Spectra Tac Voter

[jimbo5169]

Can anyone here help me locate a schematic for a SpectraTac voter Signal Quality Module TRN6091 Version C? I am interested in modifying it to accept a COR input instead of status tone detection.

Jim

[Lake Effect]

I have a scanned copy in pdf. PM an e-mail address and I'll send it to you.

L.E.

[Bill_G]

Let us know how it goes cuz the sqm is all about the status tone, and doesn't know what to do with a cor input.

[N4DES]

And if I remember correctly it uses the status tone as a level reference, so it might affect overall site selection.

[Bill_G]

And if I remember correctly it uses the status tone as a level reference, so it might affect overall site selection.

You remember right. The SQM (signal quality module) depends on the 2175 status tone for two things - carrier detect, and line level adjustment. When status tone drops, it is assumed the far end receiver muted it because a valid receive signal has arrived. That starts the voting process for only the cards with a carrier det occurring. The card with the lowest noise is selected, and its audio is passed on. There is no hardware on these cards to accept COR or E&M signaling on the SQM he specified. There were E&M cards out there. I don't have the model number off hand. They were about as rare as chicken teeth. May have been an SP.

[PETNRDX]

You can use your delivered COR signal to turn any old tone generator on and off at the back of the voter.
Used to be you could get the COM-SPEC TS-32 tone (or some similar model) to generate function tones.
That is the easiest way to "add" the status tone right at the voter and not have to modify it.
The voter did use the level of the tone to sort of like an AGC to keep old phone line levels consistent, but if your link method is stable, the locally inserted tone should not be a problem.
You could even use a tray of the Spectra-Tac status tone encoders in the same rack as a voter and put several old tone boards to use.
Just thinking outside the box...

[kcbooboo]

While you can supply your own 2175 Hz tone generator, there's a specific timing protocol that the comparator's line card needs to see in order that the AGC operates properly. Just switching a tone on and off via a receiver's COR signal won't quite do it.

Bob M.

[jimbo5169]

I do recall a problem with hearing "chirps" on a system with GE receivers that had their status tones moved up to 2175 from 1950. Maybe using the status tone generator from an old Spectra TAC RX would yield better results. Thanks for jogging my memory!

[d119]

I do recall a problem with hearing "chirps" on a system with GE receivers that had their status tones moved up to 2175 from 1950. Maybe using the status tone generator from an old Spectra TAC RX would yield better results. Thanks for jogging my memory!

That is going to be THE BEST way to do it. I highly recommend doing just that, with old SpectraTAC "Encoder" modules. They will provide the correct timing required to make the thing work the way it is supposed to.

In the meantime, I would be very interested in seeing the Rev C SQM documentation, as I have a comparator I would like to use at 1950Hz status tone.

[jimbo5169]

IT WORKS! Well, the COR input SQM has been service for a couple of weeks now and it appears to be working fine. After reading the circuit description, it appears the AGC circuit is locked out once the Status Tone stops. This means the AGC isn't functionl while the card is receiving. Sort of makes sense if you think about it as AGC would affect the voing process as the noise floor would be increased by the AGC.

Here is what I did:
Cut the Solder Side trace from IC U4 Pin 6 (Status Tone Detector) to R38 (10K). Install a wire from the R38 end of the cut trace to the solder pad for I/O PIN 21. Pin 21 is an unassigned pin on the SQM that is NOT used on the backplane of the Voter shelf. Pin 21 on the correct slot of the backplane is now the COR input for the SQM. Pulling Pin 21 LOW will cause the SQM to go into the receiving mode. If no audio is present, the activity checker will do its job and the card will fail out. (Just like if the status tone disappeared)

Note: I tried conecting Pin 21 directly to the COR output of a Motorola CDM1250 and it locked into Receive mode. It appears Q7 is very sensitive and the slighest low on R38 will cause it to turn on and switch to Receive mode. Rather than further modify the SQM, I decided to install a relay between the radio and Pin 21. This solved the problem, however, I do notice that the relay is a little slow. I will come up with a solid state solution to replace the relay. Changing the values of R38 and pull-up resistor R40 may also work.

Many Thanks to LAKE EFFECT for providing me with the documentation for the Rev C SQM.

[kcbooboo]

Yes, the SQM's AGC circuit uses a sample-and-hold system. It samples the status tone (when present) to set the AGC level, then holds that level when the status tone goes away. But the status tone has to go away before other audio enters the SQM, otherwise the AGC will readjust its gain based on the brief audio signal that it hears between the time the status tone goes away and the detector notices that and switches the AGC to hold mode. The modules in the SpectraTAC receiver provide this timing so the SQM works correctly.

Bob M.

[n3fe]

I am just digging into this as I was given some spectra tacs. How man versions of the signal quality modules are there? I still don't have a manual for this unit yet but I was following the pin 6 off of U4 and I don't see a 10K resistor. All the boards I was given were TRN6091C44. I am looking at doing the same thing and feeding them with COR.