CTM 640 Colour Monitor Repair - No Power

[pledg]

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.

[darkhalf]

I 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?

[darkhalf]

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

[Bryce]

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.

[darkhalf]

@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/

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

[pledg]

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.

[pledg]

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!

[Bryce]

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.

[darkhalf]

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

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.

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.

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!

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.

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

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.

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

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.

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

STK3708 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

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 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)

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.

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.

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.

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

Please 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

[darkhalf]

@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 

[darkhalf]

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.

LA7800 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

[darkhalf]

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

[darkhalf]

Wagner has the LA7800 and SR2M equivalents
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/

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)

[darkhalf]

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

[darkhalf]

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