Yamaha RX-A730 VFD power supply issue

[CityZen]

I've got a Yamaha RX-A730 receiver that has a problem with the VFD's power supply. Specifically, the transistors that provide the switching to convert DC to AC are overheating.

When I got this receiver, the VFD was not working at all. I noticed the burned out transistors, so I replaced those, but that didn't fix it. I found a replacement VFD display and swapped that out, and it looked like things were working fine. However, the transistors I replaced overheated again.

The schematic for the circuit is shown below. If you need more details, you can google for the service manual (same as the one for RX-V675).

What would you suggest I check out next?

[CityZen]

Re: Yamaha RX-A730 VFD power supply issue

Q3012 and Q3013 are the transistors that burned out.

[R_J]

Re: Yamaha RX-A730 VFD power supply issue

Replace the caps C3066, C3067 & C3054

[redwire]

Re: Yamaha RX-A730 VFD power supply issue

I would check the switching frequency is around 100kHz and if the previous transistors shorted, it might have cooked L3001 100uH or weakend the switch Q3011. There should be around 5.6V coming in.

Otherwise, the 2SC5964's might not be so great, having low hFE. You can't get rid of much heat in an SOT-89 package. Zetex parts have lower switching losses but even 1W is going to heat everything up. Ensure C3064 is 0.47uF