Homemade Telex Power Supply
- Thread starterEdrick
- Start date
themuzicman
Well-Known Member
It is nothing you could just throw together today unless you happened to have all the parts around and were really patient using perf board -- http://www.rcrowley.com/intercom/
I guess I missed some specifics, sorry about that. What I need is the power supply unit that provides power to the com packs. I don't need an intercom system or belt backs, there's already user stations and belt packs. It's simply the Telex PS2002, which is a two channel power supply.
http://www.rtsintercoms.com/us/rts/products/Power_Supplies/83?fam[]=172&fam[]=81
As far as I can tell it's just a 24v power supply mind you the actual power supply has some additional features for protection like in the event of a short, etc...
I'm just confused by the documentation as it says it's 24VDC +, 24VDC +, and then Common on the shield.
Connector Pinout
Pin 1 Common (audio and DC return)
Pin 2 Full-duplex, balanced intercom audio and +24 VDC output
Pin 3 Full-duplex, balanced intercom audio and +24 VDC output
http://www.rtsintercoms.com/us/rts/products/Power_Supplies/83?fam[]=172&fam[]=81
As far as I can tell it's just a 24v power supply mind you the actual power supply has some additional features for protection like in the event of a short, etc...
I'm just confused by the documentation as it says it's 24VDC +, 24VDC +, and then Common on the shield.
Connector Pinout
Pin 1 Common (audio and DC return)
Pin 2 Full-duplex, balanced intercom audio and +24 VDC output
Pin 3 Full-duplex, balanced intercom audio and +24 VDC output
themuzicman
Well-Known Member
There is a hole heck of a lot going on inside the power supply that the pinout isn't going to tell you. From the link I posted in my above post: Pin 1, the common, is returning the DC power supply voltage and return/screen for the audio. Pin 2 is carrying the audio signal and call signals as a multi-tap bus. The audio is AC coupled and the call is signified by a DC level imposed on top of the AC audio. Pin 3 is carrying the power from the central supply to all stations.
I am not certain if your specific Telex unit is doing that, but I do know that this is the principal that all ClearCom stations work under. Com systems are tricky, hence the hefty price tag you pay for them, and there is a ton going on in them. The power supply may not have any fancy features like a regular com remote station, but it is doing all the things a main station normally does, minus the buttons - including a lot of heavy filtering to make sure your signal is clean and quiet.
http://www.livesoundint.com/lab/lab/messages/archive2/41373.html Here is another link on it, a poster mentions an RDL ST-IC1 circuit which can be bought for $150 on Amazon.
I am not certain if your specific Telex unit is doing that, but I do know that this is the principal that all ClearCom stations work under. Com systems are tricky, hence the hefty price tag you pay for them, and there is a ton going on in them. The power supply may not have any fancy features like a regular com remote station, but it is doing all the things a main station normally does, minus the buttons - including a lot of heavy filtering to make sure your signal is clean and quiet.
http://www.livesoundint.com/lab/lab/messages/archive2/41373.html Here is another link on it, a poster mentions an RDL ST-IC1 circuit which can be bought for $150 on Amazon.
Looking at this documentation more mimics what I was thinking, this includes a schematic of the power supply, http://www.rtsintercoms.com/us/rts/file?i=97098&lg=eng
themuzicman
Well-Known Member
Yea, that schematic is clearly what you want- though it only powers 4 single-channel beltpacks and at that the total system length needs to be 300' or less. The biggest thing missing is what exactly you want to leave the AC transformer, and what exactly PI-1, PI-2, and PI-3 are, as those are not circuit symbols I am familiar with. The second transformer is also something you will need someone with a bit more electronics information to help spec. The Diodes seem to be just generic ones like an 1N40001 Diode, and all of the capacitors and the single resistor are clearly marked.
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JD
Well-Known Member
I remember slapping one of these supplies together about 25 years ago. Just 24 volts DC nice and filtered. To balance it on the line, I used a small 70V transformer off an old CV ceiling speaker. (Cheap little thing like pic below)
I think it was a 3 watt one, so I used the 70 volt side and metered to find a tap that was equal. Scratching my brain, the common went to pin 2, the 1 watt tap went to the 24 volt supply, and the 1/2 watt tap went to pin 3. Although the transformer lists the windings in the thousands of ohms, the DC resistance (which is all that matters) is quite low. Just cut off the secondary leads, and the other unused ones.
I think it was a 3 watt one, so I used the 70 volt side and metered to find a tap that was equal. Scratching my brain, the common went to pin 2, the 1 watt tap went to the 24 volt supply, and the 1/2 watt tap went to pin 3. Although the transformer lists the windings in the thousands of ohms, the DC resistance (which is all that matters) is quite low. Just cut off the secondary leads, and the other unused ones.
Getting the center tapped audio transformer, with a ~2.5:1 turns ratio, (T2) would take some effort and time. By the time you gathered all the parts, the replacement one will show up.
So am I correct in the understanding that supplying 24v from a regular transformer will not work?
If I understand this right, the old power supply stopped working? Or has it walked?
if it's just become non functional, then surely one could just take a well filtered 24v DC power supply and use the transformer (and connectors) of the now defunct unit?
As an aside, I'm fairly certain that PI-1 through PI-3 is just a designation for the 3 poles of the PCB input connector - screw terminals or whatever...
Transformer secondary is probably something like 24 - 0 - 24 volts - that would make sense with a 50V cap, but depending on current draw and other design factors, it could be lower.
if it's just become non functional, then surely one could just take a well filtered 24v DC power supply and use the transformer (and connectors) of the now defunct unit?
As an aside, I'm fairly certain that PI-1 through PI-3 is just a designation for the 3 poles of the PCB input connector - screw terminals or whatever...
Transformer secondary is probably something like 24 - 0 - 24 volts - that would make sense with a 50V cap, but depending on current draw and other design factors, it could be lower.
Nope, a clean source of DC is only half of the problem. The other half of the problem is to couple the DC power to the beltpacks without effectively shorting out the audio. That's what T2 accomplishes in the Telex schematic. It's sort of like mic phantom power, but it has to deliver more current.
JD
Well-Known Member
Exactly! The balanced line needs to have a high impedance on AC signals across pins 2 & 3 while having a low resistance to feeding DC from the 24 volt source to both pins. I lucked out with the old CV transformer because in addition to watts, they also listed the impedance in ohms. I chose the common, 1200 ohm tap, and 2500 ohm tap (watts listed in above post.) The DC resistance was around 20 ohms so it worked great.
In my case, I wanted to build a supply into a board that was going to be in one location for 6 months so that I could take the portable supply on the road with me. The concept of this old system was that the audio was balanced pack-to-pack to reject background noise.
Current system I have is clear-com, and that is an unbalanced system, although I have never run into a noise problems with it, so I guess time marches on!
