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Old 6th Nov 2016, 9:12 pm   #1
andrewn
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Default Zanussi 20ZA374GB stuck in standby

Since I posted a few weeks ago about this TV sadly its now gone faulty. Normally when first switched on it goes into standby showing two dashes on the LED display. Now it shows random characters or none at all. There is no response to the remote or local controls. I managed to get it to come out of standby a couple of times but can't get a picture although the EHT is working and the sound.
It seems to be a problem with the microprocessor or EEPROM. I can confirm there is 5V on the ZC8426 IC and measured 4MHz across the crystal so it seems the clock is running. Other ICs are MCM2802P, MC144111P and UAA2001P. I have a printed service manual but its a photocopy and all the voltages quoted are unreadable. Online versions of the service manual show a completely different chipset which is odd.
Can anyone advise how I can test further? I have a scope and frequency counter. Many thanks, Andrew.
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Old 9th Nov 2016, 10:38 pm   #2
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Default Re: Zanussi 20ZA374GB stuck in standby

I've found that spraying the ZC84260P microprocessor with freezer spray sort of restores operation in that the set will come out of standby using the remote control but cant do any thing else. I'll put a request in the wanted section and I've emailed a company that might have it in stock.
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Old 10th Nov 2016, 3:33 am   #3
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Default Re: Zanussi 20ZA374GB stuck in standby

If the set, when it comes out of standby, has EHT and sound but no picture it might be a supply line or luminance fault which is being sensed by the micro processor and shutting the set down.
Quite a few years ago I repaired a Zanussi ctv with stereo sound for a friend. I was impressed by the build quality of the set and the comprehensive clearservice manual. I had two faults the first was a dead set which required a new SMPS transistor and IC plus some caps, the second fault was the frame output IC which did put the set into standby.
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Old 10th Nov 2016, 4:07 am   #4
Maarten
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Default Re: Zanussi 20ZA374GB stuck in standby

It might just be that the processor isn't reset on power on properly or there is a ripple on a supply voltage. Both can be caused by faulty electrolytics.
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Old 10th Nov 2016, 10:01 pm   #5
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Default Re: Zanussi 20ZA374GB stuck in standby

How can I check if the reset is working? Is a pin held high or low for a moment at power up?
I'll connect my scope to the 5V line to check its clean, its supplied by a 7805 regulator. Its a pity I can't find a datasheet for the microprocessor. It would be nice to be able to fix it as there can't be many of these sets around. The build quality is quite good but the pcb tracks tend to lift easily.
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Old 13th Nov 2016, 2:48 am   #6
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Default Re: Zanussi 20ZA374GB stuck in standby

The reset line should be pulled high or low (depending on the circuit) during switch on.

The processor might be a mask programmed MC68xx or whatever was current for Motorola at the time. You could compare pinouts.
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Old 15th Nov 2016, 10:40 pm   #7
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Default Re: Zanussi 20ZA374GB stuck in standby

Thanks for your help everyone. As Maarten suggested I've found its similar or the same as a MC6805T2. I've tried resetting the micro by grounding pin 28 which does nothing when its in its faulty state. If I spray the IC with freezer the reset function clearly works, reverting the set to standby with two dashes on the LED as it should be. After one freezing session the LED kept working properly, changing to the correct channel number in response to the remote etc. On another attempt the screen lit up with a snowy raster so its very erratic!
I've replaced the crystal and also MC144111P because I had them in stock but it made no difference.
I'll scan the circuit diagram tomorrow just in case anyone recognises the circuit as similar to other sets. Attached is a photo of the micro part of the board.
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Old 16th Nov 2016, 12:54 am   #8
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Default Re: Zanussi 20ZA374GB stuck in standby

The MCM2802P is an EEPROM and will probably contain the tuning and preset control data. The MC144111P is a D-A converter which will output all the control voltages for the colour, contrast, brightness and volume controls. If the data in the EEPROM is corrupt it may upset the processor and prevent it from working correctly. This IC is unsurprisingly obsolete but if you can get hold of a blank one and fit that, the software in the processor may detect the blank device and program default values.

The processor will be a custom mask programmed part and I suspect the MC6805T2 would either be a blank part or have some other software programmed into its memory which would be unsuitable for your TV.

Are there any inputs, from say a keypad or channel selection buttons on the TV, which may be stuck high or low which could prevent it working correctly?

The processor, EEPROM D-A converter and the UAA2001 are all linked by a 2 wire serial interface. If you can identify this, check for any activity at power up.

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Old 16th Nov 2016, 10:56 pm   #9
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Default Re: Zanussi 20ZA374GB stuck in standby

I've attached a scan of the circuit diagram. I tried disconnecting the front buttons and there is no difference. The data lines show some activity but only if I spray the ZC84260P with freezer first. Otherwise they remain low. I think it must have some internal fault. What a shame, most of the other components in the set are easy to obtain.
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Old 17th Nov 2016, 2:24 pm   #10
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Default Re: Zanussi 20ZA374GB stuck in standby

You replaced the crystal, but dd you also try replacing the small capacitors associated with it?
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Old 17th Nov 2016, 9:26 pm   #11
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Default Re: Zanussi 20ZA374GB stuck in standby

No I didn't change them but I did connect a frequency counter and it measured exactly 4MHz so that part of the circuit seems to be fine.
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