FIXED - Yamaha RX-v475 won't power on

Some progress. I found corrosion around R352 (100x4 near CB21 on Digital Board P104), looks like maybe liquid damage. I also found no continuity between that CB21-KEY1 and pin 125 on the MPU although R352/2 measured ok at 100 ohms. Looking at the PCB layout, R352/2 goes to a via hole between R352 and R353, and the corresponding trace on the underside of the board does connect with pin 125 on the MPU. The via hole was blocked with corrosion or something hard. I carefully drilled through with a 0.1mm bit but found no connectivity to the top of the vi hole. so I used a piece of fine enameled wire through the via hole and carefully soldered it to the trace on the underside of the board, then put a 100 ohm resistor between this and CB21-KEY1.

Now when I apply power, the standby LED comes on and if I quickly press Info + Tone Control + Power for 3 seconds (protection defeat) it actually powers up but with no front display. If I don't press the buttons quickly enough, the standby LED still goes out after around 18 seconds after which its unresponsive again.

When powered up in this manner, I can play a source through headphones (not connecting speakers yet!) and can see the volume control OSD on the TV via HDMI out. And the remote works at least for changing the volume. So it does appear to be working at least partially.

Next steps will be to check connectivity between each CB21 pin and its destination, looking for open or short circuits, and examining the digital board under a microscope to look for any more corrosion.

I'm more hopeful than I was, but would still greatly appreciate any advice or guidance.

Partial success, and have gone about as far as I can go with this one. After doing all of the above and reflashing the firmware from USB, all functions are now working except the standby LED and front display. Standby LED no longer lights at all, and without a front display I can't tell whether it's on or off. So I put a 100 ohm resistor between +3.3DSP and STBY_LED so at least it lights when the receiver is on now. Sounds good too and the on screen display via HDMI is useful without a front display!

As for the front display, filaments are OK (can see them glow faintly in the dark) but no display at all. Anyone know how to test a VFD in situ?

Thanks mastertech- apologies for the length of this post, I'm not sure I'm understanding this or VFDs correctly, so I'll lay out what I'm thinking.

Looking at the schematics on P108 and 109 (Operation 1/2 & 2/2), F1 and F2 (VFD filaments) connect across a low voltage secondary winding of the main transformer, and this AC voltage is what makes the filaments 'glow' (e.g. like the heater in a vacuum tube).

-VP of -39VDC is limited by two end-to-end 4.3v zeners  (D4006/4005) to -35V at their junction, and -30.9VDC at the cathode end of D4005. This is then connected to each of F1/F2 via 120 ohm (presumably current-limiting) resistors, which gives the filaments a negative DC bias of around -30VDC.

The negatively biased filaments (VFD cathode) emit electrons, which are either repelled by the VFD grid (depending on voltage) or allowed to collide with specific phosphor-coated anode segments (depending on their voltage), causing those segments to glow. In order for a segment to glow, both the grid and anode would presumably need to have a positive voltage with respect to the filament bias.

I think that's the theory, please correct me if I've misunderstood.

In practice, F1 to F2 measures around 6VAC which appears to be OK, the filaments can be seen to be glowing very faintly in a darkened room. The following DC measurements are taken with respect to DGND, with the expected figure in brackets:

 -VP = -38.88V (39V)
Junction of D4006/4005 = -36.96V (-35V)
Cathode of D4005 = -35.11V (-30.9V)
F1 = -35.11V (-30.2V)
F2 = -35.11V (-30.3V)

The low readings make me wonder if the two zeners are doing what they're supposed to. I don't have any replacements here to swap out though.

All of the grid and anode connectors of the VFD read between -38.5V and -38.7V, lower than the filament bias voltage but suspiciously close to -VP. Presumably this is why the display remains dark. I don't think correcting the filament bias would help.

So then my question is whether the problem is with the driver chip IC401 or with its inputs which come from P104 (Digital 2/5):  FLD_MOSI (IC26 pin 3), FLD_SCK (IC26 pin 6), FLD_N_CS (MPU pin 76) or FLD_N_RST (MPU pin 77). These all read 3.3V and I've checked their connections back to the MPU pins.

Not sure where to go from here.

R4042 and R4043 each read correctly at 120 ohms when one leg lifted. I temporarily replaced the 2 diodes with a single 30V zener - the voltages at cathode and F1/F2 remained unchanged and the display didn't light, so I put the original zeners back in. Looks like the problem is elsewhere.

Out of interest I put the scope on FLD_MOSI, FLD_SCK, FLD_N_CS and FLD_N_RST, and on pins 4 and 5 of the MPU (EX_MOSI - 'FL driver / EEPROM / Expansion Flash synchronization data input' and
EX_SCK - 'FL driver / EEPROM / Expansion Flash synchronous clock output'. They all remain constant at 3.3V, no oscillation. Would I expect to see something different, like a clock signal?

Yes, both pins read correctly. Pin 18 and Pin 8 (VDD) read 3.3V and have continuity to 3.3DSP (pin 6 on connector CB402). Pin 64 reads -39V and has continuity to -VP (pin 2 on connector CB403). I've also checked the connections between pins 1-4 and their inputs on CB402 via their resistors; pin 5 to DGND, and pins 6& 7 to each other via R4028.

All of the other pins on IC401 that connect to the VFD or Q4001-4004 read the same, around 200mV higher than -VP (i.e -38.8V).

I put the scope on FLD_MOSI on CB402 (which I think is the serial input to IC401), triggered on single shot, downward slope. When I press a button or move the volume control (which would change something on the display) I get a trace like the attached jpg, which looks like a signal is being sent to IC401. I'm beginning to think that maybe either IC401 is fried, or the VFD is dead.

 

 I have a few minutes so I will write up a short explanation on how these VFD's work and maybe it will help you to troubleshoot just where the problem may be.

The VFD works very similar to an old CRT. The cathode or filiment heats up and produces electrons by way of their chemical makeup. Now at this point the electrons do not do anything, just hover around the filament. In order to move they have to be magnetically controlled. A variation in voltages will create this affect. In this case the filiment is given a negative charge of app. -30 volts. A positive charge will attract the electrons to another location. This is where the grids come into play. This display contains 18 grids layed out over the entire display and each grid connected to a pin on the driver ic (G pins). When 1 or more of these individual grids is sent a postive voltage, in this case 3.3v the electrons will be attrached to it. this is the frist step in this process. So now that the electrons are at a grid what do they do. Nothing yet.


The next step is getting these electron to a particular point of a segment, each point being covered with a Phosphorescent coating which will emit light when hit by an electron. In order to do this each of the bottom segments of this particular dislay consists of 35 points with each point being connected to a pin on the driver ic (P pins). So when one or more of these points is sent a voltage, again in this case 3.3v, that part of the segment will attract electrons and when they come in contact with the coating it will emit light. Depending on which of the 35 points of a segment are charged to attract electrons, that will determine what number or letter will show on the display.


So with this in mind, if the filament is charged and forming electrons and you were to manually charge a grid (G pin) and segment point (P pin) that point should light up if the VFD is working properly. However, as you indicated these pins are currently all reading @ -39v which would be correct since this voltage would repel electrons and keep the display dark from any stray electrons. This also means these pins are being pulled negative and if you were to apply 3.3v positive it may just pull the voltage negative and not do anything. It may even damage the rail regulator if a buffer resistor was not used. You would probably have to remove the driver ic to do the test.


Also, if you look in the service manual on pages 60 and 61 it shows much info on the VFD. And on page 61, the chart shows what will emit light if you charge a particular grid pin and segment pin. For example, if you send 3.3v to pin G16 and P31 then the "CINEMADSP" will light up.

So the function of the VFD and ic401 driver chip is really very simple. Most of the pins are just for sending voltage to the VFD and a couple control lines to the MCU. I will attach a link to ic401 datasheet. It contains some info on these controls lines that may be helpful. If anything I wrote is not clear to you just ask. Happy hunting.


www.tubeclockdb.com/media/kunena/attachments/7782/M66003_xxxxfp_brief.pdf

Thanks for taking the time to provide such a clear and succinct explanation, much appreciated. To test the VFD, rather than trying to remove either the driver IC or the VFD, I carefully snipped pins G1 and P1 on the VFD in such a way that I can solder them back again, and applied 3.3V to each of them via jumper cables. Segment 1-1 lit nice and brightly, which shows the VFD is working. So I guess the problem is either with the driver chip, or the signals it gets from the MCU telling it what to do.

The driver chip is fairly expensive, so I'm reluctant to replace it unless I'm certain it's the problem. I'll try using my cheap LOTO oscilloscope logic anaylzer on the control lines (if I can work out how to use it!) and see what I get.

If all else fails, I guess I can live with no display as most of the important info can be found on the OSD.