Discussion in 'Radio Circuits, Repair & Performance' started by KC4UMO, Apr 26, 2015.
Is the reverses polarity protection Diode good ?
Good Luck on your project.
I thought of that also. But if it was shorted it should show zero ohms like a dead short. At least everyone I seen has in the past. I will not rule it out though. When I return home this afternoon I will pull the final board so I can get to the back side to desolder stages.
Like I said It's been awhile since I sold mine. I'll be watching as well.
If you should need any help, I'm a few hours away (Anderson, SC) but you are welcome to bring it over as I'm pretty well setup for reworking such boards (including hot air rework and IR preheating for boards with large internal ground planes).
Because you have a low impedance fault, if you have a good multimeter (preferably with 4-wire Kelvin probes, such as a good 5.5 or 6.5 digit bench meter) that will reliably measure in the low milliOhms to upper microOhms range then you can go a very long ways to isolating the fault by following through the circuit and finding the component that lies across those power rails and has the absolute lowest resistance measurement anywhere on the board -- it will either be that very component that has failed or a neighboring component that is extremely close by. That is a very useful way to troubleshoot low impedance faults on a board and has really helped me on otherwise difficult projects for which there were no available schematics. Don't worry about zeroing your probe resistance because all that you care about is the absolute lowest value resistance measurement you can find on that board.
Good to know there is another person close that has the know how to troubleshoot radios.
I have yet to look through the schematic. Will have a peek at that tonight. I'm a bit of an old school tech myself but no stranger to smt.
The collector of the finals is hooked directly to the input voltage, no switch. They would blow the fuse when plugged in. The drivers are not connected to power until he radio is turned on. If one is leaky C-B at turn on it will turn on the other hard enough to draw 20a and blow the fuse. Lift the bases of both drivers first and see what happens.
Thanks for the reply Clif.
I just got home read your reply and lifted the bases of both. Problem is still there.
With the bases lifted I installed 2 10 amp fuses. Turned the current down on the supply and When touching the positive lead to the external supply I got a little spark, saw the amp meter climb fast so pulled off.
So this was with the radio powered off. Since the finals are connected to the supply perhaps they are the culprit. I want have time tonight to check them, will be tomorrow evening.
I am a bit confused on your fuse setup.
You were using 2ea 2 amp fuses ?, and now you are using 2ea 10 amp fuses ?
The original power cable has 2 fuses ? Maybe you meant 2 20 amp on your original post.
What kind of power supply are you using ?
Good Luck on your project.
Yep, the cable has 2 fuse holders. It quickly blew the 2 amps. So went to 10.
I am using smaller fuses so they blow quicker. I do not want to risk putting the 2 20 amp fuses in line.
The supply I am using to test with is a Astron VS-50. It has varable power and current.
So since I have nailed the problem down to one board I do not have to power it up again untill the faulty component is found.
The radio should not draw more than 2 amps on receive. But I saw over 10 amps.
Hope this explains a bit better.
Sounds like you are doing good.
I would disconnect the C or E to see if the transistors are shorted.
It would suck if the firmware was updated, And it puts the radio in transmit on power up, but if you disconnect the base that is ruled out.
Does the radio put out any power ?