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GRA-CO Offline OP
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Does anyone know if there are any plans out there for the circuit and components of this device? It converts Hz only from 60Hz Input to 30Hz Output, leaving the voltage the same. I have one and it works. They were always available on eBay and other sites for 10.00 - 50.00 USD. Now they seem to be getting hard to find and starting bids are $100.00. They appear to have a Bakelite (sp) shell and are epoxy potted? The weight leads me to believe there is a transformer inside, probably a group of Diodes, and a some sort of load coil. The load might be the weight of the device. Since they were always on inside the KSU they won't last forever. Thank you for this group. If I posted this in the wrong area please advise.

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It's a ring generator used in 1A2 key systems to ring the phones, no real use outside that unless you want to use it to ring phones on a movie set or something or you're an old Key System buff and need one. They're tough and will last a very long time, I can't remember ever having to replace one.


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To question about if there's a circuit and components for an 118a frequency generator, I've never seen one myself, but perhaps there's some out there somewhere. Since it seems to be a potted unit, it was never intended to be repaired, so I'm guessing there isn't a schematic provided in the BSP's (Bell System Practices). However, there might be an internal document at Western Electric.. such a document might be harder to come across. Or perhaps there's a patent that describes its design?

My guess is it's a simple ferroresonant transformer circuit, perhaps much like in the TelLabs 8101 ring generator (see below). These are well suited to divide the input AC frequency from 60hz -> 30hz used for 1A2 ringing.

In a 551 KSU like my 551a, these 118a's are indeed "powered on" at all times, but I've noticed they don't consume much power at all when idle, which leads me to think there's some additional current sensing smarts built in to power down the transformer when there's no load from ringing.

I can tell you the TelLabs 8101 does NOT have such a load sensing circuit, as it does consume some power when idle -- about 8 watts. The idle AC hum of the 8101's transformer is loud enough to be heard across a large room.

Here's a photo of a TelLabs 8101, and an excerpt from a technical manual for it showing the ferroresonant circuit and a brief circuit description:

[Linked Image from seriss.com]

Thing is I think such transformers are custom wound for this purpose, and not just simple isolation transformers with a center tap on the secondary with a capacitor tank circuit or such. But I've not really investigated this; exotic transformer designs are not my thing. But there's quite a bit of info out there on ferroresonant transformers.

There's certainly many electronic circuits for ring generators from e.g. 12VDC, 24VDC, etc. IIRC, the small "Black Magic" and "PowerDSINE" ring generator modules have fully documented circuits (that are much more complicated) and do some clever stuff to handle dead shorts and drop voltage as load increases, and work equally well in place of an 118a.

Anyway, I'd be certainly interested too in seeing a schematic circuit for the 118a. But it might be hard to replicate if it's using a custom wound transformer.

Last edited by Greg Ercolano; 04/29/22 02:30 AM.
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Greg, Thank you for your fantastic response! I actually was led here by researching many times in various search engines. I have (1) 551B KSU with 118A RG, (2) 551C KSU's with only (1) 118A RG and 20 or so 2565HKM sets in a few colors. 5 COM-KEY 416 (2) main and (3) add on sets. I also have a collection of Merlin 820, Merlin 820D2, 1030, Merlin II and nearly every style Merlin Voice Terminal. I have been a telephone enthusiast since I was 11 or 12. My grandfather worked for Sears as a TV and Radio repair technician. As a youngster he taught me how to component level solder on boards. I can do some component level testing, however that is the limit to my knowledge. Sometimes my 61 YO brain gets ideas and I just want to experiment. My parents had a sheet metal fabricating shop. I was in sales, an estimator and a buyer for his company. In the early 80s I was able to get a private guided tour of the WE chip manufacturing facility in Reading PA. It was a guarded facility on top of a hill that not too many knew was even there. We were a sheet metal components manufacturer. As large as WE was they became a customer. We made a few prototypes for them, and only 1 small run of about 500 pieces. They also payed almost immediately.

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GRA-CO Offline OP
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Thank you for your response. I thought with these becoming scarce I might venture into manufacturing some of them. Just a thought. I checked Bell Labs and WE patents with no luck. Thank you Again.

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GRA-CO,

Have you consider manufacturing some other 1A2 Parts? If you are, here are some ideas.

Manufacture a KTU card cage or card shelf. Some people who like to add more KTUs such as MOH but the KSU has no more card slots.

Also make the bracket for the 118 frequency generator for the 551 as some people want a true authentic 551 KSU and do not want use to zip ties to hold the 118 frequency generator.

If someone can make the color buttons for the phones, that would be very cool.


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