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Reverse engineering and repair: Power supply UTP3703S

What

The Uni-Trend UTP3703S is a regulated power supply for lab use. It features two adjustable voltage and current regulated 0..32V DC / 0..2A outputs, and one static 5V/2A output.


Outside case

Front side

Front side

Front panel

Front panel

Back side

Bottom side

Similar models

There is a rebranded Tenma power supply with the same internals.

The UTP3702 is an identical variant without the 5V section. It exists also as a rebrand, the Tenma 72-8690.

It is said the UTP one is a clone of a Mastech one.

Inside

The box can be opened easily by removal of five M3 screws; two pairs on each side and one at the front top middle.


Inside view

Inside view

Sections

The power supply has two identical adjustable-output sections, and one fixed-output 5V section.

Mains wiring

The power is provided through a C14 socket, and wired to the transformer through a front-panel mounted mechanical switch.


Main power switch

Main power switch

Mains wiring

Transformer

The power supply is built around a large heavy toroidal mains transformer, with multiple windings. The main winding has several taps. The windings for adjustable sections have multiple taps.


Transformer view

Transformer detail

Power transistors

Transformer outputs

Transformer outputs

Transformer wiring

Display and output setting board

The display board contains four panel meter modules; two voltage and two current, for the two output sections.

The voltage is adjusted with a multiturn potentiometer on the front panel. The current is adjusted with a regular one-turn potentiometer. A pair of LEDs indicates if the voltage or current limit is active. This is selected with a single wire; the signal on the wire lights up one LED and opens a transistor shorting the other LED.

The panel contains a switch, for selection between separate settings and matching voltage setting for both channels (for symmetrical power).

Each half of the panel is powered with a separate winding of the transformer, using a 7805 stabilizer for 5V power for the logic and displays.


Display board

Display board, switch

Display board, switch

Display board, displays

Display board, displays

Display board, displays

Display board, displays

Display board, displays

Display board, potentiometers

Display board, potentiometers

Display board, electronics

Display board, electronics

Outputs

The outputs are realized as binding posts for screw-in wires and 4mm banana plugs. The contacts are bridged with electrolytic capacitors and antiparallel diodes.


Outputs

Outputs

Outputs

Outputs

5V section

The 5V section is realized as a parallel pair of 7805 stabilizers, mounted on the bottom of the chassis for heatsinking. The output binding posts are bridged with a 470µF/16V capacitor.


5V stabilizer

5V stabilizer

Power board

The power board contains three sections.

A simple diode bridge and an electrolytical capacitor for the 5V section, connected via wires to the pair of 7805 stabilizers on the chassis bottom. It is fed from one transformer winding.

The two other sections are electrically identical. They are the regulated 0..32V, 0..2A output ones. Each section is fed with a tapped winding, where the tap is selected by relays; a common way for this class of power supplies. This limits the power loss on the regulation transistor. There is also an auxiliary winding for powering the board logic.

The adjustable sections' board layout is identical but mirrored.

Each section is connected to its half of the front panel, through a wiring harness. The front panel sections contain voltage and current indication displays. The current is sensed on a large series resistor on the power board.

The section contains a diode bridge and a large capacitor. It is fed from a selected transformer winding tap, and its output is wired to a power transistor.

The outputs are bridged with an antiparallel diode, and a 470µF/35V electrolytic capacitor.

The comparators and regulators on the board are built around a HA17324A chip, a quad op-amp, a functional equivalent of a LM324. Two of the comparators control two transistors that operate the relays for selection of the transformer winding tap. Third one is selecting between voltage and current regulation. The fourth one is operating a pair of NPN transistors in Darlington setting. The output transistor is a power one, the D1047 type.

The power transistors are mounted on a large heatsink on the back side of the chassis.

The electronics of the section is powered with its own transformer winding, through a diode bridge, capacitor, and a 7815 stabilizer. The unstabilized rail feeds the relays, the stabilized one powers the op-amp.

The comparators for the relays take a reference voltage from a pair of antiparallel Zener diodes and two series diodes.

The two series diodes are connected between the positive output and the board logic ground, raising the positive side about 1.4 volts (twice 0.7V) above the ground (the op-amp's negative power rail).

A large, low-value high-power resistor between the positive output and the power transistor emitter is used for current sensing. Signal from it is connected to the display panel, and to the current-comparator op-amp. The comparator's output (high for voltage mode, low for current mode) is connected to the display panel to select the indicator LED.


Power transistors

Power board, back side

Power board, back side

Power board, back side

Power board, back side

Power board, back side

Power board, back side, in chassis

Power board, top

Power board, top

Power board, top

Power board, top

Power board, top

Power board, top

Power board, top

Power board, top

Power board, top

Power board, top

Power board, top

Power board, top

Power board, top

Power board, top

Reverse-engineering and repair

The power supply was received in a damaged condition. The right channel was inoperative, stuck at 0.6 volts of output and indicating being in current-control mode. It was not reacting to any kind of input.

The parts were drawn with a marker to the back side of the power board, to aid with debugging an reverse engineering.

After some unsuccessful attempts to just guess, the section was drawn into a schematics. This, together with understanding of the circuit's functionality, greatly aided the debugging.

Further aid was having a functioning identical other circuit for comparisons.

It was found that the quad op-amp chip failed. The circuit behaved identically with the chip removed from the board.

The chip was replaced with a LM324 chip in a 14-pin DIP socket. The power supply was then working perfectly again.


Schematics, rough draft

Schematics, rough draft

Schematics, rough draft

Schematics, partly redrawn, nicer

LED modification

The front panel layout is confusing. The voltage/current meters are volt-amp-volt-amp, the controls and voltage/current LEDs are volt-amp-amp-volt. This leads to significant potential of operator errors.

It was decided to ease the operator's life at least a little by replacing the current-limit indicator LED from red to green. This way, at least, the voltage-limit mode (which is the usual regime in most cases) is indicated by a green light, while current-limit is shown with red color.

The replacement was straightforward. A flat-topped LED was chosen for the reasons of availability.


Disassembly

Front panel LEDs replaced

Replacement detail

Replacement detail

New front panel

After the modification, the device operation is much easier to sense at a glance, without lengthy process of getting used to the instrument's design peculiarities.

The drawback is the disappointingly low brightness of the green LED. This could be addressed sometime later.

Further modifications

The knobs for voltage and current could be painted green and red, to match the LEDs.

Alternatively, on the right side, the LEDs and the potentiometers could be swapped - a mechanical hack requiring cutting a few traces, adding a few patch wires, and swapping the potentiometers.

Todo


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