MSF5000 PA Problem

[ve9xf]

I'm having a problem with a MSF5000 UHF repeater I recently acquired. Essentially, I have gone though the tuning procedure and both the RX & TX VCOs lock, but no TX power from the PA. The PA Key, PA On & PA Full LEDs come on momentarily, then go out. The specifics on the station are as follows:

- Model C74CXB7106BT (UHF Range 2)
- SSCB v3.25
- TTRC v4.22
- No internal pre-filter or post filter

Things I have tried so far:

1. Terminated the PA into a 50 ohm dummy load, both directly out of the PA and also after the low pass filter on the junction panel and the problem still persists;
2. Turned the Power Set pot down to minimum and slowly increased it - no luck;
3. Attached a UHF HT as a driver and was able to get full power out of the PA, so it appears to work fine.
4. I tried putting the SSCB in test mode and keyed the XMIT. Still no PA output, though at a very minimal setting of the Power Set, the PA On LED remains active, through the PA Key remains off. I also measured the IPA output and it is 0 watts.

Does anyone have an ideas as to what the problem may be here? Is my IPA dead?

Thanks!

Don
VA3XFT

[d119]

Is the metering cable between the RF deck and PA connected? It's a small cable with a molex connector on each end.

If this isn't connected, the station won't "see" the PA, and will do exactly what you describe.

If it IS connected, pull out the RF tray and verify that it's connected to the interconnect board on the bottom underside of the RF deck.

It goes like this: SSCB > Interconnect Board > Cable to PA Connector > PA connector into PA.

Check all of that out, I bet that's your hangup. Please report back and let us know how you did.

[ve9xf]

Thanks for the response. I just removed the RF tray from the cabinet and verified that the metering cable is connected to both the PA and J502A on the interconnect board on the underside of the RF tray.

Regards,

Don

[kcbooboo]

Try attaching a wattmeter and dummy load to the IPA output, then turn the power adjust all the way up. You should get some brief indication on the wattmeter if the IPA is working. It's capable of around 10 watts.

I would think that in TEST mode it should stay keyed longer.

That firmware seems rather old, but I don't think there's much in the firmware that would prevent the station from transmitting, unless it's been configured for something besides a conventional station. Have you tried starting with the CONV.DEF code plug file, configure the TX and RX freqs, and write that to the station? Perhaps some other setting has been changed and it's interfering with proper station operation.

It almost sounds as if the forward power sensor in the PA isn't working, or the signal from it back to the interconnect board isn't making it back to the Uniboard. The power controller is a closed loop. The IPA will be told to make enough power such that the forward power sensor produces a voltage that can be compared to the power adjust pot, and then the IPA power is controlled to keep a balanced condition. If that loop is broken, or the IPA and PA has to put out more power than it thinks it should, it'll shut down.

Bob M.

[kcbooboo]

Further analysis shows that the power control circuit on the Uniboard checks a lot of things, then if it decides the PA is having troubles, it shuts down the PA and sends a signal to the SSCB, which unkeys the station about 30ms later.

It seems you can ground the PA KEY line going from the SSCB to the Uniboard and that should make the station transmit, but there are no safeties in place, so if there's really a big problem, the IPA or PA can destroy itself.

The feedback from the PA (forward power, temperature) have to get to the Uniboard through the interconnection board. The filters (connector assemblies mounted to the RF Tray casting) have been known to get corroded and leak to ground, causing low resistance connections that shouldn't be there. These can affect the output power in strange ways. I had one station that had the LEDs on the SSCB glowing dimly when not transmitting due to leakage on some pins.

So it really boils down to having good troubleshooting skills and tracing the various signals, or swapping things (Uniboard, interconnect board, connector filter assemblies, etc) until you find what's causing the problem.

Bob M.

[ve9xf]

So I checked the IPA power output while in TEST mode and it is 0 watts, regardless how I adjust the Power Set pot. Does this mean the IPA is dead?

Also, as if things could not have gotten worse, I unplugged the J452 (the 4-wire connector) on the IPA and the SSCB went dead. Was this the one thing all good MSF5000 owners must never do that I forgot to read?

This is turning out to be one unlucky radio.

Cheers!

Don
VA3XFT

[kcbooboo]

The IPA board in the RF Tray is also home for the 9.6V regulator, and that feeds the SSCB. It shouldn't permanently kill the SSCB, just suspend life for a while. If you plug it back in, it should work again, unless there's another problem. There is a fuse in the SSCB but I think that's only for the 5V supply. The older stations (CLB) used 9.6V to operate the 5V regulator on the SCB, and removal of 9.6V could cause the SCB to self-destruct. Not a great design. I've powered up SSCBs with just 13.8V and while they produce tons of errors because none of the analog circuitry is working, they do survive.

The TX VCO LOCK LED being lit means the TX VCO should be working, but to check, put a receiver near the station and tune to the TX Freq. If you hear a carrier, the VCO is working.

The 4-wire connector on the IPA/REG board brings +13.8VDC into the board, brings +9.6VDC out of the board, and has two signals that enable and control the IPA. This is what the power control pot and associated circuitry work with, to adjust the output power. So it still could be a failure on the Uniboard or the IPA.

Bob M.

[motorola_otaku]

I have had a recurring problem with the regulator board on the IPA dying. If you take it out, you'll see that it has a ceramic board with the power transistors resting on top of a normal PCB with a handful of jumpers between the two boards. The lower PCB is the one that develops a problem, but luckily it's not band-specific so you can swap it with the same part from a VHF, 800, or 900 MHz station (note on the 800 IPAs: I have seen two styles used, one with the ceramic hybrid and one without so keep that in mind before you grab one off Ebay.)

[mruwave]

Could it be something as simple as the reference voltage on the power set pot? Does the wiper voltage change as you change the power setting? Is the filter cap on the reference side blown or shorted? Is the dropping resistor on the reference side open? Is the low side ground open?