35AMP POWER SUPPLY REPAIR 35A
power supply repair. 24 January 2025.
A
recently purchased 35-amp power supply, lacking any manufacturing
markings, appears to be commercially constructed and
resembles the Motorola 10Amp HPN1007A unit in both design and consistency with
the schematic, however this power supply has six 2N3055 power
transistors and there are variations to the regulation board.
The
power supply functioned well for approximately a year before the
mains fuse operated, accompanied by a brief growling hum from the
transformer under load. The fuse appeared to have blown due to a
very high fault current, which completely vaporized the element
inside the glass fuse—a clear indication of a short circuit within
the power supply.
Photo
1. Top front view of the power supply.

Photo
2. Rear view of the power supply showing the six 2N3055
power transistors
.

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|
Photo
3. Transformer spec plate. |
Photo
4. Blown 240 VAC 8A fuse. |

Photo
5. Power supply regulation board (shown within the broken
line in the circuit schematic)
.

Figure
1. Circuit schematic of
a Motorola HPN1007A power supply.
To isolate the fault and verify that the transformer was not faulty, the secondary connections to the transformer were disconnected, thereby isolating the AC circuits, including the transformer, from the DC circuits. The mains supply was then connected, resulting in the fuse remaining intact. A healthy 37 VAC was measured across the transformer’s secondary windings, confirming that the transformer was operating normally and that the fault was within the DC section of the power supply.
The two rectifier diodes D1 and D2 were tested, revealing that D2 was short-circuited, creating a hard half-cycle short with
D1.
Both diodes are to be replaced with VS-70HF10 diodes, rated at 100V and 70A, as the existing diodes appear to be
automotive button diodes of indeterminate specifications.

Photo
6. Original diode attachment to the heat-sink.

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|
Photo
7. Button diode mounting |
Photo
8. Button diode location on the heat-sink. |
The
VS-70HF10 is a standard recovery rectifier diode manufactured by
Vishay. It is designed for high-current applications and will be
more than adequate for the job based of the following
specifications:
-
Maximum
Repetitive Reverse Voltage: 100 V
-
Average
Forward Current: 70 A
-
Forward
Voltage: 1.35 V at 220 A
-
Maximum
Surge Current: 1.25 kA
-
Package
Type: DO-203AB (DO-5), stud mount
-
Operating
Junction Temperature Range: -65°C to 180°C
The
VS-70HF10 rectifier diodes come in several variants, with the device
code and specifications highlighted in blue, as per the one used in
this repair. An important variant is that the device can be supplied
with reverse polarization, and as noted in reference 4 in the table,
this particular device has the cathode connected to the stud.

1 |
Vishay
Semiconductors product |
2 |
70
= standard device
71 = not isolated
lead
72 = isolated lead
with silicone sleeve
(red = reverse
polarity)
(blue = normal polarity) |
3 |
HF =
standard diode |
4 |
None
= stud normal polarity (cathode to stud)
R = stud reverse polarity
(anode to stud) |
5 |
Voltage
code x 10 = VRRM (see Voltage Ratings table in spec sheet) 10
= 100V |
6 |
None
= stud base DO-5 (DO-203AB) 1/4" 28UNF-2A
M = stud base DO-5 (DO-203AB)
M6 x 1 |

|

|
Figure
2.
VS-70HF10 rectifier diode dimensions. |
Photo
8.
VS-70HF10 rectifier diode |
The
button diode arrangement was easily replaced with the new VS-70HF10
rectifier diode by drilling out the existing heat sink holes to 6mm
and mounting the new rectifier diodes. In this arrangement, the heat
sink is at a positive DC voltage and is mounted with insulating
feet. See photo 9.

Photo
9. New VS-70HF10 diodes fitted to the heat-sink.

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|
Photo
10. VS-70HF10 diodes connected to the
transformer secondary winding. |
Photo
11. Power supply output voltage under load. |
With
the new rectifier diodes installed, the power supply was switched
on, and the output voltage was measured with about 10A of load,
showing 13.66VDC at the output terminals.
VS-70HF10
diode data sheet: https://www.vishay.com/docs/93521/vs-70hfrseries.pdf
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Page initiated 24
January, 2025
Page
last revised 1 February, 2025
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