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NOTES ON THE YAESU FP-29 POWER SUPPLY
SAFETY NOTE: Troubleshooting this unit
should NEVER be attempted without an isolation transformer -
signal grounds exist that are 165VDC above AC line neutral.
Other voltages exist on exposed parts that exceed 300VDC.
EXTREME CAUTION should be used, and only by EXPERIENCED
troubleshooters.
I recently received an FP-29 for repair. The supply had no
output (s) - no blown fuses - no evidence of smoke - no visible
indication of failed components. Application of 120 VAC resulted
in a inrush of current, which settled down to less than 2 watts
of power consumption.
Cold checking with an ESR meter, a DC ohmmeter, and a Huntron
Tracker, revealed:
No bad electrolytic capacitors.
No shorts in Q2 or Q3.
Q4 shorted.
D6 shorted.
D7 open.
These components supply the startup source for the control
module HIC-1.
Since no expected voltages were supplied on the schematic, a bit
of reverse engineering had to be employed. The defective
components would charge C18 to some voltage that would cause the
control module to drive Q2 and Q3. A datasheet was obtained for
IC-1. This is a device that senses about 4.7 volts, and changes
state. R16 and R17
form a voltage divider to supply the input to that device. This
established that the voltage expected across C18 was about 13
VDC.
There is no datasheet available for the MB2013A control module.
This is a proprietary 12 pin, epoxy coated assembly, mounted
vertically on the board.
Since D7 was open - it was a simple matter to apply an external
supply across C18, with no AC supplied to the unit.
A scope revealed that at 17VDC, drive pulses appeared at the
gate of Q3 - reducing the voltage to 13 volts removed the drive
pulses. No drive appeared at the gate of Q2.
Hope still existed that HIC-1 may be OK - since Q2 an Q3 are in
a totem pole configuration, and with no schematic of HIC-1
available, it was possible that DC might be required at pin #1
of HIC-1 to cause drive to Q2.
The defective components in the start circuit were replaced.
An isolation transformer, Variac, and wattmeter were used to
bring the voltage up slowly while observing the results.
The voltage across C18 rose to about 11VDC at 50VAC input. As
the input voltage was increased, the voltage across C18 dropped
to about 9VDC.
No output was obtained from the supply, and no drive appeared on
Ether Q2 or Q3.
D7 was now removed - with an input of 120VAC, current limited by
a 40 watt lamp -- an external supply was fed to pin #12 on the
control module - and again drive was noted only to the gate of
Q3, and no output from the supply. The control module drew about
250MA and became very hot.
At this point an interesting note was made - the voltage at the
anode of D6 was about 160VDC - much higher than expected, since
the capacitor that it charges (C18) is rated at only 35VDC. This
will become very important as we look at this design. In
simplistic terms - D11 rectifies AC line voltage - Q4 limits the
current - C18 charges towards 15 volts - IC-1 lifts the short on
Pin#10 of the control module - the control module drives the
FET's - D9 provides the DC to sustain the control module - D10
causes Q5 to shut Q4 down - D12 causes Q1 to short the
soft-start resistors R2 and R3 -the secondary side regulators
take over, and normal operation ensues. Output voltage and
current information is fed back to the control module by a pair
of opto-isolators - PC1 and PC2. These control the control
module via pins # 8, 9, 11, and 12.
This explanation was derived from observing the supply after it
was repaired. Repair involved replacing the VERY expensive
MB2013A control module as well as the earlier mentioned
components. C18 was also replaced - even though it checked OK -
it may have been overstressed - Q5 was replaced for the same
reason.
A POSSIBLE EXPLANATION OF THIS FAILURE : The supply failed to
start - Q4 continued to raise the voltage across C18 - the
MB2013A module failed - that failure took out the other
components.
MODIFICATION TO REDUCE THE POSSIBILITY OF THIS RECURRING: An 18
volt, 1 watt Zener was installed from the Anode of D6, to the
negative lead of C18. The Zener diodes Anode towards the
negative lead of C18. In the event of any failure of the supply
to start, R20, R21, and the added Zener diode should limit the
voltage applied to sensitive components.
NOTES ON THE MB2013A CONTROL MODULE: This unit is a proprietary
device. Yaesu quoted well over $100 for a replacement. Research
developed the fact that it is used in a supply of a different
manufacturer. It was obtained for about 33% of the quoted Yaesu
price. Comparing the schematics of the two supplies, they are
identical up to the transformer, leading us to the conclusion
that they were supplied by a vendor. The unknown vendor might
possibly be a source of the MB2013A control module. With the
exception of this module, and the transformer, all other
components are readily
available. The original module was marked MB2013A-1 and the
replacement was marked MB2013A-2. Any difference in the later
version is unknown.
OTHER NOTES: It appears there are multiple versions of this
supply. Some are switchable 120 - 240, others are 120 input
only. Some have a thermostatic switch to shut the supply down by
shorting pins # 11 and 12 on the control chip, others do not
show that. Some supplies have a connector CN-2 some do not.
Several components appear in one version, and are deleted in
others. The 13VDC output is derived from the 30VDC by a separate
regulator - any failure of the main supply
causes loss of 13VDC as well.
Don KA1BXB
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