atcomsystems.ca/forum Forums Other TDM Phone Systems 1A2 and Comkey Telephones & Systems WE 118A Ring Generator Circuit and components

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.

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.

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:



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.

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.

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.

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.

Following up on this thread, I did just now find a circa 1961 "Bell System Record" document that describes the circuit of a 107-b frequency generator that describes generating 20cps ringing from 115 VAC/60Hz.

Skip to page 23 in this document:
https://telecomarchive.s3.us-east-2.amazonaws.com/docs/bsp-archive/BLR/39-7.pdf

..and find the article called "Static Frequency-Generators For Ringing Power". The term "static" in this context means "no moving parts", as would be the case with a motor-driving-a-generator approach to frequency shifting. This particular design appears to be a ferroresonant design (all passive components; transformers + caps) similar to the TelLabs 8101 schematic I posted earlier in this thread.

There does seem to be a path of revisions that lead to the very compact 118a 30Hz frequency generator (aka. ring generator) we're used to seeing in 551a/b/c shoebox KSUs. The procession I'm familiar with so far is: 107b -> ??? -> 113a -> 117a -> 118a. The latter three were "potted" in a block of plastic, the 107b was (apparently) just a couple of large donut-like wound wires in a partially open metal case with a lot of exposed wiring, presumably to let one configure different frequencies, due to the numerous transformer taps available. In the potted units, the frequency is "fixed" and not adjustable.

Here's pics of the various dedicated ring generators that I've been able to find so far; these pics from worthpoint's archive of previous ebay sales:

117a (30Hz):


113a (30Hz):


107b (20Hz):


I have myself a 118a and a 117a, and I /think/ I might have a 113a (can't remember). I can't find much other info on the 107b, such as if it is indeed "configurable" for different frequency outputs.

Re 118a brackets for 551C shoeboxes. I have a friend who works for a manufacturing firm who took a genuine bracket, generously loaned from Arthur Bloom, and made a handful. I don’t have any currently, but I have asked him to make some more, now that he has it all in cad on his computer.

I would love if someone could reverse engineer and create a schematic for some sort of solid state ringing generator, either in 20cps or 30cps flavors, even better if there were 12 volt and 120 volt variants.

Sorry it’s been a million years. Life has been….. busy.

Re: 118a brackets: Phil's been selling these on ebay for $20/each for a while now. Says its for the 551B, not sure if it's the same for 551C..?
https://www.ebay.com/itm/166764563778

It’s not even close to the same. I don’t have a picture, but I’ll try to dig one up.

@ChrisRR: Just happened to see this pic of a 550C on ebay (similar to 551C I think) which shows the special bracket and 118a mounted to it, so I'm guessing this must be what you mean: