News:

Printed Amstrad Addict magazine announced, check it out here!

Main Menu

CTM 640, seemingly dead

Started by CRG, 10:53, 04 January 22

Previous topic - Next topic

0 Members and 1 Guest are viewing this topic.

CRG

Last year I picked up a nice CPC 464 with its CTM 640 as part of a massive job lot of systems.  I'm only now getting round to the CPC and its monitor but seemingly that is dead. 


When initially powered up you get the typical degause bong of all CRTs but there is no power coming out of the 5V supply for the computer and there is no action on screen.  Powering the computer separately also doesn't produce an image on screen.  The computer itself works fine. 


While I don't mind getting my hands dirty inside an old computer I'm always a bit hesitant working inside a monitor and in particular CRTs because of the voltage.  Before I go poking, does the CRT have a self discharging circuit or would it need discharged (I know in practise you should always discharge but I'd like to know how big of a bang to expect when doing so).  In terms of discharging can I do so simply to the chassis or is there another point?  To test the low voltage side of the board is it safe to power it up outside of the CRT, i.e. with the HV anode disconnected? 


From a quick look last night the fuses appear to be ok, I will of course test them.  Some of the caps do look like they might have leaked, there's at least 1 spot that looks wet potentially from electrolytic spill.  Are leaking caps common in these monitors?  I do need to dig into more detail but before doing that I need to pull the board out, hence the questions above.   

Bryce

#1
Some quick answers:

- No, it's not self discharging, you'll have to do it yourself.
- If you discharge a tube properly (through a load) there shouldn't be a bang.
- There's no special point, use the chassis.
- NEVER power up a CRT with the Anode flapping about loosely, you're asking for trouble!
- Capacitors of that age can always be an issue.
- If it made a "boing" but there's no 5V present, it may just be an issue with the 5V output. Can you feel static on the screen when it powers up?

Bryce.

CRG

Thanks for the answers.  CRTs are a bit of mystery to me, the only one I've work on before was in a MAC classic hence my questions about the discharging and powering up the board without the CRT connected. 


One thing I don't have is a proper tool to discharge the CRT.  When working on the MAC I just ensured that was discharged by using the screwdriver and wire method but since that CRT does discharge itself it never sparked.  I suppose I'll have to see about getting a proper tool because as well as this CRT I have a few old VGA monitors that I need to get to in the future. 

I didn't think about feeling for static but I can try later, there was certainly no crackle of static that you would sometimes get but I can check later to be sure.  I did power the computer separately with the video cable connected and it didn't display any image on the screen although I'm not sure if the missing 5v from the monitor is also required to drive some internal circuitry.


I get the impression from reading other threads that the failure tends to lie in or around IC401 (LA7800), could be anything though.  Need to go digging. 


CRG

Finally found time to open the monitor. 


It's obviously had a lot of use, it's absolutely caked in dust, several of the diodes are blackened and 1 of the larger resistors the coating is flaking off.  Presumably all this is from a build up of heat. 


I've not tested much of it yet but certainly no shorts to be found, all the diodes etc test fine and the fuse is good.


One obvious issue is that the ground strap around the CRT is broken.  Not sure just how much of a problem that is, it's snapped from corrosion but there is still continuity from ground on the neck board to same beside the flyback. 


Next job was going to be power it up and look for voltages around the la7800 but is there anything else you'd recommend to check first? 


Bryce

No, just blow as much dust out as possible. Damp dust is a resistor.

Bryce.


CRG

Sorry its been a while I was waiting on parts arriving and then other things got in the way but over the last couple of nights I've recapped the entire board of the 640 (well the electrolytic caps) and and replaced the STK7308 which I discovered had a short between pins 10 and 11. 

But still no dice and thats because I've found more shorted components, D505 and D507.  Both out of circuit are more or less a dead short  in either direction.  Thing is I'm not 100% sure what I need to order as replacements.  They are listed in the manual as Zen. RD 3.6FB Part No 170458. 

Could someone kindly point me in the direction of what I need to order? 

Nworc

#6
Seems to be a bit tricky.

Before you continue, make sure you have read the safety measures that you can find here: https://repairfaq.org/ also be sure that I take no responsibility - you are responsible for anything you do. Doing this repair is very dangerous (also for your life) and should only be done by a trained technician. I suggest to give this repair to an expert, because doing it on your own is very dangerous, and chances are that you spend more money instead of giving it to someone who does this professionally.

Only if you can't resist ...

Please check the schematics, without that you are lost. I would progress step wise (just my opinion): First replace the identified defective components. Check the components on the live side of T501, especially Q501, D501-D504, R501-503. Put a 100W lamp in series to the mains power to limit the current in case there still is a problem, before you power up. Even with that lamp in series you should get a 5V on the connector to the CPC. If that works, then repeat the test without that lamp.

Update: As I have checked, opening B502 is not a good idea as the switching circuit blow if there is no load.

Bryce

Quote from: CRG on 00:17, 12 February 22
Sorry its been a while I was waiting on parts arriving and then other things got in the way but over the last couple of nights I've recapped the entire board of the 640 (well the electrolytic caps) and and replaced the STK7308 which I discovered had a short between pins 10 and 11. 

But still no dice and thats because I've found more shorted components, D505 and D507.  Both out of circuit are more or less a dead short  in either direction.  Thing is I'm not 100% sure what I need to order as replacements.  They are listed in the manual as Zen. RD 3.6FB Part No 170458. 

Could someone kindly point me in the direction of what I need to order?

They are 3.6V Zener Diodes. Something like a BZX85C3V6-T should be fine as a replacement.

Bryce.

CRG

Sorry for again the long delay in getting back to this, been very busy working in the house and not had time for retro goodness.  

Anyway... I finally got the chance to fit the new diodes this evening.  Powering up the monitor it runs for only a few seconds while giving a painfully high pitched squeal then switches itself off.  During these few seconds the 5V line comes up to 5V then drops back to 0 as the monitors turns off.  

Blown transistor or maybe even the flyback itself is gone?  

One component I changed is R427.  On my board this resistor was badly burnt so going by the schematic I swapped it out for a 3.3k 2w metal oxide resistor.  The old part despite its cooked exterior measures 1.7k.  Just wondering if by chance there was a different revision to the monitor that had a different value resistor in this position?  I mention this because R427 seems to sit on the horizontal circuit and the high pitch squeal would remind me of maybe a fault in the horizontal output transistor.  Not that I know that much about CRTs so could be barking up the wrong tree here.  

Nworc

#9
I suppose the test with B502 open was successful. In case you did'nt blow anything again in the power supply, I vote for the HOT (Q504).

The strange noise comes from high voltage (starting from above a few hundred volts) finding it's path in a way which is not intended. Can be inside the DST (it's not a FBT) but I also saw cases where the cause is a simple short by a carbon bridge in the insulation of the horizontal transistor. That one is hard to measure as you do not see it when probing the transistor with a multimeter. You could not even test that with a bridged DST, as the voltage in the horizontal circuit was not high enough to cause the problem so that you thought it must be the DST, but it wasn't.

I saw cases where the leakage of an old cap lead to a short on the PCB, the only way to fix that was to remove a bit of the PCB physically using a drill. And with the number of dust that accumulates over time in these old monitors, the chances that the short is not within the DST are rising.

You know that this is a difficult repair, rushing things is contra productive. Before closing B502 I would have bridged Pin 6 and 11 of the DST (you need to desolder these pins before, and bridging goes by a 60W lamp) to check step by step which circuits are working, this is how servicemen are doing the work, I call it baby steps, and had more success with that instead of applying power to everything ;)

BTW: My schematics say that R427 is a 1.8k 2W.

PS: From your initial post "is it safe to power it up outside of the CRT, i.e. with the HV anode disconnected?" I would not do that, if the anode capacitance is missing, the anoge voltage will be higher than designed.

CRG

I replaced the HOT but it made little difference. 

Taking a step back though I thought I'd take your advice @Nworc so I lifted one leg of B502.  The fuse immediately blew.  R501 and 502 went with it (they were fine before) and I think Q501 is also bad.  Perhaps this was always bad, I'm not sure.  IC501 is also possibly blown again. 

Unfortunately I don't have a 100w lamp to put in line with the thing but I'm a little disappointed that so much more damage has been done.  I have another space 501 I can order a 502 cheaply enough as I can also a Q501 but if IC501 is indeed dead again.... I'm thinking of just calling it a day. 

I don't know enough about monitors and I'm honestly not comfortable working with mains and HV.  I could keep chasing the fault but I think there comes a time when you need to draw a line under something. 

I might gut the chassis and install an LCD in place of the CRT. 

Nworc

#11
You said you replaced R427 by a 3.3k resistor - did you see that I mentioned that in my schematics it's a 1.8k, and have reverted that ?
Repairing a switching power supply can be tough, especially if it's unclear what else is possibly broken. I would recommend to hand it over to a serviceman, at least ask how much a repair would be, sometimes you be lucky and it's not that expensive.

Update: I have tried my suggestion to open B502 in order test the power supply, but regretfully this does not work with this design, I'm sorry. That blew me F501, IC501, R511, D507, R501. Q501 was still ok but I replaced it anyway just to be on the safe side, monitor is now working again. I'll PM you for spares.

Powered by SMFPacks Menu Editor Mod