HP4262A Power supply problem

[profun]

Recently, I got an HP4262A LCR in the second-hand market. When I turned on the power after cleaning, I saw a flash from the back of the device!!! The fuse is blown! I can't replace the fuse immediately until I find the fault. So I looked up the service manual, found the circuit of the power supply part, and then opened the chassis. I found the junction box of the transformer's primary winding, and then welded it down to measure the resistance value of the primary winding. The resistance value measured by the two primary windings is too low, only a dozen ohms. It is obvious that there is a short circuit inside the transformer.

The only thing I can do is replace the transformer. It is difficult to find the same transformer in the second-hand market, so I decided to order a new one. But I can't find the details of this transformer in the circuit diagram of the manual. The secondary output voltage, current and rated power, etc. Do you know who has the HP4262A to help measure the output voltage of the transformer?

[alm]

I don't have one of these, but from the schematic the transformer has an electrostatic shield to reduce capacitive coupling between the windings. So keep that in mind when ordering a replacement.

Is the unit wired for 115 V or 230 V? Are both primary windings shorted? Otherwise you could try feeding 115 V to just one of the primary windings and measure voltages and currents.

It would probably not too hard to figure out the minimum load voltages (including ripple) from the schematic, and then feed each rail DC voltage from bench supplies, which would allow you to measure dropout voltage and also current draw under various circumstances.

Note that after rectification the DC voltage is higher than the AC voltage but the maximum current is lower. By how much depends on the rectification scheme and amount of buffer capacitance.

[exe]

I just measured resistance of primary of my 120VA transformer designed for 230V (PN RT120-2015). It measured ~14.2 Ohm.

What I'd check first is that the voltage of the transformer is selected correctly for your country.

[profun]

Thank you for your response.
I have measured some similar transformers and found that their primary resistance values do not differ significantly, approximately 10-30 ohms. And I don't believe that two secondary windings are short-circuited at the same time. So I focused on the thin film capacitor connected in parallel with the power lines L and N, a capacitance of 0.22uf. Through measurement, it was found that this capacitance had been broken down and short-circuited. When I replace it with a new capacitor, the device can work normally.

[alm]

Congratulations on the simple fix!