toshiba crt tv model mv20fm3 100% dead

[ptcriserx3]

hi
ive got an old 2002 toshiba crt tv

its 100% dead

no power on
no standby light
no click sounds

nothing

ps main fuse checks ok
caps not bulged

what would be most common failure to cause this?

[R_J]

Re: toshiba crt tv 100% dead

You could at least provide a model number.

[ptcriserx3]

Re: toshiba crt tv 100% dead

model mv20fm3

[ptcriserx3]

Re: toshiba crt tv 100% dead

hrre pic of ps board

ii saw a repair on diff crt and it was simply a cap that was bad

from the photos which cap should i test..

if

[ptcriserx3]

Re: toshiba crt tv 100% dead

another pic of ps

[ptcriserx3]

Re: toshiba crt tv 100% dead

i found a service manual with voltages so when i get time i will do some probing ..

[ptcriserx3]

Re: toshiba crt tv 100% dead

here is ps schematic

[R_J]

Re: toshiba crt tv 100% dead

If you do not have any B+ voltage on the secondary side, check the primary side.
Check the resistance of R501, if it is ok, check the resistance of R527, If they are good, check for DC voltage across C507, If that is about 165vdc, Check the voltage across C542, and I would just replace C542, it is likely bad

[momaka]

Re: toshiba crt tv 100% dead

Looks like the set uses mostly Rubycon capacitors, which are likely going to be OK. But nevertheless, a cap failure is not excluded yet.

First, before touching anything too much with your bare hands, make sure the large black 400V (or 200V?) cap next to the transformer (the one seen on the bottom-center of this picture) is discharged. Take a multimeter and read the DC voltage across its terminals. It should be less than 30V, preferably close to 0V. If non-zero, use a 1 kOhm resistor and put it across its terminals for about 10-20 seconds to discharge it. Alternatively, you can use something like a 25-40W "dumb" soldering iron (one with no temperature control or electronics) or a 10-30W hot glue gun - just stick the plug end directly onto the cap's terminals for 10-20 seconds, and that will discharge the cap too.

Next, take a small paint brush and remove as much of the dust you can so that you can see things a little better.

Then, use your multimeter to check the following:
- Resistance of the large white 7 Watt 0.1 Ohm resistor --> should read close to 0 Ohms or whatever is the lowest your MM can measure.
- Resistance of R529 --> looks like a 0.22 Ohm resistor on the schematic, so it should also read very low resistance like the 7 Watt resistor above.
- Resistance of R511 --> should be 22 Ohms if I'm not mistaken, and read close to that value even in circuit.
- Diode value (use diode check on your MM) of D508 - should show a normal 0.5 - 0.7V (or 500-700 mV) diode reading one way (in the diode's forward direction) and high resistance or open-circuit when you reverse the probes.
- Diode values of all of the output rectifiers. That is, D505 (or is it D508?), D509, D510, D512, D514, D513, D523, and D519. These should show a normal diode reading in the forward direction. Alternatively, and just to verify, use lowest resistance scale on your MM to measure across each of these diodes. If any of them show a very low resistance (under 100 Ohms) steadily, note which diode(s) this is and report back here.

If all of these check out OK, take a hair dryer and heat up capacitor C542. It's a small 25V, 47 uF (according to the schematic) electrolytic cap that's used for starting up and running the PWM IC (IC501: TEA1507p) and is right next to it. Then try plugging in the TV and see if it starts up. If it does, that cap is probably bad/exhausted. Alternatively, if you have an ESR meter, remove C542 and test its capacitance and ESR out of circuit. If this "startup" cap is bad, the PSU won't start.

Other things that can cause the PSU not to start up: shorted HOT (Horizontal Output Transistor) and/or shorted "damper" diode on the HOT. The HOT should be mounted on a heatsink and not too far away from the flyback transformer. It might be the transistor seen on the lower-left of this picture. Note that CRT circuits often place a low-resistance resistor (typically 8-47 Ohms) between Base and Emitter of the HOT. So if you test the HOT diode junctions in-circuit, you might erroneously conclude that B-E junction is shorted.

*EDIT*
Looks like R_J beat me to it, as some of the checks above are a repeat of what he stated already. I'll leave the information in this post as-is anyways.

[ptcriserx3]

Re: toshiba crt tv 100% dead

thank you
when i get time i will just replace the c542 cap as it is very simple and cheap...im not looking to do anyhting major...

any other cap i should look at or start with c542 and go from there?

like i said i have 0v at standby pin so if this cap is responsible for supplying that voltage im hoping its the problem..although from my bried tinkering experience the caps that typically fail are 1000uf or 220uf or 470uf caps

ive never seen a failed 47uf cap

[ptcriserx3]

Re: toshiba crt tv model mv20fm3 100% dead

as i suspected the tiny 47uf cap not the issue...i have never had a cap that small fail in a tv...usually 1000uf 470uf 220uf....is there a cap that directly supplys the standby voltage?.

[R_J]

Re: toshiba crt tv model mv20fm3 100% dead

And the voltages are? There is no separate standby circuit. The power supply will be in burst mode when in standby, you should have AT+12v.

[ptcriserx3]

Re: toshiba crt tv model mv20fm3 100% dead

hi rj ...i did not check voltage at c542...i simply replaced it because its easy enough to do... i have no voltages on the 10pin connector
ill do more tinkering another day

where does the voltage that goes to the standby pin on the 10pin connector come from?

[R_J]

Re: toshiba crt tv model mv20fm3 100% dead

The STAND_BY pin voltage comes from D617 which is supplied through R532 from the unregulated 8v. This unregulated 8v also supplies the voltage regulators for the main processor etc. (page 63, syscon pcb)
This is an always on power supply but in standby (tv/vcr off) it only supplies a small amount of current to keep the micro etc. alive.

also check R529, this supplies the voltage to the drain pin of the ic which seems to have a misprint I believe pin 8 should be 165vdc.

[ptcriserx3]

Re: toshiba crt tv model mv20fm3 100% dead

thanks
any of the other secondary voltage larger caps should i look at?...

[R_J]

Re: toshiba crt tv model mv20fm3 100% dead

Did you check the resistors?

[ptcriserx3]

Re: toshiba crt tv model mv20fm3 100% dead

no..i may try over weekend

[ptcriserx3]

Re: toshiba crt tv model mv20fm3 100% dead

hi rj..im going to just give up..i was hoping it was something simple like a common bulgef failed cap but its not....thanks anyways...ill just toss tv in trash.