Batboard

I have a Lowband Motorola radius D51MJA97A3AK and it has a weird problem. I bought it with a blown 2A pico fuse and replaced it, then that one and the subsequent one also blew (thought maybe just a defective fuse). Well it turns out I am getting continuity between the fuse end nearest to the Accessory connector and the chassis. I know that is not how it should be. Any ideas where to start looking to find the fault?? I have never run across this....

Mike

May be a multi-layer board with a short between layers. If you're using the internal fuse then I assume you're not using an ignition cable. You may have to try some micro-surgery on the board to cut/isolate the faulty trace and do a jumper to by-pass it.

It's possible that there's an issue in the power to the PA as well, either ground or A+. 30A = blown 2A fuse.

It shouldn't be a problem in the PA A+ as that's connected the other side of the fuse.

The side of the fuse that connects to the ACC connector is B+, this connects to the volume control board through pin 6 of J8, goes through the power switch, and back to the radio through J8 pin 5 as SW B+.

So the next thing to do is see if the short goes away if you turn off the power switch. If it doesn't, try unplugging the volume board from J8, if the short is still there it's going to be somewhere between the fuse, ACC connector IGN SENSE pin, and J8.

If the short goes away, it's somewhere in the circuitry after the power switch. Likely suspects would be VR807 which is a 27V zener diode connected between J8 pin 5 (SW B+) and ground, and U402, the 5 volt regulator IC.

Last but not least, tantalum capacitors.

I ran across one shorted cap where I noticed that the resistance between ground and the power bus went up as I moved away from it. Eventually, I spotted it on visual inspection, and when I crushed it, the power busses opened.