Hi,
I am restoring a Bridgeport milling machine and trying to repair the table power feed system. The table is driven by a DC brushed motor with 100V DC going to the field and a variable up to 100V DC to the armature. Although I have not fully tested the motor its field DC resistance and armature resistance at approx 500 ohms and 10 ohms seem reasonable. My problem is in the control board that provides the field DC supply and the armature variable supply.
It is 40 years since I did any Electronics but I believe it is a PWM (Pulse width modulation) unit. The circuit diagram is attached along with a separate list of component.
It is being fed by a 110V AC supply and I get 100V DC across the field output at Tp 8 and 9 but 0 volts across the armature at 7, 8.
My initial attempts at diagnosing the fault centred around the various electrolytic capacitors but all tested ok using an ESR tester. I also checked R1 and R3. I then found that the secondary windings of the pulse transformer T2b and T2c were open circuit and prematurely jumped to the conclusion that this was the problem without thinking why they should have failed in the first place. I was not able to identify the pulse transformer part number as the number shown in the circuit diagram and the number stamped on it appears to refer to a Mullard E ferrite core only and does not give any clue to the windings. So I made a bit of a "guess" and went for a 1:1+1 unit from Vigortronix VTX-110-006 on the basis that it had roughly the same DC primary winding resistance, the only thing left form me to measure, a 200mA max pulse current and that it would physically fit the board.
I fitted it applied power and bang the pulse transformer blew immediately. So lesson learnt, why did I not find out why the original transformer had blown in the first place! To do that I turned my attention to the Thyristors (SCRs) Th1 and Th2 and tried to test them with a multimeter.
There is an open circuit between anode and cathode as one would expect but briefly connecting the gate to the anode does not switch the thyristor on. The multimeter is putting out about 2.2V so it should be enough to turn the SCR on. I then put the meter across the anode and gate and get a virtual short circuit either way the leads are connected. Surely this cant be right and think it is why the pulse transformer blows as it is getting best part of 110V AC across its secondary windings.
Sorry this is rather long but my questions are:
1. Is my diagnosis of faulty SCRs correct and if so what might have caused them to both fail? Sorry but I have no history of the unit at time of failure.
2. Is it possible to specify the pulse transformer rather than me just guessing.
Any help / suggestions on how to proceed further most welcome.
David
I am restoring a Bridgeport milling machine and trying to repair the table power feed system. The table is driven by a DC brushed motor with 100V DC going to the field and a variable up to 100V DC to the armature. Although I have not fully tested the motor its field DC resistance and armature resistance at approx 500 ohms and 10 ohms seem reasonable. My problem is in the control board that provides the field DC supply and the armature variable supply.
It is 40 years since I did any Electronics but I believe it is a PWM (Pulse width modulation) unit. The circuit diagram is attached along with a separate list of component.
It is being fed by a 110V AC supply and I get 100V DC across the field output at Tp 8 and 9 but 0 volts across the armature at 7, 8.
My initial attempts at diagnosing the fault centred around the various electrolytic capacitors but all tested ok using an ESR tester. I also checked R1 and R3. I then found that the secondary windings of the pulse transformer T2b and T2c were open circuit and prematurely jumped to the conclusion that this was the problem without thinking why they should have failed in the first place. I was not able to identify the pulse transformer part number as the number shown in the circuit diagram and the number stamped on it appears to refer to a Mullard E ferrite core only and does not give any clue to the windings. So I made a bit of a "guess" and went for a 1:1+1 unit from Vigortronix VTX-110-006 on the basis that it had roughly the same DC primary winding resistance, the only thing left form me to measure, a 200mA max pulse current and that it would physically fit the board.
I fitted it applied power and bang the pulse transformer blew immediately. So lesson learnt, why did I not find out why the original transformer had blown in the first place! To do that I turned my attention to the Thyristors (SCRs) Th1 and Th2 and tried to test them with a multimeter.
There is an open circuit between anode and cathode as one would expect but briefly connecting the gate to the anode does not switch the thyristor on. The multimeter is putting out about 2.2V so it should be enough to turn the SCR on. I then put the meter across the anode and gate and get a virtual short circuit either way the leads are connected. Surely this cant be right and think it is why the pulse transformer blows as it is getting best part of 110V AC across its secondary windings.
Sorry this is rather long but my questions are:
1. Is my diagnosis of faulty SCRs correct and if so what might have caused them to both fail? Sorry but I have no history of the unit at time of failure.
2. Is it possible to specify the pulse transformer rather than me just guessing.
Any help / suggestions on how to proceed further most welcome.
David
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