Spectra Tac Vote Comparator not locking up question?

[FMROB]

Hey all, want to see if anyone has had this issue before.

Spectra tac voter hooked to a quantar with in cabinet fall back repeat turned on.

Sometimes when a marginal signal is generated the spectra tac shows rx'd but will not vote and the quantar goes into in cabinet repeat. When we shut the other two sites off by the SQM's and leave only the quantar RX site on it will vote everytime even on the noisy, marginal signal. Once we introduce the other SMQ's into the mix, the voter appears to be confused and will not vote on a site and then causes the ICFBR condition in the quantar. All of our line levels are great and equalized properly. The status tone and voice levels are proper and it works a majority of the time but occasionally shows this symptom with marginal signals??

It appears as the voter logic circuit might be screwy. Has anyone had this issue before, I am pulling my hair out with this one.

Thanks, Rob

[Bill_G]

Swap your sqms around to see if it is just the sqm associated with the local Quantar. If the problem follows the sqm, then you get to fix that sqm. Probably a problem in the high pass filter, signal quality det, or dc comparator. But, if the problem stays with the Quantar, then you need to look at the high freq noise content from the Quantar - might be too high, and you might have to cut the high end equalization. There is also a possibility that one of the sqms is loading the ref gen on the command module. You can discover that by pulling one of the other sqms one at a time until the Quantar is voting correctly.

[xmo]

If the voter is co-located with the Quantar do you have the required roofing filter installed for local receiver operation?

Also, are all your receivers the same type?

Spectra-Tac can have issues with configurations of multiple receiver types.

[FMROB]

All of the Rxrs are astro tacs and we are using the quantars RX'r. The voter is located remotely at headquarters and there is an astro tac co located with the Spectra tac. We performed line equalization and have the same audio characteristics and tone levels (-13 system). We are utilizing a combo of phone lines and microwave to get signals back and forth.

What concerns me is that there is sometimes that the spectra tac sqms are unsquelching but refuse to vote? So what is happening is the repeater rxr sees the signal but doesnt see a line key tone and defaults back to a repeater, which then causes the dispatcher to lose console RX audio. So this is not a good situation. I am going to try to jockey some sqmss and swap the command module.

Thanks, Rob

[Bill_G]

It's doubtful it's a command module problem unless the reference generator itself is bad. More likely a sqm problem if there is a hardware failure. You should verify that upon loss of status tone the individual sqms are voted immediately. It's an easy test by shorting across the line input.

If it passes that simple test, then place a 1k resistor series'd with a .1uf cap across the input to the local Quantar sqm. That should cut the high freqs enough to improve voting with normal equalization. If it does, then re-equalize the local Quantar cutting the high freqs above 2700 at least 3db and more likely 6db.

[RKG]

My take on this is a tad different, though any long range diagnosis is an exercise in speculation.

A clue lies in the report that, when the problem occurs, one or more SQMs show unsquelched, but the Quantar goes into FBIC mode. The entrance condition for FBIC is that the Quantar's local receiver unsquelches but the transmitter does not receive a wireline transmit command within a specified time.

A second clue lies in the report that, if two of three SQMs disabled, all works fine.

The SpectraTac comparator, when it detects one or more unsquelched receivers and exercises its vote selection, drives two pins to ground. One is the receiver vote pin (don't recall the number from memory), which is unique that the SQM slot of the voted SQM. The other is the "vote bus," which appears on all slots, but they are electrically connected. It is the vote bus signal that causes the Tone Keying Module to send the keying tones to the transmitter.

My guess is that one or more of the SQMs has a failed component or bad solder joint that is keeping the vote bus high despite a vote determination by one of the errant SQMs.

[Bill_G]

I like this analysis. I know the first pin - rcvr voted (or whatever it is called) is not connected on the backplane. You have to wirewrap/solder your own lead on, and take it to an aux i/o in the CEB if you want to display voter activity in dispatch. The second pin, vote bus, is common to all the sqm's, and both pins are derived off the dc comparator circuit on the individual sqm's. There is no master circuit looking at each sqm determining which is best. Essentially, a sqm votes itself by pulling the reference generator lower than another card, and I have seen a sqm screw up the voting as you described.

My take on this is a tad different, though any long range diagnosis is an exercise in speculation.

A clue lies in the report that, when the problem occurs, one or more SQMs show unsquelched, but the Quantar goes into FBIC mode. The entrance condition for FBIC is that the Quantar's local receiver unsquelches but the transmitter does not receive a wireline transmit command within a specified time.

A second clue lies in the report that, if two of three SQMs disabled, all works fine.

The SpectraTac comparator, when it detects one or more unsquelched receivers and exercises its vote selection, drives two pins to ground. One is the receiver vote pin (don't recall the number from memory), which is unique that the SQM slot of the voted SQM. The other is the "vote bus," which appears on all slots, but they are electrically connected. It is the vote bus signal that causes the Tone Keying Module to send the keying tones to the transmitter.

My guess is that one or more of the SQMs has a failed component or bad solder joint that is keeping the vote bus high despite a vote determination by one of the errant SQMs.

[FMROB]

OOOOhhhhh Baby.. A bad SQM it is. I have now completely lost all of my hair, and my pile of "for repair" sqm modules gorws larger by the moment. Bill and the guys, another kick butt job on the tech help. The spectra tac is alittle before my time, as I am from the age of diagnose to the neaerest board, remove and replace. I am slowly making up for not learning "core" electronics skills, but with help it grows more and more each day.

Thanks again, Rob

[Bill_G]

You're welcome Rob. Don't feel bad about being a board swapper on a comparator. It's a live system, and nobody likes their tech fiddleing around. Restore service, and then get outta the way. Unless you have a test chassis to plug the cards into for diagnosis, you end up throwing away possibly simple to fix boards and buying good used ones off the web. You do what you can, and then off to the next service call. The nice thing about this work is there is always something else that needs your attention.

[RKG]

Glad it worked out.

My guess is that there are two possibilities: with a SQM schematic and a meter and 45 minutes of time, you could find and repair what is wrong with that SQM -- or you could spend a lifetime and never find it. So stop after 45 minutes.

[d119]

OOOOhhhhh Baby.. A bad SQM it is. I have now completely lost all of my hair, and my pile of "for repair" sqm modules gorws larger by the moment. Bill and the guys, another kick butt job on the tech help. The spectra tac is alittle before my time, as I am from the age of diagnose to the neaerest board, remove and replace. I am slowly making up for not learning "core" electronics skills, but with help it grows more and more each day.

Thanks again, Rob

If you are using Quantars and ATAC receivers, might I suggest you upgrade to AT LEAST a DIGITAC at the next opportunity. Seeing as how you have all ASTRO-type gear (and are running in Analog), you could even make use of an ASTRO-TAC 3000 if you could find one. If you have no plans to go to digital, just upgrade to a DIGITAC. Remember, the SpectraTAC is MICOR-era stuff.

If you insist on staying with the SpectraTAC, you should look towards upgrading all the SQM's to "C" rev boards. Again, if you can find them.

On that note, I seem to recall having heard something about "A" and "B" rev SQM's not playing nicely with "C" rev SQM's... Don't know for sure though, never had that situation.

[FMROB]

My next exact question. We know that we need to pruchase a new voting comparator. We are budgeting money for other projects and was looking towards the JPS SNV unit, which is about $13,000 equipped the way we need it.

What else is there. I would not mind using the Motorola equipment, but I am under the impression that I have to use modems attached to the analog lines in order for it to fly. Do you know if I can directly connect to a digitac or astro tac?

Thanks, Rob

[Bill_G]

If you agency has patience, there should be plenty of Digitacs coming on the market as folks move out of their conventional systems into 700M. I can think of eight of them off the top of my head. They will interface directly with phone lines out to your receivers and base station(s), and have with a console interface. The physical connection is different from a Spectratac. Requires a standard 25pr cable to a 66 block on the wall to punch down your lines. And they are physically larger than a Spectratac. If I recall correctly, you will have to cough up 8ru for the chassis, and another 4ru for the power supply.

Or, the JPS is a great choice. The same physical size as the Spectratac, all connections to screw terminals on the back, and a lot of features not available in any conventional Motorola comparator.

[RKG]

I second the motion with respect to the JPS Voter. It is an extraordinary device that, once you have learned it, performs quite nicely. We are in the process of converting all the legacy SpectraTacs to JPS Voters.

Two caveats:

One: the JPS Voter has umpteen features for applications you'll never use, and its terminology for functions is not the same as we're used to, both of which mean investing a fair amount of time not just reading, but also absorbing, the highly detailed JPS Voter manual. It also helps to keep Bennie Hillman's email address handy.

Two: a minor point, but it has confounded more than one person: the "disable" pin on the JPS Voter (presuming you handle console disabling of the repeater via a console contact closure the same way you did with the SpectraTac, which I recommend) is reversed: active for "repeat" and inactive for "disable." Once one realizes this, one easily solves this issue with with an inverting relay.

[RFguy]

Does the JPS still lack any provision for line equalization?

The one we installed about 10-years ago had no line equalization provision and we had to install external Telabs line interface units that provided the line equalization function.

[Bill_G]

They have some limited line equalization built in now. As always, the majority of the equalization is done at the rcvr site. You have to "push" the curve you want at the comparator. But, they do have some equalization you can throw on the line at the comparator.

[FMROB]

If I am not mistaken, The astro tac Rx'r and Quantar have "Line equalization" adjustments in their own software, hence me not needing it. I would assume if one were using spectra tac rx'rs or low cost CMD radios with a pilot tone generator you could use some line equalization? If I am not mistaken about this.

Thanks, Rob

P.S. The JPS is the way I am leaning, do you guys recommend having a second voter racked up and off as a back up? What happens if the JPS goes down, what about parts availabilty? For me a second voter is kinda costly.

[Bill_G]

I have never fixed a JPS.

[RKG]

1) I've never had to repair a JPS Voter. On the one hand, that's good news. On the other hand, that means I have no experience re: parts availability, factory response, or like issues.

2) Interesting you should mention sparing the comparator.

A) To my knowledge, no one else does it.

B) When we replaced our SpectraTacs, however, it occurred to me that for zip cash, we could do exactly that. What we did:

1) Take the various lines that went into the comparator (e.g., receivers, transmitter, console) and move them to one side of a split-66 block with an RJ-21 jack on the other side.

2) Run a new set of wires from a second split-66/RJ-21 block to the old SpectraTac.

3) Run a new set of wires from a third split-66/RJ-21 block to the new JPS Voter.

4) Run a short 25-pr RJ-21 Telco cable from Block 3 to Block 1.

5) Pull the fuse from the SpectraTac power supply.

Should we ever need to, the theory goes, we move the JPS end of the Telco cable from Block 3 to Block 2, put the fuse back into the SpectraTac, and we're back on the air.

Of course, we've never had to do that.

Total cost: 3 split-66 blocks and a 3-foot length of Telco cable. Under $150.

[FMROB]

Staying on the air when the JPS blows up = Priceless

[d119]

This is all opinion here, but every JPS voter I've ever heard sounds just awful...

If I can recommend one thing, it would be to listen to what systems sound like on a DIGITAC and on a JPS.

[RKG]

This is all opinion here, but every JPS voter I've ever heard sounds just awful...

If I can recommend one thing, it would be to listen to what systems sound like on a DIGITAC and on a JPS.

The JPS Voter requires level setting to be done carefully and by the JPS book. The book method is different from what old timers (like me?) are used to, but it works. Once done, the JPS voter is crystal clear, both in general and on vote shifts.

We actually spent considerable time comparing the JPS Voter and the DigiTac, once the decision was made to replace the aging SpectraTacs before they died. The DigiTac is what Motorola touts as the SpectraTac replacement for analog systems, but it wasn't really designed for 4-wire analog systems and Motorola (whose interest seems to be confined to Astro systems) doesn't plan to have a true SpectraTac replacement. We concluded not only that the JPS Voter sounded better, but it was going to be easier to install and maintain over time than the DigiTac would be. So far, at least, this conclusion has proved to be accurate.

[FMROB]

Further opine this subject of level setting.

How does the JPS voter work. I am using a mix of micro (carslon) and RTNA phone lines, so I can really only use a -13db system, as with the 0db system I could possibly exceed the harmonic response limit fo the rtna circuit.

So with that in mind, how exactly does the jps system level out? and can I mix the micro and phones.

BTW: My real first experience with the Carlson wireless trailblazer leased line emulator. We are running a 4.9ghz 4 4 wire circuit unit, and I am running low band paging TX and RX, fire ground rx'r, repeater TX and a Rx site for the spectra tac main channel and the unit is rock soild. Super easy to set up and provision and less than a 1/2 db loss (which I attribute to meter inaccuracy). Highly recommend for medium applications, it is cost effective as we have racked a complete redundant back up set hop.

- Rob

[RKG]

I don't know anything about the Carlson unit. All of our receivers, save one on each channel, are returned over owned fiber optics (using TC Comms fiber muxes); the "one" is co-located and returns on a short length of dry copper.

One limitation of fiber is that one has to be careful of overdriving. As a result, we run our system at -13 dB and have had no problems.

The JPS book wants one to set levels based on the LEDs on the face of the receiver cards. There is a test point on each card, but this apparently samples post- or at least mid-processing by the board's DSP, and the values you get are high to the point of meaninglessness (at least to me). Since I'm old fashioned, we started by setting levels using a Lineman or TIMS at the punchdown block at which the receiver channels are landed from the fiber mux. We then tweaked using the JPS lights and, surprise of surprises, it sounds terrific.

Edit: I should add -- or, maybe, I shouldn't add but I'm going to anyhow: the reason we converted all of our phone lines to fiber was because of really bad experience, growing worse with the passage of time, with the phone company's product. We've used both RTNAs and FDDAs, but neither is reliably stable. In our case, the fiber conversion was aided by the fact that the Town has its own Electric Light Board, so access to the poles and ducts was not a problem (or a cost). Unlike some fiber stuff, the TC Comms series 8000 muxes are quite shallow (1 T1 per strand pair), but the units do not require complex programming or a resident EE to keep them running. Both the Tx/Rx boards and the fiber pairs are redundant. If a fiber card goes south, it sets off an alarm (which we have remoted), but the system continues to work. At that point you wait until the next morning, swap in a spare card, and send the bad one off to California. At least, that is the plan; we haven't had a failure yet.