So, I've just popped the top off this CTM640 I've inherited. It apparently died after briefly working a few months ago. I've not tried turning it on myself, and having looked inside (see attached) I'm rather glad I haven't. Several of the caps look very corroded on top, as do some of the large resistors, it's utterly filthy and smells a bit like wee...
I've worked on CRT PCBs before, leaving them in situ and the HT side of things well alone, however this one's going to have to come out in its entirety. What's the preferred method of rendering it safe to get out? I've seen several videos involving croc clips and screwdrivers and leads, but frankly it all looks rather sketchy.
Any thoughts/advice gratefully received!
d.
If the monitor has been unpowered for more than one month, it's highly unlikely that any high voltages will remain.
More later after I have looked at the pictures on a large screen (I'm mobile at the moment).
Mark
Ugg, that's crusty. Even if it has been left off for months, I wouldn't touch anything without checking that everything has been discharged, especially the HT side. With the capacitors you can easily discharge them with a bulb with some wires attached. For the HT, the trick with the screwdriver / croc clips is the only way to go unless you want to invest in a discharging tool.
Bryce.
It's utterly revolting. The inside of the 464/fd-1 were similar, but at least I'm happy cleaning that sort of grunk up when there aren't ridiculous voltages around. And they didn't smell of wee...
Ok... Assuming I go the suicide screwdriver route, where should be grounding to?
d.
I usually connect (just) the earth pin of the monitor lead to the earth pin of the nearest socket and then short to the TV chassis.
Bryce.
With the EHT for the CRT, you need to short to the grounding strap (for the CRT) point on the chassis.
For the capacitors, discharge using either a suitable filament lamp (as Bryce says) or a suitable 1 Watt or greater power resistor (*) across the terminals of each capacitor.
(*) I have one with leads soldered on just for this purpose, IIRC mine is 1.2k ohms. Power resistors normally have a higher voltage rating than 1/4W resistors. For the lower voltage capacitors, I use a 12V or a 24V vehicle lamp in a holder that has wires attached. Or just a screwdriver...
The main advice when working on high voltages, is only use one hand and keep your other hand out of the way in your back pocket. That way, if you do touch a live high voltage, a lot less current will flow across your chest.
Mark
Thanks guys. I think I'll back-burner the monitor for now - the tape deck seems to have died, the motor started sounding horrendous then finally gave up, taking out a capacitor with it by the looks of things (weirdly) - C322. Smokey :(
Is this the same as the tape deck in the speccy +2 etc? Just thinking about options...
Here you go, everything you need to know about electrical safety >>>> link (https://youtu.be/wR6g38Pxwog).
Mark
Quote from: danielj on 19:47, 07 January 17
Thanks guys. I think I'll back-burner the monitor for now - the tape deck seems to have died, the motor started sounding horrendous then finally gave up, taking out a capacitor with it by the looks of things (weirdly) - C322. Smokey :(
Is this the same as the tape deck in the speccy +2 etc? Just thinking about options...
Motors are easily obtainable. Had my last one shipped from China. Just a few quid. A clean, new belts and then if it needs a new motor take it from there.
The 464 I have here was horrific before I gave it a clean and new belt. Good as new now.
Perfect - thanks chinny :D . The motor's dead, there's absolutely no resistance across the terminals and it was shorting the whole motor drive section of the PCB (hence the toasted components). Everything else is spick-and-span and I've got a new belt on order. Do you have a link to the motor you ordered from China, or equivalent?
Quote from: danielj on 17:06, 09 January 17
Perfect - thanks chinny :D . The motor's dead, there's absolutely no resistance across the terminals and it was shorting the whole motor drive section of the PCB (hence the toasted components). Everything else is spick-and-span and I've got a new belt on order. Do you have a link to the motor you ordered from China, or equivalent?
Been through my Ebay history but I buy so much on there I can't find it. Just check the motors specs and go from there. They are all pretty standard.
All I know is it's 5V (at least that's what's delivered to the main board) with the mounting holes about 10mm from the centre pin, which is about 8mm long. Not sure what the RPM is or anything (and can't find this information online after a fair bit of digging)...
They seem to vary rather wildly in RPM so finding where to start is rather elusive. I shall continue searching! Many thanks!
d.
C322 can't be destroyed by any other failure on the PCB, so if it's "smokey" then it was either somehow connected the wrong way around or it just died of its own accord. Swap it before you do any further tests.
Bryce.
Thanks Bryce. I've replaced it alongside the burnt out inductor - although now you say that I'm starting to wonder if it hadn't gone and it was just the inductor - it didn't look "gone" but given the amount of smoke I assumed it'd decided to have a bad day down one of its legs... Anyway, no harm in replacing old electrolytics.
d.
Did the CPC by any chance get connected with the 5V polarity wrong? Or possibly someone connected 12V to it? That's the only thing that could have caused damage like that.
Bryce.
So, it's been connected to the right polarity since I've had it. The relay is working OK. I've checked it all through, the "shorting" item was the motor, I'm pretty definite now it was just the inductor that smoked - everything else tests fine.
As short could damage the inductor, but not the capacitor, so possible the capacitor was fine.
Bryce.
Back to the task in hand...
Some horror pictures!
Am I best just replacing all the caps and resistors from the off? I get the impression doing anything else is probably just storing up issues... Any thing else I should replace/test too before powering it up?
Or is this potentially a hiding to nowhere?
d.
You can probably check the resistors before replacing them :) . Another thing I would do is to put new thermal paste. This can prevent overheating in case the old one does not work so well anymore.
Oh! I forgot. Regarding the PCB, I recently repaired a CTM that was equally dirty and what I did was to use the vacuum cleaner first and then a soft brush and isopropanol 100 per cent to clean it.
It's had the vacuum/brush treatment now. I need to remove some of the chunkier components to get at the board properly to give it a good going over with IPA. Some of the resistors are crumbling. Utterly minging...
Replace all the electrolytic capacitors. Replace any high voltage disc capacitors.
Replace any component where there are signs of corrosion, overheating or which have bad dry joints.
Test all resistors (you may have to desolder one leg to test if there is parallel electrical paths).
Remove the solder on any large component, or any component that gets hot, and then resolder the joint. You may have to clean the pad and component leg/pin.
Remove devices from heatsinks, clean off the heatsink paste, then apply fresh heatsink paste.
Check the board carefully for any track damage, testing with a meter to confirm.
Check and resolder any suspect or dry joints.
Mark
If you are very keen, you can remove components from the worst areas, then clean the top of the PCB with a damp (new) washing up sponge.
Make notes and take pictures so you know what went where...
Mark
Thanks. The LOPT seems to have been "got" - but I'm loathed to start getting into replacing that?
I take it all those HV ceramics then should also go (eg C513), that diode (D510). I'm tempted to do all the ceramics in the affected area as I've had corroded ceramics just disintegrate before even when they look fine.
Does anyone know why there are certain resistors missing from the parts list in the service manual? e.g. the resistor you can see on a diagonal in the attached image, R452, 1/2W, 1K2, doesn't appear in the parts list, but is on the PCB and Circuit Diagram. Additionally things like VR501 seem to be missing from the parts list ???
Special inductors and custom transformers are a problem, as they were most likely made only for this set, or similar size / type TVs... so spares may be hard to come by :( .
The LOPT may be available, as spares (often by third party's) were made to replace failed units. As back then, TV LOPT failure was sometimes a problem.
Mark
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
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.
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.
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 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
Quote from: danielj on 20:49, 17 January 17
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.
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.
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.