Batboard

Of all the radios I have worked on this one has me stumped!!
Radio worked at one time but now the PA shuts down as soon as it starts to Xmit. Although all looks ok, I get the double beep over the speaker to alert me the PA is down. Any help on how to start of if there is a way to bypass shutdown circuit so I may test path??

Thanks
~john~

Check the wiring to the forward/reflected power sense circuit in the PA. If the controller in the MSF does not sense good power, or senses high reflected power, it kills the PA and does exactly what you say.

Bat

I thought that might be the problem. But is there a way to bypass the shutdown circuit so that I can measure the test points with the xmitter xmitting.

~john~

On an older model there is a jumper JU1 i beleive that will diable the safties in the control. I had the same problem in one on my MSF5000's. I found two things. 1) A bad power supply was putting a spike into the A+ on transmit ans the unit was above rated out power. Try a bird on the output of the PA and see if you are over the rated output.

<font size=-1>[ This Message was edited by: ulmspec on 2001-10-22 07:38 ]</font>

Nope, JU1 didn't bypass supervision circuits. Thanks for trying. I wish these service books explained things a little more in detail. As soon as I hit the Xmitt switch, the PA led lights and shuts down instantly. To fast to measure anything. I still need to force it to run long enough to see where the problem is.

~john~

Try turning your power down via the pot (If a CLB series). Sounds like the post filter has gotten out of tune and is sending some reflected power back to the PA. You can verify this by bypassing the post filter and going through a wattmeter to a 50 ohm dummy load.

For the CLB series the power control circiuts are on the Uni Board in the receiver drawer.
The power control is looking at tempature (PA and circulator), froward and reflected power, overvoltage on the control voltage line. I remember in MSF school the instructor talked about a jumper, but I have'nt found one. Check the voltages comming from the PA to the receiver Uni Board's power control circuit on P402A. The power control sends two signals, PA FULL POWER and PA ON to the Control board that is processed by the microprocessor and lights the TX led's.

I thank you sir!

I bypassed prefilter and it keyed up. Now I have the task finishing it up. Again thanks!

~john~

By the way, it is a digital but with the talent out there you guys got me going!

When tuning the post filter be sure to have the coax connected to both the transmitter and the receiver. The coax is cut to the proper length depending on whether your offset is + or -.