Power supply blowing fuses in Zebra 2746e thermal printer.

[torin3]

Re: Power supply blowing fuses in Zebra 2746e thermal printer.

It's low but it is the min Vcc accepted by TL494.

I'm still curios to know where the voltage comes from.

Well, I orded a replacement one anyway, just to be on the safe side.

R1 is to far from IC1 I think it is not the start up resistor, it's value is 820ohm probably 1W, the value is too low for start up.
If you read 50Kohm be sure all caps are discharge, you can lift on leg and measure it disconnected. If it is 50Kohm then replace it with 820ohm 1 or 2W.

Disconnected it has an even higher resistance. I'll order a replacement tonight.

Looking at the pictures I see IC3 near D13: have you identified it?
Maybe it is another part to check.

Ciao
Gianni

Yep, BT137-600D. Triac. Datasheet is here:

[torin3]

Re: Power supply blowing fuses in Zebra 2746e thermal printer.

Oh, and the only larger resistor on the board is R4, which if I'm reading it right is 2.7Kohm. (Red, violet, red, gold).

It reads lower resistance than that. It is hard to tell on my analog meter, but I'm getting the same reading as when I touch the probes together, so I should assume it is shorted?

Edit: Nevermind, I borrowed the DMM, and I lifted the leg. 2675ohms.

[Toasty]

Re: Power supply blowing fuses in Zebra 2746e thermal printer.

R1 = White-Red-Orange-Gold = 92k ohm 5%

[Gianni]

Re: Power supply blowing fuses in Zebra 2746e thermal printer.

Well R4 is just a bleeder on the output, measure it disconnected because it is in parallel with the output and maybe it takes time to charge the output caps. It is a dummy load to keep the PSU in tolerance with no load/light load and to discharge the output caps when it is disconnected from mains.

So IC3 it is a triac, probably it is part of a crowbar to blow the input fuse for input overvoltage protection.

The bigger resistor on this PSU is the white one behind the trafo, it is 5W but can't read the value. probably a snubber on the primary side, I can't follow the tracks.

Let's wait til you have replace IC1.

Ciao
Gianni

[Toasty]

Re: Power supply blowing fuses in Zebra 2746e thermal printer.

Big white resistor is 22 ohm.

(Lower left) What about R2(3?) and Q1? Also, what is DIC1? I can make out 125v on the side...?

>>It reads lower resistance than that. It is hard to tell on my analog meter, but I'm getting the same reading as when I touch the probes together, so I should assume it is shorted?<<

You likely forward biased a nearby component. When that happens, reverse the leads and see.

Toast

[torin3]

Re: Power supply blowing fuses in Zebra 2746e thermal printer.

R1 = White-Red-Orange-Gold = 92k ohm 5%

Wow! Good eye. I would have sworn it was brown until I compared it to the other resistors of the same type.

Big white resistor is 22 ohm.

Pulled and checked. 22.2 ohm.

(Lower left) What about R2(3?) and Q1? Also, what is DIC1? I can make out 125v on the side...?[/QUOTE]

R2 and R3 are both 1000 Mohms. (brown-black-grey-gold). Q1 is a transistor that was bad. I've replaced it already. This part: http://www.mcmelectronics.com/product/2SC2625

DIC1 has 3 markings on it. There is a logo type ST symbol, followed by a block letter E. Next line down is DB3, and the line below that is X527 (what looked like 125V).

[Toasty]

Re: Power supply blowing fuses in Zebra 2746e thermal printer.

Ok, my mistake. I can't see the markings directly. It's the large gray resistor next to SQ1, as I was directing your view to the lower left. I went to an earlier pic, it's SR3. Then I also need to go back to the previous question about Q1 in which I really should have said SQ1.

DIC1 is a Diac:

"Functioning as a trigger diode with a fixed voltage reference, the DB3/DB4 series can be used in conjunction with triacs for simplified gate control circuits or as a starting element in fluorescent lamp ballasts"

Once the voltage across it rises to a value of about 32v, it conducts and remains on until the current value drops below a certain point. It's in the gate circuit to that triac IC3. So, as Gianni said, it's part of a crowbar circuit.

When checking resistors in circuit, they will present as the value marked or lower. If it reads higher, then it's failed. So 50k on that 92k resistor was fine. If it had read 150 k then you would be suspicious.

You are replacing the 494, correct?

Toast

[torin3]

Re: Power supply blowing fuses in Zebra 2746e thermal printer.

Ok, my mistake. I can't see the markings directly. It's the large gray resistor next to SQ1, as I was directing your view to the lower left. I went to an earlier pic, it's SR3. Then I also need to go back to the previous question about Q1 in which I really should have said SQ1.

No problem. SR3 is yellow, violet, orange, gold, for 47 Kohms. I'll pull SQ1 out and test it. (That would be resistance, switching pos/neg on all the leg combinations, plus the part number?)

DIC1 is a Diac:

"Functioning as a trigger diode with a fixed voltage reference, the DB3/DB4 series can be used in conjunction with triacs for simplified gate control circuits or as a starting element in fluorescent lamp ballasts"

Once the voltage across it rises to a value of about 32v, it conducts and remains on until the current value drops below a certain point. It's in the gate circuit to that triac IC3. So, as Gianni said, it's part of a crowbar circuit.

When checking resistors in circuit, they will present as the value marked or lower. If it reads higher, then it's failed. So 50k on that 92k resistor was fine. If it had read 150 k then you would be suspicious.

You are replacing the 494, correct?

Toast

Yes, the 494 should be in about 10am tomorrow.

[torin3]

Re: Power supply blowing fuses in Zebra 2746e thermal printer.

Forgot to say SR3 tested about right on with the resistance.

SQ1 is part number MPS2907A and positive probe on the center lead reads about 3 Kohm to leads 1 and 3. Positive probe on leads 1 and 3 reads infinite on all other combinations.

[torin3]

Re: Power supply blowing fuses in Zebra 2746e thermal printer.

Ok, I replaced the 494 IC and it still doesn't work.

No lights come on at the front of the printer, and the little LED1 on the board doesn't light up. I haven't tried putting a fan in the socket for it yet, but I can if someone wants me to try. Any suggestions?

[torin3]

Re: Power supply blowing fuses in Zebra 2746e thermal printer.

I'm heading home in a few minutes. Should I take the printer with me, or are people running out of troubleshooting ideas?

[torin3]

Re: Power supply blowing fuses in Zebra 2746e thermal printer.

Ok, taking home just the PSU.

Fan header has 1.6-1.7 volts going to it.

That is about 1/7 of the voltage it should have. Does that lend itself to any suggestions?

Edit: Found a fan with the right header. Fan doesn't move when the PSU is turned on.

[Toasty]

Re: Power supply blowing fuses in Zebra 2746e thermal printer.

I have some voltages I'd like you to check, but I need to mark the test points on the photo(s).

In a bit...

Toast

[Toasty]

Re: Power supply blowing fuses in Zebra 2746e thermal printer.

Okay, here's the pic.

Please take readings for each of the 6 numbers at the points indicated and report.

I marked the IC pins 1 & 9 so I (we) don't get confused when tracing.

I also marked the board between the Hot (mains) side and isolated side. You should be doing any measurements on the Hot side while using a isolation transformer (trafo). Please be careful not to come in contact with any components on the left side of the yellow divider.

@Gianni - The supply for the IC and the other components there comes from the C26 cap and the diodes D23 & D24. It's a center tap, full wave arrangement off the large trafo.

Toast

[torin3]

Re: Power supply blowing fuses in Zebra 2746e thermal printer.

I don't have an isolation transformer. I should skip the reading for 1 then?

[Toasty]

Re: Power supply blowing fuses in Zebra 2746e thermal printer.

Not at all, just be very careful when you do so.

Toast

[Gianni]

Re: Power supply blowing fuses in Zebra 2746e thermal printer.

or are people running out of troubleshooting ideas?

I have no more suggestion since I can't figure out in which way the TL494 starts.

I see SCR1 and some diodes around IC1, check if they are OK; I don't know what SCR1 does in the circuit but you can identify it and check the connection/voltages, maybe it is damaged and it block the circuit. I think it could be a latch for output OVP.

I hope Toasty will be able to help you.

Ciao
Gianni

EDIT: Toasty you are right about C26/D23/D24 and trafo but before the trafo creates the voltages, the TL494 must starts. C26 must be charged through a resistor or something else. I hope my doubt is clear for you

[torin3]

Re: Power supply blowing fuses in Zebra 2746e thermal printer.

Not at all, just be very careful when you do so.

Toast

0 Volts DC on 1 through 6.

[Toasty]

Re: Power supply blowing fuses in Zebra 2746e thermal printer.

I thought you had 7 volts on IC1...?

Try it on AC scale or reverse the leads on DC.

[torin3]

Re: Power supply blowing fuses in Zebra 2746e thermal printer.

I see SCR1 and some diodes around IC1, check if they are OK; I don't know what SCR1 does in the circuit but you can identify it and check the connection/voltages, maybe it is damaged and it block the circuit. I think it could be a latch for output OVP.

SCR1 is

CR02AM
8 5F