Rotel 855 Display & Keypad - Non-Functional

[dtm1962]

Rotel 855 Player functions OK but Display (no illumination) & Keypad (no response to contact) are non-functional.

Anything common between the 2 systems that would cause these malfunctions?

Remote does work!

Any help would be appreciated.

Thanks in advance,

 

[Mooly]

First guess would be cracked print 🙂

Has it worked OK before and just suddenly stopped, or has it a "history" ? Ebay ? 😉

 

[ide2003]

hi there Dan, funny that I also having that problem with friend's marantz SA8400, everything works but no display, took me by surprise during tweaking of this thing.

Turns out one of the 27V zener and one transistor for the display supply (approx -26V) just needs replacement. They are located near one of regulator in my case. Schematic helps a lot..such as this one (sorry its in Indonesian)

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[dtm1962]

Voltage Measurements - Narrowed it down to Display/Keyboard PCB

Checked all tthe voltages going to the Display/keyboard circuit board and the PS voltages are correct. The voltages on this board are all wrong. I have a couple transistors/1n4148's/resistors/capacitors & the controller chip (40 Pin IC).

I will post the schematic with the actual versus published voltages for you to have a look. I am thinking the controller chip IC101 (TMS3763CNL) is at fault.

 

[Mooly]

It's never the "chip"... or lets say 99.999% of the time it isn't.
Don't guess... go around every pin with a 'scope and meter and make sure every input/output/supply/clock/reset line etc is correct. Then and only then even think about replacing the IC 🙂

 

[ide2003]

99.999% agreed Mooly 🙂

 

[dtm1962]

Review my findings & let me know if I solved the issue with Display & Keyboard

It looks like a 15V Zener on the ground leg of the 78M12 12V regulator has shorted. This normally would lower the ground leg to -15V and the 12V regulator would act like a 27V regulator. I didn't notice this until I reviewed the schematic as the 12V regulator was giving a 12V output so I was fooled until I seen this. Zener is reading 0V on anode (tied to ground) and the cathode is reading only 0.4V versus -15V.

Does this make sense?

Let me know.

Thanks,

 

[Mooly]

27 volts... is that a negative 27 volts for the display ? Would have to see the circuit.

I would suggest checking or better replacing any electroylitics that feed the regulator.
Replace the reg and the zener and try again.

It's sounds a typical fault tbh... usually faults like this are caused by the caps drying out and allowing high levels of AC.

 

[pkseek]

Where is the location of the zener diode as I have a similar non functioning control panel as well for my rotel 855? I used the remote to operate it for 5 years. Anyone with a service manual?
Thanks