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MSF-5000 VHF repeater keys up then immediately drops out of TX

rono

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Any MSF5000 experts out there that might have seen this issue before?

I'm servicing an MSF5000 that is our local ham radio clubs main VHF repeater. The unit has had it's uniboard caps replaced within th epast couple months due to it developing an audio tone in it's transmissions. Seems that this symptom is generally related to bad caps, so I pulled the uniboard and found that almost all of the caps were leaky or swollen. After the cap replacement all seemed to be back to normal. Sunday morning, the unit went off the air. could be keyed but only remained in TX a second or so.

It's currently on the bench. Unit is a digital logic board (programmable with the MSF software on a DOS computer) version. The tag on the cabinet identifies a UHF machine, so I cannot provide a proper model number as the guts and cabinet don't match. The unit will key up come to set power (60W) and then drop out of TX. Same thing happens when the TX switch on th elogic board is switched on: PA Full and PA ON LEDs light green and PAKey LED lights yellow. TX starts comes to full power, then all lights go off and TX deops out. I believe I hear a relay keying and unkeying at TX initiation and drop. Currently Running into a dummy load SWR 1:1

I have a diag panel, so can look at muxbit conditions, but don't know what I'm looking for! Any help would be appreciated.

ron
 

Astro Spectra

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Is the power supply OK when it's keyed?
 

Notarola

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Also when on the bench does it show the same behavior? I would also check to see if the repeater is still repeating even at low or no RF output. This might help identify a PS or PA issue.
 
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rono

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Yes, same behavior on the bench. Power supply is OK. One additional finding: I tried to reduce the PA power with the PA power set pot. Initially all was OK below 40 W, then it failed to even key up. turned PA Power set pot fully ccw, and now holds TX with 1W transmitted power. As soon as I turn the pot clockwise , the machine refuses to key. Looks like it's supposed to be a 10K pot. I measure 6k across it in circuit, and continuity to the wiper
 

Notarola

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sound like an issue in the APC or related component. your loosing the PA bias somewhere
 
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rono

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Not losing PA bias. If I inject a 1W signal into the PA input from my HT with PA enabled, I get stable power out of the PA , this problem seem to be logic related. Just hung the diag panel and can see that the TX act Muxbuss bit goes active under 1W, does not activate above 1W. I'm thinking a power sense issue, maybe related to the power set pot circuit. Unfortunately don't have a replacement and the originals are all discontinued, so need to find an appropriate replacement. For tomorrow, going to just rip it out and run leads to another 10k pot to see if operation restores.
 

Notarola

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sounds good let us know what happens.
 

kf8yk

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I've had a VHF MSF with this symptom before. A dirty power set pot was the culprit, applying contact cleaner spray & rotating the pot back and forth restored operation.
 
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rono

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That was the first thing I tried after identifying that the pot created a behavior change. Come on guys, I'm not a noob. Been maintaining comm equipment since 1975. If it's simple and stupid, I've already tried it. Pot is fine and measures perfectly. I've just swapped the uniboard with a known good one. Problem remains, so I now know the problem is NOT related to the PA (already checked that independent of the rest of the repeater) or the uniboard power set feedback loop (replacing the entire board places it out of the problem loop). Now focused on the IPA as a potential issue. Please, if you have no experience with the MSF and don't even know what a mux bus is, don't waste time responding to this thread. I'm looking for hints from someone who maintained this equipment back in its era and is uniquely familiar with its quirks.
 

knickerlessparsons

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I know F### all about fixing MSF's, but what I do know with some confidence is that a new member (4 days) of this forum might do well to consider the attitude they display in their posts. You could just as easily ended that last post after the first sentence and maybe prefixed it with "Thanks,"

The guy was freely offering advice based on experience. While it might not be useful information to you, it may help someone else in future.

You may have 45 years experience with radios, but during that time you seem to have neglected your people skills. In my experience, peoples' willingness to offer help has a strong correlation with the attitude of those seeking help.
 

Notarola

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I agree with knickerless. There are a lot of people here with huge amounts of experience. As a professional in industry you should be used to people asking or suggesting the obvious like "is it plugged in???" LOL. You also have to be tolerant as we cant see the problem first hand but must rely on your hands and eyes. Often you will get people like kf8 suggesting a repair that worked for them. The reason that I wouldnt ignore it out of hand is because its remotely possible of a 'resistive' connection to the pot. The pot would read fine but under operational conditions it would fail due to the bad connection. Sometimes a suggestion will lead you to investigate something and from there to find the cause of the failure. Anyway since you have swapped out the board we can eliminate the whole APC circuitry. Either way a small apology would go a long way and show that like everyone else frustration sometimes makes you bark at the hand that is only trying to pet you.

Nest thing I would look at is the COR circuitry perhaps something is dropping and telling the TX that there is no input signal. Im also thinking along the lines of PL/DPL decode. Since from your description the drop is sudden it has to be a component failure (heat?????) or a gate.

If you wait for the failure and unkey and immediately re key does it stay failed or does it go thought the normal operation-failure sequence observed.
 

CARC383

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I have very little experience with MSF5000s but have seen similar behavior on other equipment and presence of correct power supply voltages are the first thing that come to mind when status lights abruptly go out when there is suddenly a large change in current draw like going into transmit. You also have to ask what has changed from the point in time it was working well to when this problem started.

Besides the physical replacement of PS caps, were power supply cables disconnected and maybe they didn't go back in the same spot or they are loose or?? I've also seen 5V logic very sensitive to the 5V buss where a droop to 4.8V or something lower than 5V causes things to blink out or reset. Are all voltages at proper levels on the RF deck circuit boards besides right at the power supply?

Sorry if this sounds juvenile but I've had some repair jobs where I get wrapped up in certain things and someone comes along as asks a few simple questions and sure enough, I overlooked some stupid basic stuff.