Batboard

I'm trying to troubleshoot yet another MSF5000 that won't transmit (groan) and in doing so, I think I've discovered a method to bypass the power control cutoff in a Digital Capable (CXB) station:

1) Press and hold the Select/Set switch to Set.
2) Press and hold the Xmit switch. "tSt" will pop up in the LED display.
3) Release the Set switch while continuing to hold the Xmit switch. The Disable LED will begin flashing slowly.

On my station, after performing this procedure the PA Key LED will illuminate when I press Xmit while the PA On and PA Full LEDs flash briefly. Since this isn't documented in any manual I have access to, my question for the MSF gurus is:
-is this actually a power control bypass test procedure, and
-if I'm metering 0 watts out of the IPA into a dummy load while doing this, can I safely assume the IPA is dead?

1. Sorry. It is documented, although very hard to find, in some manuals, and that IS the procedure to enter power control bypass test mode. In the Motorola MSF section of repeater-builder, search for 6881082E05 and go to PDF page 26.
2. No. The power control circuit, which includes the front panel pot that often goes bad, controls the output of the IPA. To thoroughly check it, you have to make sure you've got RF drive power coming into the IPA (it comes from the VCO and may be buffered on the Uniboard), then make sure you've got some voltage on the controlled DC power input. I'd have to do some research to find out what that voltage should be, but I'd expect between 4 and 9VDC should produce some RF from the IPA.

Bob M.

Thanks, Bob! Can you refresh my memory on what the value for the power control pot is? (edit: nvm, found it... R426, 10k)

I know the VCO is good because it tuned up OK and I can hear the carrier with CW ID and PA Fail beeps on an unmuted receiver, but beyond that it could go either way on the Uniboard or IPA being bad. I can say the station makes zero power from the IPA while turning the power control pot, and I've already done the basics like shooting it full of contact cleaner and running it back and forth.

Additional info: find 6881082E20 in the Motorola MSF section and open the VHF IPA and Regulator PDF file. It has troubleshooting procedures for both assemblies. The test procedure even documents how to put the station into tSt mode.

The pot could go open. That's often how they fail. The arm doesn't make contact with the carbon substrate. If you have three hands you can put an ohm-meter from either end to the center and rotate the pot and look for a value change. Usually the pot can't be fixed; it must be replaced. It's a poor design.

Over on repeater-builder, in the Motorola MSF section there's an article on replacing the Uniboard pots. The values are listed there.

Bob M.

Did you find the problem?

Bob M.

kcbooboo wrote:Did you find the problem?

Yeah, the IPA is Tango Uniform. The power control pot was fine, if a bit out of tolerance.

Control voltage, A+ and 9.6V are all present at J452, and the TX Enable line swings high to 12V when the front panel toggle is flipped. The control voltage swings low from 12V to 1V on transmit, and varies by a few tenths of a volt based on the power control pot setting. Following the troubleshooting flowchart form this manual there is no A+ at Q459 and no control voltage at Q460, Q461, or Q462. That indicates multiple failure points, and the manual just stops at "replace the IPA" at that stage.

You probably can't fix anything on the ceramic IPA substrate but the surrounding discrete parts might be easy to replace. The style of IPA/REG in that manual is the "old" type. The newer ones use a small RF brick and the majority of the circuit board is the regulator and an interface to the brick. I also think that the output coax is permanently attached to the substrate on the "old" style but goes through an SMA connector on the "new" style. It still terminates with an N-male connector though.

Could just be an open resistor or transistor causing the problems, or a burnt foil. Pull the IPA and trace the A+ line.

Bob M.