Help needed to stop blowing fuses

[Loquah]

I don't think so, Doc. Can you tell me from where to where I need to measure?

[Doc B.]

Start by making sure all the wires are disconnected from the power transformer. First check for shorts between windings.

For the primary to high voltage secondary measure the resistance from power transformer terminal 1 to terminal 6 - should be infinite.

Primary to heater secondary measure from 1 to 4 - should be infinite

high voltage secondary to heater secondary measure from 6 to 10 - should be infinite

Now check the DCR of each winding. I will give you what I measure, your numbers may be a little different.

Primary (1 to 2) 36.8 ohms

High voltage secondary (6 to 7) 40.8 ohms

The heater secondary will be too low a resistance to measure above 0 ohms with pretty much any regular meter and this is why PJ didn't suggest it for the heater secondary. So - and again I'm not sure if this has already been covered - check very closely what PJ suggested, make sure that you have two wires coming out from under the coil wrapper to each of terminals 4 and 5, not three wires to one of them and one wire to the other. If you could take a photo from directly over the terminals it could help us to help you verify the wire count. I'm not saying it's impossible for the transformer to have been mis-terminated in production. We went through our entire stock this morning from two different production batches and we didn't find any here that were mis-terminated. So we can certainly send a replacement with a high degree of confidence if the one you are working with was miswired.

It is also possible that there could be a short in the IEC socket wires coming from the socket to the transformer. I see a fair amount of melted wire insulation on that build, and it might be worth comparing the build with the assembly manual photos of the power switch and power entry wiring on pages 23 and 24.

[Loquah]

Thanks Doc. I'll do all that and report back.

I do believe there has been excess heat and excess solder applied at various times during the initial build. I've replaced anything that I can that looks too far gone and have tested the IEC socket for shorts with no issues found.

I'll do all the testing, take notes and report back soon.

Thanks again for your help everyone - I feel like this poor kit was cursed from the outset!

[Loquah]

Start by making sure all the wires are disconnected from the power transformer. First check for shorts between windings.

For the primary to high voltage secondary measure the resistance from power transformer terminal 1 to terminal 6 - should be infinite. Infinite 

Primary to heater secondary measure from 1 to 4 - should be infinite Infinite

high voltage secondary to heater secondary measure from 6 to 10 - should be infinite Infinite

Now check the DCR of each winding. I will give you what I measure, your numbers may be a little different.

Primary (1 to 2) 36.8 ohms 9.5 ohms
High voltage secondary (6 to 7) 40.8 ohms 40.9 ohms

The heater secondary will be too low a resistance to measure above 0 ohms with pretty much any regular meter and this is why PJ didn't suggest it for the heater secondary. So - and again I'm not sure if this has already been covered - check very closely what PJ suggested, make sure that you have two wires coming out from under the coil wrapper to each of terminals 4 and 5, not three wires to one of them and one wire to the other. If you could take a photo from directly over the terminals it could help us to help you verify the wire count. I'm not saying it's impossible for the transformer to have been mis-terminated in production. We went through our entire stock this morning from two different production batches and we didn't find any here that were mis-terminated. So we can certainly send a replacement with a high degree of confidence if the one you are working with was miswired.

It is also possible that there could be a short in the IEC socket wires coming from the socket to the transformer. I see a fair amount of melted wire insulation on that build, and it might be worth comparing the build with the assembly manual photos of the power switch and power entry wiring on pages 23 and 24.

The transformer terminals are attached with the following:

T1: 1 thin wire
T2: 1 thin wire
T4: 2 thick wires (I believe - hard to see clearly)
T5: 2 thick wires
T6: 1 thin wire
T7: 1 thin wire
T10: 4 thick wires

The following photos will hopefully show what I am seeing. I've taken them in order from terminal 1 to terminal 10 with a full shot at the end.

[Paul Joppa]

Once again, Doc gets to keep the corner office!

No idea what the problem is at this point. The measureable windings are fine, the unmeasureable one looks fine, the primary wi..

wait a minute. 10 ohm primary? That's a 120v PT-7, not a 240v as shown on the label. Crap, we should have seen that a lot earlier!

[mcandmar]

Good job it was blowing fuses or you would scraping capacitor goo off the ceiling

[Loquah]

Once again, Doc gets to keep the corner office!

No idea what the problem is at this point. The measureable windings are fine, the unmeasureable one looks fine, the primary wi..

wait a minute. 10 ohm primary? That's a 120v PT-7, not a 240v as shown on the label. Crap, we should have seen that a lot earlier!

Oh, so it's a mis-labelled US power transformer? (Just checking I'm unsderstanding correctly)

[Grainger49]

Yup, that is what Paul indicated.  Somehow I would expect the 240V transformer primary to be a higher resistance, and impedance.

[Chris65]

Yup, it is Grainger. 120V PT-7 = 10 ohms,  240V = approx 36 ohms.

[Kris]

IMO, this info should be some sort of sticky note or part of FAQ, so that in the future everyone can easily determine whether a transformer received with the kit is the correct one or not... Just my 2 cents.

[Loquah]

IMO, this info should be some sort of sticky note or part of FAQ, so that in the future everyone can easily determine whether a transformer received with the kit is the correct one or not... Just my 2 cents.

Nice idea.

[Doc B.]

OK, now you guys see where I was headed. I have to base these troubleshooting procedures upon what I have found to be the most likely scenario. Here it was that you had a 120V transformer, as that was in my experience far more likely than a miswired one.

What I have found is that it is often more expeditious to start with the user collecting data in a  methodical fashion rather than offering three or four guesses as to what may be wrong. I have lead folks down a twisted path and created frustration at times when I have thrown too many possible scenarios into the mix.

Our apologies on the mix-up, and we will send the proper transformer.

[Paul Birkeland]

IMO, this info should be some sort of sticky note or part of FAQ, so that in the future everyone can easily determine whether a transformer received with the kit is the correct one or not... Just my 2 cents.

This is less than a 1 in 1000 situation, and as we rewrite manual, we tend to put a section in about measuring AC voltages once the transformer primary is connected.

The number of posts, however, about whether there is something wrong with a 120V PT-7 with a primary that isn't exactly 10 Ohms would come up a lot and provoke unnecessary hysteria.

[Kris]

and as we rewrite manual, we tend to put a section in about measuring AC voltages once the transformer primary is connected.

brilliant idea 

[Doc B.]

provoke unnecessary hysteria

He may mean that about the staff here.