Hi guys,
I'm a bit of a noob to the Amstrad world. I've repaired quite a few C64s and an Apple ][. I got hold of a CPC 464 recently, and the computer itself is fine, but the monitor is dead.
What happens: Upon switch-on, there is a loud "clack" noise, which I suspect is the degauss coil. But the tube doesn't come on - there's no static and no high pitched whine.
I started checking voltages on the board. Got hold of a schematic. Voltages seemed as expected from the mains, through the filter and into the bridge rectifier, but then on the other side of the R501 resistor (the big white wire-wound one packed in cement) there was 0 volts. I de-soldered the resistor and found it to be failed, open circuit.
So I ordered another of the same type (5W, 5.6 ohms, cement) and installed that today. Powered up the CTM640, and got ... nothing. Same "clack" noise and no tube output. Fuse is intact, voltages are the same at the bridge, and .. oh, the resistor is open circuit again! It has failed.
So my question is, what can I check from this point? What could be the next component to check?
I'm guessing replacing the 5.6 ohm resistor with a bit of wire would probably be a bad idea, or would it?
Thanks for any help :)
Hi,
For sure replacing R501 with a Wire link is a VERY BAD idea, and will only cause more damage / Fire.
Be VERY careful, this section of the circuit will have around 380VDC when powered (when working correctly) and is NOT isolated from the AC Mains supply (This could kill you), so I strongly suggest if you are not skilled with working on Switching PSU to avoid working on this section of the monitor.
For your own safety , I suggest giving this monitor to someone who has the equipment and experience working with HV Non isolated PSUs - this is NOT a "safe" Low voltage "Repair".
Agreed. There is a reason why (!) is against all those components. They are safety critical. Quickly looking at this the line goes to T501 which may potentially have shorted windings (check ohms with a mutimeter on all windings) or C506 (400V) is shorted (check with ESR meter). Either way like what RetroCPC said, this is not an area of the monitor circuit to mess with live and those caps will hold power after power off (not to mention the tube)
Well, an update. After doing some more investigation I found that C506 (1 uF) was open circuit, but that (and the resistor) was probably a result of a short somewhere else. I found diode D507 was a dead short, and so was resistor R509.
However, while I was testing a bunch of components, I noticed there's almost no resistance in the coils of the input side to the T501 transformer. So I figured no matter what I replaced on the board, it was probably going to fail again.
Sure enough, after I'd replaced all the failed components, I switched it on, and got a bright flash from the fuse.
Given other comments about the difficulties in replacing this transformer, I think it's game over for this 640 :(
Okay old thread but same issue here with D507 (dead short)
https://www.cpcwiki.eu/forum/hardware-related/ctm640-dead/ (https://www.cpcwiki.eu/forum/hardware-related/ctm640-dead/)
So D507, the STK7908 and R501 have been taken out in both our monitors.