Another Oltronix power supply – D400-007D
Another Oltronix power supply – D400-007D
This is another 1960s-era laboratory power supply from Oltronix. This article provides an overview of the design and highlights key considerations when repairing or restoring the unit.
To ensure reliable operation on modern mains voltages, several components require upgrading or replacement.
Below is a screenshot from the original LABPAC 30-series product presentation:
Rear view with the EL34 series pass element visible.
Overview
This instrument is a regulated high-voltage laboratory power supply intended for vacuum tube (valve) testing and development. It provides either 0–400 V / 70 mA or 0–200 V / 140 mA DC output, selectable by a range switch.
The design is characteristic of the 1960s: a hybrid regulator using an EL34 power pentode as the series pass element, controlled by a transistorized error amplifier. Two independent 6.3 V AC outputs are also provided for external heater supplies.
I haven't found a service manual for this power supply, but the schematics are available. Here's a cleaned up version:
A full size version of the schematics are available here: https://johanh.net/Oltronix/Oltronix_D40-04d_d400-007d_psu-serie_1966_sm-001.png
Component identification on the PCBs is shown further below.
Circuit description
1. Mains input
The unit is designed for 220 V, 50 Hz operation. The mains enters via fuse F1 (2 A) and power switch S1. As the transformer has only a 220 V primary, operation on modern 230–240 V systems increases all secondary voltages and should be considered in calibration and stress analysis.
2. Transformer
The transformer includes multiple isolated secondaries:
- 205 V winding — main HV supply (reconfigured for 200 V / 400 V operation)
- 220 V winding — auxiliary HV supply for control circuitry
- 6.3 V (×2, 2 A each) — external heater outputs
- 6.3 V — internal EL34 heater
- 12 V — panel indicator lamps
The relatively high winding resistances contribute to inherent current limiting and reduced inrush stress.
Winding resistance of the transformer:
Primary 9 Ω
205 V winding 24 Ω (naturally limits inrush current to a safe level)
220 V winding 58 Ω
6.3 V windings about 0.6 Ω
3. Range selection (S2)
Switch S2 (multi-section) reconfigures the supply between 200 V and 400 V modes:
- Reconfigures rectifier topology (bridge vs. voltage doubler)
- Changes filter capacitor configuration (parallel vs. series)
- Adjusts feedback divider scaling
- Selects range indicator lamps
This switch effectively alters both the power stage and the regulation loop gain.
4. Rectification and filtering
Auxiliary Supply (220 V Winding)
The control circuitry is powered from a bridge rectifier followed by a CRC filter (C82–R82–C83), producing a low-ripple DC rail.
R82 serves both as the CRC series element and as the current-setting resistor for the zener reference.
Main HV Supply (205 V Winding)
- 400 V mode: configured as a voltage doubler, producing ~600–630 V unregulated DC
- 200 V mode: conventional full-wave rectification, producing ~300 V DC
Filter capacitors are switched between series (high voltage) and parallel (high capacitance) configurations. In doubler mode, the midpoint connection provides natural voltage balancing across the series capacitors.
5. Voltage Reference
A 24 V reference is generated using two series-connected 12 V zener diodes. The operating current (~13–16 mA) is defined by R82.
The voltage setpoint network (R1, R1a, P90) is derived from this regulated auxiliary supply, ensuring that reference stability is independent of load variations on the main output.
6. Error Amplifier
The regulation loop consists of two interacting stages:
Primary Control (T2)
T2 (PNP) directly senses the output voltage and drives the EL34 control grid. This forms the fast feedback loop:
- Output ↑ → T2 conduction ↓ → EL34 conduction ↓ → output ↓
- Output ↓ → T2 conduction ↑ → EL34 conduction ↑ → output ↑
This direct sensing path provides the dominant regulation action.
Secondary Control (T1/T3)
T1 (PNP) and T3 (NPN) form a low-current comparator stage:
- T1 receives the setpoint voltage from P90
- T3 senses the EL34 voltage drop (feedforward term)
This stage injects a correction current into the output node, indirectly influencing T2. It improves dynamic response and compensates for pass element voltage variations.
Loop Compensation
C92 provides phase compensation and stabilizes the loop, particularly where multiple active stages introduce phase shift.
7. Series Pass Element (EL34)
The EL34 operates as a variable series element between the unregulated HV rail and the output:
- Anode: unregulated DC
- Cathode: regulated output
- Grid: driven by T2
It continuously adjusts its effective resistance to maintain regulation.
Worst-case dissipation occurs at low output voltage with significant load current, where the full input voltage is dropped across the tube.
8. Output Voltage Adjustment and Compensation
- P90 — main output voltage control
- P1 — feedforward compensation adjustment (EL34 drop contribution)
P1 affects regulation consistency across the output range and should be optimized for minimal load-dependent variation.
External programming terminals allow insertion of a series resistance to define a minimum output voltage.
9. Output Filtering and Protection
- C90/C91 (series-connected) form the output filter
- R92/R93 provide voltage balancing
- D90 suppresses inductive transients at the output
- D1/D2 protect against reverse voltage conditions
10. Output Metering
A single meter is switched between voltage and current measurement:
- Voltage mode: scaled via divider and calibrated with P92
- Current mode: measured across shunt R71 and calibrated with P91
11. Auxiliary Outputs
Two independent 6.3 V / 2 A AC outputs are provided for heater supply.
The supply should be treated as high impedance with significant stored energy; fault conditions can still be hazardous despite transformer resistance.
Necessary circuit modifications
Due to modern mains voltages, several components in the supply operate close to or beyond their original voltage ratings. Corrective modifications are therefore required for reliable operation.
Mains Voltage Range (SFS-EN 50160)
The standard specifies a nominal voltage of 230 V with a tolerance of ±10%:
- Minimum: 207 V
- Nominal: 230 V
- Maximum: 253 V
All worst-case calculations should be based on 253 V mains.
C82/C83 — Error Amplifier Supply (220 V Winding)
The 220 V secondary scales directly with mains voltage:
-
Secondary voltage:
V_secondary = 253 V × (220 / 220) = 253 V AC -
Peak rectified voltage:
V_peak = 253 V × √2 ≈ 358 V DC
This exceeds the 350 V rating of the original C82 and C83 capacitors. These should be replaced with 450 V or preferably 500 V rated capacitors.
Higher capacitance values may be used if space permits. The relatively high winding resistance of the transformer limits inrush current, so increasing capacitance does not significantly stress the rectifier or associated components.
In this unit, 22 µF / 500 V radial capacitors were installed. This requires drilling new mounting holes. Axial capacitors could be used to fit the original footprint, but are typically more expensive.
C80/C81 — Main HV Supply (205 V Winding)
In both 200 V and 400 V operating modes, each capacitor can be subjected to voltages up to approximately 330 V. Therefore, 450 V or 500 V rated capacitors are recommended.
The original dual 32 + 32 µF capacitors are effectively configured in parallel. Suitable replacements include:
- 64 µF axial capacitors (direct form-factor replacement), or
- Higher-value radial capacitors, if space allows
In this case, 100 µF / 500 V radial capacitors were selected. The diameter and length must not exceed the original components to ensure mechanical fit. When using radial capacitors, some other method of mounting them is required, e.g. by drilling holes in the PCB and using cable ties.
As with the auxiliary supply, the transformer’s winding resistance limits inrush current, allowing the use of larger capacitance values without adverse effects.
Component substitution
Original components may be difficult to source. The following substitutions are suitable alternatives:
Transistors
- T1 (2N3702) → BC556A, 2N2907A, 2N3906, 2N2905A
- T2 (2S302) → 2N4036, 2N4037, possibly 2N2907A (gain may be too high)
- T3 (2N3710) → BC546A, 2N2222A, 2N3904, 2N2219A
Always verify pinout before replacement.
T1 and T3 form a complementary NPN-PNP pair in a long-tailed configuration. When selecting new transistors, choose a similar complementary transistor pair. For stable operation, moderate and well-matched hFE is preferred. Very high-gain devices should be avoided. The BC556A / BC546A pair (specifically the “A” gain group, not B or C) provides suitable characteristics and adequate voltage margin.
T2 Considerations
The original 2S302 is uncommon and has no direct modern equivalent. Its key feature is an unusually high base–emitter voltage rating (V_BE ≈ 20 V), whereas most modern PNP BJTs are limited to approximately 5 V. Due to this physical construction, its gain is correspondingly low (hFE ≈ 15).
Under normal operation, low V_BE rating is not an issue because the output voltage remains below the error amplifier supply. However, the high V_BE rating provides robustness against transients during switch-off, range switching, and rapid adjustment with capacitive loads.
When substituting T2 with a modern device, a protection diode (e.g. 1N4148) could be connected between base and emitter (anode to base) to limit reverse V_BE. Protection against inductive load transients is handled separately by D90.
Diodes
- Z1, Z2 (ZD12) → BZX85C12 or 1N4742A
- D1, D2 (10D2) → 1N4007
- D90 (SK 0.5/10) → 1N4007
- D80 (W06) → W10G
- D81 (W04) → W10G
Capacitor
- C92 (220 nF, RIFA) → 220 nF polypropylene, ≥630 V DC rating
This must be a DC-rated capacitor suitable for at least 450 V operation. X-rated capacitors are not appropriate in this position.
Capacitors with the original 20 mm lead spacing are uncommon; 15 mm or 22.5 mm types can be used instead. For example, Kemet R71 series (e.g. R71PI32204030K) is suitable, though mechanical adaptation (e.g. drilling new holes) may be required.
Resistors
The original high-power resistors are generally reliable, with the exception of R5 (mechanically fragile). Voltage rating is often the limiting factor; standard 0.5 W resistors are typically rated for ~350 V.
Recommended replacements:
- R1 (15 kΩ) → 0.25 W, ≥400 V
- R2 (220 kΩ) → 1 W, ≥400 V (2 W preferred)
- R4 (1 kΩ) → 0.5 W, ≥400 V
-
R5 (3 kΩ) → ≥2 W, ≥350 V
- The original uses a 10 W resistor, which appears excessive.
- Estimated operating conditions (~16 mA, ~50 V) imply ≈0.8 W dissipation.
- A 5 W replacement provides adequate margin.
- R6 (2.2 MΩ) → 0.25 W, ≥500 V (750 V preferred)
Alternatively, R6 can be implemented as two series resistors to increase voltage rating:
- 2 × 1.1 MΩ, 0.5 W → ~700 V rating
- 1 MΩ + 1.2 MΩ is also acceptable
A look inside
- Burn marks are present in the area around R1, C92, R4, D1, D2, and R2.
- R1a is missing (possibly not populated in all units).
- Several additional holes have been drilled in this region.
Component condition:
- C92 (0.22 µF RIFA): Cracked and heat-damaged. Measured at ~300 nF with ESR ≈ 12 Ω, indicating significant degradation. Replacement is required.
- R5 (3 kΩ power resistor): Cracked at the base with intermittent connection. The original 10 W rating appears excessive; estimated dissipation is ~0.8 W under normal operation (≈16 mA, ≈50 V). Replaced with a 5 W resistor for adequate margin.
- R4 (1 kΩ): Severely damaged; measured value is 82 Ω. Replacement required.
- R2 (220 kΩ): Measured at 260 kΩ. This is outside typical tolerance unless a 20% tolerance is assumed.
- R1 (15 kΩ): Measured at 16.7 kΩ. Deviation affects maximum output voltage, as R1/R1a define the upper adjustment range. Final value should be verified against output calibration.
- The 16 µF electrolytics measure within capacitance tolerance, with ESR in the range of 2–4 Ω, which is acceptable for components of this age.
- However, the voltage rating of C82 and C83 remains insufficient, as discussed previously.
Diodes:
- D1 and D2 have been replaced with 1N4007 devices.
Broken R5 resistor.
The issue with the PCB edge connector
The edge connector shows visible burn marks between pin 6 and pin 7. These correspond to the incoming 220 V AC (pin 6) and the unfiltered positive DC rail (pin 7), approximately 320 V DC under nominal conditions, or up to 253 V AC and 358 V DC under worst-case mains conditions.
The clearance between the PCB pads in this area is less than 0.6 mm, which is clearly insufficient by modern standards. For reference, IPC-2221B specifies approximately:
- 2.5 mm clearance for uncoated conductors up to 300 V
- 4.0 mm clearance for 300–500 V
Achieving such clearances with the original connector and PCB layout is not feasible without redesign.
The connector uses a double row of contacts, where each contact forms a U-shaped structure, electrically connecting both sides. The original PCB is single-sided and utilizes only one side of the connector.
A practical improvement is to redesign the PCB as a double-sided board, alternating pads between the top and bottom layers. This effectively doubles the creepage distance, resulting in more than 5 mm separation between adjacent pads.
With narrower pads (e.g. 2.4 mm width), the geometry can be further improved:
- Creepage distance to the opposite-side pad is approximately 2.3 mm along the PCB edge (for 1.6 mm board thickness)
- Distance from a pad to the adjacent contact increases to approximately 1.6 mm
Additional creepage margin can be achieved by moving the pads slightly away from the PCB edge.
Using narrower pads imposes tighter mechanical tolerances. The PCB must fit accurately within the connector, with minimal lateral play. This is not a concern with modern manufacturing capabilities.
Typical edge connector clearance in common PCB edge slots is approximately 0.15 mm per side. Given a connector slot width of 63.6 mm, a PCB edge width of 63.3 mm provides an appropriate fit.
Conclusion
Operating a vintage power supply of this type on modern mains voltages presents several challenges. In addition to age-related component degradation, suitable replacements are not always readily available, and modern equivalents may differ significantly in electrical characteristics.
Higher present-day mains voltages also result in increased secondary voltages, pushing components closer to—or beyond—their original ratings. In some cases, this necessitates not only component replacement but partial redesign to ensure safe and reliable operation.
A follow-up article may be published to evaluate and validate potential improvements.
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