General Category > Lisa Troubleshooting and Repair
Help needed for Apple Lisa PSU Transformer 120VAC resistance
greniu:
Hello,
I am asking for help if anyone can measure the value of resistance between 2 and 5 pins of his working T2 Transformer (attached picture)?
This is for Apple Lisa PSU 120 VAC, 60 Hz, 1.2 Amp, 150W which isn't working.
I have also another working PSU 1.2 AMP but dedicated for 220-240 V~, 50 Hz, .7 A, 150 W. The transformer looks almost the same, but it has different number, so it may have different resistance. Resistance between 2 and 5 pins in this PSU is 1 ohm and I am looking this value for 120VAC version.
I appreciate any help.
Thank you
sigma7:
--- Quote from: greniu on August 04, 2023, 09:41:27 am ---I am asking for help if anyone can measure the value of resistance between 2 and 5 pins of his working T2 Transformer (attached picture)?
This is for Apple Lisa PSU 120 VAC, 60 Hz, 1.2 Amp, 150W which isn't working.
I have also another working PSU 1.2 AMP but dedicated for 220-240 V~, 50 Hz, .7 A, 150 W. The transformer looks almost the same, but it has different number, so it may have different resistance. Resistance between 2 and 5 pins in this PSU is 1 ohm and I am looking this value for 120VAC version.
--- End quote ---
When I measure the DC resistance between the power input pins (from outside the PSU), the resistance is about 247 ohms. With the power off, I think the DC path between the terminals is essentially the primary of T2.
If the resistance between those pins of your transformer is 1 ohm, then I suspect there is a short circuit... connecting 1 ohm to 220V will draw enough current to blow a fuse vigorously.
I expect you are asking about this to repair a PSU; is it the 120V or 220V variant that you are repairing?
HTH, James
greniu:
Hi James,
Thank you for the answer.
--- Quote from: sigma7 on August 04, 2023, 12:17:15 pm ---When I measure the DC resistance between the power input pins (from outside the PSU), the resistance is about 247 ohms. With the power off, I think the DC path between the terminals is essentially the primary of T2.
If the resistance between those pins of your transformer is 1 ohm, then I suspect there is a short circuit... connecting 1 ohm to 220V will draw enough current to blow a fuse vigorously.
--- End quote ---
I have 1k Ohm on those pins in working 220 V PSU when it is powered off. This is the same value thatI get on 2 and 5 pin on T2 transformer.
--- Quote from: sigma7 on August 04, 2023, 12:17:15 pm ---I expect you are asking about this to repair a PSU; is it the 120V or 220V variant that you are repairing?
--- End quote ---
Yes. My non working PSU is 120V version. I have two of them and both have the same problem and values on those T2 transformer pins. When I measure ohm with multimeter on powered off PSU it shows OL. So yours has 247 ohms and its is 120V PSU right?
Greg
sigma7:
--- Quote from: greniu on August 04, 2023, 02:15:28 pm ---I have 1k Ohm on those pins in working 220 V PSU when it is powered off. This is the same value that I get on 2 and 5 pin on T2 transformer.
My non working PSU is 120V version. I have two of them and both have the same problem and values on those T2 transformer pins. When I measure ohm with multimeter on powered off PSU it shows OL. So yours has 247 ohms and its is 120V PSU right?
--- End quote ---
Yes, I measured 247 ohms across the primary winding of T2 in a 120V 1.2A PSU.
I removed the cover to check at the transformer itself (in circuit) and it was the same. I did find that there is varnish or other insulation on the transformer pins, so had to move the probes around to get good contact. If you are getting infinity ohms (usually what OL means in resistance mode, although it may mean "out of range" if your resistance range is manually set to a smaller value than you are measuring), then double check you are making contact, or measure at the power inlet and verify the fuse is intact.
I imagine that connecting the PSU with the 120V version of T2 to 220V may cause T2 to fail open... is that your suspicion too? Is there any visible damage such as scorched varnish?
greniu:
--- Quote from: sigma7 on August 04, 2023, 03:14:36 pm ---I imagine that connecting the PSU with the 120V version of T2 to 220V may cause T2 to fail open... is that your suspicion too? Is there any visible damage such as scorched varnish?
--- End quote ---
I think that thermal protector can be damaged in this T2 (if such exists inside). That's why I am getting OL in multimeter when I am measuring resistance. I have automatic multimeter Testo 760-3 ant it is definitely working. I cannot see any visible damage on T2.
I have also noticed that 220V T2 version has different number than 120V.
120 V is Excel Transformer 157-0015-A, 220v is Excel Transformer 157-0016. So they may be two different transformers with different resistance.
Do you know when I can get spare T2?
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