Mag 780 LT776S Not working....cap question on inverterboard

[Welchs101]

Re: Mag 780 LT776S Not working....cap question on inverterboard

Bill,

I measured both transformers and got similar results for both (see below):

With Black Meter probe on output terminal (black arrow) and red meter probe on each of the 7 pins:
Pin1: 370.5Kohm
Pin2: 370.2Kohm
Pin3: 369.9Kohm
Pin4: 368Kohm
Pin5: 367Kohm
Pin6: 366Kohm
Pin7: 0.48Kohm



With Red Meter probe on output terminal (black arrow) and Black meter probe on each of the 7 pins:
Pin1: 8.5Kohm
Pin2: 7.97Kohm
Pin3: 7.97Kohm
Pin4: 7.97Kohm
Pin5: 7.97Kohm
Pin6: 8.48Kohm
Pin7: 0.48Kohm

[Welchs101]

Re: Mag 780 LT776S Not working....cap question on inverterboard

ok. I made the Vgnd/Vreturn measurements.

Note: I replaced the "bottom" set of lamps with newer ones. The old ones had issues and needed replacing.

Red probe from meter on Vreturn pad and Black lead connected to Vgnd wire.


Vreturn(Top lamp): 3.01V AC

Vreturn(Bottom lamp): 2.13V AC

[PlainBill]

Re: Mag 780 LT776S Not working....cap question on inverterboard

Bill,

I measured both transformers and got similar results for both (see below):

With Black Meter probe on output terminal (black arrow) and red meter probe on each of the 7 pins:
Pin1: 370.5Kohm
Pin2: 370.2Kohm
Pin3: 369.9Kohm
Pin4: 368Kohm
Pin5: 367Kohm
Pin6: 366Kohm
Pin7: 0.48Kohm



With Red Meter probe on output terminal (black arrow) and Black meter probe on each of the 7 pins:
Pin1: 8.5Kohm
Pin2: 7.97Kohm
Pin3: 7.97Kohm
Pin4: 7.97Kohm
Pin5: 7.97Kohm
Pin6: 8.48Kohm
Pin7: 0.48Kohm

I would say that pin 7 is the other end of the secondary, and their resistance matches to within 3%. So an open secondary is not the issue.

PlainBill

[PlainBill]

Re: Mag 780 LT776S Not working....cap question on inverterboard

ok. I made the Vgnd/Vreturn measurements.

Note: I replaced the "bottom" set of lamps with newer ones. The old ones had issues and needed replacing.

Red probe from meter on Vreturn pad and Black lead connected to Vgnd wire.


Vreturn(Top lamp): 3.01V AC

Vreturn(Bottom lamp): 2.13V AC

OK, that looks like a big difference in current.

First test: Switch connections - hook the 'bottom' lamps to the connectors the top lamps plug into. And hook the top lamps to the connectors the bottom lamps normally plug into. Let's see if the difference follows the lamps.

PlainBill

[Welchs101]

Re: Mag 780 LT776S Not working....cap question on inverterboard

ok. I think my nomenclature on the last post was a little wrong so i am going to re-do the measurements (see below with correct nomenclature):


Red probe from meter on Vreturn pad and Black lead connected to Vgnd wire.


Note: See enclosed pic for defn of the following
Bottom-lamp
Top-Lamp
Bottom-connector = connector on invert. board that goes to bottom lamp
Top-connector = connector on inverter board that goes to top lamp
Vreturn bottom
Vreturn top



Experimental setup:
Bottom-lamp connected to Bottom Connector
Top-lamp connected to Top connector:

Vreturn(Bottom): 3.01V AC
Vreturn(Top): 2.15V AC



Experimental setup:
Bottom-lamp connected to Top Connector
Top-lamp connected to Bottom connector:

Vreturn(Bottom): 3.01V AC
Vreturn(Top): 2.15V AC


Voltage values "STAY" with the lamps..........
Switching the connectors does not change the voltage readings.

[Welchs101]

Re: Mag 780 LT776S Not working....cap question on inverterboard

In my last post i made this statement

"Voltage values "STAY" with the lamps..........
Switching the connectors does not change the voltage readings."


I dont think this is right. The "return pads" essentialy stay with the connectors.

Top-connector/Top-lamp: Vreturn top measures return voltage of top lamp.
Bot.-connector/Bot.-lamp: Vreturn bot. measures return voltage of bot lamp.

Top-connector/Bot-lamp: Vreturn top measures return voltage of bot lamp.
Bot.-connector/Top-lamp: Vreturn bottom measures return volt of top lamp.


The "return" voltages stayed the same as i swapped lamp connector combinations......so as the lamps change the return pad voltages stay the same so the measured voltages follow the board NOT the lamps.

Did i get this right? I think so......helps to use the picture.

[PlainBill]

Re: Mag 780 LT776S Not working....cap question on inverterboard

In my last post i made this statement

"Voltage values "STAY" with the lamps..........
Switching the connectors does not change the voltage readings."


I dont think this is right. The "return pads" essentialy stay with the connectors.

Top-connector/Top-lamp: Vreturn top measures return voltage of top lamp.
Bot.-connector/Bot.-lamp: Vreturn bot. measures return voltage of bot lamp.

Top-connector/Bot-lamp: Vreturn top measures return voltage of bot lamp.
Bot.-connector/Top-lamp: Vreturn bottom measures return volt of top lamp.


The "return" voltages stayed the same as i swapped lamp connector combinations......so as the lamps change the return pad voltages stay the same so the measured voltages follow the board NOT the lamps.

Did i get this right? I think so......helps to use the picture.

That is correct; the difference follows the board. Now the problem is to figure out why the voltage reading is different on the two sides.

PlainBill

[Welchs101]

Re: Mag 780 LT776S Not working....cap question on inverterboard

I remeasurd the secondary transformer resistances:

PT1: had 0.474kOhm
PT2: had 0.477kOhm

That seems close..........not sure how close it needs to be. I wonder why they have the other connection as one of the 7 pins.

[PlainBill]

Re: Mag 780 LT776S Not working....cap question on inverterboard

I remeasurd the secondary transformer resistances:

PT1: had 0.474kOhm
PT2: had 0.477kOhm

That seems close..........not sure how close it needs to be. I wonder why they have the other connection as one of the 7 pins.

That matches within 1%; I'd expect that much variation in the wire.

Having one end of the secondary at the same end as the primaries is common. The only answer I have is that it increases isolation from the high voltage output.

By the way, on my list of 'research to do' is finding a schematic for an inverter that uses a capacitive voltage divider to monitor output voltage. 'Stealing' this design should allow determining if the difference is due to different output voltages. The alternative would be to start swapping components between sides.

Speaking of that, if you trace the return lines, you will see several resistors and a diode (on the back of the card). It would be worth while checking those (in circuit measurements should suffice).

PlainBill

[Welchs101]

Re: Mag 780 LT776S Not working....cap question on inverterboard

I am enclosing a pic for reference.

The resistors in group A and group B. Each resistor separately measures 0.971KOhms.


The resistors in group C and group D. Each resistor separately measures 0.496KOhms.


All resistors have the same package marking.....so i dont know why groupA,B are different than C,D.

Diode A7:


Diode1:
Pin1-Pin3 diode voltage:0.45V
Pin3-Pin2 diode voltage:0.59V


Diode2:
Pin1-Pin3 diode voltage:0.52V
Pin3-Pin2 diode voltage:0.59V

[PlainBill]

Re: Mag 780 LT776S Not working....cap question on inverterboard

I am enclosing a pic for reference.

The resistors in group A and group B. Each resistor separately measures 0.971KOhms.


The resistors in group C and group D. Each resistor separately measures 0.496KOhms.


All resistors have the same package marking.....so i dont know why groupA,B are different than C,D.

Diode A7:


Diode1:
Pin1-Pin3 diode voltage:0.45V
Pin3-Pin2 diode voltage:0.59V


Diode2:
Pin1-Pin3 diode voltage:0.52V
Pin3-Pin2 diode voltage:0.59V

By any chance are the resistors marked 202? If so, they are 2K resistors. It looks like those in groups C and D might be in parallel.

Do you have Chip-Quik on hand? That would make it easy to remove the diodes and test them out of circuit. I don't know if the difference is due to a failure of the diode, or further downstream in the circuit.

PlainBill

[Welchs101]

Re: Mag 780 LT776S Not working....cap question on inverterboard

Yes, they do have the markings of "202" on them.

I dont have the Chip-Quick material......was thinking about getting some because soldering those little guys is "tough" for me anyway. So you would recommend this Chip-Quick stuff.

I can remove the diodes if you think i should........but they seem to measure ok.......but i can remove them if you think it would help solve the problem.

[PlainBill]

Re: Mag 780 LT776S Not working....cap question on inverterboard

Yes, they do have the markings of "202" on them.

I dont have the Chip-Quick material......was thinking about getting some because soldering those little guys is "tough" for me anyway. So you would recommend this Chip-Quick stuff.

I can remove the diodes if you think i should........but they seem to measure ok.......but i can remove them if you think it would help solve the problem.

Chip-Quik allows you to easily remove small surface mount components (or large ones, for that matter) without damaging them or the board. Without Chip-Quik I don't suggest removing the diodes from the board unless you can determine that the outputs of each side (pin 2) are tied together.

I've attached a schematic for a theoretical inverter SIMILAR to the one you are dealing with. CR1, R15, R16 form the circuit equivalent to what you are working on. Note that your inverter has two of these circuits. They are identical, you should be getting identical readings, but you aren't.

Check if pin 2 of the diodes in group A and B are connected together.

PlainBill

[Welchs101]

Re: Mag 780 LT776S Not working....cap question on inverterboard

Resistance from Pin1 of Diode1 to Pin1 of Diode2: 0.3ohm
Resistance from Pin2 of Diode1 to Pin2 of Diode2: 190Kohm
Resistance from Pin3 of Diode1 to Pin3 of Diode2: 1.975Kohm

Just to make sure we are talking about the same pin configuration i have included a pic of what i think the part pin configuration is for this part.

So the Pin1's are shorted together.

The Pin2, pin2 and Pin3 readings shown above are exactlly the same regardless of how i measure (red/black probes).

Does this help?

[PlainBill]

Re: Mag 780 LT776S Not working....cap question on inverterboard

Resistance from Pin1 of Diode1 to Pin1 of Diode2: 0.3ohm
Resistance from Pin2 of Diode1 to Pin2 of Diode2: 190Kohm
Resistance from Pin3 of Diode1 to Pin3 of Diode2: 1.975Kohm

Just to make sure we are talking about the same pin configuration i have included a pic of what i think the part pin configuration is for this part.

So the Pin1's are shorted together.

The Pin2, pin2 and Pin3 readings shown above are exactlly the same regardless of how i measure (red/black probes).

Does this help?

Sort of. What is the pin 1 to 3 RESISTANCE in both directions on the two diodes?

PlainBill

[Welchs101]

Re: Mag 780 LT776S Not working....cap question on inverterboard

A7 Diode1:
Pin1 (red probe) to Pin3 (black probe):0.971kohm
Pin1 (black probe) to Pin3 (red probe):0.984kohm

A7 Diode2:
Pin1 (red probe) to Pin3 (black probe):0.991kohm
Pin1 (black probe) to Pin3 (red probe):0.991kohm

[PlainBill]

Re: Mag 780 LT776S Not working....cap question on inverterboard

A7 Diode1:
Pin1 (red probe) to Pin3 (black probe):0.971kohm
Pin1 (black probe) to Pin3 (red probe):0.984kohm

A7 Diode2:
Pin1 (red probe) to Pin3 (black probe):0.991kohm
Pin1 (black probe) to Pin3 (red probe):0.991kohm

It's definite. There is something funky going around in that area. I don't know if it's contamination on the board, a bad diode, or somewhere downstream from there.

PlainBill

[Welchs101]

Re: Mag 780 LT776S Not working....cap question on inverterboard

How do you know its definite? Is that reading not a good one?

[PlainBill]

Re: Mag 780 LT776S Not working....cap question on inverterboard

How do you know its definite? Is that reading not a good one?

I'm of the opinion that the slightly low resistance is caused by either a leaky diode or a problem with the circuitry connected to the diode.

PlainBill

[Welchs101]

Re: Mag 780 LT776S Not working....cap question on inverterboard

ok. So do you think i should replace the A7 diodes? Or continue searching for other issues on this board?

When i made the measurement in the previous post i was thinking that the low resistance equaled the resistors next to the transistors.....