Repairing the Oltronix D400-007D - part 2

In the previous article, I covered the functionality and issues of the D400-007D 400V DC linear power supply by Oltronix. 

In this second part, I'll describe the repairs and improvements carried out and also provide a link to the complete KiCad PCB project.

Main high-voltage board 

On the main high voltage board, the two main filter capacitors (C80, C81) were replaced with two 500 V, 100 µF capacitors. Although the original capacitors were still within specification, they were replaced to ensure reliable operation under modern mains conditions. Equivalent axial capacitors are expensive, so two more cost-effective radial capacitors were used (high-quality 105 °C types). 

Two mounting holes were drilled in the PCB, and the capacitors were secured using cable ties. Jumper wires were then used to connect them to the original capacitor pads. The increased capacitance is not an issue due to the high resistance of the transformer secondary. It does not introduce significant inrush current, and it helps reduce output ripple.

The two trimmers used for the voltage and current calibration were replaced, as was the bridge rectifier.

The remaining resistors on the board were measured and found to be within tolerance, so no replacements were necessary.

The area around the wiring requires thorough cleaning. During initial startup, intermittent sparking was observed near the wires at the edge of the board. This was traced to contamination from dust and residue. The spacing between wires on the PCB edge is not sufficient by modern clearance standards, and with over 600 V DC, flashover can easily occur between wire ferrules on the board. No structural modification was made beyond thorough cleaning.

  

 

Regulator board redesign 

As discussed in the previous article, I ultimately chose to replace the regulator board (error amplifier) entirely to resolve several design and reliability concerns.

Schematic 

The circuit was redrawn in KiCad.

 

Several minor design changes were made compared to the original circuit: 

  • R82 was split into two 9 kΩ resistors, as no suitable 18 kΩ component was available. 
  • A 2N4037 was selected as the main grid driver transistor (T2). This device appears to have been used in later revisions of the supply.
  • BC556A and BC546A were selected for T1 and T3. These provide adequate voltage ratings and suitable gain characteristics as substitutes for the original transistors.
  • An additional protection diode (D3) was added across the base-emitter junction of T2 to prevent excessive V_BE from damaging the transistor under fault conditions. This compensates for the originally used 2S302 transistor, which reportedly had an unusually high V_BE tolerance (up to 20 V).
  • The capacitance of C82 and C83 was increased to 33 µF. This was done simply because space was available on the PCB. The transformer’s winding resistance limits inrush current, and the increased capacitance reduces ripple on the auxiliary supply.
  • Output capacitors C90 and C91 were selected as 22 µF. Higher values are not recommended on the output stage due to stability considerations.
  • C92 should remain unchanged to preserve loop stability, although a high-voltage polypropylene capacitor is recommended.

BOM 

The bill of materials is listed below. Note that several resistors must withstand relatively high voltage and power dissipation, so standard low-cost parts are often unsuitable. Components not explicitly listed can generally be standard types (e.g., 1N4007 and 1N4148 diodes).

C80, C81
KEMET 100 µF, 500 V
Mfr. Part No. ELD107M500AQ5AA
Mouser Part No. 80-ELD107M500AQ5AA

C82, C83
Panasonic 33 µF, 500 V DC
Mfr. Part No. 500LXW33MEFR16X25
Mouser Part No. 232-500LXW33MEFR16X2

C90, C91
Rubycon 22 µF, 250 V
Mfr. Part No. 250BXC22MEFC10X16
Mouser Part No. 232-250BXC22MEFC10X1 

C92
Kemet 220 nF, 630V DC
Mfr. Part No. R71PI32204030K
Mouser Part No. 80-R71PI32204030K

R1
Yageo 15 kΩ, 1%, 1 W, 400 V
Mfr. Part No. MFR1WSFTF52-15K
Mouser Part No. 603-MFR1WSFTF52-15K

R1a
Yageo 200 kΩ, 1%, 1 W, 400 V
Mfr. Part No. MFR1WSFTF52-220K
Mouser Part No. 603-MFR1WSFTF52-220K

R2
Yageo 220 kΩ, 5%, 2 W, 500 V
Mfr. Part No. FMP200JT-52-220K
Mouser Part No. 603-FMP200JT-52-220K

R5
Ohmite 3 kΩ, 5%, 5 W, 460 V
Mfr. Part No. 45J3K0E
Mouser Part No. 588-45J3K0E

R4
Vishay 1 kΩ, 1%, 1 W, 500 V
Mfr. Part No. MBE04140C1001FC100
Mouser Part No. 594-E0414C1K000F1A

R6
Yageo 2.2 MΩ, 1%, 0.5 W, 3.5 kV
Mfr. Part No. HHV-50FR-52-2M2
Mouser Part No. 603-HHV-50FR-52-2M2

R82, R83
Ohmite 9 kΩ, 5%, 5 W, 330 V
Mfr. Part No. 25J9K0E
Mouser Part No. 588-25J9K0E

R92, R93
Yageo 100 kΩ, 1%, 2 W, 500 V
Mfr. Part No. FMF200FTE52-100K
Mouser Part No. 603-FMF200FTE52-100K

P1
Amphenol 1 kΩ trimmer, 10 mm
Mfr. Part No. PT10LV10-102A2020-S
Mouser Part No. 531-PT10V-1K-S

D80, D81
W10G Bridge rectifier 1.5 A, 1000 V
Mfr. Part No. W10G-E4/51
Mouser Part No. 625-W10G-E4

Z1, Z2
Onsemi 12V 1W Zener
Mfr. Part No. BZX85C12
Mouser Part No. 512-BZX85C12

T1
BC556A
Mfr. Part No. BC556A 
Mouser Part No. 637-BC556A

T2
2N4036
Mfr. Part No. 2N4036 PBFREE 
Mouser Part No. 610-2N4036

T3
BC546A
Mfr. Part No. BC546A 
Mouser Part No. 637-BC546A 

PCB

As discussed in the previous article, the weakest part of the original design is the edge connector area. One example is where the incoming 220 V AC (pin 6) sits directly next to the unfiltered high-voltage DC rail (pin 7).

On the original PCB, the clearance between these pads is less than 0.6 mm. This is far below modern expectations. For reference, IPC-2221B suggests roughly 2.5 mm clearance for up to 300 V, and around 4 mm for 300–500 V systems. In practice, the original layout leaves very little margin, especially in the presence of dust and aging.

The connector itself is not the main limitation. Internally it has a pin cavity width of about 2.45 mm and reasonable spacing. The issue is that the original single-sided PCB layout does not make good use of this geometry, resulting in insufficient creepage along the board edge.

The redesign addresses this by using a double-sided PCB, alternating connector pads between top and bottom layers. This effectively increases creepage distance to over 5 mm and significantly improves high-voltage robustness without changing the connector system.

With slightly narrower pads (~2.4 mm), the layout also improves mechanically and electrically. The creepage along the PCB edge becomes about 2.3 mm (for a 1.6 mm board), and spacing between adjacent contacts increases to around 1.6 mm inside the connector.

The PCB must be manufactured accurately to fit the connector slot, but this is not a practical issue with modern fabrication. Typical edge clearance is around 0.15 mm per side, so a board width of 63.3 mm for a 63.6 mm slot remains a good fit.

Overall, this change significantly improves safety margins while keeping full compatibility with the original mechanical design.

Some resistors operate at elevated temperatures (R5, R82, R83), and were therefore positioned to allow unobstructed vertical airflow.

As noted previously, R1a (or Ra1 in KiCad notation) may need adjustment to set the maximum output voltage. In my implementation, this was achieved using two resistors in series.

 

 

The board was conformally coated to further improve clearance and creepage.

 

Finally, P1 must be adjusted as described in the previous article.

The complete KiCad project is available on GitHub if you would like to build your own PCB:

https://github.com/hevanaa/Oltronix_D400 

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