failed transformer workaround - Page 1
Forum section: electronics / repair

author: mian2zi3
author_rank: Contributor
timestamp: December 09, 2015, 04:36:30 am
content: I recently picked up a cheap dead HP 3478A on the 'bay. The first section of the primary winding is broken. I'm in the US (120). There are unused sections for 240. To fix it, I had the idea of moving the line voltage from between 0-120 windings to the 120-240 windings. Anyone see a reason why that wouldn't work?

author: poot36
author_rank: Frequent Contributor
timestamp: December 09, 2015, 05:46:14 am
content: It may work but I would try repairing the original winding first. How to do this is disconnect the output windings and into the broken input winding feed a high voltage low current supply (I have used a dc powered ccfl transformer) and watch the current consumption of the high voltage supply and when it increases you know that you have effectively arc welded the broken wire inside the transformer back together. If it does not work try your idea of the winding swap, my only concern is that the winding will be in a slightly different location on the transformer and would be made for 50Hz instead of 60Hz.

author: mariush
author_rank: Super Contributor
timestamp: December 09, 2015, 05:56:00 am
content: You should also check if there's a thermal fuse hidden somewhere under the tape of the transformer, the transformer may have overheated at some point and blew the fuse.If that's the case, then also check the bridge rectifier and linear regulators or whatever is connected on the secondary winding, something may be shorted there and pulling too much from the transformer overheating it.

author: mian2zi3
author_rank: Contributor
timestamp: December 09, 2015, 06:03:37 am
content: Interesting idea! I had no idea that was possible. I don't have a readily available high-voltage supply, but I can probably cook something up. I guess there is some danger of damaging the components on the secondary side? One set of secondary windings is soldered directly to the PCB. It will be some work to remove the transformer.There is a soft switch that feeds through a buffer to the one of the processor data lines. It seems like any 50/60 Hz sensitivity is handled in software. The specs say it supports 120/240 at 50 or 60.

author: poot36
author_rank: Frequent Contributor
timestamp: December 09, 2015, 07:04:14 am
content: 1+ for the thermal fuse. I have only zaped a audio output transformer for a tube amp so it is a bit different then a standard line voltage transformer (close to 10 K ohms of resistance versus the sub 300 ohms on most standard transformers). My ccfl transformer was out of an old scanner film slide adapter and runs off anywhere from 3V to 24V (standard single transistor based oscillator with feedback winding) very simple to use just slowly increase the input voltage and watch the current consumption. It could be possible that someone tried to run the unit on 240V and poped the transformer. I would check all the outputs for shorts if this fault is suspected.

author: kripton2035
author_rank: Super Contributor
timestamp: December 09, 2015, 07:12:59 am
content: ++1 on thermal fuse at the primaryhad this once for a battery charger. most difficult was to remove kindly all the transformers flat iron piecesthere are videos opn youtube for that, then the fuse is not very far you dont have to rewind all the transformer only a few turns.

author: MarkL
author_rank: Unknown
timestamp: December 09, 2015, 07:39:08 pm
content: Quote from: mian2zi3 on December 09, 2015, 04:36:30 amThat's an interesting idea and I think it would work, at least in theory.I would verify that the windings for the 120-240 primary are of the same wire gauge. It's possible they made them thinner because it's only carrying half the current at 240V, but admittedly not likely since they'd have to interrupt the winding in manufacturing to do this.A DC resistance reading of the primary can tell you. My 3478A has a primary of 58.9 ohms on 120V and 121.3 ohms on 240V. Other data points are 110.3 ohms on 200V and 49.8 ohms on 100V. So, everything being proportional says it's a single winding of the same size wire, and therefore would have the same rating on the 120V-240V tap.And it does power on with the 120V-240V tap. I tried it.However, the other posters have a point about a possible thermal fuse. I've never seen them in this type of equipment, but if there is one and you did the 120V-240V trick, you'd be overriding it. You'll have to be the judge of the risk.

author: edpalmer42
author_rank: Super Contributor
timestamp: December 09, 2015, 07:55:44 pm
content: One time I had a piece of equipment with dual primaries and since I'm in 120V territory, the primaries were in parallel. Butbothof them were open. I looked at the transformer and found that both wires had broken right at the point where they were soldered to the terminals. A little careful soldering got both sections working again.You might want to inspect your transformer carefully. Maybe you'll be lucky.Ed

author: Jeroen3
author_rank: Super Contributor
timestamp: December 09, 2015, 08:30:37 pm
content: Also make sure there is no short between primary and secondary, or primary and earth. Due to excess heat the isolation varnish might turn brittle and crack risking short.This can make some spectacular fireworks.