Batboard

We have a couple of MSF5000's that we purchased used and we have a problem with them remaining disabled when we hook them up to our central site controller. We have a TSTAT error that is happening and I feel I have narrowed it down to a problem with the op-amp buffer circuit that reads the dc voltage from the rf forward/reverse wattage sensor assy. Everything checks good going into this circuit (compared to one of our working stations), but the voltage coming out of the forward wattage buffer amp in the LM2902 (U806 located on the SCSCB circuit board) stays at about 3volts whether the station is keyed or not. I then swapped out the SCSCB board, reprogrammed the station and it came up fine, so I know there is a problem on this board, and in I feel it is in this Forward/Reverse wattage buffer circuit. I changed the LM2902 on one board, and it had no effect. Supply voltage to the op amp is good too. I'm hoping someone else has seen this since we have two stations here acting identical.

I checked the schematic. It looks like the non-inverting input (+) would have around 3v on it if the reflected power sensor wasn't dragging it to ground. Make sure the forward and reflected sensor leads are in the proper jacks. Verify the setting of JU21 and JU22 - I don't see them on the pictures of the board but they are listed in the installation manual appendix. One setting is for normal power stations, the other reduces the gain for higher power stations. Of course, they don't say where the thresh-hold is.

The output of U802 feeds the microprocessor directly. I suppose it's possible that the input of the uP got zapped at some time. It's socketed but you might need a special tool to get it out. If you can, swap them and check the voltage again. There's nothing else that it goes to.

My initial thought was that you have too much reflected power, or your load is such that the PA is putting out too much power initially, then it cuts back a bit, but by then the reflected has gone too high. But since you mentioned another board cures the problem, this probably isn't a factor.

Can you adjust the reflected power value via RSS or the front panel switches so the station is less sensitive to this? There is a sequency you can go through with the front panel switches to access these two values. Also, they appear on the RSS EEPot adjustment screen but on PAGE 2 - the first 16 pots are on page 1, and that's normally all you would see. These would be carried around if you cloned the codeplug, but who knows what value they have on a different board. Worth swapping boards and checking, or just setting them to values that make sense (00 or FF should disable the power checking entirely). I've only worked with 450 MHz radios and they don't have the power sensors so I've never had to go here.

Bob M.

Thanks Bob for the reply.

I verified the forward and reflected leads, even measuring the voltage on the RCA cables. I have virtually no reflected dc voltage, and the forward dc voltage I have about .7 volts dc when I run about 75 watts out (this is out of the RCA jack with the leads hooked up to the board). I also noticed JU21 and JU22 in the documentation as you talked about, but I have been unable to find them on the board, as you noticed too. Maybe these jumpers are on a different version of the board?

I am beginning to lean towards the micro as you suggested, and feel game to swap the micro.

The board replacement should narrow it down quite a bit, and yes, I can adjust the forard and reflected wattage with the front panel switches. I've played with the levels and pulled the reflected RCA cable to disable the reflected wattage. But of course, all of these parts are unchanged, when I changed out the SCSCB board and the station comes up. I am checking out the base stations into a load, and the SWR's are great. I have even reset the forward and reflected power levels after cloning the code plug, as you suggested.

Again, I think you're right... it's leaning towards the micro. I'll let you know what I find out when I get a chance to dig some more. Thanks!

Mike

As another test, you could swap the FWD and RFL RCA plugs and measure the output of the associated op-amps to see if the symptom moves when the input voltage changes. I know that these stations aren't easy to probe around, especially if they're still on the floor.

The uP is probably very generic so even if you had to buy a new one from someplace like Jameco or Mouser, it'd probably work just fine.

It's possible that the missing jumpers are actually zero-ohm component jumpers under the board, but the schematic sure makes them look like all the other user-selectable two-pin jumpers. I'll have to check my other schematic sets and see if they show up somewhere. Unfortunately I only have a VHF and UHF manual, nothing for 800 or 900, and trunking only happens above 800.0 MHz.

There are special IC pullers for those leadless chips. They have little hooks that fit into two of the four corners, and when you squeeze the tool, they grab the chip and pull it out straight up. You could probably make something with a couple of paper clips.

Bob M.