CPC6128 only starts up when switched on from monitor / main current

Started by kikolin, 12:39, 02 June 22

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kikolin

Hi everyone! I've got an Amstrad CPC 6128 fully functional but there's something wrong about it. If I switch it on and switch on monitor (or external current) it starts up. But if I switch it off and on (external or monitor current already on) it only starts up properly 1 out of 20 times or so. I have checked the switch even in another computer and it's fine. I have measured voltage in main places (5v connector, Z80, CRTC, Gate Array... And it seems to be ok even when the switch doesn't seem to have started up the computer. Sometimes switching off and on ends in a troubled screen (light background, black pararel lines)  or a low whistle somewhere in the pcb. No problem. Switching it off and on from the monitor solves the thing. Any idea/s what it could be, what to check next? Thank you very much.


dragon

Thats rare. I don't have idea really. But... Made a some reflection. One thing that works switching from nothing and not work when is loaded/unloaded inmediately...

Sound as how work a capacitador switching from not load case... And switching when it's charged but don' take time to full discharge when is switched off/on.

Try the thing that don't work. But wait a few minutes. Which the monitor on. And view if then  it start up o.k

I found this old threat but I  don't know about 6128
https://www.cpcwiki.eu/forum/amstrad-cpc-hardware/c101-on-cpc464/


WacKEDmaN

yup ...the low whistle sound gives it away...bad electrolytic cap in the reset circuit...theres only 2 on the board, so probably best to change them both...

Bryce

Quote from: WacKEDmaN on 06:46, 03 June 22yup ...the low whistle sound gives it away...bad electrolytic cap in the reset circuit...theres only 2 on the board, so probably best to change them both...
Since when is a low whistle sound a sign of a bad capacitor in the reset circuit? The reset circuit only contains one capacitor and it's a ceramic capacitor and they don't tend to go bad unless they are physically broken. It could still be a reset circuit issue, but my guess would be that the clock isn't starting up properly.

Bryce.

WacKEDmaN

Quote from: Bryce on 10:00, 03 June 22
Quote from: WacKEDmaN on 06:46, 03 June 22yup ...the low whistle sound gives it away...bad electrolytic cap in the reset circuit...theres only 2 on the board, so probably best to change them both...
Since when is a low whistle sound a sign of a bad capacitor in the reset circuit? The reset circuit only contains one capacitor and it's a ceramic capacitor and they don't tend to go bad unless they are physically broken. It could still be a reset circuit issue, but my guess would be that the clock isn't starting up properly.

Bryce.


whistle is usually sign of a cap failing.. (could also be inductor, but none on the board afaik).. but sorry my bad..i was thinkin 464 reset..

Bryce

Really? I must be deaf. I've never heard a whistle sound on circuits with failing caps. Transformers whistle when they get older, but not caps. What exactly is producing the whistle sound?

Bryce.

kikolin

Hi and thank you all.
About the whistle, I can't really tell where it comes from. I'll try to find out. About waiting to switch on to find out if is about capacitator, it still can't start up. In fact, if I try to start first time switching on monitor, then computer, fails. The only way to start up is directly with monitor button. Everything else fails.
I will try to dig in reset circuit and check clock before unsolder any part.

Thank you again!

dragon

The schematic of the 6128 is caotic.

I don't found the cpc switch. Take a look was between 5v connector and the switch if have a capacitator there. 

Bryce

Quote from: dragon on 14:26, 03 June 22The schematic of the 6128 is caotic.

I don't found the cpc switch. Take a look was between 5v connector and the switch if have a capacitator there.


Yup, the power switch wasn't in the original schematics. The capacitor you're referring to is the main electrolytic (C136). You could try replacing this and seeing if it helps. It's right behind the power socket.

Bryce.

kikolin

One thing I have just tried: I have connected Dandanator and tried the whole thing with it on. Behaviour still the same BUT if i switch on computer, then monitor (we have reached the no start up state) and then use Dandanator reset button (on its own or combined with button to force computer's Basic start) it starts up correctly.
Does it solve some of the doubts? The clock should be working correctly, I guess. Don't know about the reset circuit, Dandanator reset may not work the same as switching on/off.

Bryce

Yes, that points to C136 as being the problem.

What seems to be happening is that the reset delay is finishing before all IC's have reached a stable voltage. It may be that the reset circuitry is reacting too fast (unlikely) or that the capacitor is leaking so much current that the IC's aren'T being fed properly by the time the delay ends and reset get deactivated.

Bryce.

kikolin

I will be replacing C136 and come back to share how it goes.
Thank you very much for your help and support!!!!!!!!

gerald

Quote from: Bryce on 15:00, 03 June 22Yup, the power switch wasn't in the original schematics.
Page 29 of the 6128 service manual. It's part of the Cassette control circuit diagram  ;D
Logical knowing that the power switch was connected to the tape drive on the 464.

Bryce

Quote from: gerald on 19:08, 03 June 22
Quote from: Bryce on 15:00, 03 June 22Yup, the power switch wasn't in the original schematics.
Page 29 of the 6128 service manual. It's part of the Cassette control circuit diagram  ;D
Logical knowing that the power switch was connected to the tape drive on the 464.

Ok, the switch is there, but where is C136? :)

Bryce.

dragon


gerald

Quote from: Bryce on 21:26, 03 June 22Ok, the switch is there, but where is C136? :)
With all the other decoupling caps maybe  :D
You cannot view this attachment.

Bryce

Wow, how many years have I stared at that schematic and not noticed that they were there!!

Bryce.

kikolin

So finally I replaced the C136 and... everything remains the same. Switch on issue is still there. Any ideas on what to try next? Thank you all.

Bryce

Do you have an oscilloscope? Any further tests would require one.

Bryce.

kikolin

I'm afraid I don't.  Will try to borrow one. What should I check? Clock?

Bryce

The length of the reset delay and the voltage rail fluctuations would be interesting to see.

Bryce.

kikolin

That's a point to start from :-) Thank you very much!

JupiterJones

#22
Hi! I don't know if this may help. My 464 did start with the monitor, everything worked seamlessly, but when starting it up using a power supply from RetroComputerShack plus a SCART cable to connect to a colour TV, it showed no life at all. In my case it was the barrel connector in the PCB that has bad solders in the GND rail. I guessed that when using the monitor: GND from video connector was used effectively closing the circuit and switching on the CPC.

Maybe it is worth to refresh barrel connector solders and as told replace C136 (I also did that for CPC464's C101 just in case).

kikolin

Any idea is more than welcome. Will check that too. Thank you!

GUNHED

It may be of help to clean the switch in den CPCs keyboard itself too.
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