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Hi Everyone,


It's been a while since I've posted on the forum but I finally got hold of CTM 640 monitor to go with my restored Amstrad CPC464's.


I was fortunate to find a green screen GT 65 monitor a while back dumped at the bottle bank? I thoroughly dried it out as it had been outside for some time and cleaned it internally.
Fired it up and it worked!!!


Good as this is Iv'e always been after a colour screen and finally picked up spares or repair CTM 640 as a repair project. As you do???


I was told it worked initially, the owner walked away for a short time came back and it was dead. He told me he'd replaced a few parts but being as the unit was full of dust I couldn't see anything that looked like a replacement i.e. clean! everything was covered in dust.


So here's what I've done and found so far:


Checked the plug fuse all okay.


Thoroughly cleaned the unit blown out the dust.


Checked the fuse on the board okay.


There was a burn mark under R502 but it seemed the burn flash mark emitted from the bottom of C505. (I will check this resistor again)


I removed C505 100uf 400v capacitor and underneath was a black burn mark and damage to the negative pin and the wrapping on the neg side of the capacitor.


I replaced this capacitor.


IC401 the LA7800 looked like it maybe it had some discolouration to some of the legs so I just replaced it as I had some available.


I put the case back together and powered the unit on. Not the usual kind of static sound at power on and no 5V from the CPC power connector.




I'll refer to the schematic and check the power supplies.


Any suggestions for points to check appreciated.


Cheers


Pledg.

So today I removed and checked R502 which was okay but I replaced with new anyway.


I will remove R501 and check that also.



D501 and D502 removed and checked okay, D503 and D504 to do next.


I've started checking voltages and they don't seem correct but i'll do a more methodical check when time allows.


Next I'll check the following:


IC401 (New I replaced this)
IC402
IC502
Q405
Q404


I'll run through these but I'm suspecting there's a problem with IC501 STK7308


Any thoughts please chip in...

Quote from: pledg on 20:54, 07 September 21
So today I removed and checked R502 which was okay but I replaced with new anyway.


I will remove R501 and check that also.



D501 and D502 removed and checked okay, D503 and D504 to do next.


I've started checking voltages and they don't seem correct but i'll do a more methodical check when time allows.


Next I'll check the following:


IC401 (New I replaced this)
IC402
IC502
Q405
Q404


I'll run through these but I'm suspecting there's a problem with IC501 STK7308


Any thoughts please chip in...

I'd suspect the STK7308. These tend to die first.

Bryce.

Hi Bryce,

Hope you're keeping well and thanks for your valued input.
Sometimes things are just staring you in the face an it takes a while to register. I'm no expert but took some time to understand what the rectifier circuit is supposed to be doing and how to check it's input and output voltages AC and DC.
I realised that this section of the board is right under the front of the screen and didn't get caked in dust as the rest of the board so I didn't spot that a diode and a smoothing capacitor had been previously replaced.
On reading this rectifier I'm getting 240v into the AC and 280v out DC?
This seems like a fail to me and what is next component down the line I should be checking for over voltage damage? Voltage regulator???

Pledg

Quote from: pledg on 10:09, 09 September 21

On reading this rectifier I'm getting 240v into the AC and 280v out DC?
This seems like a fail to me and what is next component down the line I should be checking for over voltage damage? Voltage regulator???

Pledg

Why does that sound like a fail to you? Those are the values I'd expect.

Bryce.

I incorrectly assumed that it would be lower than that, clearly I need to research this more.  :doh:

Given it's the correct voltage output should I now check R501?


Would this need to be replaced with a cement resistor or could I use a metal oxide film resistor of the same value?

Quote from: pledg on 12:06, 09 September 21
I incorrectly assumed that it would be lower than that, clearly I need to research this more.  :doh:

Given it's the correct voltage output should I now check R501?


Would this need to be replaced with a cement resistor or could I use a metal oxide film resistor of the same value?

Measure the voltage across C505. If it's anywhere near 250V or above, then R501 is fine. R501 is a high power resistor, so if you replace it, you'll need to use a resistor with the same or higher power capacity.
Regarding the higher voltage reading, this is due to the fact that it's AC (with at least a minor load) on the input but DC (possibly without a load) on the output. I also don't know what meter you are using, so RMS (Root mean square) may be a factor too.

Bryce.

Thanks.


I removed and tested R501 and it was in fact dead so I replaced it with new.


On power up I had the main 2amp power fuse blow.


I replaced the fuse and powered on again.


I'm now getting 240v DC across C505.

If the fuse blew the first time, there was a short which you have now nuked out of existence. So you are now looking for a component with an open circuit rather than a short circuit. Unfortunately I would guess that it's the HV transformer (LOPT).

Bryce.

So is there an easy way to check whether the flyback is perhaps in open circuit or could it be an associated transistor that's blown?


My next question would be if needed are replacement flyback transformers available?

I'm not going to give up on this just yet.


I'm in no hurry and the case of this unit is in such mint condition that if I end up buying another for parts then so be it. I just want the complete CPC464 package finished.


I'd like to look at why I'm not getting a 5v supply out of the monitor. As you said earlier STK7308 is a suspect and I do have a spare for that but I'll check a few other parts before replacing it.


I'm going to follow the schematic and the voltage I do have going to T501 and the components around the voltage regulator IC502 LM78MG.


Also I'm going to remove Q405 to test.

Transistor Q405 2SD1397 is a fail so I've ordered a replacement.


Question is when did it fail? I feel I should have tested it earlier on in the repair.


Am I correct that if this part was defective from the start it would have stopped the operation of FB401?

The problem is that these fails probably weren't the initial cause of the non-functionality, they failed because something else had already failed. So don't just blindly swap the failed parts out, because if something else is still broken they will fail again.

Bryce.

It may not appear so to a trained technician but I am trying to understand the theory behind how the CRT works and follow the voltages on the schematic etc.


I'm not going to replace and just power on again. I'm sure your right the flyback has gone so I'd be interested in any method to test this on the board. I'll give it another visual inspection for any damage first. I can test for shorts between windings etc. I think.


I'm interested to find out the lack of 5v from around T501 and IC502.


If nothing else it's all good practice reading schematics, removing components safely and without damaging them to test.

Measure the resistance of each coil in the LOPT, but make sure everything has been completely discharged before you start!

Bryce.

You are right of course it seems that the flyback has blown.


I get steady and similar resistance across all the coils until I get to down to 8,1,9


I'm also getting a short between 1, 11, 5 & 6.


Its's real shame that these parts are virtually impossible to replace with new.


I'll keep my eye out for a replacement flyback but as you've previously commented Donberg advertise them but they'll obviously be well used examples no doubt if they ever send one.


In the meantime I'll keep my eye open for spare parts but I'm thinking of a mad project of mounting an old flat panel inside the CRT case? 

Thanks for your help!!!

Had a bit of a revisit to this...


I replaced the power resistor R501 again after what I had used previously failed, It was the same rating but for some reason it had failed.


I replaced all the diodes in the rectifier. This had been repaired by the previous owner but I noticed one of the diodes was a different rating.


The main fuse has not blown at all after this...


So I'm now getting a steady 320v across C505 and on until pin 14/15 of IC501


Don't seem to be getting any voltage into or from IC502 and have checked back through Q504, Q505, Q506, Q503 and Q502


Probably clutching at straws but anywhere else to check voltage around T501 or IC502?


Thanks...

Hi Bryce,


Can you give me a bit of pointer please.


I've recapped the board, replaced a couple of dead diodes and one zener diode around IC501.


Previously I was getting 0v from pin 11 and 6 of the flyback transformer. I'm now getting 15v but I trace that back to T501.


Should I be getting 110v out of this side of the transformer? If so is that a fault with the transformer or it's associated components?


Any other component I should be checking?


Thanks...

Update!


Progress maybe???


I found a very small piece of copper wire fragment bridging pins 4 & 5 of IC501. When I removed it and powered on I get power noise/buzzing, the first time Iv'e heard anything like this!


No static to monitor front though?


I then stupidly accidentally made a short checking Q502 and the main power resister blew rather annoying so I need to replace and test more carefully.


I then discharged the CRT and got a small crack sound so that tells me that some HV has finally reached the CRT tube.

So I replaced IC501 STK7308 again after stupidly shorting it the first time and I now have a stable system again ie. the main fuse isn't blowing.


I'm back to getting a high pitched whine and no voltage out of T501 8142004. The vibration and noise is definitely emitting from this part.


When I power down the system I get the winding down of this transformer audibly.


I'll try and see if I can find a replacement part but in the meantime if anyone has a replacement kicking around on an old board I'd be happy to purchase.

Okay so I removed T501 and tested all the coils but I can't see anything wrong with it.


I can't understand why I'm not getting the 110v and 9v out of it. Must be something not right around the STK7308.


The buzzing sound must mean that T501 isn't getting the correct voltages?

Quote from: pledg on 15:49, 10 November 21
Okay so I removed T501 and tested all the coils but I can't see anything wrong with it.


I can't understand why I'm not getting the 110v and 9v out of it. Must be something not right around the STK7308.


The buzzing sound must mean that T501 isn't getting the correct voltages?

Maybe a silly question, but I'll ask it anyway: You are aware that these are all high frequency AC voltages? So if your meter is on DC you'll measure nothing? And if the frequency is higher than what your meter can handle on AC you will be getting inaccurate readings. The buzzing sound is normal for an old transformer. The coils loosen over time and vibrate at the frequency on the input voltage. Unfortunately, this vibration is also what causes shorts inside the coil, killing both the coil and the part feeding it (STK7308).

Bryce.

Thanks for the input Bryce, I was trying to measure the voltage 110v going to the HOT Q405 but getting nothing.


I understand what you're saying re the transformer outputting AC not DC. How does this actually work as the AC goes through the rectifier before it reaches the transformer. I guess I'm not understanding how this is being powered at all. Maybe an idiots explanation is required for me?


Anyway I have made progress which I'll detail separately.

Update!

So I had a mint CTM640 case with non working internals, what to do? I found a working CTM644 online but it was in a smashed split case. I transplanted/swapped all the internals including the tube/neck board so I now have a mint CTM640 case with the internals from the working CTM644. Although the PCB was working it did need a serious clean up. The internals are swappable even though there are slight differences in the internal tray mouldings for the PCB in the monitor shell. I had to 3D print a new mounting plate for the brightness control as this section is different but it works really well. So I now have a hybrid CTM640/644 and yes it has the 12v output so I can power my CPC6128 recently repaired with a RAM fault. Whilst working on the CPC6128 I was using my GT65 green screen restored some time ago (found abandoned at the bottle bank?) This led me to discover that the 12v out from that monitor did not work so I replaced the voltage regulator and capacitor so now all is working. After doing that I discovered that the 12v out from the new CTM644 PCB also did not work so the same process again brought that back to fully working. So in the end I have a fully working CTM640/644 monitor and a CTM640 board still to come back to and repair. I feel it's cheating slightly to say I repaired the monitor but I suppose I have brought back into use some internals from a monitor that could have well all ended up down at the tip. The other upside is that I think the flyback for the CTM644 is actually still more readily available thus future proofing it slightly??? Anyway I've had the monitor up and running and burning in with some classic Amstrad games and so far it's working perfectly. Although the kids do complain they can hear I high pitched whistling when it's on but I can't hear it! :)

I'm still sure I can repair the original CTM640 board so will go back to that and have a look at Q502 and the LM78MG voltage regulator having already swapped the STDK7308. I still need to get my head round how this thing is powered properly first though!

I pulled the old abandoned Amstrad CTM640 monitor down from the shelf for another repair attempt after gaining some knowledge on the workings of CRT's.

A few final replacement parts and it finally fired up but with vertical collapse. I traced and replaced a final offending resistor and that was it, job done!!!

I'll follow up with a more detailed post listing parts replaced etc.

QuoteI get steady and similar resistance across all the coils until I get to down to 8,1,9I'm also getting a short between 1, 11, 5 & 6.

Seeing the same problem with both of my flybacks

@pledg so was the flyback the cause of this issue? What parts did you need to replace?

Some parts arrived.

@Bryce of course was correct, in that if you are swapping parts but not the main cause, then it will take out other parts. In my case subsequent power on took out the STK, R501 and D508 on the second monitor

Getting the same mid pitched whine out my my T501 transformer on my CTM644. Not sure why that is (no flyback fitted yet) and not seeing 5V either

Whining, as you call it, on an old transformer happens because the coils have started to loosen up (glue getting old, coil no longer so tightly wrapped) and are vibrating at some harmonic of the 50Hz supply. Happens to most old transformers.

Bryce.

@Bryce I have measured the frequency of output at 50Hz but it was not making that sound prior to the issue. Double checked capacitors and ESRs on those are all in range

@pledg interested if you had info on if your whine went away and what you ended up on replacing

I've been documenting parts as I went in the other thread:
https://www.cpcwiki.eu/forum/hardware-related/ctm640-dead/ (https://www.cpcwiki.eu/forum/hardware-related/ctm640-dead/)

So far the list is for power:
F501 Fuse (2A, 250V)
D501,D502,D503,D504 Rectifier diode (1N700N)
R502 Cement/Wire wound resistor (5.6 ohm 5W)

For the regulator side:
IC501 STK7308 regulator
D507 Zener diode (RB36F-B-2)

Flyback side:
FB401 37140041 / HR6355 (CTM640, difficult to source). 3714009 / HR7674  (CTM644 Donberg, HR etc)

Also check:
R444 8.2 ohm 1/4 fuse type
Q405 2SD880Y

Hi, Just seen your post.
Bryce is the main man but I'll prepare you a checklist/guide from my amateur experience in getting my monitor up and running. 
It was a while ago so please bare with me while I fact check it and make it easy to follow.
One tip though don't bother trying to source the flyback it'll be a waste of your time and money.
I'll try and get something back to you later this evening.

I haven't had the chance to read all of your posts so forgive me if I go over anything already tested.

I'm not going to guess what your issue is but I'll give you a list of likely failed components and where you can purchase them in the UK.

A failed flyback transformer is often the first suspect but you may be lucky it's not always the culprit. Inspect it for damage, cracks in the shell and also check the solder pads below. If it looks good then proceed to check the following components/sections.

PSU check this is working correctly check the rectifier diodes, main power resisters and the filter cap. Again any damage is normally obvious with physically broken or burnt components.

My unit had a power section with signs of failure burnt components etc. Some diodes in the rectifier circuit and main resistor having blown. Interestingly the main filter cap was fine.

The problem with CRT's is that one seemingly small failed component can lead to other catastrophic component failure down the line and I think this is what had happened to my monitor.

Firstly check/replace the voltage regulator LM78MG it supplies the 5v output for the computer but also forwards other voltage into the flyback circuit. It's cheap and easy to replace and it's old so just replace it.

STK3708 available from Cricklewood Electronics widely used in TV's this IC is a regular failure part along with the connected zener diodes. This section passes voltage onto the transformer and if faulty may be the cause of your whining noise. I had this noise with a failed STK3708, easy to check this IC for shorts across the pins.

LA7800 check the voltages out of this if in doubt socket and replace for ease of testing, again easy to source common part.

Also check the Horizontal Output Transistor Q405 they're also available from Cricklewood Electronics a std part.

Generally go over the whole board testing all large resistors and capacitors, Normally most will test okay in the board if in doubt just pull one leg to check properly.

Also check replace the SR2M zener diode near the flyback. This was the last thing I replaced that stabilised power before I found the final failed resistor that was giving me vertical collapse.

If you want a good guide on repairing CRT's and how they work lookup Mike's Amateur Arcade repairs on YouTube. He's an arcade owner operator and posts videos on fixing his CRT's. Lots of handy techniques and methods for component replacement and general repairs etc.

Cricklewood Electronics are a good source for most parts you'll need and have very fast delivery.

Please don't try and source a replacement flyback transformer as this will be a waste of money and time.

Hope this is of some help!

Wow! Cricklewood Electronics still exists!!!! That was one of the first places I used to buy components from via "Postal catalogue" (For the young: That's like an internet shop, but made of Paper).

Bryce.

QuoteA failed flyback transformer is often the first suspect but you may be lucky it's not always the culprit. Inspect it for damage, cracks in the shell and also check the solder pads below. If it looks good then proceed to check the following components/sections

I have 1 x CTM640 and 2 x CTM644 here. During July the CTM640 and CTM644 have failed. They took out different supporting components. Thought measuring the resistance of the flybacks, I've confirmed these are the culprit causing the chaos of damage to supporting components. There were no obvious marks of damage here.

QuotePSU check this is working correctly check the rectifier diodes, main power resisters and the filter cap. Again any damage is normally obvious with physically broken or burnt components.

The PSU circuit showed no power out in both cases. One was a result of blown fuse and faulty rectifier diodes. The other was blown resistor,  STK and supporting diodes were damaged. After replacing the fuse and rectifier diodes on monitor 644 it then also took out the resistor STK and supporting diode. Bad mistake, as the problem was the flyback for both!

QuoteMy unit had a power section with signs of failure burnt components etc. Some diodes in the rectifier circuit and main resistor having blown. Interestingly the main filter cap was fine.

That main 400V filter cap is storing about 310V (DC of course) and should be okay assuming it doesn't get AC input via the bad diodes. The diodes were open circuit when testing

QuoteThe problem with CRT's is that one seemingly small failed component can lead to other catastrophic component failure down the line and I think this is what had happened to my monitor.

This has happened the same to both of mine, and looking at other posts here, the paths of destruction are similar. So good to have this in one place for people seeing this in future. My friends Commodore monitor blew a flyback and we just replaced that and the driver transistor, and all was good. These orion monitors have more of a flow on effect

QuoteFirstly check/replace the voltage regulator LM78MG it supplies the 5v output for the computer but also forwards other voltage into the flyback circuit. It's cheap and easy to replace and it's old so just replace it.

Thanks. I'll check the voltage going into the LM regulator, just in case that was a problem. CTM644 also has a 12V reg

QuoteSTK3708 widely used in TV's this IC is a regular failure part along with the connected zener diodes. This section passes voltage onto the transformer and if faulty may be the cause of your whining noise. I had this noise with a failed STK3708, easy to check this IC for shorts across the pins.

I am in Australia so purchased one STK3708 from a proper supplier (France) on ebay, and also 2 x STK3708 from AliExpress. The AliExpress one so far on the CTM644 has the whining noise, so this is a brand new item causing it. I'm waiting for zeners to arrive for the CTM640 and see how the second AliExpress unit goes before using the ebay one

QuoteLA7800 check the voltages out of this if in doubt socket and replace for ease of testing, again easy to source common part Also check the Horizontal Output Transistor Q405 they're a std part.

Thanks for the info. I've done a diode check on the Q405 so far and both measure the same, with no short. I'm also waiting flyback for the CTM 644 (CTM640 one arrived yesterday)

QuoteGenerally go over the whole board testing all large resistors and capacitors, Normally most will test okay in the board if in doubt just pull one leg to check properly.

Thanks been doing that. I'm trying to check the power circuit (130V, 12V, 5V) works before I move to the 130V side and put the flyback back in at the end of this.

QuoteAlso check replace the SR2M zener diode near the flyback. This was the last thing I replaced that stabilised power before I found the final failed resistor that was giving me vertical collapse.

Will do. I was wondering which part caused your vertical collapse. I'll check over this one

QuotePlease don't try and source a replacement flyback transformer as this will be a waste of money and time.

Confused why this would be the case. I have a confirmed short with two of them, so this is definitely the cause of the problem. I just need to get the power circuits on both working first before installing the replacements. @pledg thank you so much for the info

@pledg thankyou again for the summary

I've managed to find a path of destruction with my CTM640 and CTM644. Both due to failed flybacks. Refer to image. Most of the failed parts are on the power side. I'm hoping the IC401 (LA7900) was not affected

Additional things to check are:
- R521 (fuse type 1 ohm resistor) this failed on my CTM640. On the CTM644 this is called ICP501 (but was ok!) so much for In Circuit Protection!  Closest I can find is 4.7 ohm on Digikey
- R511 (fuse type 10 ohm resistor) this failed on my CTM644 (one which was whining). 10 ohm available on digikey
- SR2M zener (130V 1amp zener). Like you suggested this has failed on my CTM644. Replacements are difficult to find, but Donberg has them. 
- D405 (Silicon TGR06K diode). These are obsolete. RGP10K appears to be replacement with same specs from Digikey

Got more parts to order before check the power is ok and and then install the new flyback 

Some good news today. I've used my CTM640 for donor parts (diodes and fusable resistors) with the CTM644 and now have 5V, 12V rails working, and over 100V to the flyback transistor.

I've installed the new flyback and powered on. However we have a white screen with lack of vertical hold. Turned down the intensity (screen pot) on the flyback to just above grey on full bright. Even with the CPC6128 plugged in, we have no screen. So something else is damaged.

QuoteLA7800 check the voltages out of this

Forgot to check this one, but wondering if it could be the culprit. I'll have to look into this next

Okay did some reads:

IC401 (LA7800)

Expected (Pins 1-16)
6.4V ok
6.5V ok
0.3V ok
0V ok
0V ok
0.85V ok
3.21/3.3V ok
0.25/0.33V measured 0.38V (8: Vertical driver)
-1.1/0.93V measured -0.66 (9: Vertical Blanc)
5.6/5.55V measured 6.4V (10: Vertical oscillator)
1.1/0.96V measured 0V (11:  Vertical oscillator)
11.8/11.04V measured 12.5V (12: )
0.5/0.89V ok
8.9 measured 10V (14: Sync sep)
12.5V ok
4.1V ok

IC402 (LA7830)
0V ok
12.74V/12.5V ok
24.V measured 25.5V
0.84V/0.8V measured 0.91V
0V ok
24.2V/24.5 measured 25.1V
2.5V/1.4V? measured 1.1V

Both showing variation. Voltages listed in schematic / table in the CTM644 have a bit of a mismatch

Wagner has the LA7800 and SR2M equivalents
https://www.wagneronline.com.au/la/ics/semiconductors/electronic-components/la7800-24318/959437/pd/ (https://www.wagneronline.com.au/la/ics/semiconductors/electronic-components/la7800-24318/959437/pd/)

Not sure if I need the LA7830 just yet but wagner have them also
https://www.wagneronline.com.au/la/ics/semiconductors/electronic-components/la7830-24326/959437/pd/ (https://www.wagneronline.com.au/la/ics/semiconductors/electronic-components/la7830-24326/959437/pd/)

They also have the replacement fuse resistors (but 0.5W instead of 1W)
https://www.wagneronline.com.au/0-5w-fusible-resistors/resistors/electronic-components/5f1-2267/184/pd/
https://www.wagneronline.com.au/0-5w-fusible-resistors/resistors/electronic-components/5f10-2279/184/pd/

The next part is the diodes TVR 06K is obsolete (800V reverse, 1.4V forward, 600MA diode)
https://www.digikey.com.au/en/products/detail/vishay-general-semiconductor-diodes-division/TVR06K-E3-73/8627765

Replacement part# RGP10K-E3/54 or RGP10K-E3/73  (Farnell, Mouser, Digikey etc)
https://www.digikey.com.au/en/products/detail/onsemi/RGP10K/1048960 (800V, 13.V volt forward 1A is the equivalent)

I'll do a better update later but found the following things

Flyback dies and takes out a bunch of other components with it. Depending on the failure (CTM640 died whilst in use, whilst CTM644 blew rectifiers and fuses on power on) depends on which components

In a running order
- Check fusable resistors in spec
- Check diodes. Particularly the ones in the diagram. I lost several of them (around the regulator or near the flyback)
- IC401 (LA7800) is most likely going to suffer, but IC402 was fine on both. I replaced both anyway
- CTM640 only will not get 18V power to the regulator without the flyback fitted. It gets power from this to convert down to 5V. I was avoiding fitting the flyback until the end
- My CTM640 was missing brightness and the red gun was not active. The gun transistor and diode was dead. One of the brightness transistors also failed. Using diode check on the multimeter and comparing helped find the problem ones
- Thought I had a failed regulator LM78MG , but was a transistor supplying 5V to that. Similar part# are UA78MG/T78MG

Other parts:
D901 6ZA6.2Y diodes can be replaced with 1N4735 or 1Z6.2 (Wagner)
Q403 KTA950Y PNP can be replaced with 2SA950 (Wagner)
Q801 2SC3417 300V NPN can be replaced with 2SC2611 (Wagner)

I got CTM640 flybacks from Tulli Electronica and Netashop.gr but stocks were limited. No one else had them. CTM644 flybacks were in a few more placed. Of course remembering caps in these hold upto 400V and these things make 20KV and the tube stores this voltage, all precautions were taken by myself (discharging tools, rubber gloves, second person in room when powering up etc). Even after discharge, the tube may still hold voltage

@pledg thankyou for your info. Managed to get both monitors working  :)