Power transformer trouble

[bandade]

I have a MC 18 watt PT and OT I used in a Lite IIb build, I converted the amp to a superlite and now it seems like I have PT issues when i tried to start it up. All my heater voltages are ok and my wall AC is 125. The problem is the high voltage leads are putting out 350V on one and 0 volts on the other which after the EZ81 is giving me only around 150V on my standby switch.

I tried switching the leads on the tube socket and it seems to be the transformer lead. I tried a new tube and a solid state rectifier with the same results.

Any Ideas?

Thanks

[passfan]

Is the high voltage winding center tapped and if so is it grounded ?

[bandade]

[quote="passfan"]Is the high voltage winding center tapped and if so is it grounded ?[/quote]

Yes its center tapped and it is grounded with the heater center tap.

[Phil_S]

Lift the center tap from ground and measure resistance from each end to the center tap. The ohms should be close but not equal. You can get the ends at the tube sockets without desoldering. Just pull the power tubes.

[bandade]

[quote="Phil_S"]Lift the center tap from ground and measure resistance from each end to the center tap. The ohms should be close but not equal. You can get the ends at the tube sockets without desoldering. Just pull the power tubes.[/quote]


I have 86ohms on one lead and 1.8Mohms on the other. Not good.

[ToneMerc]

Lift the center tap from ground and measure resistance from each end to the center tap. The ohms should be close but not equal. You can get the ends at the tube sockets without desoldering. Just pull the power tubes.

I have 86ohms on one lead and 1.8Mohms on the other. Not good.

Yep, looks like it's open

[bandade]

I've been emailing a tech rep all day performing tests and he says it sounds like the HV winding is open. How does this happen?

It was working fine before I changed the circuit from a Lite IIB to a superlite. I didn't even desolder any leads except for the primaries...

We will see what he says tomorrow.

[Phil_S]

86 is reasonable. 1.8M is not open, it is shorted. Not that it matters. The PT is hosed. It shorts when the varnish on the winding melts from overheating. This could happen on account of any number of things. You may have done something you didn't see, maybe a whisker to ground, tubes acting up, who knows? This is hard news to take, but the only reasonable thing is to get another PT.

Unless you think you can get it replaced for free (unlikely), here is a good quality, economical replacement for an 18W: http://www.musicalpowersupplies.com/4.htm

275-0-275 is close enough to the typical 18W.

Or you could go for this one if you think the 15V on each leg actually makes a difference http://www.classictone.net/40-18035.html

[Richie]

Also pull the recto tube and measure again with it out.if your measuring the pins or connections on the socket.

[M Fowler]

It was working fine before I changed the circuit from a Lite IIB to a superlite. I didn't even desolder any leads except for the primaries...

You mean you unsoldered the secondaries when you change it to superlite right?

Are you using the MC 40-18035 HT secondaries are (red)290 CTred/yellow 290(red)

Yellow and white are used for the EZ81 rectifier 6.3v

Green for filament for the rest of the tubes with CT grn/yellow


Mark

[bandade]

[quote="M Fowler"][quote]It was working fine before I changed the circuit from a Lite IIB to a superlite. I didn't even desolder any leads except for the primaries...[/quote]

You mean you unsoldered the secondaries when you change it to superlite right?

Are you using the MC 40-18035 HT secondaries are (red)290 CTred/yellow 290(red)

Yellow and white are used for the EZ81 rectifier 6.3v

Green for filament for the rest of the tubes with CT grn/yellow


Mark[/quote]


That is the MC transformer I am using. I disconnected the primaries because I had to move my power switch to the back panel and needed to move around some wires. I do have the other secondaries connected as described above and the voltages are normal. I swapped the two leads of the HV secondary to see if there was a problem with my tube socket or the leads.

[M Fowler]

Well crap now I don't know, I was thinking it was the rectifer wiring.

[bandade]

MC is sending me a new PT. They have great customer service from what I've experienced. Triode took 3 days to respond to my email and phone call.