This scenario (below) applies to a time 0... the time you plug the mains cable into the unit IEC socket (CN201):
Have a look at the IC210 block diagram.... for IC201 to start "oscillating" i.e. for that IC201 to start working correctly at time 0, the Vstr pin 5 needs to see sufficiently high voltage to fool the IC into thinking that the Vcc is present. If the current through that R233 (100K) is just right & sufficient, this will happen. Now, this will happen at time 0 if the current through that NTC is correct... because the voltage at the R233 top end depends on the mains voltage rectified by the D202-D205... and ultimately... by the current running through that NTC.
So, at time 0, the IC must be powered up by the voltage at the junction of the R233 and pin 5. If this voltage is correct, the comparator will create Vref, which in turn will start the proper oscillating operation of that IC201. At the exact same time, the Vstr is not needed any more... and the Vcc is now derived by the ongoing oscillating operation of that IC201... via D207, R203, R236, ZD201 and ZD203 (and C205). Once the IC201 is working, you can turn the power to the unit ON... because finally -> you will have the standby rails sitting at +5 volts:
Once the STB +5V is stable and of a correct value, the front ON button will be able to turn the RLY201 on.... which will in turn power up the power transformer... and the whole unit will start working correctly.
The NTC 3D-15 has (I think.. just check the spec sheet...) cold resistance of 7 ohms at +25 degC. If you cool it down to +8 degC, Its cold resistance will be probably something like 10-15 ohms and its current capabilities will decrease as well. So, the fact that you can get that IC201 to kick-start by cooling the NTC probably means that the NTC is out of spec....
I wrote probably because... you also need to check the PCB eyelets!!! That NTC would be dissipating a fair amount of heat... the solder joints may be cold. When you cool down the PCB, the eyelets may contract closer to the NTC legs (squeeze the legs more tightly)... ensuring good contact and ample current capability.
By replacing the NTC, you will automatically address the possible cold solder joint situation
Good luck, and keep us posted...
That IC201 is in DIL8... I'd put a little heatsink on top of it...
Have a look at the IC210 block diagram.... for IC201 to start "oscillating" i.e. for that IC201 to start working correctly at time 0, the Vstr pin 5 needs to see sufficiently high voltage to fool the IC into thinking that the Vcc is present. If the current through that R233 (100K) is just right & sufficient, this will happen. Now, this will happen at time 0 if the current through that NTC is correct... because the voltage at the R233 top end depends on the mains voltage rectified by the D202-D205... and ultimately... by the current running through that NTC.
So, at time 0, the IC must be powered up by the voltage at the junction of the R233 and pin 5. If this voltage is correct, the comparator will create Vref, which in turn will start the proper oscillating operation of that IC201. At the exact same time, the Vstr is not needed any more... and the Vcc is now derived by the ongoing oscillating operation of that IC201... via D207, R203, R236, ZD201 and ZD203 (and C205). Once the IC201 is working, you can turn the power to the unit ON... because finally -> you will have the standby rails sitting at +5 volts:
Once the STB +5V is stable and of a correct value, the front ON button will be able to turn the RLY201 on.... which will in turn power up the power transformer... and the whole unit will start working correctly.
The NTC 3D-15 has (I think.. just check the spec sheet...) cold resistance of 7 ohms at +25 degC. If you cool it down to +8 degC, Its cold resistance will be probably something like 10-15 ohms and its current capabilities will decrease as well. So, the fact that you can get that IC201 to kick-start by cooling the NTC probably means that the NTC is out of spec....
I wrote probably because... you also need to check the PCB eyelets!!! That NTC would be dissipating a fair amount of heat... the solder joints may be cold. When you cool down the PCB, the eyelets may contract closer to the NTC legs (squeeze the legs more tightly)... ensuring good contact and ample current capability.
By replacing the NTC, you will automatically address the possible cold solder joint situation
Good luck, and keep us posted...
That IC201 is in DIL8... I'd put a little heatsink on top of it...
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I think Vstr at the time 0 needs to be between 8 and 12V... so while the front LED is blinking (i.e. the unit is not powering up), measure this voltage at pin 5 and let us know the value.
CAREFUL!! the top end of R233 will be sitting at around 350V DC...
... by the way... is that R233 100Kohms? Lift one side and check with the ohm meter. I'd re-flow the whole PCB with soldering iron and rosin core solder.
CAREFUL!! the top end of R233 will be sitting at around 350V DC...
... by the way... is that R233 100Kohms? Lift one side and check with the ohm meter. I'd re-flow the whole PCB with soldering iron and rosin core solder.
Sure but I will wait until the TH201 is changed. I soldered the area again and sometimes it comes up sometimes not. My observation is that it works better if the AC was plugged out for a while. I still rely on the TH201. I will change it next week and keep you posted.
Just to eliminate the obvious - are you measuring pin 5 against pin 1 or chassis ground?
But what could be the explanation? If R233 is shorting I would assume that the device would never come up.
That might be okay... because there is no current running through the R233.... so Pin5 will see the full DC voltage after D202-D205. The problem is that IC210 never starts so the voltage drop stays high (floating). The moment IC205 starts operating, the current through R233 will increase, and you should get around 8-12V DC at Vref.
Which means... R233 will dissipate a fair amount of heat, and so would IC201 (which may have developed a secondary breakdown-like problem... very tricky to identify..)
DIL8 for IC205... is just crazy small; is that R233 1/4W?
So I just measured R233 and it is ok with 100kohm. Could you explain a bit more what to buy? Are we talking about an 7805 IC ?
All you need is to provide 5V at 0.5A to the main board via CN205. So, a 5V 1A (5W) SMPS (MeanWell is reliable) will do fine. You can even get a smaller SMPS.
https://www.digikey.com.au/en/products/detail/mean-well-usa-inc./PS-05-5/7705819
I will post below what to do on the original SMPS PCB (which unfortunately you'll have to keep as well because of all of the connectors... and that important RLY201...)
https://www.digikey.com.au/en/products/detail/mean-well-usa-inc./PS-05-5/7705819
I will post below what to do on the original SMPS PCB (which unfortunately you'll have to keep as well because of all of the connectors... and that important RLY201...)