Repair of a Keithley T179 multimeter / Transformer rewind - UK Vintage Radio Repair and Restoration Discussion Forum

dave cox · 2nd Oct 2020, 4:46 pm

I put out a 'Parts Wanted' request about a month ago for a transformer for this multi-meter. Given that 'Parts Wanted threads have a limited life I am replicating the information here while I try for a rewind

The original item had gone intermittent on a low voltage winding. This is a PCB mounted low profile custom made transformer and was very generously lacquered. I assumed rewinding was not realistic as getting it apart looked impossible. There is little space in the case, not nearly enough for even the smallest Tx I had to hand (6VA) let alone any way of fastening it down if there was any space.

There are two secondary windings on this Tx. One for a 5V linear regulator, and a second one for 15-0-15 linear regulators. The '5v' winding also has a tap which is switched when changing the internal battery pack. I don't have the battery pack and the idea of lugging this instrument around when I have a 1/2 decent hand held meter fills me with the intention of never having one!

dave cox · 2nd Oct 2020, 4:47 pm

I measured up the transformer, as detailed below ...

dave cox · 2nd Oct 2020, 4:58 pm

Laminations.
With help from Ed Dinning, this Tx has EI-48 laminations, with a 16.8 stack thickness.

Disassembly / winding details.
Attacking this Tx with a dremel cutting disk I managed to snap off the laminations at one end and started to unwind. First is the '5V' winding with the extra charging boost 'layer and a bit', this wire measure 0.47 to 0.5mm - with the lacquer. Next comes the 15-0-15, this measures 0.1mm but this is right at the limit of my calipers - flippin' fine for sure! . Next comes a copper tape connect to the 0V on the 15-0-15. I assume this doesn't make a shorted turn but my brutal disassembly makes this less than obvious After that comes another isolated copper tape, this time connected to earth and the lams (soldered on) but plenty of isolation from the other shield.

dave cox · 2nd Oct 2020, 5:11 pm

The primary winding consists of two equal windings, in series for 240V and in parallel for 120V. It would have made senses not to unwind this but some monkey with a dremel had trashed the windings deep enough to damage the primary

It proved to be quite difficult to remove the 'core' from the empty bobbin even after snapping off the remaining parts of the laminations, but by supporting the edges and given the center of the stack a smack with a hammer I eventually got it out.

dc

2nd Oct 2020, 4:46 pm	#1
dave cox Nonode Join Date: Jan 2009 Location: Bristol, UK. Posts: 2,061	Repair of a Keithley T179 multimeter / Transformer rewind I put out a 'Parts Wanted' request about a month ago for a transformer for this multi-meter. Given that 'Parts Wanted threads have a limited life I am replicating the information here while I try for a rewind The original item had gone intermittent on a low voltage winding. This is a PCB mounted low profile custom made transformer and was very generously lacquered. I assumed rewinding was not realistic as getting it apart looked impossible. There is little space in the case, not nearly enough for even the smallest Tx I had to hand (6VA) let alone any way of fastening it down if there was any space. There are two secondary windings on this Tx. One for a 5V linear regulator, and a second one for 15-0-15 linear regulators. The '5v' winding also has a tap which is switched when changing the internal battery pack. I don't have the battery pack and the idea of lugging this instrument around when I have a 1/2 decent hand held meter fills me with the intention of never having one! Attached Thumbnails

2nd Oct 2020, 4:47 pm	#2
dave cox Nonode Join Date: Jan 2009 Location: Bristol, UK. Posts: 2,061	Re: Repair of a Keithley T179 multimeter / Transformer rewind I measured up the transformer, as detailed below ... Attached Thumbnails

2nd Oct 2020, 4:58 pm	#3
dave cox Nonode Join Date: Jan 2009 Location: Bristol, UK. Posts: 2,061	Re: Repair of a Keithley T179 multimeter / Transformer rewind Laminations. With help from Ed Dinning, this Tx has EI-48 laminations, with a 16.8 stack thickness. Disassembly / winding details. Attacking this Tx with a dremel cutting disk I managed to snap off the laminations at one end and started to unwind. First is the '5V' winding with the extra charging boost 'layer and a bit', this wire measure 0.47 to 0.5mm - with the lacquer. Next comes the 15-0-15, this measures 0.1mm but this is right at the limit of my calipers - flippin' fine for sure! . Next comes a copper tape connect to the 0V on the 15-0-15. I assume this doesn't make a shorted turn but my brutal disassembly makes this less than obvious After that comes another isolated copper tape, this time connected to earth and the lams (soldered on) but plenty of isolation from the other shield. Attached Thumbnails

2nd Oct 2020, 5:11 pm	#4
dave cox Nonode Join Date: Jan 2009 Location: Bristol, UK. Posts: 2,061	Re: Repair of a Keithley T179 multimeter / Transformer rewind The primary winding consists of two equal windings, in series for 240V and in parallel for 120V. It would have made senses not to unwind this but some monkey with a dremel had trashed the windings deep enough to damage the primary It proved to be quite difficult to remove the 'core' from the empty bobbin even after snapping off the remaining parts of the laminations, but by supporting the edges and given the center of the stack a smack with a hammer I eventually got it out. dc Attached Thumbnails