Rotel RCD-975 display panel and controls not working

[newdiyer]

I first asked about my problem on this thread Rotel RCD 971 Readout panel doesn't work
but as suggested by a member, I start a different thread here.

So my problem: I recently replaced the tray gear on my rcd-975. I must have messed with something, as the display panel does not work anymore, and push buttons do nothing, making my CDP useless.

As suggested by Mooly, I checked continuity of all pins in the connector between the main board and the display/control board. All are fine. I did my tests directly on the PCBs, not the connector, to make sure the problem is not a wire not properly connecting to the PCB.

Then I removed the display board. No capacitor seems to be leaking (haven't tested them though), resistor seem to have proper values and all diodes test ok (I haven't disconnected them, nor any component).

There is an IC chip underneath the display. Haven't tried to test anything regarding it, as I don't really know what to test

I attached the schematic of the display board. I have the complete service manual (got it from Rotel), but it's too large to upload. Let me know what else can be helpful and I could upload additional pictures or PM the complete PDF. I also attached a picture of the whole open unit, and one of the display/control PCB.

Any help to isolate and fix (if possible) the problem would be greatly appreciated.

[stephensank]

Considering when the fault happened, I'd be looking for a blown fuse device, i.e., fusible resistor, "IC protector"(ICP-Nxx). There is a 2.2ohm fuse resistor right on the display board for the +5V supply feed, which would explain the fault if blown. It would even shut down the remote receiver, preventing all user operations. If your power LED is lit, then it means that the +5V supply for the display pcb is working, but not getting to the display chip.

[Mooly]

Checking all the rails has to be a first step as Stephen says.

Have you got 5 volts on BOTH sides of R482 (on the front panel) when measured from ground ?

Depending on the size of the pdf (if it's below around 1mb) you might be able to put it into a zipped folder and post that.

[newdiyer]

Thanks, Stephen and Mooly. I'll investigate those fusible resistors tonight and post results.

*edit* The pdf is 3.5MB - I just put it on dropbox, so you can access it here:
http://dl.dropbox.com/u/55817844/RCD-975-TM.pdf

[stephensank]

I had found the SM on elektrotanya, which is why I sounded quite so informed about the fault. That 2.2ohm FR is the only fuse device in the related circuitry, on display or main board. So, if it's not that, and power LED is lit, and remote does not work, you must have a cable problem. If remote works, than you may have a bad display controller chip, which seems unlikely, unless you zapped the hell out of it with static discharge.

[Mooly]

Got the manual... see how you get on checking the 2.2 ohm Stephen mentions.

If the 5 volts isn't present then check the -5 and +5 volt rails on voltage regulators U110 and U111 (page 9 pdf)

[newdiyer]

First, thanks again for your help. Every input brings some hope, when I thought I was good to part out my sweet CDP.

I just did the tests you recommended.
R402 (looks like R482 on the fuzzy PDF) - fusible resistor on the 5v rail: looks good. It's not open (I measure about 2.5ohm). When power is on, I get 5v on both sides, relative to ground.

There are also two other fusible resistors, which also seem ok:
R404 (looks like R484 on the pdf) on the -vf rail: I measure 1.5ohm and, when power is on, I get -20.5v on both sides, same as the connector's -vf pin.
R405 on the +vf rail: 1.5ohm, -18v on both sides, same as the connector's +vf pin.

When I hit play on the remote control, the laser head moves for a second, so the remote receiver still does something. If I hit the play button on the unit's display board, nothing happens.

As for a static discharge, I don't remember getting any spark, and I always discharge myself before touching anything. But maybe.

Any idea what I should check next?

[stephensank]

Next thing I would do is check that the xtal oscillator for both the display & syscon uprocessors are both running. Then check for proper voltage on the reset line, pin 6 of the connector from main to display board. Looks from my low res schematic that it's weirdly a 3.3V reset line, which does not appear to have a pulse cap in the reset drive circuit, that TR1(blob)7 transistor circuit, right above & to the right of the U1(blob)7 syscon uproc ic on the schematic. My schematic download is horribly disorganised, so hard to follow without printing out the whole thing.

[Mooly]

Lets go back and start again... without seeing the unit for real it's difficult so if any of these questions sound daft then that is why.

You replaced the tray gear. Did you remove any plugs to the mech ? Asking because I can see two small plugs in pictures. Not mixed up ?

Does the mech appear OK and the tray is free to move ?

If you partly open the tray manually and then switch on does the tray go in automatically as the player powers up ?

Can you manually load a CD. What happens if you press play on the remote.

When the player has been on a while does the display feel "warm". There are horizontal heater wires in these displays which you can see in total darkness as glowing orange.

Check these resistors marked in picture.

[newdiyer]

Originally Posted by stephensank

Next thing I would do is check that the xtal oscillator for both the display & syscon uprocessors are both running. Then check for proper voltage on the reset line

Sounds good - a processor can't do much without a clock. With your help, things start looking a little less like a big black box to me.
But my (very basic) multimeter doesn't measure frequencies. So I'll first check that reset line, and then might get a new multimeter.
For the oscillator, the only one on the display board is X201, and the parts list says "ceramic resonator, 4MHz", not "crystal oscillator". Still, this is the one I should check, right?
The cheapest way to go (instead of getting a new DMM), would probably be to try a new oscillator. However, it's located under the display readout, so kind of hard work to replace.

Originally Posted by Mooly

You replaced the tray gear. Did you remove any plugs to the mech ? Asking because I can see two small plugs in pictures. Not mixed up ?

Does the mech appear OK and the tray is free to move ?

If you partly open the tray manually and then switch on does the tray go in automatically as the player powers up ?

Can you manually load a CD. What happens if you press play on the remote.

When the player has been on a while does the display feel "warm". There are horizontal heater wires in these displays which you can see in total darkness as glowing orange.

Check these resistors marked in picture.

No, I didn't unplug anything (besides the power led) when replacing the tray gear.
If I manually open the tray and turn on the unit, the tray closes poperly.
If I manually load a CD and press play on the remote, the head moves for a second, and than nothing.
I'll check if the display feels warm, and those resistors.

But I don't think I'll have time to test anything this weekend. Will post my results...
Thanks!