CTM640 dead

[darkhalf]

My CTM640 has just suddenly stopped working. Just made a 'click' sound and died

Checked the CPC464 with another monitor and that is working fine. Have pulled the thing apart, discharged the caps and the tube. 

Diagnosis so far is that 
- F501 fuse is okay
- no power on the outputs of T501 transformer

So the problem appears to be on the hot side. Any ideas where to start on this one? 

Not a massive fan of probing the hot side live, so any pointers of parts to check appreciated

[lordheavy]

I have a similar issue with a CTM640. I'll bet on the STK7308

[darkhalf]

I'm hoping not, as that will be the most difficult to replace part. I've probed all the parts and measured ESRs etc.

So far first part is confirmed dead R501 (5.8 ohm) resistor showing open. This is one of the things which went pop but is not the cause

Measuring both sides C506 (100uf) shows there is a short still, so I removed that capacitor and no change. Cap ESRs ok

Next we remove R502 (1 ohm) which goes directly to Pin1 and Pin11 of STK7308 and pin 4 (input coil 2 of the transformer) and that removes the short shown at C506 pads

From other side of R501 we trace through the 4 ohm coil (pins 1-2) and then direct to pins 14 and pins 15 of STK7308

Whilst transformer coil pins 1-2 and 3-4 are 4 ohm we have 4-5 measure in 1 ohm. Unsure if it is meant to be like this

STK7308 shows pins 1, 11 and 14-15 measure between about 1 ohm. This indicates a short inside the STK7308

I need to double check the coil in my other monitor and see if the coil shorted first and then blew the STK7308 and then  the R501 resistor

Any possible place we can get parts. Anyone have any dead monitor boards?!


Edit: Okay so STK7308 (voltage regulator) and 5W 5.6 ohm resistors available on ebay/aliexpress. Just need to confirm the transformer

Jaycar has the exact one here in Australia
https://www.jaycar.com.au/5-6-ohm-5-watt-wire-wound-resistor/p/RR3244

[darkhalf]

Okay this is from one of my other CTM644 (with the 12V). So HV circuit is the same, but the transformer has the extra outputs for 12V

Removed the input side from the PCM and measured this one. Coils on HV side are around 0.1 - 0.4 ohms so are less than two of the coils power supply in the CTM640. Will get another STK708 and see how we go in the CTM640

[darkhalf]

Found another issue. Given my other CTM644 just died, I now have both units open for comparison. I ended up desoldering all the diodes on both units to check them properly. D507 zener diode on this one was reading 0.4 both ways around. This may have been the culprit responsible for frying my regulator! I'm glad I found this prior to the replacement going in

I should note that even though it was left off overnight and never powered on, Whilst discharging all the caps etc, I still managed to get visible (high voltage) discharge on one of the flyback transformer pins from over 48 hours ago! This was after discharging the tube. So always be aware when opening CRT monitors!

[darkhalf]

RB36F-B-2 is the part number for the zener diode.
https://www.alldatasheet.com/datasheet-pdf/pdf/6552/NEC/RD36F.html
About 34-36 forward volts, 27V reverse volts

Tried putting the regulator, and diode from the CTM644 to the CTM640. One of the legs didn't go through right on the regulator and got a snap on power on. After fixing that, one side of transformer shows 180VAC and DC (to 5V) side is 0V. Likely regulator is cooked

[darkhalf]

From the above measured about 170V on the transformer high volt side, and 0V on the 5V output. So coil isn't getting proper voltage on the 3 part coil. Should be about 18V going into the LM78M6 from the coil here. 

Okay so I'm the third person with the same issue it seems
https://www.cpcwiki.eu/forum/hardware-related/ctm-640-help

I'm going to order another two regulators (this is starting to get expensive) assuming this one isn't or wasn't working and there are regulators on Aliexpress
https://vi.aliexpress.com/i/1005001751084814.html

[darkhalf]

Several problems happening. Did some more searching on this forum

https://www.cpcwiki.eu/forum/hardware-related/ctm640-has-power-but-no-picture-5v-out
https://www.cpcwiki.eu/forum/hardware-related/ctm644-problem-blown-flyback

I'm going to order another two regulators
https://vi.aliexpress.com/i/1005001751084814.html

 https://www.cpcwiki.eu/forum/hardware-related/ctm-640-colour-monitor-repair/
Okay after seeing this, I checked both flybacks, and pin 1 is shorted on both to the other coil same as this post

One is 37140041 / HR6355 which I'm hoping WES still stocks. (12 pin)
Other is 3714009 on the part but CTM644 service manual has 3714004 for the 10 pin part (HR7674) which I only see at donberg

[robcfg]

I would suspect your electrical installation, I've never seen in my life any CPC monitor go boom...

[darkhalf]

I've never seen in my life any CPC monitor go boom

The bang in the CTM644 (other thread) was the fuse and two Zener diodes getting taken out. Literally the fuse is in three pieces!

So the path of destruction seems to be coils in flyback shorting, and nearby driver transistor (maybe) and then power circuits. So its taking out parts as it goes. I've highlighted the parts in the images

red=dead parts found and orange = things to check which have been reported by others. The first thing to check is the flyback at pin 1 to other pins. If there is a short to the drive winding side  (pins 6,5,11) then that seems to be the primary cause for everyone posting about these so far
https://www.repairfaq.org/sam/flytest.htm

I notice that the 2SD1397 has a diode from emitter to collector. So if I am diode testing this, would that be base > emitter and emitter > collector only should show a reading?

So I'm waiting STK7308 to arrive for the CTM640, but depending on sourcing flybacks it may be used in the CTM644 instead if I cannot get a flyback for it

[darkhalf]

Schematics for the HR6355 (CTM640) 12 pin
https://www.hrdiemen.com/reparation/flyback/scheme/6355

Schematics for the HR7674 (CTM644) 10 pin
https://www.hrdiemen.com/reparation/flyback/scheme/7674

Did think about using HR7674 to retrofit into HR6355 foot print but there are too many differences

[darkhalf]

Working now will put details in here
https://www.cpcwiki.eu/forum/hardware-related/ctm-640-colour-monitor-repair/25/

Basically flyback went through various circuits and took a bit of time to work out which ones. Took out fusable resistors, diodes, and a couple of transistors plus the regulator

[darkhalf]

Images of the repair. Schematic in other thread