Telco line question

sportsvoice · Post by **sportsvoice** » Sat Jul 15, 2006 10:31 pm

Got a telco mixer that is having issues when seizing the line. I get a quick burst of dial tone and then nothing. The switch is responding like it's getting a pulsed "1" digit when I seize the line.

I'm assuming bad filter caps in the hybrid circuit causing the POTS line not to see the proper load when the line is first seized.

Have tried swapping the hybrid IC's and also removing dialer IC in case it was glitching on line seize. I can receive an incoming call OK, but with the switch left open, I still get the "1" glitch once switch drops battery and then comes back with battery and dial tone.

lastone · Post by **lastone** » Sun Jul 16, 2006 7:22 pm

The obvious question is have you tried this on another line? If not what happens if you plug in a telephone? Could just be a bad interface port at telco. If it does this on any line...try putting a milliamp meter in series with the unit to see if the hybrid coil is going open under current. If so you will see your pulse. You may also want to look closer to see if your unit uses the switch to connect or the switch telling something else to through a relay to seize the line. In this case it could be a bad relay contact or coil. If the switch is the means of connect, by-pass it with another of some kind and try it again. You may even wish to watch the meter once the line is seized as you "thump" the unit. If you see your pulse, the'res a loose wire, bad connection, etc. Just don't assume it's your unit until you try another line!

sportsvoice · Post by **sportsvoice** » Mon Jul 17, 2006 9:02 am

lastone wrote:The obvious question is have you tried this on another line? If not what happens if you plug in a telephone? Could just be a bad interface port at telco. If it does this on any line...try putting a milliamp meter in series with the unit to see if the hybrid coil is going open under current. If so you will see your pulse. You may also want to look closer to see if your unit uses the switch to connect or the switch telling something else to through a relay to seize the line. In this case it could be a bad relay contact or coil. If the switch is the means of connect, by-pass it with another of some kind and try it again. You may even wish to watch the meter once the line is seized as you "thump" the unit. If you see your pulse, the'res a loose wire, bad connection, etc. Just don't assume it's your unit until you try another line!

Haven't had another line at my disposal yet. Regular phones and a Telos hybrid all work fine on it. I checked it with a test set and voltage and loop current are both in acceptable ranges.
There's no relay for line-seize...tip and ring pass through a double-pole switch. I tried jiggling the switch around which doesn't seem to make much difference. There's also no hybrid coil as the hybrid circuit is on an IC chip. (I already tried swapping the IC out with no luck). Haven't tried it with the ammeter yet.

There are some filter caps on the line as well as some more caps around the hybrid IC, so I'm suspicious that I might have a bad cap somewhere.

Waveguide · Post by **Waveguide** » Sun Nov 05, 2006 11:56 pm

Hay Sportsvoice,
Did ya ever get a real answer about your porblem??

sportsvoice · Post by **sportsvoice** » Mon Nov 06, 2006 8:29 am

Waveguide wrote:Hay Sportsvoice,
Did ya ever get a real answer about your porblem??

Not really. The only additional thing I discovered was that I could go off-hook just fine if I had a telephone off-hook somewhere on the same line. It has to be something about the load that the mixer is presenting to the line. I tried it with a test set meter on the line but couldn't see anything funny going on.

I gave up and used a different unit for football season. I should try this one on our hookup in the basketball gym just to see what it does. Come to think of it, I should probably test that line anyway before our first game to avoid any rude surprises. Things like jacks being run over with the bleachers or school phone guys tinkering with the switch have a bad tendency of happening during the off-season.

Waveguide · Post by **Waveguide** » Tue Nov 07, 2006 1:02 am

OK.....
Let start with step #1

What king of remote unit is this?
Not something "home made" is it???

sportsvoice · Post by **sportsvoice** » Tue Nov 07, 2006 9:44 am

Waveguide wrote:OK.....
Let start with step #1

What king of remote unit is this?
Not something "home made" is it???

Telfax (later Marti) GX-440.

Waveguide · Post by **Waveguide** » Fri Nov 10, 2006 9:45 pm

#2...
Anybody do any kind'a "mods" to this unit?
Or is it stock..... out of the box?

( be truthful, now......)

sportsvoice · Post by **sportsvoice** » Fri Nov 10, 2006 9:52 pm

Waveguide wrote:#2...
Anybody do any kind'a "mods" to this unit?
Or is it stock..... out of the box?

( be truthful, now......)

Looks stock. I replaced a couple of the components (phone line surge protector circuit) in case it had gotten zapped. No special mods or jumpers on any of the boards.

I was able to get service manuals from Marti, so I have parts list and schematics.

Waveguide · Post by **Waveguide** » Mon Nov 13, 2006 9:37 pm

What did ya replace??
Just to point this fact out:
There should be NO polarized caps in the path with the telco line.
For typical line loading, no more that 2.16uF MAX for coupling.
Electrolytics are a no-no on the line side. Use NON-polerized caps such as mylar.
I have never used one of the unit that ya have.
But too much cap can cause the C.O. line equipment to "bounce" and break dial tone.
I've seen this happen when equipment connected to a line was really non standard.

sportsvoice · Post by **sportsvoice** » Tue Nov 14, 2006 8:27 am

But too much cap can cause the C.O. line equipment to "bounce" and break dial tone.
I've seen this happen when equipment connected to a line was really non standard.

...which is exactly what happens with this unit. I try to go off-hook and the telco thinks it's seeing a dialing pulse. Don't have this problem with my other unit (JK Audio RemoteMix 3) at all.

It's weird because there's nothing really exotic going on in the line interface. There are a couple mylar shunt caps in there (small ones but I'll compare the capacitance to what you quoted), a diode pair for overvoltage protection (which is what I replaced), and a bridge to get DC off the line independent of the line polarity. The hybrid is on an IC (MC34014 I think).

I can scan in a schematic if you're curious.

lastone · Post by **lastone** » Tue Nov 14, 2006 8:15 pm

a diode pair for overvoltage protection (which is what I replaced)

Are you certain you didn't get hold of a low voltage zener instead?

Just a thought.

sportsvoice · Post by **sportsvoice** » Tue Nov 14, 2006 9:08 pm

lastone wrote:
a diode pair for overvoltage protection (which is what I replaced)
Are you certain you didn't get hold of a low voltage zener instead?

Just a thought.

I was able to get an identical replacement part to the original. No change, still getting the "bounce" digit before and after.

Waveguide · Post by **Waveguide** » Tue Nov 14, 2006 10:10 pm

Hmmmm......

This is too simple to be a problem.
Yea, scan the diagram and PM me.
I got'a see this.....

Waveguide · Post by **Waveguide** » Thu Nov 16, 2006 1:44 am

Thanks for the PM.

Have you tried pulling IC7 then connecting to a line?
That IC in the dialing chip. May be the problem.
Which line is used for the program audio back to the station?
There are 2 phone line ports.

This deal is different......
Makes ya wonder how this unit got passed the type
acceptance to connect to the PSTN???

sportsvoice · Post by **sportsvoice** » Thu Nov 16, 2006 8:34 am

Waveguide wrote:Thanks for the PM.

Have you tried pulling IC7 then connecting to a line?
That IC in the dialing chip. May be the problem.
Which line is used for the program audio back to the station?
There are 2 phone line ports.

This deal is different......
Makes ya wonder how this unit got passed the type
acceptance to connect to the PSTN???

Line 1 is the conventional program line. Line 2 can be used either as talkback only or to patch a call-in guest into the mix that will then go out on Line 1. I think the way it's set up Line 1 has to be in use for you to use Line 2...I think that's because the dialer IC is actually powered by the voltage taken off Line 1.

I have no idea how (or if) this thing passed Part 68...but the design lives on in the Marti GX-500 telco mixer that is still made today.

I can try pulling IC7...that may have been something I didn't try at first. I did try swapping the MC34014 hybrid IC's just in case the Line 1 chip was bad...no change.

Waveguide · Post by **Waveguide** » Sun Nov 19, 2006 10:29 pm

pull the chip ( IC 7 )...... tell me how it react.........