Ringing Circuit Failure

The Fault & The Fix

Symptom The unit powers up and operates normally in all other respects, but no extension will physically ring when dialled. Calls connect correctly but the telephone handset bell or ringer remains silent.

A relatively common fault with the Ultimate 106 is the failure of the ringing circuit, causing the extensions not to ring when dialled. The root cause is an age-related failure of two components in the high-voltage ringing generator: capacitor C22 and resistor R20.

The Ringing Generator Circuit

The ringing generator produces a high-voltage AC signal superimposed on a DC rail, which is distributed to each extension via an optoisolated triac (MOC3021). The MCU selects which extension to ring by gating individual triacs. The circuit topology uses a closed-loop regulated voltage source built around a TL084CN op-amp, a Darlington transistor pair (BF469 × 2), and a feedback network of resistors and capacitors.

Ringing generator circuit schematic. R20 (highlighted in red) sits between the 1N4004 cathode and R4, forming part of the feedback loop. C22 is the smoothing capacitor on the regulated ringing voltage node.

How the circuit works

Q1 (BF469) operates as a common emitter stage with a large emitter resistor. The junction of R1 and C21/C22 forms the regulated ringing voltage node. C22 (47µF 200V) smooths the output, while C21 (1000pF) is the AC coupling and timing capacitor feeding back to the op-amp. Each extension is served by a MOC3021 optoisolator whose triac gate is driven from the C22 node via a 470Ω SMD resistor.

With R20 sitting between the 1N4004 cathode and R4, together with R1 and the Darlington pair, it forms a feedback loop creating a closed-loop controlled voltage source for the ringing signal.

The Failure Mode — A Cascade

The failure is not a random component failure but a predictable cascade driven by the age of the electrolytic capacitor C22.

Electrolytic capacitors of this age (mid-to-late 1990s manufacture) dry out over time, losing capacitance and — critically — exhibiting a rise in their ESR (Equivalent Series Resistance). The sequence of events is:

C22 (47µF 200V electrolytic) ages, loses capacitance, and its ESR rises.
Higher ESR causes more ripple current through C22, which in turn increases current on the supply rail.
Increased rail current places greater stress on resistor R20.
R20 overheats and fails open circuit.
With R20 open, there is no current limiting — the voltage across C22 rises uncontrolled.
The uncontrolled voltage accelerates C22's failure completely.

In short: C22 probably started the cascade, and R20 was the victim. The characteristic "domed" appearance of C22 confirms the internal pressure build-up typical of a failing electrolytic.

Domed C22 capacitor showing failure — C22 (47µF 200V) showing the characteristic domed top — a clear sign of internal pressure from electrolyte degradation. This capacitor must be replaced.

Damaged R20 resistor — heat discolouration visible — The resistor cluster in the ringing generator area. R20 (centre) shows significant heat damage and discolouration. In the unit documented here, R20 had failed open circuit and its original value could no longer be determined from its markings.

Calculating the Replacement Value for R20

Because R20 had been so badly damaged that its colour bands were unreadable, its value had to be calculated from the circuit topology. The ratio of R1 to R20 determines the feedback fraction and thus the regulated output voltage at the C22 node.

Supply rail: ~183V DC
R4: 100Ω
Q1 V_CE(sat): ~1–2V
1N4004 forward drop: ~0.7V
R1: 100KΩ
Target output (C22 node): ~75V

V_out = V_supply × (R1 / (R1 + R20))
75 = 183 × (100K / (100K + R20))
100K + R20 = 183 × 100K / 75
100K + R20 = 244K
R20 = 144KΩ → nearest standard value = 150KΩ

Power dissipation check

Voltage across R20 = 183V − 75V = 108V
P = V² / R = 108² / 150,000 = 11,664 / 150,000
P ≈ 0.078W

This is comfortably within the rating of a standard ¼W resistor. However, given the history of thermal stress in this location, fitting a 1W or 2W rated resistor at 150KΩ would significantly improve long-term reliability. The unit documented here was repaired with a 200KΩ resistor as that was the closest available value to hand — this results in a slightly lower ringing voltage but the unit functioned correctly.

Safety Warning The ringing generator operates at high voltages (up to ~183V DC on the supply rail). Always disconnect the unit from the mains supply before opening the case or working on the PCB. Allow time for capacitors to discharge before touching components.

Summary — What to Replace

Component	Value	Action
C22	47µF 200V electrolytic	Replace — look for the domed top as confirmation of failure
R20	150KΩ (calculated) — 200KΩ fitted as nearest available	Replace — heat damage will be visible; original value likely unreadable

Note on component markings Some component references on the PCB are printed on the silkscreen underneath the component and are therefore not visible without first removing it. References such as R1, R2, R4 in the circuit diagram above are not necessarily the actual PCB identifiers — locate components by position relative to the ringing generator area (bottom right of the PCB) rather than by reference designator alone.