MSF5000 SSCB Not powering up

[Pump3]

Hey folks

Looking for some input...

I have a VHF CXB MSF5000, got it at a hamfest, got the usual speel on how it was a military hot standby unit on a voting system and was just like new...

Anyways I purchased it a few months ago and never had time and finally hauled it out of the shed, hooked 110 to it and as soon as I did I heard a pop and the unit wouldnt power up, power supply was reading normal, fuses were good and the fans go, but nothing more

Under the hood the SSCB is getting power on the main pwr connector and the fuse is good but I wasnt reading any 5v or 9.6v on TP5 or TP6

So off comes the RF tray cover and I can immediately smell the hint of magic smoke and see this



Popping the main receiver/exciter board out I see the previous owner was kind enough to leave a complimentary screw floating around loose under the board not sure if thats what blew the cap or or not but good chance of it

Does anyone know what value C461 is and also what it feeds? could it be why im not getting any 5v into the SSCB?

I dont have a service manual and am not overly fimiliar with these units, I do have a UHF one in ham service but aside from tuning it, i havent had to touch it once

[Pump3]

Guess its somewhat of a common problem?

http://www.repeater-builder.com/motorol ... pa_cap.jpg

Says its a 22uF, 20V tantalum capacitor

[Pump3]

Further tracing shows that Pin 2 of the power cable on the IPA, J/P454A, on the main rf deck, the trace burnt off the board

[kcbooboo]

Yup. Those tantalum caps short out when they go bad, taking the A+ foil on the Uniboard along with it. Luckily it's a fairly simple fix: replace those three tantalum caps, add an insulated wire over the burnt foil (you might as well go from P454A to the connector that the IPA/REG plugs into), and you're probably good to go. The SSCB gets 9.6V from the RF Tray IPA/REG board. You can power up just the RF Tray and check that you've got A+ (13.8V coming in), 9.6V, and 5V on the various connectors. The 5V comes from a 7805 regulator on the interconnect board under the RF Tray. It only feeds the synthesizer logic; everything else on the Uniboard uses 9.6 or 13.8V.

Bob M.

[Pump3]

I can get A+ once I soldered a wire over the burnt up foil like you suggested, getting 15v but im not reading anything on the other P454A pins

[kcbooboo]

I presume you also replaced those capacitors and checked for obvious short circuits on the A+ and 9.6V lines on the IPA/REG board.

Are you getting A+ at the IPA/REG board?

Are you getting 9.6V at the IPA/REG board?

The other signals probably don't matter right now.

Bob M.

[Pump3]

Replaced the caps

VR451 blew and a little magic smoke came from the SSCB, not sure from where, but I replaced VR451 and now the SSCB will continuously power cycle giving error code D82

[Pump3]

Anyone know whats making it power cycle? any ideas