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Gx 4000 voltage drop from 5 to 3? And no output

Started by Rexus, 11:05, 05 September 17

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Rexus

Hello everyone,


This is my first post here, so nice to meet you all and thanks for all the interesting info that you've posted over the years.


I just got an 'untested' (i.e. non-working) gx4000 from ebay. The machine didn't turn on. I opened it up and saw that the 78L05 was blown and that the L7805 wasn't working.


Well, I've changed them all and I've also checked that the diodes were OK.


The thing is that I get no video output. I've checked the CXA1145 and while it is getting RGB input (even if quite low I guess--around 0.5), it doesn't output anything at all. The Vcc in-line for the CXA (the 5v mod) seems OK.


Before I swap the CXA1145:


I've seen that under load the L7805 output drops from 5v to 3v! I've even changed the new regulator for another new regulator and I get the same drop. Is that normal? Could caps 183 and 181 have something to do with it? (Haven't changed them yet, new ones are on their way). Could this 3v be underfeeding the Ram chips, asic etc.. so that the RGB signal is not high enough for the CXA to work?




Any ideas will be more than welcome!




Thanks in advance,




R

Bryce

If the 5V is dropping to 3V it means that either the regulator is under-dimensioned (unlikely if you used the same part as the original) or you have a short somewhere. Dropping 2V is quite a bit, so whatever has the short must be getting hot. I very much doubt that the CXA has a problem, it (and every other chip on the board) just can't run on 3V.

Bryce.

Rexus

Thanks, Bryce. There's one of the two RAM chips that gets pretty hot in no time. I just saw in another post that you had changed them both after a gx4000 got fried. I'm gonna order a couple of them and swap them up. I'll keep you posted!


R

Bryce

Yup, that's definitely your problem then. They shouldn't get hot at all.

Bryce.

Rexus

Well, here an update on my gx4000. I finally got the two new RAM chips. I've changed them and now I don't have the short anymore, the regulator is issuing 5v. However, still no output signal when connected with the scart (haven't tried RF, I don't have a cable handy). Since I had RGB input voltage in the CXA1145 but no RGB output (pins 21,22,23 if I remember well) I swapped the IC and put a new one. I've got the same result. No output. Any ideas?


Thanks very much in advance!


R

dragon

#5
You can check if the console is working inserting the burning rubber and connectting a headphones in the 3,5mm.


If he sound the problem should be in the video part circuit.

You can check the rgb output in the 8 din connector,it go directly from the dac to the 8 pin din. So you jump the scart circuit.




If it works here, problem is the scart conversion zone. If not problem is more back














gerald

Quote from: Rexus on 23:27, 07 October 17
Well, here an update on my gx4000. I finally got the two new RAM chips. I've changed them and now I don't have the short anymore, the regulator is issuing 5v. However, still no output signal when connected with the scart (haven't tried RF, I don't have a cable handy). Since I had RGB input voltage in the CXA1145 but no RGB output (pins 21,22,23 if I remember well) I swapped the IC and put a new one. I've got the same result. No output. Any ideas?


Thanks very much in advance!


R

Check that you have a clock signal on the Z80 (pin 6). For that you either need a oscilloscope or a multimeter with frequency measure capability.
Alternatively you can check that the pin is not constant, ie not tied to gnd or 5V. A working clock should measure close to 2.5V DC.
If the 4MHz is not there, check that the main oscillator (~40MHz) is working on pin 1 of the ASIC.
If that one is working there are big chances that the ASIC is dead, and the only way to fix this is by getting a new GX4000.

chinnyhill10

Out of interest do we think this is the usual fault of either someone shoving 12v up the 5v port or has it failed due to the original PSU going into meltdown?
--
ChinnyVision - Reviews Of Classic Games Using Original Hardware
chinnyhill10 - YouTube

gerald

Quote from: chinnyhill10 on 09:46, 08 October 17
Out of interest do we think this is the usual fault of either someone shoving 12v up the 5v port or has it failed due to the original PSU going into meltdown?
I only had one 12V abused GX4000 and ASIC and RAM passed away.
Most of the GX4000 I fixed only had the RAM or RAM+7805 (sometimes the diode as well).
Is the RAM acting as a fuse or a dying RAM could kill the 7805 + PSU, that's the question.
I personally had one CPC not stating one day due to a failing RAM. It was powered with a PSU from a well know LAB equipment manufacturer (that changed 3 time it's name in the last 20 year ;)

Bryce


Rexus

Thanks for your answers,




I'm afraid I don't have good news. The ASIC has around 2.5V in pin 2 (osc 1) but nothing in pin 1 (osc 0). The CPU has stable 5v in pin 6. Dead ASIC? :picard2:


About chinnyhill's question: I've no idea of why this one was fried, just bought it like that. I just can tell you though that the PSU that went with it had the fuse gone. Hard to say whether it was a PSU failure or not. I can perfectly imagine someone trying all the PSUs they have at home in all the console's holes until smoke put them off from trying even more! Gosh...






Best,


R

gerald

Quote from: Rexus on 17:41, 08 October 17
I'm afraid I don't have good news. The ASIC has around 2.5V in pin 2 (osc 1) but nothing in pin 1 (osc 0). The CPU has stable 5v in pin 6. Dead ASIC? :picard2:
Look like it's dead indeed. However, other bits and pieces may still be OK.

chinnyhill10

Quote from: Rexus on 17:41, 08 October 17


About chinnyhill's question: I've no idea of why this one was fried, just bought it like that. I just can tell you though that the PSU that went with it had the fuse gone. Hard to say whether it was a PSU failure or not. I can perfectly imagine someone trying all the PSUs they have at home in all the console's holes until smoke put them off from trying even more! Gosh...



Really stupid design by Amstrad to have two voltage inputs with nearly the same connector. Baffles me why they just didn't go for 5v on a standard CPC monitor connector and use a converter inside to generate the voltages needed for the SCART. Well apart from money which was always the answer with Amstrad.
--
ChinnyVision - Reviews Of Classic Games Using Original Hardware
chinnyhill10 - YouTube

dragon

#13
Quote from: gerald on 18:59, 08 October 17
Look like it's dead indeed. However, other bits and pieces may still be OK.


Osc 1(pin2) and osc 0(pin 1)  Are the same i think remember i folow it in the decapped picture and at certain point they go to the same site. Thats why osc 0 its not connected phisically




Maybe he can try reroute osc1 to 0?. If the problem is in the entrance of the clock route maybe he can jump it.

Bryce

OSC0 shouldn't have a signal on it. The Plus uses a TTL oscillator which goes to OSC1. OSC0 is only used if a crsytal/capacitor circuit has been used instead and I've never seen a plus where this was the case.

Bryce.

Rexus

I was now taking a look at the schematics and, in theory, the CLK in the Z80 (pin 6, where I have no signal) is the CLK4 that comes from pin 125 in the ASIC, right? To measure that with a multimeter, should this CLK give around 2.5v too? I'll test this point later on the day when I come back home. If the ASIC weren't outputting CLK4, could I use any other CLK (like CLK16 that goes to the DAC) and rewire it to the Z80? Maybe it's a stupid Q, but I really don't know if these are different CLK signals, or just the same signal outputted through different places.




Thanks everyone


R

Bryce

No, you can't use CLK16 instead. Even if you could it wouldn't help. If CLK4 is missing, then the ASIC has issues and you can be pretty sure that CLK4 isn't the only issue it has.

Bryce.

Rexus

Thanks, Bryce. I'm gonna test CLK4 in the ASIC later on the day. I don't have much hope to find any signal in there, tbh.


I'll have to give up my reluctance to give up! and look for another gx4000 I guess.








R

Bryce

Have you tried the burning rubber cartridge to see if sound comes out?

Bryce.

Rexus

Well, I give up. The ASIC outputs stable 5v in CLK 4 (and other clocks). It must have melted. And no, there's no sound on the headphones. I'll start looking for a PCB or another gx4000.




Thanks all for your help






R

dragon

Never buy a plus a gx4000 or a cpc made in taiwan that not was tested.


The only can be repaired are the original.


Is much risk to get it cheap.

Bryce

Quote from: dragon on 10:14, 10 October 17
Never buy a plus a gx4000 or a cpc made in taiwan that not was tested.


The only can be repaired are the original.


Is much risk to get it cheap.

?? If the ASIC is fried, it doesn't matter where it was built, you won't be fixing it.

Bryce.

dragon

#22
Quote from: Bryce on 10:22, 10 October 17
?? If the ASIC is fried, it doesn't matter where it was built, you won't be fixing it.

Bryce.




Thats i tell, not buy a plus computer,gx4000(asic) or cpc made in taiwan(preasic) if not was tested becase the ic can't be repaired as this case.


The cost down is only made in taiwan.

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