noisy Marquis II PABX

[Dave Moll]

I have recently acquired a Marquis II PABX. It seems to be in working order (apart from appearing to disconnect my ADSL when I plugged it into my telephone line!) but I get a loud mains hum when I select an external line (whether connected to a line or not).

Having noted that there is an earth terminal in the connections box, I did try connecting this to mains earth, but with no effect. Is it worth creating an independent earth rod and connecting to that - or my problem likely to reside elsewhere?

I suppose an earth rod might be a useful thing to have for vintage radio purposes anyway.

[AndiiT]

Hi,
Whilst not being too familiar with the Marquis II, I believe that the Earth terminal was for signalling purposes if the Marquis was being "Piggy Backed" on to a larger PBX.
At a guess I would say that your problem may lie with either dodgy electrolytics somewhere in the power supply to Line switching circuitry, or possibly a faulty bridge rectifier in the same area.

You mention that you have ADSL, when you connect the Marquis to your telephone line are you connecting a filter between your phone line and the PBX line port?

Regards
Andrew

[McMurdo]

The earth terminal will be for lightning and ESD protection, its normally connected via cream wire to the main earth point in the consumer unit but most BT installations I've seen just shove it in the back of a fused spur or ring socket and often in green-yellow.

I had a similar fault with a PABX that was caused by mains hum being picked up off one of the external extension cables: the hum only appeared on the exchange line, not during internal calls!

edit, just seen Andrews note about ER (simultaneous post!) and would bow to his greater knowledge

[Dave Moll]

My setup is that ADSL is split off directly after the master socket, with all telephone sockets (including the one into which I plugged the Marquis) being after the single filter.

The earthing consideration was very much an afterthought, and (as you say) the earth terminal is presumably for signalling purposes rather than earthing in the sense of "screening".

I think I would be reluctant to start poking around in the circuitry unless I can obtain a circuit diagram. Though I will probably open it up to see whether there are any components showing physical signs of distress, such as bulging electrolytics.

[AndiiT]

Hi,
A circuit diagram for any/most PBX's are the stuff that dreams are made of, they were all intended to be "non - serviceable" at least from a component level repair point of view.

Opening it up and visibly checking for dodgy electrolytics and perhaps metering across any large bridge rectifiers wouldn't do any harm, taking in to account any static precautions as I think that this series of PBXs used at least some static sensitive devices.

Andrew

[Dave Moll]

Originally Posted by McMurdo

The earth terminal ... normally connected via cream wire to the main earth point in the consumer unit but most BT installations I've seen just shove it in the back of a fused spur or ring socket and often in green-yellow.

Indeed, there is a wire (looks white rather than cream to me) going from the earth terminal back to the main unit as part of the multi-core cable, though there is no continuity from this to the earth terminal of the (moulded-on) mains plug. This and the green wire from the terminal to the metalwork in the connection box are quite heavy duty, which contributed to my thought that they may be for "earthing" in the traditional sense.

[Dave Moll]

Originally Posted by AndiiT

A circuit diagram for any/most PBX's are the stuff that dreams are made of

I rather suspected as much!

[Dave Moll]

It may or may not be relevant, but this morning I replaced the short bit of cable which came attached to the 2/3C socket with a length of cable from stores. Lo and behold, the hum has now completely disappeared!

Could it be as simple as a dodgy core or poor connection in the previous cable? If so, I'm surprised that the hum was only present when an external line was selected - and that it was otherwise working normally.

[McMurdo]

No, a cream telecom earth wire wont be wired to the mains lead as it's meant to be a permanent and very seperate bond to a permanent and seperate earth facility. If it used the mains earth, and was to be unplugged, it might leave the CCU susceptible to lightning strikes on the external wiring which wouldnt have anywhere to go. These could serve long stretches of extension cables across factory buildings! You'll probably find the earth terminal proveides a return for the MOVs or GDSA's / spark gaps inside the CCU if fitted.

[AndiiT]

Originally Posted by Dave Moll

.....I'm surprised that the hum was only present when an external line was selected ......

If the cable that you replaced had even a high(ish) impedance leak to earth this would have an effect on the exchange line and perhaps cause the symptoms that you were experiencing.

A PSTN exchange line should never have either of it's 'legs' connected to true earth; although the A leg is often referred to as earth it is not at true earth potential but sits some 4 to 5 volts above this.

Regards
Andrew