sundance-communications.com/forum Forums Other TDM Phone Systems 1A2 and Comkey Telephones & Systems Melco Intercom with Bell Systems 551C help needed

No, what I am saying (and what the label on the ICM says) is that the A supply from the shoebox goes to the A terminals of the ICM, and the B supply of the shoebox goes to the B terminals of the ICM.

The 10Vac supply from the shoebox goes to the AUDIBLE SIGNAL terminals on the ICM (or whatever Melco calls them.)

That is the way it is hooked up. The A and B terminals on the ICM are connected to the corresponding supplies in the shoebox

Nonetheless there is some hum. Not severe but noticeable. That was why I was asking if I should use the filtered supply for both.

I'll double check but I am certain it is hooked up correctly.

Anyway I think I need to check the ripple on the filtered supply. The capacitor has a 1970 date so it may be a bit past it's prime.

Just take another 1000uF electrolytic and wire it in parallel with the A battery output, and see what happens. Remember to observe polarity, or you'll let all the smoke and noise out.

Thank you Arthur. I will check my wiring and give that extra filter a try. Trust me I know the consequences. I work on vacuum tube equipment. You don't get second chances at 450 volts.

Connect the 10VAC to buzzers/sig pins. Connect A battery to A pins on Melco. Connect B bat to sig pins on Melco.

Disconnect the 25pr connector on the melco. On the power supply, put your buttset on the A Battery to A ground. You go off hook and should hear no hum. IF you hear hum in this condition, your power supply is bad. This is very likely due to it's age. If you know how, just replace the filter capacitor or get a tech to do it. Good luck

Yeah I picked up another power supply cheap and it definitely is the ps that's the problem. So it must be a bad capacitor. Now that I have a spare I can take the original one apart and rebuild it.

So a question is there any reason not to make the b battery supply filtered while I'm at it? It would be easy since I'll be in there already. But I hesitate and ask because in the 70s there were plenty of circuits that relied on ripple on the supply circuits for various purposes and if you filtered them too well things would misbehave. Bally pinball machines were notorious for that.

Filter away. A battery is for talk, so it needs to be filtered to eliminate hum. B battery operates relays. The relays really don't care if it's filtered or not, but, like you said, since you're already in there, adding a 1000 uf 100 VDC capacitor will not harm anything.

Rcaman

All the central offices and PBX's that depend upon a battery bank use filtered battery for the relays.

Thanks guys, I figured as much.
I've looked for a BSP for the 28A1 power supply for a schematic and haven't had any luck can anyone point me in the right direction? The master list had an entry for KB518-010-106 regarding key power supplies but I don't seem to be able to find anything searching on that.
Not a big deal, dont go out of your way. Its a pretty simple box.

It's a pretty simple power unit. A transformer, a full-wave bridge and a filter cap. There might be an inductor, too, but I don't remember. The cap is probably marked as to its value. I suspect around 1000uFd. A quick poke around inside with a DVM will tell you the output flavors and voltages.