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

[Joe Mann]

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