Help with fuses blowing at startup

[Ankaret]

I requested 4 replacement diodes from bottlehead today, so hopefully that invoice will come through soon and I can try replacing them in hopes of that solving things.

Also, I believe it was Josh(?) that I talked to, and he mentioned that I should double check I got the right transformer (120V, even though he thought mine was accidentally labeled as 240V). One way to check he said was to look for about 12ohm resistance between transformer terminals 1 and 2. But Im getting a reading of only 6 ohms here, what do you all think? Something wrong with my power transformer?

 

[rlyach]

I just measured my transformer, and I get ~12 ohms of DC resistance on the primary winding of the mains transformer. If you are getting 6 then I would assume you have half the windings and thus you probably have a 240V transformer. I will let one of the guys at BH confirm this.

 

[fullheadofnothing]

That is incorrect. A 240V PT-3 measures about 45Ω. I think Kyrill has a 120V transformer, and the exact reason that it is measuring lower than normal is unknown (quite possibly related to the method of measurement). What is important to know here is that it is not a higher reading, indicating that it is a 240V.

I knew that a few 120V PT-3 were mislabeled, and I was asking for confirmation that this was why a 240V label was visible.

 

[rlyach]

If that is the case... Then would a 6 ohm reading indicate a partially shorted primary winding?

 

[fullheadofnothing]

Some meters don't do well measuring low resistances. Some don't do well measuring inductive resistances. Placement of probes on the terminals has a major influence on the measurement, particularly when not using clip leads. Solder and flux on the terminals will influence measurement. There will be some variation from sample to sample of what a transformer will measure.

It is not measuring high, and that was my concern.

 

[Ankaret]

Thanks for weighing in Josh, and catching the reason to test this out just in case in the first place. Ill rest assured that Ive got the right transformer and wait for the diodes to arrive and keep my fingers crossed this does the trick.

 

[Ankaret]

Got replacement diodes (thanks for the speed Bottlehead) and replaced all four just to be safe, all oriented correctly, carefully soldered etc. and STILL won't start without blowing the fuse...

What makes sense to try next? Getting a little flustered...any help is greatly appreciated.

 

[NightFlight]

You replaced the cap that was powered the wrong way around?  I can't see from the photos how the power switch is wired. One pin should have L on it and the other go to pin 1 of the transformer. I pretty certain that is what you have, but I couldn't tell for sure.

 

[Ankaret]

It was actually a rectifier diode that was wired the wrong way. And yes, Ive replaced all four of the diodes, now positioned correctly. For the power switch, one pin goes to 1 on the transformer, and the other is connected to a power cord socket pin. Should be correct.

 

[Ankaret]

In terms of build errors, from my original post this might be noteworthy:

"I found that I had forgotten to solder one end of the 270 5w resistors (15L)" and something started to smoke around that area when I first tried to power it up. What might need replacing or checking there? If something was smoking, probably a good chance another component there is blown?

 

[NightFlight]

Well, in the power supply, there are just caps and resistors left. I woudn't have a clue how to ensure the transformer is still good.  If you like toys, it would be a good excuse (for me) to purchase a cap testing equipment.

Or you can put a small charge on the caps and see if they charge/discharge. They would have to be removed from the circuit to test. A PITA, but possibly necessary.  They also can't maintain heat on their legs for a terribly long time. The datasheet might have more in that regard. If  you do pull them, make sure everything is discharged. I found several residual voltages the other day while I was poking around.  I found 183V on my one output cap the other day. Don't know how it got there - possibly the multimeter, but I certainly found it when I touched post #6 by accident.

https://www.youtube.com/watch?v=FDuN7QHn1-s

 

[fullheadofnothing]

Far more useful then testing capacitors would be doing the resistance checks from the manual. If there is a terminal that reads less than 1Ω and is not specified as being 0Ω, that would be the problem.

If that fails to find the problem, go through the wiring section of the manual, carefully, step by step. Confirm that every single component and wire is soldered EXACTLY where and how it is specified in the manual. Close is not close enough.

 

[Paul Birkeland]

Josh is quite correct.  If you pass the resistance checks, let us know, as I have another idea about how to systematically eliminate potential issues.

 

[Ankaret]

Ive been through resistance checks and they all check out perfectly. Last time I went through it all was just after replacing the four rectifier diodes.

I have also been through the manual step by step again, but I guess Ill spend some time doing that again.

Any other ideas?

Thanks all

 

[Doc B]

You probably need to disconnect the power transformer from the rest of the high voltage circuit first and run it by itself to be sure it wasn't damaged.

 

[Ankaret]

How would I go about doing that? Just unsolder everything connected to the power transformer minus the LED coming from the on switch?

And how would I know that it's running correctly?

 

[NightFlight]

Sorry guys. Don't listen to me. I was attempting to be helpful, but might have missed the mark entirely.
Watching this thread... its an interesting bug.

 

[Doc B]

The power switch we supply does not have an LED so you have lost me there. You want to disconnect the red wire that goes from power transformer terminal 9 to t-strip terminal 19L. Disconnect it at the power transformer. Fire up the amp and see if the fuse holds. If it does, turn tthe am off, set your meter for AC volts, at least 250VAC. Connect the red meter test lead to power transformer terminal 9 and the black test lead to power transformer terminal 10. Turn the amp on an measure the AC voltage across those two terminals.

 

[Ankaret]

So I disconnected the wire that goes from power transformer terminal 9 to 19L, at the power transformer. Note: I left the other red wire connected to 9, the one that is part of the double wind that goes to terminal 6 on the transformer, correct? Then tried to fire it up and fuse still blew...does this mean the transformer is acting up?

 

[Doc B]

My hunch is that the 6 ohm reading instead of 12 indicates that the high voltage secondary has some shorted turns from the backwards diode. To be thorough I should have mentioned removing the tubes to take them out of the equation too. If you had the tubes in the sockets you might want to try it again with the tubes out. I suspect it will still blow the fuse. The only other possibility left after that is a short somewhere around the IEC power inlet. But that would be more likely to blow the fuse as soon as the amp was plugged in whether it was switched on or not.