[Drake] TR-7 Short on the DR-7 Board - Resolved

[Joe Mann]

Hi again,

Private e-mails from several list members suggested replacing the 7805
Regulator.  Unfortunately, replacement of the original MC7805C made no
difference.  8-( 

The short (5V buss to ground) was about .2 Ohms most of the time.  I did
find that it would vary up and down a tenth or so, but not in a consistent
fashion.  During sessions where I would try all the ICs and caps, the lowest
readings seemed to indicate the short was closest to the 7805 and not at one
of the chips/caps.  Resistance readings on my other "good" TR-7's DR-7 board
were between 600 and 700 Ohms for this 5V bus.

As an aside the DR-7 board actually has 3 separate +5V busses.  My short was
associated with the counter portion. 
 
I then detached the 7805 regulator from the DR-7 resulting in almost
identical results.  

I finally decided to pull the DR-7 from the radio.  The short went away!
?????

To make a long story a little shorter, I swapped the DR-7 in and out several
more times.  In = shorted, out = 600 - 800 Ohms, just like the functioning
TR-7's DR-7.  

The culprit ends up being a loose screw (Where have we heard that before?)
holding the shield for the several boards UNDER the DR-7.  The screw head
was shorting to the underside of the DR-7.  The screw's location is just
about under the connector for the 7805.  

Seat the DR-7 into the TR-7 loosely, no short; tighten it down , short.

Apparently, I had failed to fasten the shield properly at some point and
later on, paid the price.  8-( 

I managed to break off all but 1/8 in. of one of 'legs' of the original
7805.  I'll wire it up to 12V and see if it failed or not.  I suspect not.

So the good news is that the short has been lengthened, the bad news is that
I fell on my sword.  8-) 

Joe K9HDE

> -----Original Message-----
> From: drake-bounces at mailman.qth.net 
> [mailto:drake-bounces at mailman.qth.net] On Behalf Of Joe Mann
> Sent: Tuesday, February 22, 2005 8:58 PM
> To: Drake at mailman.qth.net; drakelist at www.zerobeat.net
> Subject: [Drake] TR-7 Short on the DR-7 Board 
> 
> 
> Hi,
> 
> Can someone point me in the right direction my DR-7 problem? 
> 
> Apparently there is a short to ground on the counter +5V bus. 
>  The input to the MC7805 (U9033) is about 13V but the output 
> is somewhere in the neighborhood of .2V rather than the 
> expected 5V.  I am assuming that the 7805/U9033 is in some 
> sort of shutdown mode due to excessive current being drawn.  
> The 7805/U9033 itself is quite warm/hot.
> 
> The other MC7805 (U9034) seems to be functioning normally, 
> i.e. 5V out. 
> 
> Other symptoms are:
> 
> LED readouts show only their decimal point. 
> 
> Rx does not tune, signals "wander" past the IF in a fashion 
> similar to when the RCV button is pressed. 
> 
> As far as I can tell all circuitry from the 2nd mixer thru to 
> the 2nd IF/audio sections is functioning correctly.  
> 
> On the PS/REG board, the +25V line is at +22.4V and the -5V 
> is at 3.12V, and the +5V is at +4.81V and all other voltages 
> are nominal.  
> 
> I am not seeing the 500 kHz waveform on R9041.  Hmmm, maybe 
> there's more wrong than I originally thought.
> 
> 
> 
> My real problem is finding the short, which is probably a bad 
> IC, maybe a tantalum, without unsoldering and removing all 
> the ICs on the 5V buss. 
> 
> I have another correctly functioning TR-7 for making voltage 
> comparisons, board swaps, etc.  
> I also have a set of extender boards, a 40 mHz scope, a 0-10 
> mHz sig gen and a Fluke 8050A digital voltmeter to bring to 
> bear on the problem.  
> 
> While I am not afraid to go in there with a soldering iron, I 
> really don't want to.  8-( 
> 
> I'm sure I have another 7805 regulator in the junk box; are 
> there any concerns about swapping that first?  [Actually, 
> there's one in the 'good' TR-7.]  8-) 
> 
> Joe, K9HDE
>