After about 4 years (things move slowly out in the country), I finally have begun wiring in my Tone Commander intercom. It's a common talk path, dial-selective unit and I'm connecting it into my Stromberg KSU (for which I have no documentation, unfortunately).

The intercom requires from the KSU only A battery, B battery, audible signal source (could be ringing voltage or buzzing voltage), lamp battery, and associated grounds. I did not have enough spaces in the KSU for the B ground, so I used A ground for that, since they are tied together in the KSU. Inside the KSU, I used AB, AG, BB, AG, 18V, 18G, and 10V respectively.

I have these punched down using solid copper CAT-5e, since that's all the pairs I need for this, and for a small residential installation it seems like the best option. I document my color selections in order to keep my brain straight and not reverse anything. The intercom breaks out from an Amphenol connector on the side to an M-style block. From there, it goes directly to the blocks built into the KSU.

I have one phone in the basement, about four feet away from the intercom and punched to the T/R, Lamp, and audio output pairs marked on the intercom unit as station 0. I have another phone in my office on the first floor punched down on the M style block as station 1. The office phone also has lines configured for two 400 cards handling our C/O home line and a VoIP line.

When I pick up either phone and select the button I've designated for Intercom, the lamp lights on both stations. In the basement receiver, I can hear the other party (my daughter, who thinks this is great fun!) but there is a roughly 120Hz hum that almost obliterates the audio. Tone dialing doesn't work to signal the basement station, but rotary dialing the office code from the basement buzzes in the office. I am assuming at this point tone dialing doesn't work because of the hum on both phones.

I don't think I'm inducing electrical noise into the wiring, but only because I have two other lines to the office phone that work fine. This sounds to my ear like I'm restoring one of my antique radios and there's a bad power supply filter. Has anyone seen/heard anything like that, and is it possible I'm getting unfiltered voltage into my talk path?

Thanks for any guidance and help you can provide. Once I get this licked, I'll be 8 pairs out to my garage and finishing off the garage/workshop phones and have intercom in all three locations. Then I'll probably unlock a spare line key on the phones and wire them to open the garage door.

Hum on the ICM line is often caused by using B battery instead of A Battery. Try testing the output from the power supply with a butt set. Go across B Battery and B ground and listen to the power. it should be noisy.

Now try the same thing with A battery and A ground. It should be clear. If it's not, then you could have a defective power supply. If it's clear, then make sure you are actiually wired to A ground and not B ground. Make sure that all your grounds a re commoned.

Let us know what develops.

Sam

I was worried I may have mixed up A and B, though I had checked myself a few times. Was wondering if perhaps Stromberg decided to use opposite nomenclature, even.

Lacking a butt set (I need to get one at some point!), I re-punched the power pairs on their opposite rows on the block, and still had the hum.

I then took a quick peek into the power supply, and found what looks like a leaking capacitor. I would suspect this is the cause; if not, it's certainly an important aspect to get fixed. Photo linked here:
[img]https://imgur.com/a/7KSaFIN[/img]

I have an identical KSU in the attic...I think I'll start by checking that one over to see if the caps look okay. If yes, I can swap it in quickly and test things out. If not, I'll have to disassemble and get the capacitor specs.

Thanks for the reply and advice!

I was able to swap the power supply with the spare KSU I have in the attic. I noticed that one of the main caps in this replacement power supply was newer looking, so it looks like perhaps there was a field replacement at some point in this unit's past. (Both KSUs came out of a hospital...I could have had three, but they're bulky).

Once I swapped in the new power supply, I was able to successfully test with a clean talk line and successful tone dialing on the intercom. So, we'll call this one fixed! The malfunctioning power supply will go on the bench for new capacitors when I have time.

Thanks again! Looks like I'll be running wire to the garage this weekend...if the upstate NY weather stays fair.

A Capacitor is what was used to filter the DC, so that's a likely suspect. Good catch!

If you don't have a butt set, you can use a single line (500/2500) phone. Connect alligator clips (or just loose conductors) to T&R and use that. If A battery is not perfectly clear coming out of the power supply, that's your problem.

A battery is really only used (As I Recall) on Intercoms (manual & Dial) and Ring Down circuits (manual & automatic). If you don't have any of those circuits in your system you'd never notice a problem with A battery.

Good luck!

Sam

That explains why I never noticed an issue before going with the intercom. Now, it's clear as a bell with the other power supply!

Thanks again!

Good deal!

Glad you got it working!

Sam