transformer failure question

john blackburn · 6th January 2007, 07:54 PM

I'm on with my 1st chipamp, a chipamp.com LM3875 kit and I've had a bit of a disaster.
I hooked the transformer (2x22v) to the rectifier board to quickly check the output voltage and it all went horribly wrong resulting in no output from either secondary pair (both now showing open circuit when checked with the meter)
All the diodes are in the right way round, as are the caps and I can find no short circuits anywhere. The secondaries were paired as per the label.
The only thing that wasn't done, other than using the light bulb in series and a proper fuse (I know now!) was to solder the wires into the board. Could the poor contact have "spiked" the transformer?
Obviously I would like to find out what killed it before I connect it's replacement with the lightbulb this time.
I'm a newcomer to electronics but I've learned the price of hastiness!
Any suggestions please

Nordic · 6th January 2007, 08:20 PM

Strange that your mains didn't trip...

You getting 0v with nothing connected to the secondaries?

john blackburn · 6th January 2007, 09:07 PM

The mains didn't trip, there wasn't a bang, flash, smoke or even a buzz, just nothing happening.
I know the transformer was good as I checked the output on both secondaries before connecting it. It was around 25v with no load so I hooked the wires through and switched it on.
When I found no output from the rectifier I checked the transformer and I now had a 2x0v special paper weight.
Whatever happened, it happened instantly!
I have the second pcb and diodes from the chipamp kit to replace the rectifier completely, but until I learn where my mistake was theres a good chance I will make it again.
Im more than puzzled

Original Burnedfingers · 6th January 2007, 09:20 PM

Never,never,never put something together without a fuse. Could you post some pictures of the PC board? Also, what size filter caps do you have?

Netlist · 6th January 2007, 09:30 PM

Depending on the transformer, some have internal fuses build in. Because these are thermal fuses, your transformer should have been exposed to much heat before they melted. So this option is unlikely.

Some come with copper wire still shielded with lacquer.
This is needed to isolate the windings but the wire ends, where you make the connection, should be stripped and lacquer free.

Disconnect the transformer from the mains and the amp.
Again measure the primary and secondary with an ohm meter and make sure all lacquer is peeled off from the end of the wires. I use a sharp knife for the job.
Make sure the mains voltage is present and re-measure both primary and secondary voltages.

/Hugo

john blackburn · 6th January 2007, 11:00 PM

YOU SUPER STARS!!!
I now realise I twisted and cropped the leads to length between the 1st and 2nd measurings and as you so rightly said, they were lacquered!
Two minutes with a penknife blade and we have a wonderfull 24.4 Volts on each.
Thanks so much, I would never have considered that. The wire was lovely and clean and shiny and must be giving a good connection, how wrong can I be?
Many thanks to one and all
JB

Nordic · 7th January 2007, 06:38 AM

Stinking Teflon I guess....lol

Glad to hear you got sorted... I've done some silly stuff, but the mains always trips before my trannies die, even with fragile small pcb transformers...

AndrewT · 7th January 2007, 07:48 AM

More likely to be polyurethane.
It is formulated to melt back when dipped into a solder pot. Thus avoiding fatigue inducing scratches that are inevitable when scraping off the insulation.

Enamelled wire can be specified that has a higher temperature that does not self remove during soldering.

For testing & safety I use a terminal strip and secure EACH transformer wire to a SEPARATE terminal. That way nothing can inadvertently short or electrocute.

Have you built that light bulb/socket/plugtop yet?

Netlist · 7th January 2007, 08:33 AM

Its always nice to see my ramblings corrected and translated into proper terminology.

/Hugo

john blackburn · 7th January 2007, 06:42 PM

Netlist, there was no translation needed. You hit the nail on the head and did it in a second language.......... Impressive!

6th January 2007, 07:54 PM	#1
john blackburn diyAudio Member Join Date: Sep 2006 Location: Wirral UK	transformer failure question I'm on with my 1st chipamp, a chipamp.com LM3875 kit and I've had a bit of a disaster. I hooked the transformer (2x22v) to the rectifier board to quickly check the output voltage and it all went horribly wrong resulting in no output from either secondary pair (both now showing open circuit when checked with the meter) All the diodes are in the right way round, as are the caps and I can find no short circuits anywhere. The secondaries were paired as per the label. The only thing that wasn't done, other than using the light bulb in series and a proper fuse (I know now!) was to solder the wires into the board. Could the poor contact have "spiked" the transformer? Obviously I would like to find out what killed it before I connect it's replacement with the lightbulb this time. I'm a newcomer to electronics but I've learned the price of hastiness! Any suggestions please

7th January 2007, 07:48 AM	#8
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	More likely to be polyurethane. It is formulated to melt back when dipped into a solder pot. Thus avoiding fatigue inducing scratches that are inevitable when scraping off the insulation. Enamelled wire can be specified that has a higher temperature that does not self remove during soldering. For testing & safety I use a terminal strip and secure EACH transformer wire to a SEPARATE terminal. That way nothing can inadvertently short or electrocute. Have you built that light bulb/socket/plugtop yet? __________________ regards Andrew T.

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6th January 2007, 08:20 PM	#2
Nordic diyAudio Member Join Date: Sep 2005	Strange that your mains didn't trip... You getting 0v with nothing connected to the secondaries?

6th January 2007, 09:07 PM	#3
john blackburn diyAudio Member Join Date: Sep 2006 Location: Wirral UK	The mains didn't trip, there wasn't a bang, flash, smoke or even a buzz, just nothing happening. I know the transformer was good as I checked the output on both secondaries before connecting it. It was around 25v with no load so I hooked the wires through and switched it on. When I found no output from the rectifier I checked the transformer and I now had a 2x0v special paper weight. Whatever happened, it happened instantly! I have the second pcb and diodes from the chipamp kit to replace the rectifier completely, but until I learn where my mistake was theres a good chance I will make it again. Im more than puzzled

6th January 2007, 09:20 PM	#4
Original Burnedfingers diyAudio Member Join Date: Nov 2002	Never,never,never put something together without a fuse. Could you post some pictures of the PC board? Also, what size filter caps do you have?

6th January 2007, 09:30 PM	#5
Netlist diyAudio Moderator Join Date: Jan 2003	Depending on the transformer, some have internal fuses build in. Because these are thermal fuses, your transformer should have been exposed to much heat before they melted. So this option is unlikely. Some come with copper wire still shielded with lacquer. This is needed to isolate the windings but the wire ends, where you make the connection, should be stripped and lacquer free. Disconnect the transformer from the mains and the amp. Again measure the primary and secondary with an ohm meter and make sure all lacquer is peeled off from the end of the wires. I use a sharp knife for the job. Make sure the mains voltage is present and re-measure both primary and secondary voltages. /Hugo

6th January 2007, 11:00 PM	#6
john blackburn diyAudio Member Join Date: Sep 2006 Location: Wirral UK	YOU SUPER STARS!!! I now realise I twisted and cropped the leads to length between the 1st and 2nd measurings and as you so rightly said, they were lacquered! Two minutes with a penknife blade and we have a wonderfull 24.4 Volts on each. Thanks so much, I would never have considered that. The wire was lovely and clean and shiny and must be giving a good connection, how wrong can I be? Many thanks to one and all JB

7th January 2007, 06:38 AM	#7
Nordic diyAudio Member Join Date: Sep 2005	Stinking Teflon I guess....lol Glad to hear you got sorted... I've done some silly stuff, but the mains always trips before my trannies die, even with fragile small pcb transformers...

7th January 2007, 08:33 AM	#9
Netlist diyAudio Moderator Join Date: Jan 2003	Its always nice to see my ramblings corrected and translated into proper terminology. /Hugo

7th January 2007, 06:42 PM	#10
john blackburn diyAudio Member Join Date: Sep 2006 Location: Wirral UK	Netlist, there was no translation needed. You hit the nail on the head and did it in a second language.......... Impressive!

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