Kit 34 at it again

[Richard]

Just to add that when the transformer connections are all undone it may be possible to run some resistance checks on the windings to confirm a fault.

I don't have any values but would expect to find a good one to have the 2 primary windings isolated from each other, and from the earth shield, and be very similar with ohms values in the tens or possibly hundreds of ohms.

[bob orbell]

You may need a mega to confirm that the transformer is bollocksed or not, if you do not find any problem with Richards tests, it may pay to have an electrician check the transformer with a mega. BOB PS dose the transformer smell burnt, please pull the power lead out before sticking your noes in the amp.

[John Caswell]

Jiten,
You say "I have disconnected the 2 black wires, 1 white wire, 1 purple wire and 1 grey wire. Even with the 5U4s out, the fuse blows".
No! you need to disconnect all the secondary wires - 2 black, 1 white, 2 purple, 2 grey, and 1 blue, to totally isolate the secondaries from the following circuits.
This will then just leave the primary connected to the power line.
If the fuse still blows when powered up, it is pretty certain that you have a faulty power transformer. Resistance measurements may help, but I doubt it.
Please follow the instructions to the letter otherwise you will be chasing your own tail and/or misleading others, then report back here.

John

[bob orbell]

Just to add, the blue wire may be yellow, it is on mine, it is the C/T from the 6.3v to the heaters, Jiten, please do as John has asked. BOB

[jiten]

Dear All,
I have now cut all the secondary wires from the power transformer. 2 grey, 2 black,1 white, 1 yellow(blue) going to earth, 2 purple.
I have checked the wiring and soldering to the iec , which has been undisturbed since the original build and the wires are secure.
I have taken off the insulation from the orange/brown and re-soldered just for completion and re-insulated.
The 5U4s are plugged in.

The fuse blows instantly!

There is no burning smell from the transformer and in fact no smell of heat from anywhere near the amplifier even before the first fuse blow. Now of course it does not remain warm for long enough as the fuse pops the moment the amp is switched on.
Apologies for not following all the instructions earlier.
Jiten.

[Richard]

Hi Jiten,

Back to post 20 then; test the transformer, or have it tested by someone, or replace it from Amethyst, indeed they may be able to test or rewind it for you.

The fuse blows with nothing connected to the secondaries. This could be shorted primary turns or shorted secondary turns (which may then draw an excessive load).

There are only the 2 windings on the primary side and their possible leak to earth, as you can see on the psu drawing, so I'd start with them as you can compare one with the other.

Separate them into brown/orange and red/blue and check the resistances of both. For a guide I just tried a small transformer here and found 24 ohms on each of its primary windings so you are looking in that range.

If you have an inductance meter you can check their inductances; the small one I have here is 0.6H on both primary windings. If one winding was short I'd expect the inductance to be less.

If you have a megger check them for insulation leaks at 500V as Bob suggests.

If you have a scope arrange the winding with a small capacitor and check the trace for ringing.

If nothing seems wrong move to the secondaries.

[John Caswell]

Hi all,
It is pretty certain that the power transformer is faulty I suspect shorted turns in the primary. There is no solution but replacement or rewind.

John

[Richard]

Yes, if the previous tests don't show any fault though, before scrapping it I would hook the transformer to a variac (via a fuse) and wind up the voltage gradually whilst monitoring the unloaded output on the secondaries. Start at 24V AC and look for a tenth the expected voltage on the output (if the fuse doesn't blow immediately) and wind it up from there.

[John Caswell]

Hi all,
As Richard says but better still, if you have it an ac ammeter/DMM place it in series with the primary of the txand wind the variac up
Usually if the transformer is u/s it will draw 2-3A when fed about 5 v ac.

John

[jiten]

Dear All,
Thank you for your help. I do not have the equipment to go into a detailed test of the transformer. Plus my knowledge of electronics is limited and would need help all the way.
As suggested, I contacted Amethyst who can supply a new one to original spec for £135.
I think I will just have to get the new one.
If that fails.......
Thank you again for all the help.
Kind regards,
Jiten.