atcomsystems.ca/forum Forums Other TDM Phone Systems 1A2 and Comkey Telephones & Systems 1A2 and some old WECO stuff

Yesterday was The Day!

551C KSU from 1981 is converted to 230V, wired and made operational. Now in more details.

I managed to find a matching size power transformer with necessary secondary voltages. Original Western Electric 110V transformer is on the left (removed) and the new one is on the right (installed).



Below is a new transformer in closer look. I was lucky enough to find this one on eBay here in Germany. The dimensions are really very close to original WE one, it is only slightly narrower so I have to drill another hole for fixing it. One may see now the original small hole in the power supply unit faceplate, however it will be used later for a lamp indicating activated ring generator.

The transformer has 3 secondary coils: 10V, 18V and 24V. Later one is used to provide DC 24V output for signalling and talk. I've installed a precision regulated power board (behind the transformer) as I use 24V AC coil of transformer to provide power for 24VDC circuits (551C scheme requires common ground on positive side of DC and one of the sides of AC voltages, if 18VAC and rectified 24VDC are taken from the same secondary coil, then diode rectifier bridge need to be modified to 3 diodes scheme instead of 4 diodes full bridge. I've decided not to bother with that and install a precision DC power supply board to have regulated and precise 24VDC). Original filter capacitors block has been removed from service to save space within the power supply unit.

Also installed Siemens & Halske power varistors for 20V and 14V to protect buzzer and lamps outputs from possible transformer's inrush current or other bad things. They are soldered directly to the 8-pin blue Amphenol connector of power unit (see upper left corner of the photo below).



A small bonus - I've ordered on eBay a regulated power board with no LED, as I don't need it and don't want to waste energy. However, I received an alternative model with blue LED indicating power on state. Decided not to cut it out for now. It actually gives a nice deep blue background in the power unit making it look fun.



New transformer fits well the housing of 28D2 power unit, have some space inside for ring generator board and potentially control unit for audibles.



If I were a marketer of Western Electric / Bell System I would have called it "Mystery Blue" and probably would charge customers $1.99 per month for this additional feature. Mention the small hole behind the blue voltage selector wire (it is not used any longer, just left it for cosmetic purposes to keep the look of original unit). This is the original transformer mounting point. New one is slightly to the right from it. I will later use this hole to bring up on the panel a miniature lamp, which will be run in parallel with ring generator, so lamp will indicate when the ring generator is powered on (I don't want to power it continuously, it takes power in idle state and also goes warm. So it will be controlled by a power MOSFET transistor from interruptor's motor start lead. Thus ring generator will receive power - rectified 10VAC which is 12VDC - only when interruptor is activated, and this is when ringing voltage is needed).



Siemens & Halske power varistors on the Amphenol output connector within the power supply unit. I have become superstitious to line transformers after an integrated rectifiers bridge in my 108A loudspeaker unit of 4A speakerphone has been killed by a secondary 18VAC coils of a big transformer (the one which will be used for powering 584C panel later) presumably during power on/power off cycle. Now varistors are everywhere and Zenner diodes are in stock to provide more precise protection (not installed in this version).



Nitty-gritty wiring job on KSU's 66 block. I'm happy I've got a punch down tool, since I planned originally to install wires manually. What a pleasure I had with a punch tool instead for $8.50. This is really a great thing for dealing with 66 blocks. I was surprised that limited space for cabling within 551C KSU is in reality more than enough to make a neat termination. For now only two 25-pair cables with female Amphenol end are terminated here, I plan to add two more when they arrive. These ends are made from old Amphenol male-female extender cables with plastic ends. I've ordered some cables with original metal shell female Amphenol plugs. I will re-terminate these cables as well with metal Amphenols. Strange enough, but female Amphenols are in much bigger shortage than their male counterparts. I have a supply of plastic black AMP connectors here for ~$2.50, but I like original metal ones more. And those are possible to find as male, but female almost not exist on eBay.

I managed to find a Western Electric gray color insulation tape (of course it is not WE brand, I'm only talking about the color. Lines pairs are divided by this tape. So if anyone needs a grey insultaion tape I can mail it from the nearest Bauhaus (this is a German construction outlet like Home Depot in States).

Small rust brown unit in the right cable tray is PowerDsine 3W ringing generator (12VDC to 70VrmsAC). This tiny unit is the heart of my 551C KSU audibles. It will power all ringers in my telephone sets and as well Western Electric Chime bell to provide service of central ringer. This is a highly recommended unit, small and powerful it is potted in ceramic providing necessary protection and heat dissipation properties. It can be powered from 10VAC lamp battery via a rectifier (red and black wires). Output 70VAC ringing voltage (white and blue wires) is routed to RG and RB terminals of 551C KSU. I sourced these units from a guy in Israel were they were produced till 1996. This is a NOS supply, looks they produced more than they could sell at that time.



Four 400-series line cards (San/Bar 4000F) are installed into line card jacks of the KSU. I like San/Bar more as they are well designed and made, and suitable for repair. WE 400D are good, but they are on the metal board and really look like from stone age. I have some in stock, but they will be used on 584C panel. Haven't tried yet WE 400H cards on solid state circuits, which originally came with this KSU. Those should operate silently, since they have no magnet coil relays (all is solid state) and only one reed relay, but that one is with mercury wetted contacts and only can be installed vertically. So I can mount those cards only when KSU is on the wall.



For the nearest future the following improvements are planned:

1) Produce and install ringing generator power control board and install it into the KSU, so that ringing generator is activated by motor start voltage of interruptor and shut down after ringing cadence is not needed any longer.

2) Produce a feature block based on one 4-switching contacts relay to be able to easily switch between common audible based on ringers to buzzers and back across the whole network. This is an easy task, I plan this switch to be activated by pressing a signal key on any of the telephone sets connected.

3) Extend 551C KSU service to 5 lines. There is indeed a designed place for 5th line card below the standard cards slots. This space has thread holes for a jack and even mounting holes for a retaining bracket like on upper card slots (of course there is no bracket provided). I will take 18-pin line card jack from 584C panel (I don't need 13 lines there and 13 is not a good number, I like 12 much more) and wire it up. The only engineering reason I see from finally abandoned idea to furnish 551C KSU with 5 lines service (which would have been fantastic, since these combined commercial/residential shoe box in this case would have matched the most popular layout of 1 hold and 5 pick up buttons of standard 1A2 telephone sets) is that they could squeeze in a 66 block with enough terminals for all 5 lines. And this reason is stupid for me. I've ordered a 6-pin small additional 66 block and will terminate line 5 there (will need to figure out where to place this additional block - probably on the bracket above the interruptor where 118A ringer generator is installed, since I don't need a 118A generator.

Trouble encountered:

Somehow have problem with Tip and Ring polarity. San/Bar 4000 line cards are not sensitive to CO line polarity, however they don't want to detect ring when I put polarity correctly (tip is positive to ring). Reverse polarity works well for ring detection, so I have to keep it as is ("plus" of CO line is on ring terminal). However this cause a problem with one of my 565 telephone sets - touch tone dials is inoperative in this condition. Reversing CO line polarity to normal allows dialing out but disables ringing detection by all four line cards. If anyone has an assumption of possible cause, would be great to know.

Here's the first video of 1A2 system testing on 2872A2M and 564HK telephone sets. Will keep you posted on updates. Comments and recommendations are welcomed.

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