CTM640 remove main PCB, discharge tube.

[danielj]

Well, the LOPT is available, but I think I might see how we're doing before venturing into spending $s on one. I get the impression this isn't going to be super-cheap to fix


http://www.donberg.co.uk/catalogue/line_output_transformers/hr_6355.html

[Bryce]

I've mentioned it sooo often here, but here we go again: Donberg DO NOT have this LOPT. They are a reseller that take your money, sit on it for six months to a year and when you get pissed off enough with them, they will eventually return your money. They have no stock, just links to probable sources, most of which they don't seem to have reconfirmed in a loooooong time.

Regarding the rest of the components: Resistors: extremely unlikely to have failed. Measure them if you like, if the aren't open circuit or short circuit they are fine, no need to unsolder a leg. Ceramic caps: Extremely rare that they go bad, only replace them if there is obvious physical damage (ie: a corner chipped off). Electrolytic caps: Swap them all.

Bryce.

[danielj]

Thanks Bryce - some of the resistors are disintegrating, or rusting, or both. Those are the ones I'm ditching. Several diodes are very corroded. Some of the ceramics are again corroded around the legs (from experience I know this can creep up inside them) - those are getting swapped out too.


I'm assuming the LOPT is OK. I don't have any reason to think it's not at the moment.  Out of interest, have you ever tried contacting HR Diemen directly?


d.

[danielj]

I note the monitor uses an L78MG to provide the 5V for the CPC, and it uses the variable resistor VR501 to set the output voltage. Is there any reason why a 7805 (of appropriate current rating) can't be used instead? This would remove the need for me to replace the rusty variable resistor and tweak it to 5V...

[1024MAK]

The regulator drop-out voltage as well as the current capacity need to be taken into account...

Look at the schematic to see which supply voltage controls and supplies the 5V control chip.

And then you need to work out how much heat will be dissipated at full load.

Mark

[Bryce]

I note the monitor uses an L78MG to provide the 5V for the CPC, and it uses the variable resistor VR501 to set the output voltage. Is there any reason why a 7805 (of appropriate current rating) can't be used instead? This would remove the need for me to replace the rusty variable resistor and tweak it to 5V...

The 5V circuit in the CTM640 isn't working like you possibly think. The L78MG isn't creating or carrying the entire current of the 5V rail. It is only being used as a voltage reference to bias Q502. This is the transistor that actually takes all the current, the L78MG is only a 500mA part. The output of the L78MG is actually around 5.4V at a few mA. The variable resistor is part of the feedback circuitry to tweak the output of the L78MG which in turn increases the bias on Q502 and raises the voltage of the 5V rail. To use a fixed 5V regulator you would have to change the entire circuit.

Bryce.

[danielj]

Thanks Bryce. That's the explanation I needed. The L78MG is somewhat corroded too, so I was trying to work out my options without having to buy stacks of them from UTsource or pay through the nose to littlediode...


d.

[Bryce]

If they still work then don't replace them. A little bit of rust may not look great, but if it's not effecting the function, who cares? Give them a clean and make sure you check the voltage on the 5V output after you've cleaned the variable resistor.

Bryce.