Hi!
I read your post about 4 posts back in which you thought of trying to attempt to get it to work without the voltage feedback–DON'T!
All you will do if you attempt this is very rapidly destroy replacement MOSFETs/switching transistors in milliseconds because you'll prevent the thyristor switching on to discharge the MOSFET gate charge at the switch–off point in each operation cycle – the reason is that when the MOSFET drain current reaches a high enough point to saturate the transformer core, the induced voltage from the feedback winding that normally swings negative to turn off the MOSFET/transistor pair is prevented from being developed by transformer saturation, resulting in the devices being left turned on across the rectified mains h.t. supply, and they'll go bang, spectacularly!
The control regulation works by using the opto to sense when the output from the appropriate secondary of the transformer reaches the desired value set by the preset output control and the designed o/p voltage, the opto will then conduct sufficiently between the emitter and collector of it's photo–transistor to pass a gate trigger current into the gate/cathode junction of the thyristor, causing it to fire, which then short–circuits the gate charge and the base current from the switching devices V6014 and V6018, which switch off and the primary current falls to zero. When the magnetic field in the windings of the supply transformer has collapsed, no further feedback voltage is supplied from the windings, however, a very small bleed current from the positive mains h.t. rail is then fed into the gate of V6018 and the base of V6014 via resistors R6002 and R6003, causing the switching devices to commence to conduct, and a rising current flows in the primary of the power supply transformer, producing a gradually rising positive voltage across the feedback winding again, until the point is reached where the opto H6001 starts to conduct again, and the cycle repeats with the thyristor firing as before!
It's never necessary to attempt to run this design of self–oscillating power–supply open loop – if the transistors, thyristor and the feedback opto are all in good condition along with all the small components in the loop, the feedback loop is quite fast enough to take control of the switching stage before any damage to the transistors occurs!
Properly repaired, these thyristor controlled blocking–oscillator supplies will come on quite safely with no need for any special precautions other than that provided by Philips for normal surge–limiting purposes!
They are also capable of withstanding a short–term momentary output short–circuit, under these conditions the operating frequency drops to a very low value, causing a very obvious loud rattle or squealing noise from the power supply transformer, that immediately gives away an overload or short–circuit fault!
The correct way to repair these type of power supply circuits is ONLY by checking each item meticulously against the maker's diagram for burnous, shorts or open–circuits as you go along!
You can also use the plastic T0-92 case BRY55/BRY56 or the X0102DA thyristor in this circuit as the anode gate terminal is not used in this circuit – it is simply connected to the anode via R6027 to keep the junction held off!
If you try one of the plastic–case thyristors, you want one advertised as "200V 0.8A fast switching sensitive–gate type", connect the anode lead to the Main FET gate/test point T45, the gate lead to the junction of R6010/R6011/R6012 and pin 4 of H6001, and the cathode to power supply H.T. – line (negative side of h.t. reservoir capacitors C6007/C6008) or common negative for the mains rectifier diodes.
A common cause of failure of this type of self–oscillating power supply is the high–resistance feed from the positive rectified mains h.t. going open circuit or high resistance – this feed is provided by the two series resistors R6002 and R6003 – you can replace these with 390k standard values provided you fit ±1% tolerance components!
If the chopper FET V6014 and/or the series switch transistor have been destroyed, you will need to source the correct replacement devices for this circuit (eBay) and check carefully all the small diodes and resistors around these two transistors against the diagram, replacing any damaged or open/circuit items.
The LED part of the opto–coupler H6001 can go low–emission to the point where the control is no longer effective to control the h.t. output from the power supply, and it will try and increase the drive more and more to try and restore the missing feedback until something goes bang, but replacement with a new one will eliminate any doubt on that score!
As long as you use the correct transistors for V6014 and V6018, take care over replacement of the thyristor V6006 with the exact type or a suitable replacement of the type I suggest, and pay careful attention to all the smaller components throughout the primary circuit, you will be rewarded with success – I have repaired many hundreds of monitors and other items of equipment using this type of design over a good few years!
Don't forget, you MUST also replace all the Electrolytic Capacitors on the output side of the power supply transformer and the smaller capacitors in the primary side of the power supply once you have replaced all the damaged components!
Chris Williams
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