on the AVR (X43-1500-00) is putting out 28.5V just fine, 
> but there is a smaller supply that provides some lower voltages and it 
> seems to be turned off.  Specifically, the base on Q104 is low, cutting off 
> the low voltages.  Tracing that  signal off the board to the control unit 
> (X53-1420-11), I find that it should be coming from a relay, which is open 
> because a signal, TMY, is not low (21 V).  Following that to the switch 
> unit (X41-1600-00) I find that I can't trace further, since the board is 
> mostly inaccessible without really taking things apart.
> 
> Examining the schematic, I found that the CPU on the switch unit is powered 
> by a couple of lithium cells.  Hmmmm.  Given the age of this radio, that 
> could be the source of the trouble.  Here come the questions:
> 
> 1) Is a dead radio a symptom of lithium battery failure?
> 
> 2) My schematic shows the cells in parallel, and they are 3-volt 
> cells.  With the power switch off, I measure 3.00 volts on the cell I can 
> get to, but the forward drop across D4 is 0.33V (Germanium?), and there is 
> 2.67 V on VDD.  With the power switch on, VDD rises to 4.4V.  Is this 
> correct behavior?  Is there any indication that the batteries might need 
> replacing?
> 
> 3) Assuming the batteries need replacing, I am considering getting CR2032 
> replacements and mounting them off the board.  Does anyone have experience 
> regarding that?
> 
> 4) Regarding the service manual, there are some signals highlighted in red 
> and some parts are highlighted in red.  Besides meaning they are important, 
> what is the significance of that?  I will probably feel stupid when I get 
> the answer.
> 
> Thanks in advance for your help,
> Rob
> 
> 
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