Cap question - GPO Series 300

[els1967]

A few years back I made a Frankenstein phone out of a lot of series 300 spare parts I had lying around. I recall it worked ok but on resurrecting it a few days ago for a project I'm not 100% sure I wired in the cap properly. In the picture, is the 2uF cap wired across the arrowed contacts? Issue is I get a lot of sparking when dialing a number so I'm not sure if the spark suppression cap is working properly. Could be the cap but woanted to check I had the wiring done properly first.

Thanks!

[russell_w_b]

Originally Posted by els1967

'Issue is I get a lot of sparking when dialing a number so I'm not sure if the spark suppression cap is working properly. Could be the cap but wanted to check I had the wiring done properly first...'

I think your problem is more likely to be your dial off-normal contacts, which should shunt the entire speech network when dialling. The 2uF section is for the bell and won't be necessary if you've modified the instrument to plug and socket, but will be needed if you haven't.

The 0.1uF section is a radio suppression shunt for the carbon mic and prevents the telephone acting as a demodulator in the presence of radio transmitters. You can probably get away without that, too.

According to the paster diagram for a 332, the 2uF section is connected by a blue and an orange/white wire and the 0.1uF section is connected by a slate and an orange wire.

[els1967]

Ok thanks - looks to have been a faulty mic.

[russell_w_b]

Originally Posted by els1967

Ok thanks

You're welcome. I never told you the full story - the bell cap does form part of the dial suppression cct via one of the dial-off-normal contacts - typical GPO being economical with components - but if connected à la plug-and-socket, the job befalls the master-box capacitor, not the one within the telephone. The next question I was going to ask was 'is your bell ringing OK'?

Anyway... You're sorted, and that's the main thing.

[els1967]

Yup all good. You might be interested to know I've built and exchange based on this one but used an Arduino Mega instead of the solid state logic. This is the third exchange I've built based on the design and I think I've just about got it right this time. It works extremely well for pulse dial GPO phones but I have also added DTMF. From top left clockwise - first board is the power supply. Have done a lot of work on this to get rid of mains hum using capacitor multipliers for a 40v for the phones and 30v feed for the logic etc using commercial VR's to reduce the voltage where required. The 40v feed uses a simple Zener diode setup for regulation. Also on this board are the ad9833 tone generation modules and relays for call progress signals. Next board has a LM386 amp to drive the speaker to monitor calls (more of a diagnostic thing really) and the circuitry to connect an external line as well as external call ring detection. Next board is the source and destination relay arrays and the feeds for lines, 8 of them. Next board is the line pick up detection optocouplers, call bridging and a/c ringing relays, source phone pulse detection and destination phone off hook detection. The final board is the DTMF detection and generation side of things along with a simple autogain amp to help with reading DTMF codes. It also uses call progress detection to monitor calls, ie to detect call hang-up, ring cadance, that kind of thing on external calls.

Top middle is a 16x2 LCD display that shows whats going on. Ignore the one by the lamp that's a different project.

[russell_w_b]

Originally Posted by els1967

'Yup all good. You might be interested to know I've built and exchange based on this one but used an Arduino Mega instead of the solid state logic....'

Yes - that's very impressive, and the solid-state design original looks good too. Something like this (and looming retirement) might prompt me to do something on similar lines. Maybe. Moreso, the pending change-over to fibre-to-the-premises being rolled out, even to rural dwellers like me in the infrastructure backwater, means that in order to keep our house dial-and-magneto-bell telephones going, I'll need to do something, and it would be a good project.

I've never considered building an exchange before, although I seem to remember one in (I think) Practical Electronics from the '70s. I'm afraid the only telephonic non-network interchange I've put together at home involved a couple of Type 'F' field-telephones, a 4109B (Ericsson N2185A similar), and an Irish P&T 100M 552/2 connected on a party line (shed, office, living room, living room) for when our girls were little. 'What's in the freezer for tea, dad?' Well, it was for me, really.

[Oldcodger]

For anyone looking for a replacement 2uF cap, look in a master phone socket. It's only 1.8, but thats close enough.

[Roger Ramjet]

Originally Posted by russell

I've never considered building an exchange before, although I seem to remember one in (I think) Practical Electronics from the '70s. I'm afraid the only telephonic non-network interchange I've put together at home involved a couple of Type 'F' field-telephones, a 4109B (Ericsson N2185A similar), and an Irish P&T 100M 552/2 connected on a party line (shed, office, living room, living room) for when our girls were little. 'What's in the freezer for tea, dad?' Well, it was for me, really.

Practical Electronics detailed an electronic exchange in their October issue this year.

Rog