To make this text as easy to follow as possible, I have separated the testing and fault diagnosis sections. Each specified test includes hyperlinks to the associated diagnostic section, so you can move between them easily.
You will require at least one and preferably two telephones. If you have only one telephone available, you will need in addition a telephone line plug with pins 2 and 5 shorted together, for which you may use the plug you removed from the socket doubler. When inserted into either socket, this plug simulates a telephone that is off-hook. To simulate making a call, insert the plug into either socket. To simulate the answering of a telephone, after the ringing signal has started, insert the plug into the ringing socket.

I recommend you do not connect any form of modem to the unit until functional testing is completed.

Perform all tests in the order shown, and do not proceed further until each test is satisfactory.

Pre Power-Up Check

With the unit disconnected from the power supply, use a multimeter to check that the power rails on the circuit board are not shorted.

Setting Up

It is assumed that whichever method of construction you have used, the completed unit is on the bench, connected correctly to the 12-volt power supply.
Switch on the power supply and plug a telephone into socket A.

Off-hook Detectors.
Monitoring IC5 pin 7, adjust RV1 so that with the telephone on-hook (hung up) the voltage is +12, and when the telephone is off-hook the voltage swings to 0v.
Remove the telephone and connect it to socket B. Repeat the above test monitoring IC5 pin 1. It should not be necessary to re-adjust RV1. Monostables
Connect telephones to both sockets. Using a logic probe or oscilloscope, monitor IC8 pin 10, which should be low. Check for a positive pulse on pin 10 whenever either telephone is taken off-hook, when the other telephone is on-hook and when it is off-hook.

Monitor IC4 pin 14. With both telephones off-hook, pin 14 should be low. Check for a positive pulse on pin 14 when each telephone is placed on-hook while the other is off-hook. With both telephones on-hook, pin 14 should be at a high level, sourced from IC9 pin 14.

Fault diagnosis consists simply of following the signals back to their source at IC5 pins 1 and 7.

Call Answered Detector
Hang up any connected telephones. Monitoring IC5 pin 8, adjust RV2 to a point where the voltage is low. Typically, pin 9 will be at around 0.4 Volts. This amplifier detects answering by monitoring for an increase in ringing current, therefore during functional testing, RV2 may require re-adjustment.

Astable
Check that IC1 is oscillating by monitoring for a pulse train on pin 3. If a fault is holding pin 4 low, disconnect temporarily the wire to pin 4. With IC1 working, adjust RV3 until the frequency on pin 3 is 102.4 kHz +/- 5%. If using an oscilloscope, the periodic time should be 9.76µS. For location specific settings, see the table below. If your modem can be set to recognise different dial/busy tone detection ranges (register S96 on some models), set the appropriate register value for UK and use the setting given above. UK approved modems will already have this set as the default value.

If you do not have a frequency counter or oscilloscope, there are alternatives:

• If you are in the market for a new multi-meter, many of the latest models include a frequency counter.
  
  

• There are applications available that allow a sound card to be used as an oscilloscope. Try these links which will open in a new window:

  UNIX	LINUX	Windows	Mac

  
  I have not tested these utilities.
  To discover others, run a web search on:
  
  | oscilloscope +"sound card" |.
  

• Some sound file editing utilities, may allow you to read-off the time period of a tone fed into the sound card's microphone.
  
  

• Download the QBasic file SOUND.BAS. When the rest of the unit is working properly, connect a telephone to one of the sockets, then run this routine under QBasic and compare audibly the frequency generated by your PC to the dialtone produced by the unit. When there is no beat note heard, the frequency may be assumed correct. For an alternative way to perform the above test, download the DIALTONE.WAV file and play it in a continuous loop.
  
  

• If all else fails, with the unit otherwise debugged, connect your modem and attempt dialling at different frequency settings. When you have found the range of adjustment within which the modem recognises the dialtone and the busy tone, simply set the potentiometer mid way between the two extremes. The frequency of the astable affects all the frequencies and time periods used by the unit, therefore using the latter method must result in a ringing frequency of at least 20 Hz. If the called modem fails to detect ringing, a small increase in frequency may be required.

Functional Test

The following is a description of normal operation.
With the unit powered, connect telephones to both sockets, which will be referred to as A and B. Pick up A. You hear a dialling tone for approximately 5 seconds . At the end of the dialling period, the appropriate relay will operate and you will hear a ringing tone through the receiver . B will be ringing in single rings, not paired . Hang up A. The relay drops out and ringing stops. Repeat the above, this time lifting B when it starts to ring. With both telephones off-hook, voice communication is possible between them with no tones or pulses being heard . Restore both telephones then pick up A. Before the dialtone finishes, pick up B also. At the end of the dial period, the dialtone changes to a busy tone, audible in both telephones . Hang up B, in A you will hear a dialtone followed by a continuous busy tone. Pick up B, in both telephones you hear a dialtone followed by a busy tone. Repeat all of the above treating B as A, and A as B. If all is well, the unit now can be used with any telephone apparatus. If operating the unit using communications software, the program should be set to dial an innocuous number like 1471 (in the UK), or a local equivalent that will do no harm if dialled accidentally on a real line.

Diagnostic Section

Off-Hook Detectors. If the outputs of IC5 pins 7 & 1 do not respond when a telephone is lifted off-hook, check for a change in level on the corresponding input pins 5 & 3. If there is no change there either, follow back to pin 6 of the corresponding relay. Verify that neither relay is energised at this point by checking for a low on the output of the driver gates. If the relays are OK, there must be a wiring fault associated with the line sockets. Ensure that pin 11 of both relays is connected to 0v.

Clock Frequency. Normally the oscillator IC1 should be halted only by a low level on pin 4, due to the Answered latch (IC4/D) being reset. If there is no output from IC1 with pin 4 high, check the timing components: C1, R1, R2 & RV3. With both telephones either hung up or disconnected, IC4 pin 14 should be high from IC9 pin 4 (via D7). The output of the answer detector (IC5 pin 8) should be low. If it is high, adjust RV2 until it goes low.

Dialtone. When either handset is lifted, regardless of the position of the other handset if connected, there should be a positive pulse reaching IC4 pin 4 from IC8 pin 10, causing IC4 pin 2 to go high. If no pulse occurs at IC4 pin 4, follow the signal back through IC8/C and the monostables that drive it. At this point IC4 pin 2 should go high. Pin 3 of IC4 should have been low throughout, only going high to signal the end of the dial period.
If IC4 pin 2 does go high, check the path of the dialtone frequency from IC2 pin 13, through the gating system to IC8 pin 4.

Busy Tone. All the tones heard in the handset are derived from the dialtone signal from IC2 pin 13. In the busy state, IC4 pin 10 is high and IC4 pin 2 is low. This allows the Busy Break signal to pass through IC6/C and IC9/C to IC8/B where it breaks up the dialtone on pin 6.
If the modem you are using is unable to recognise the busy tone, try taking the ring break signal from IC3 pin 6 instead of IC3 pin 7.

Ring Voltage Generator. Bring the unit to the point at which ringing should start and ensure that the appropriate relay operates. With no telephone connected at the called socket, check for 48-56 volts at the cathode of D8. When there is a telephone connected at the called socket, this voltage will drop by a few volts typically. If OK, go to the next section. IC7 pin 6 should be low from IC4 pin 9.
Check for a pulse train on IC7 pin 4 derived from IC2 pin 9 (50 kHz). If OK, check for a pulse on the emitters of Q1 or Q2. If OK, check for voltage multiplication along the four stages of the multiplier. If multiplication is taking place but the output is lower than expected, it is probably being loaded by a fault associated with the inverter.

Approximate Voltages Measurable on the Positive Terminals of C5-C12 During Ringing
C6 - 10v	C8 - 19v	C10 - 29	C12 - 38
C5 - 15v	C7 - 24v	C9 - 34v	C11 - 43v
Alternate High-Output Ring Voltage Generator If a higher ringing voltage (or power) is required, the suggested guideline circuit below can provide a solution. Although seemingly simpler, its use of an inductor makes exact component values difficult to specify. There are no globally available standard inductors that can be specified, dictating that constructors must wind their own and the results of so doing can vary widely, which can effect the optimum required switching frequency. L1 needs to a high Q factor inductor enclosed in ferrite core, but I cannot specify a number of turns or a target inductance. As an example I wound a 22mm ferrite bobbin to capacity with something like 0.5mm wire. This inductor gave satisfactory operation switching at around 3KHz. I would expect that a smaller device would serve well, albeit perhaps working at a higher frequency. Fortunately the outputs of IC2 provide a wide range of frequencies up to 50KHz, one of which will always be suitable. The chopper transistor Q1 should be capable of switching a few hundred milliamps, but will dissipate negligible power when operating properly. If the transistor becomes hot to the touch, then it is not saturating and a higher switching frequency may be required. D8 must be a Shottky type. ZD1 must be rated to match the required output voltage. C4 must have a voltage rating exceeding that of ZD1. In the main schematic, this circuit replaces all components from IC7B pin 4, up to and including D8 and C4. It may be helpful to consult online articles on switched-mode psu design. This circuit should be developed separately prior to final installation.

Ring Voltage Inverter. During the ringing period, monitor for an intermittent 25 Hz pulse at IC8 pin 11 and on IC8 pin 3. IC7 pin 8 must be low. There should be a constant pulse on IC7 pin 9, interrupted by the ring break signal on IC8 pin 12.
In ringing mode, the intermittent a.c. ringing voltage should be measurable across the collectors of Q3 & Q4. This voltage will be present whenever the ring tone is heard in the connected handset.

Answer Detector. Referring to IC5, during ringing, the voltage on pin 10 is around 0.2 volts. When a ringing telephone is answered, this voltage rises to the point at which pin 8 goes high, which sets the ring latch and resets the answered latch. If ringing pulses continue after answering (loud rumble heard in handset), lower the voltage on pin 9 using RV 2. Conversely, if the ring signal cuts out immediately after it starts, increase the voltage on pin 9.

Ring Tone. The ringing tone is derived at IC9/D by interrupting the dialtone using the ringing frequency. To attenuate the ringing tone, its amplitude is clamped by d2 & d3. The reason for this is that if the amplitude of this tone is too high, the modem may interpret it as an attempt at connection from an answering modem, leading to premature disconnection and a 'NO CARRIER' message.

The following data is provided for completeness, as in practice most constructors are unlikely to need it.

The table above is based on data taken from a modem manual. I cannot attest to its accuracy.