Rotel 855 Display & Keypad - Non-Functional

dtm1962 · 6th April 2010, 04:09 AM

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 · 6th April 2010, 07:18 AM

First guess would be cracked print

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

ide2003 · 04:06 PM

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)

[IMG][IMG=http://img530.imageshack.us/img530/1899/displayproblem.th.jpg][/IMG][/IMG]

dtm1962 · 7th April 2010, 04:20 PM

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 · 7th April 2010, 04:27 PM

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 · 8th April 2010, 03:06 PM

99.999% agreed Mooly

dtm1962 · 9th April 2010, 01:20 AM

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 · 9th April 2010, 06:55 AM

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 · 29th May 2010, 06:27 AM

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

6th April 2010, 04:09 AM	#1
dtm1962 diyAudio Member Join Date: Apr 2003 Location: Sarnia, Ontario, Canada	Rotel 855 Display & Keypad - Non-Functional 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, __________________ Dan

6th April 2010, 07:18 AM	#2
Mooly diyAudio Member Join Date: Sep 2007	First guess would be cracked print Has it worked OK before and just suddenly stopped, or has it a "history" ? Ebay ? __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*

6th April 2010, 04:04 PM	#3
ide2003 diyAudio Member Join Date: Oct 2008 Location: Jakarta	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) [IMG][IMG=http://img530.imageshack.us/img530/1899/displayproblem.th.jpg][/IMG][/IMG] Last edited by ide2003; 6th April 2010 at 04:06 PM.

7th April 2010, 04:20 PM	#4
dtm1962 diyAudio Member Join Date: Apr 2003 Location: Sarnia, Ontario, Canada	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. __________________ Dan

7th April 2010, 04:27 PM	#5
Mooly diyAudio Member Join Date: Sep 2007	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 __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*


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8th April 2010, 03:06 PM	#6
ide2003 diyAudio Member Join Date: Oct 2008 Location: Jakarta	99.999% agreed Mooly

9th April 2010, 01:20 AM	#7
dtm1962 diyAudio Member Join Date: Apr 2003 Location: Sarnia, Ontario, Canada	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, __________________ Dan

9th April 2010, 06:55 AM	#8
Mooly diyAudio Member Join Date: Sep 2007	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. __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*

29th May 2010, 06:27 AM	#9
pkseek diyAudio Member Join Date: May 2010	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

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