Batboard

I'm running a UHF MSF 5000 "RLB" (MSF Limited) with SpectraTAC attached to it.

If I put the station in in cabinet repeat (voter disconnected), I can talk on the station as normal. If I'm using an HT that doesnt provide reverse burst, and I unkey, the squelch tail is at a much lower volume than the rest of the station audio.

This is also occuring in weak-signal situations where someone is noisy into the repeater or voting system. If the voter has selected the MSF 5000 recevier as the prime site, but the user is noisy into it, the audio level drops way down.

This also occurs in standard repeat when the voter is disconnected.

Thoughts?

Well, I do know that a station with no other options does NOT exhibit that phenomenon. You get the same audio level out of it whether the signal is strong, weak, noisy, or entirely gone and the receiver is unsquelched.

My RLB was the same as a CXB - PC programmed and digital-capable.

If you listen on the metering panel's speaker, what does the squelch noise sound like if you open the squelch from the front panel?

The RF tray fully demodulates the audio and sends it right to the SCCB, where routing, de-emphasis, PL/DPL decoding, squelch, and amplification take place. I think there's a test point on the SCCB that's raw receiver audio. You can hook an AC voltmeter or a scope to it, feed a signal into the receiver with a sig gen, and see what's happening.

There's also a test point where the audio feeds the transmitter section of the RF tray. Depending on the programming and front panel switching, you can also monitor that point to see what's going through the station.

If you have the repeater gain turned way up, the clipping/limiting circuit in the transmitter section of the SCCB will cut way back on the high frequencies. This is something you'd only hear - you wouldn't see it on a deviation monitor. Try setting EEPot #5 to a value around 20-30 if it's above 60 now.

There's a whole page on adjusting EEPots 8, 9, and A in the RSS manual; these all pertain to the Spectra-TAC option, and 9 and A are equalization settings. Maybe they're all screwed up.

I wonder if something is acting as a limiter somewhere, or if the SpectraTAC unit is doing weird things to you. Can you unplug it and still use the station? There's no audio processing in the MSF5000's SCCB other than de-emphasis on the receiver and pre-emphasis on the transmitter. These can be enabled or disabled with the V5 firmware and a modern board; the older boards didn't route the signals through switches so the receiver is always de-, and the transmitter is always pre-emphasized.

Is the voter using some kind of high frequency signal to indicate an idle channel, and maybe dropping the audio level down when it sees squelch noise?

Bob M.

Hi Bob,
This is in the station, because it does it with the voting comparator physically disconnected.

Can the Spectra-TAC unit be disconnected as well? What is that supposed to do - I'm not familiar with it at all.

Do you have any kind of MSF5000 metering panel? If not, one of the pins on the CONTROL/MIKE jack has headphone audio and you could monitor that with a scope and/or DMM.

Best way to attack this, if you have the right equipment and place to fool with it, is to feed a known signal into the receiver and check the raw audio that feeds the SCCB. Then see what comes out the wire interface, if present - this implies a TTRC board in the station. The audio there should match what you're hearing elsewhere and would be de-emphasized if that option is enabled.

Bob M.

Interesting... I programmed the repeater to key on SQUELCH ONLY (CSQ), turned the squelch all the way down in RSS, and OH BOY did I hear some CRAP!

Hissing, howling, buzzing, DEFINITE VCO noise.

I opened the station up and removed both VCO's, took the lids off, inspected them and cleaned them up, reinstalled everything, and the thing works BETTER than it EVER did!

The audio and squelch tails sound 100% normal now, and I was able to bring the squelch level of the station DOWN a good 35% and still have everything operating normally. That tells me something was awry from the start, because I had my thresholds set at 75% +. Repeater Squelch is now at 50, receiver squelch is now at 42.

Something was obviously screwy from the getgo on this thing, so now it's all straigtened out. Quite interesting what VCO problems can cause to happen in a station - glad all mine needed was cleaning/reseating.

Thanks for the help!!

Cool ! Did you SEE anything dirty or corroded inside the VCOs?

I've taken a couple apart, mainly for erratic tuning, but never found any whiskers or other stuff inside. I did find one with several springs missing and the plastic spring holders all broken. The springs are cheap and come in quantities of 10. The plastic spring retainers, however, are NOT cheap! Buy only as many as you need (8 per station). They do help cut down on noise if you have a fan in the station.

Those squelch settings still sound high, but it depends on how you adjust them. The book says to inject a signal of a specific strength and set the pots to squelch the radio. I usually do it on noise only, then go up by 5 and leave it there. I also have PL enabled so this setting is more for sanity than usefulness.

Bob M.

Well I woke up this morning and the noise was back. A quick tap (we'll call it a tap) on the top of the RF deck cleared up the problem, so I'm going to have to tear this down sometime soon and clean everything. It doesn't live outdoors, but it does live in a shed in the back yard up here on the hill.

Depending on the age and environment, you could have whiskers growing in the mixer coil assembly and/or the front end. LOTS of screws to take the front end all apart. It still could be a VFO problem too - I had one that I had to remove the tuning core and hardware and clean all that out.

Bob M.

what are these whiskers? how do I get the VCO that far apart to clean them out? Seems to be defnitely VCO related.

Whiskers are a growth of metallic strings that seem to occur on tin-coated metal parts, like the kind used in MSF5000 (and others) front end assemblies. They're conductive and nearly invisible. They're about the size of spider-web threads. They can be removed by moving a plastic or wood stick all around the coils in an attempt to break or remove anything that could possibly short the coil or connector to ground. Compressed air can sometimes be used too.

See http://nepp.nasa.gov/whisker/ for more than you'd ever want to know about them ! Apparently they're a serious issue.

These whiskers can crop up in the two VCOs, the front end, and the mixer coil assembly on the MSF5000.

The VCOs are easily removed from the RF tray by unplugging the coax cable RCA plug and the 4-conductor control cable. They then pull straight up and out of the chassis. (A pair of Vice-Grip pliers grips the tab quite nicely.) They ARE different, so pull only one at a time and work on it. The end opposite the tuning slug has four Torx screws holding it on - remove these. Make a scratch mark on the cover and chassis so you can put it back in the same way you took it out. Then with a pair of big needle-nose pliers, grab the back casting and wiggle it a bit and pull it straight out. You'll then have access to the VCO coil. You'll see a small pickup thing hanging down from the circuit board on top of the VCO. Run something all around the coil and the pickup device to make sure everything is nice and clean, even if you can't see anything. Reassemble in reverse order.

The tuning slug on the VCOs should be snug and should have a thin nut holding it in position, but you should still be able to turn it with the 5mm Allen wrench. You can removel both the slug and nut, clean any metal filings from both of them, and put them back. You'll have to align the VCOs when you return them to the station anyway.

Hope this helps.

Bob M.

As an update to this, I can't make this problem go away and STAY away, so I bought a replacement RF tray from batdude to start swapping parts out of.

I'll keep the group posted.

My two bits worth:
Open the end cover of the VCO by removing the 4 screws, and gently twist, with a needle nose pliers in the central opening.

Wash the cover, as well as the interior of the cavity with contact cleaner, rub the walls with a lint free rag, such a worn hankey, remove the adjusting screw and clean it also. Put a dab of oil or grease on the threads.

The "wiskers" may be impossible to see with out strong magnifacation.

Monitor the injection frequency with your modulation meter to see if the noise persists. It is also possible to use the Transmit VCO in the receiver, but not the other way around, since the RX VCO has no modulation circuit.

Note that solvents will detune the VCO or filter, so it will drift until entirely dry.

You may be able to isolate the "VCO noise" by lisitening to the accual VCO frequency on a service monitor or receiver. If VCO noise is there, it gives a good troubleshooting clue. I have seen both AM and FM noise on 'bad VCOs'.

Also check the superfiltered DC used in the Synth stage for noise.

BTW, this works on most receivers.