I hoping to get some help with some wiring on a installation I'm working on of a Bell Systems 551C and a Melco KT19. I've never worked on 1A2 and am stuck on wiring for the Melco Intercom. I've completed the 551C install and it's working fine. I installed the Melco today but couldn't get much working. When I was done I was only getting a single click on the buzzer (KS-20419 buzzer on 2564 phones)of a two of eight phones and I had a had hum on the talk path on all phones. I hooked up 110v from 551c (RG & RB) to AUD SUP and GRD on Melco, then 24vdc from 551c (GB and BB)to A BAT and A GRD, B BAT and B GRD, then T & R to each 2564 phone Line 5 T & R. The Melco instructions say filtered 24vdc to A BAT. Is filtered needed? Any ideas on clicking problem with buzzer or problems withhum on talk path?

Filtered DC will make the talk path clean and silent, otherwise it will have a hum on it. I do believe that filtered DC lives on your 551 power supply. I'll check, but I'm sure it's there.

The reason that the buzzer is clicking and not buzzing is that buzzers use 10VAC or 18VAC ( depending on whether they have blue or white wires). Also appearing on your power supply. 110VAC is for the ringers in the phone.

Sam

Thanks! Buzzers have blue wires so they are 10VAC right? Can lamps be used when buzzing phones from Melco?

Blue =10 Volts.

Lamps can be used, though depending on the number of lamps lit at the time, they may dim slightly.

I looked at a old power supply from a 551(may not be the same as yours) and it has two 24VDC supplies, one marked Signal and one marked Talk. Talk is filtered and Signal is unfiltered.

Sam

The buzzers won't cause any noticeable lamp dimming unless you buzz every buzzer at the same time. Watch your polarity on the 24VDC. Ground is positive. "A" Battery is -24 VDC Filtered Talk battery. "B" Battery is -24 VDC Unfiltered Signal battery. If you get your talk battery connected to "A" Battery on the Melco, the talk path will be silent.

You have 10V AC buzzers. There are 90 VAC (or ring current) buzzers available. They will have an "ingrown" lump on one of the leads which is a dropping resistor. Those would be connected to RB and RG.

Rcaman

Thanks for your help. OK. I see filtered and unfiltered on the schematic now.

Sam, I have a follow up question. My Melco has two battery supplies on the schematic A and B. They are hooked up to the shoebox correctly but there is some hum on the intercom line. Are you suggesting both be connected to the filtered supply? Anyone know why the intercom schematic wants filtered and unfiltered ? Just curious

Most subscriber (as opposed to Central Office) telephone equipment relays use unfiltered DC because it's cheap to make. Filtered DC is more expensive (requires a choke and a capacitor at every power supply) so it is generally reserved for talk paths.

You fastened wires from terminals in your shoebox that are labeled BB and GB to terminals on your intercom that are labeled A battery and Ground. That should be you first clue as to what you fouled up.

Use A battery where it calls for it and you will not have (much) hum. Is it required? Yes. Take a test set and listen to AC, unfiltered DC ("signal") and filtered DC ("talk'). You will hear the difference.

You can find A-ground / A-battery on the right side of the terminal strip in the shoebox, on terminals 43 and 44 (4th pair up from the bottom.) Pull the power plug when doing the terminations to avoid popping the fuses, or blowing out the full-wave bridge in the power supply. The geniuses who designed it put the power terminals at the least convenient spot, in a KSU that usually is mounted in the least convenient spot.

Do you have the BSP for the shoebox?

click

Yes I do have the schematic. Thank you.
When I hooked the Melco up I followed the instructions on the label. There were 3 power supplies listed. The signal supply which I have to the lamp supply for blue wire buzzers. And then two more inputs that are labeled A battery and B battery so that's what I hooked them to.

So basically what we are saying here is that both should be connected to the A battery supply correct?

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.

Like the other guy said, you can add a filter cap to the B bat but it will not improve anything. The A bat has a choke and a cap to keep the audio off out of the power supply and create a "battery-like" quiet supply.