Edcor XPWR059 power problem

[webby]

Hi All,
Tried in the Tubelab forum first, sorry for double posting.
My Simple SE has been running without a problem for about a year now, then a couple of days ago the fuse blew while listening to music after a few hours.
It has never done this before, now the fuse constantly blows (1A slow blow)
I tested the amp as follows
No FRED diodes. I use the 5AR4
XPWR059 wired for 240v

Blowing fuses suggests that something is shorted to ground,
checked for continuity;
The hot & neutral tabs of the IEC receptacle
Both tabs on the fuse holder
power switch

None of these indicate continuity, regardless of whether the power switch is open or closed.

Measured the XPWR059
The 6.3V winding 0.4ohm
The 5.0V winding 0.4ohm
The filter inductor (choke) & run cap not connected.

HT secondary coming from the power transformer.
The two red wires 25ohm

CXS8E 25-8-5K
Primary = 4.6Ohm
The secondary windings of the output transformer 0.7 ohm
primary 83 ohm.

So I disconnected all the XPWR059 secondary & Centre tap from the PCB, so the only connection is the power switch + Primary + red HV to my DMM.
I was expecting to test the HV then 6.3v & 5v.
Fuse blows!
The tubes are out at this stage, and the PCB is not connected.
Also the Xfrmr is removed from the chassis, free standing.
just using the switch,fuse & power connector.
I was checking for shorts on the leads.

Does this mean the power xformer is faulty?

Any further tests of the Xfrmr i can do to to determine if its faulty?
Many thanks
Ian

[M Gregg]

webby,

You need to eliminate everything. So take the Tx out of the chassis do not use the power connector and connect a 3 core mains lead with a 1A-3A fuse in the plug. Connect all the output secondary windings to a connector block so they are isolated from each other then power up and see what happens. It may be the Tx it could be the mains filter etc. so test the Tx on its own. Remember to Gnd the chassis of the Tx! If it blows with a 3A fuse its stuffed. You could remove the end bells to check you have no trapped cables.

If the mains filter has flashed over it may not show without a megger test! A multi meter output voltage is not enough to recreate the flash over!

Regards
M. Gregg

[kevinkr]

Check the power switch as well, I've had more than one instance where the insulation between the contacts and switch bat failed. Turn on the amp, fuse blows since I use a safety ground. This is a good argument for making sure the safety ground connection is there and working correctly.

Not unlikely that there is a short in the high voltage windings, the clue is whether or not there is a gross difference in winding resistance between the two outer legs of the winding and the center tap. Incidentally 25 ohms sounds a bit low to me, OTOH I have no direct experience with this particular power transformer.

[webby]

Thank you both for the suggestions, very helpful.
Last night i tried the TX wired direct to the power cord, it blew the 3A fuse instantly.
I earthed the TX and used a connector block as advised, and used the mains switch to turn on power for safety.
If I measure from the centre tap and each of the red HV leads, they read 18ohms & 43ohms. one is more than double.
Can I conclude this TX is faulty?
Many Thanks
Ian

[Doz]

Yep, but well worth removing the bells first to see if there's a wire trapped.

[webby]

I did have a look under the covers, nothing amiss with the wires i could see.
Cheers

[M Gregg]

I would contact Edcor and see what they say!
You can E-mail them. You have nothing to loose!

I know its been 12 months, however I would expect it to last longer than that with no manufacturing defects!

Regards
M. Gregg

[ryuji]

Transformer primary windings are generally more sensitive then secondaries, but the resistance difference between the secondaries is rather telltale.

my guess: you have a short in your amps HT circuit which melted the secondary windings, and then they shorted together. check the rectifier for shorts before you try another TX. lightbulb trick might be worth while if you do not have a tester

does the fuse blow with nothing wired to the secondaries at all? its a totally trashed transformer in this case, otherwise you have a potentially partialy functioning transformer. for instance if just one HV winding is shot you could use it for running a SS rectifier on the remaining winding, or use the 50v, 5v, or 6.3v windings for future projects.

Edcor is a excellent company, its definitely worth writing or calling them up. they might be interested in seeing what failed about it and give you a new one

[boywonder]

Originally Posted by ryuji

my guess: you have a short in your amps HT circuit

Simple enough to check...since the mains transformer is out of the circuit, just measure resistance from the B+ connection to ground.

[ryuji]

Originally Posted by boywonder

Simple enough to check...since the mains transformer is out of the circuit, just measure resistance from the B+ connection to ground.

yea real easy, could also be a sorted tube rectifier diode too tho stop caution is in order