Batboard

Hi,
I finally found the Motorola battery pack option for my R2550 monitor and re-packed it with new cells. I work on a lot of equipment in the agriculture industry so being able to check radios in tractors would be a nice feature for me. So, I charged it up and it looks like the pack works like a champ!
I've had the monitor about a year and have used it on the bench day after day with no issues (the antenna input level is low, but I don't use that input much so I have yet to dig into that... fuse is good). But, it seems to have issues running off the battery pack or even external dc supply. When first hooking up the pack to the unit, it will only "flash" the power light. If you leave the external dc pack hooked up for a few hours (monitor off) and then try to power it up, it will finally take and run. (symptom of a lazy cap??) I ran it on the Motorola battery pack and was rather surprised it only ran about 20 minutes before it ran the pack down (this ends up being about a 10AH pack when repacked with the same original cells). Seemed rather short to me and it did not take long to start flashing the low battery message.
So I left the monitor over night to recharge up and check again the next day. But I came back to a blown 10 amp dc fuse on the back of the monitor. Even though I had not used the monitor at all. I had some more bench work to do that day so ran it on the AC with no problems at all. Had a chance to check the dc later and then, the monitor only ran long enough on dc to get booted up and it popped the 10 amp dc fuse again.
I am thinking with the symptoms I am having is there is maybe a power supply cap on the dc input side that is at fault (as the AC input runs fine without a hitch it seems). I pulled a couple of the caps right on the 12 volt input on the monitor, and checked them on my Z Meter from Sencore and they seem to be ok. (I think I am going to replace them anyway though since I have it apart) I don't have a schematic of this unit so I am running blind now. It appears the 12 volt line heads to a relay that is normally closed. I am thinking it's a relay that switches between the battery pack or over to AC operation when you plug the unit in. There are a couple of rather "large" blocks from Noreast on the board and wonder if these are just dc-dc converters? I am thinking they appear that they would also be used on AC operation though so they should not to be of blame (remember, AC operation is fine).
Anybody have any thoughts on what I am up against? Or a copy of the power supply circuit would be of some help too if there is one out there.

An update on the problem. After walking away from my project I sat down with it again. I thought I had better put it back together for the week and when re-mounting the power supply PCB, I happened to notice a loose screw on the case (collector) of one of the TO-3 transistors on the rear of the board. Needless to say, they were both loose on one of the transistors (switcher MOSFETS or transistors I am thinking... but no schematic). The design of the board relies on connection of these two screws for the case of the transistor to the board. The other transistor next to it seemed to be snug.
So I put the unit back together and hooked up the battery pack and fired it up. I was out of 10amp fuses and only had an 8 amp quick blow so I threw that in. After 50 minutes of running, I finally got the "low battery message". Better than before! With intermittent duty usage in the field, it should work out well.
I did hook it back up to the power supply and checked current consumption for curiosity. When running at about 12.0 volts, the current draw was around the 7 amp range. If I ran the power supply up to about 15 volts, my current dropped down to about 5.2 amps. Makes sense I guess when you look at the wattage consumption on the supply.
I'll have to keep my eye out for a good deal on a service manual on eBay. It could come in handy someday, as I should look at my ANT input since it is reading a bit low (12-15dB if I remember correct).
Hope my findings might be of help to another one someday.