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FT-736R PSU repair | Radioaficion Ham Radio

FT-736R PSU repair


Yaesu FT-736R PSU repair

The built in AC/DC power supply on the FT-736R commonly fails after about 10 years of normal use. The reason is usually faulty electrolytic capacitors (dried out) or bad solder joints on the switch mode power supply board FP-1274A.

The first symptoms of the failing internal power supply are that it fails to start properly and only a few LEDs on front panel are dimly lit. If the power supply doesn’t fail outright, the problem ultimately leads to a rig that will not power up at all with the internal power supply. The most likely sources of this problem are C9 (a 220 µF/35 V capacitor with a defective ESR of 450 ohms or more!), and C12 1 µF/50 V (with a defective ESR of 60 ohms or more). There are likely other capacitors that are suffering from similar aging problems.

Step #1 - Remove the PSU from the main frame and disassemble the unit.

  • a. unplug the AC cord
  • b. remove the radio's bottom cover
  • c. remove the 3 long M3 screws from the back securing the PSU heat sink bridge plate to rear chassis plate
  • d. remove the 2 screws from the bottom side of the base (in the middle, holding the PSU base plate on the chassis)
  • e. lift out the PSU unit
  • f. remove the PSU cage screws and the 4 PCB securing screws holding the base plate.

Step #2 – Visual inspection of the PSU board

The board is usually discolored between the two 470 µF/200 V electrolytic capacitors and the transformer. The reason for this (and the short life of the capacitors) is two very hot 33 ohm, 2 W resistors (R17 and R18). See figure 1.

FT-736R PSU repair

Fig 1. - Check C8, C9, C12 (C12 is removed here) and C22. They are likely out of spec.

Step #3 – Repair & replacement

a. I suggest you replace at least C8, C9, C12 or preferably all electrolytic capacitors on the PSU PCB with +105°C versions.

b. Replace R17 and R18 (optional) with a pair of 5 W, 33 ohm resistors (DigiKey p/n 45F33RE-ND). Install the new resistors on end, and do not trim the long lead…instead coil it so that it can dissipate some of the heat from the resistor (wrap it around a drill bit to coil it).

c. Re-solder (with fresh solder and flux) all the cracked or “cold” solder connections on the PCB (especially around the transformer).

d. Remove solder splashes (and possible solder bridges) with brush carefully.

e. Reinstall the base plate, cage and secure the unit in place in the chassis and temporarily connect the AC cable and see if the unit now works. If it does, unplug AC cord and now reassemble the rest.

FT-736R PSU repair

Fig 2. Thoroughly inspect the solder side of the board for cracked solder joints - the next picture shows several bad joints.

FT-736R PSU repair

Fig 3. Close photo showing cracked solder connections

DigiKey carries low ESR replacement capacitors (Nichicon FC Series):

Ref. Existing value recommended alternative ELFA-number /Europe DigiKey-p/n /North America
C6, C7 470 µF/200 V 470 µF/250 V 67-046-71 493-2763-ND
C8 56 µF/50 V Existing value 67-187-20 P10322-ND
C9 220 µF/16 V Existing value 67-150-56 P10297-ND
C12,C25 1µF/50 V Existing value 67-211-04 P10312-ND
C21,C22 1000 µF/25 V 2700 µF/25 V 67-229-20 P10286-ND
C23 0.47 µF/50 V 0.47 µF/50 V 67-124-91 P13466-ND
C26 680 µF/35 V 1000 µF/35 V 67-229-87 P10305-ND
R17,18* 33 ohm, 2 W 33 ohm, 5 W, 1% 60-463-12 45F33RE-ND
Fan* n/a 6.8CFM (w/o zener) 54-125-56 CR015-ND
Zener* n/a BZX85/C5V1 70-061-74 1N4733AFSCT-ND

Table 1. Replacement Component list (* = optional items)

Step #4 – Optional Cooling Fan Installation

You can also install a 12V fan 40x40x10 mm inside the perforated box to reduce the component temperature and improve their lifetime.

You may install the fan in the middle of the PCB long side on the perforated cover - see pics. Test for sufficient room between the fan and inside PCB components.

FT-736R PSU repair

Fig 4. Modified cover with two oval shaped holes for the fan.
These holes were enlarged up to 3.8 mm before filing them into an oval shape.

FT-736R PSU repair

Fig 5. Fan with 2 x 3.5 mm screws

FT-736R PSU repair

Fig 6. Fan at site, wires are cut 12 cm from fan

hamradio FT-736R PSU repair

Fig 7. A 5V1/1.3W zener diode plus a piece of shrink tube are added to the minus wire
(the fan speed is reduced to improve lifetime and reduce noise)

yaesu mods FT-736R PSU repair

Fig 8. The wire and diode is ready to insert the PCB-holes and solder

yaesu mods FT-736R PSU repair

Fig 9. Notice where and how the wire and diode are inserted into the PCB

yaesu mods FT-736R PSU repair

Fig 10. Showing the fan through the perforation

yaesu mods FT-736R PSU repair

Fig 11. Completed power supply ready to install into FT-736R.

Be sure to roll and shake the unit to check for any loose parts inside. Remove any objects that do not belong there.

If you have need to exchange TR4 (2SC3536) PA0FRI recommend BUV48A as equivalent.

Updated for DigiKey (North American) sources & data
I gratefully acknowledge AE6HC and N1MIW for their update of my repair documentation
and DL7VHF for his schematic diagrams.

DL7VHF's FT-736R PSU schematic

DL7VHF's FT-736R modified PSU schematic

PAØFRI's FT-736R new PSU schematic with comments

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