CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted


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CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted

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t0nito
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May 12, 2016, 9:52 PM

Post #1 of 24 (4022 views)
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CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted	Can't Post

Hi guys, I have a new one for you, it's a TV with a 11AK37-8 chassis.

Original symptom was when TV was plugged in there was a chirping sound from the power supply no standby light. There was a cold solder joint on C631 the joint was charred from arcing. Removed the HOT and confirmed that it was completely shorted. Upon removing the HOT from the chassis, and resoldering the joint, red standby light turned on, I turned on the TV and green light turned on as normal, having 149.8V on B+ at the collector pad.

Ordered a new HOT, and I replaced it. Standby light turned on as normal, when I attempted to turn on the light turned green for a second, heard the HV for a second and immediately started chirping again. Removed the new HOT and lights turn on as normal without it.

I checked continuity on the pads of the HOT on the board and I'm getting 0 Ohm between emitter pad and base. Checked surrounding diodes and they check out fine.

Please help, thought it was an easy fix I could do on my own, and it didn't turn out to be that way. Not sure if the new HOT went kaput either...

jts1957
Veteran

May 13, 2016, 1:54 AM

Post #2 of 24 (4018 views)
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Re: [t0nito] CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted [In reply to]	Can't Post

1) Replacement HOT is SAME type as Original?
2) Replacement HOT tests shorted?
3) "Continuity on pads" = Driver transformer secondary winding resistance (NOT a defect, but be sure to resolder all pins of TR600).

Where is C631?

-------------------------
Location: Far, Far Away

(This post was edited by jts1957 on May 13, 2016, 2:02 AM)

t0nito
User

May 13, 2016, 10:56 AM

Post #3 of 24 (4000 views)
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Re: [jts1957] CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted [In reply to]	Can't Post

Yes, the HOT is the same type it's a BU2508AF. The original tested a dead short between all pins.

I check with the diode function on the multimeter,

red lead on base gives 0.468V to collector and 0.468V to emitter
black lead on base gives same resultes´
between collector and emitter gives me 0V either way.

I'm sorry I meant C621 it's a 1.2KV capacitor with one lead connected to collector pad.

You can download the schematic here:

http://elektrotanya.com/vestel_11ak37-8_sch.pdf/download.html

retiredtech30
New User

May 13, 2016, 9:42 PM

Post #4 of 24 (3989 views)
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Re: [t0nito] CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted [In reply to]	Can't Post

im quite familar with this chassis its old school 4sure if ur ho was shorted best to replace the flyback with it or it will just short out again. as far as the voltage readings u gave ur tv is in shutdown. also try to locate the location of the chirp if its comin from the HOT (horiz-out-transformer)replace them as a pair.

Ron.M
Veteran

May 14, 2016, 4:34 PM

Post #5 of 24 (3950 views)
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Re: [t0nito] CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted [In reply to]	Can't Post

t0nito;...

Before you go off & try to find a flyback for that old TV , let's make sure that it IS bad !!!

Quote

I check with the diode function on the multimeter,

red lead on base gives 0.468V to collector and 0.468V to emitter
black lead on base gives same resultes´
between collector and emitter gives me 0V either way.

Checking transistors or diodes does NOT give you voltage readings !!!
I suggest you use the ohms scale when reading transistors...
PLEASE recheck the 2nd HOT Transistor using the ohms scale...
PLEASE post readings..

I suspect that the flyback may be good because you said that the HV came up for a second or so after you replaced the HOT...

Do not get your hopes up too high as the flyback can still be bad...

We need to clarify some things here...

Was there ANY damage to C621 from the arcing ???...
Do you have a cap checker OR an ESR meter ??..
I'd like you to check C642 in the HOT driver circuit...
That cap is in the primary of TR600 & is a 470 mfd @ 16 vdc cap...
After 10+ years of use that cap IS a major suspect for failure..
It CAN cause the HOT to blow out after a second or so..

BTW;...
I'm a retired analog TV tech with FIFTY years experience !!!.... Wink

...

Later...Ron.M... Cool

...
Veteran & Retired TV technician

jts1957
Veteran

May 14, 2016, 7:42 PM

Post #6 of 24 (3945 views)
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Re: [Ron.M] CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted [In reply to]	Can't Post

Quote

Checking transistors or diodes does NOT give you voltage readings !!!

Sorry Ron, mine DOES give DCV reading. Having said that ...
Digital multimeters can test diodes using one of two methods:
1) Diode Test mode: almost always the best approach.
2) Resistance mode: typically used only if a multimeter is not equipped with a Diode Test mode.

http://en-us.fluke.com/...ital-multimeter.html

-------------------------
Location: Far, Far Away

(This post was edited by jts1957 on May 14, 2016, 7:48 PM)

Ron.M
Veteran

May 14, 2016, 10:14 PM

Post #7 of 24 (3939 views)
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Re: [jts1957] CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted [In reply to]	Can't Post

jts1957;.,...

Quote

Sorry Ron, mine DOES give DCV reading.

I have a Fluke Model #75...
When I test diodes or transistors I get a number displayed...
I had NO idea that that was/is a voltage reading..
I always used the ohms scale for reading diodes & transistors & for 50+ years that method has never lied to me..
When reading diodes front to back readings have to have at least a 10:1 ratio or it is bad...
With transistors it was pretty much the same depending on which type they were...

But whatever works for you &/or t0nito is OK with me.. Wink

Have a GREAT day & weekend... Smile

...

Later...Ron.M.. Cool

...
Veteran & Retired TV technician

t0nito
User

May 17, 2016, 10:10 AM

Post #8 of 24 (3912 views)
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Re: [Ron.M] CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted [In reply to]	Can't Post

Hi Ron,

I checked the C642 it reads 469.8 uF 0.38 Ohm

C621 had it's pin a bit charred from the arcing but I cleaned it and resoldered, it measured 8.4 nF it's a bit on the low side should be 10, but I don't think it would cause the HOT to blow instantly, would it?

I measured the HOT with the resistance setting, 50Ohm between base and collector and base and emitter, swapping leads show same results. 1Ohm between collector and emitter, swapping leads, same results.

It's fried.

Ron.M
Veteran

May 17, 2016, 3:48 PM

Post #9 of 24 (3899 views)
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Re: [t0nito] CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted [In reply to]	Can't Post

t0nito;....

Yep , it's fried !!!...

Quote

I checked the C642 it reads 469.8 uF 0.38 Ohm

HUH !!!...
The cap value (469.8 uF) is OK...
But , what is "0.38 Ohm" ???
Is that an ESR reading ??

Since C621 is a bit off value & it was arcing , diodes D605 & D606 could have been damaged...
Also , since C621 is a bit off value (more than 10%) , the possibility does exist that that cap could blow the HOT rather quickly..
Please use the ohms scale , unsolder a leg & lift them from the board when you check them..
Reverse leads & take readings..
Also , using the ohms scale , take a reading from the junction of D605 & D606 to chassis ground...
Take a ohms reading on both sides of R612 to chassis ground...
That resistor can be found at the junction of those diodes..

Please post readings..

THANKS...

Later...Ron.M... Cool

...
Veteran & Retired TV technician

(This post was edited by Ron.M on May 17, 2016, 3:52 PM)

t0nito
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May 17, 2016, 8:00 PM

Post #10 of 24 (3885 views)
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Re: [Ron.M] CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted [In reply to]	Can't Post

Yes, 0.38 Ohm is the ESR value

I checked both diodes out of circuit.

D606 Black lead on cathode reads 8.5MOHm, red lead on cathode reads Open

D 605 Black lead on cathode rads 5.5 MOhm, red lead on cathode reads Open

Both on diode settings and on a universal component checker read fine.

R612 is actually a jumper lead, reads 6.05 kOhm to ground.

Do you want me to take the measurements from the diode junctions to ground with or without the diodes in circuit?

Ron.M
Veteran

May 17, 2016, 11:21 PM

Post #11 of 24 (3879 views)
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Re: [t0nito] CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted [In reply to]	Can't Post

t0nito;....

Quote

Do you want me to take the measurements from the diode junctions to ground with or without the diodes in circuit?

That won't be necessary...
You took a reading from R612 jumper to ground..
That reading is about right (6.05k) ...
Well , diodes are good , readings I asked for are good...
I can't think of anything else to check..
So , it looks like it's flyback hunting time... Unsure

..

GOOD LUCK !!!... Tongue

...

Later...Ron.M... Cool

...
Veteran & Retired TV technician

t0nito
User

May 18, 2016, 12:02 AM

Post #12 of 24 (3875 views)
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Re: [Ron.M] CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted [In reply to]	Can't Post

Thanks for the help Ron. Is it possible that the transistor was a dud? I ordered the transistor it had the same reference on it but the lettering and form factor was slightly different from the original.

Ron.M
Veteran

May 18, 2016, 3:56 PM

Post #13 of 24 (3865 views)
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Re: [t0nito] CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted [In reply to]	Can't Post

t0nito;....

You're welcome...
Sorry it didn't turn out better..

It's possible that the one you got may not have been the right one..
I did a search for "BU250BAF" & could not find a datasheet...
I could not find an exact replacement OR a sub !!!.. Mad

..
Where did you get yours from & what is the exact numbers on the transistor ???..
Can you post a pix or a link to a pix showing the original & the replacement side by side ???...

Later...Ron.M... Cool

...
Veteran & Retired TV technician

(This post was edited by Ron.M on May 18, 2016, 4:00 PM)

jts1957
Veteran

May 18, 2016, 6:31 PM

Post #14 of 24 (3858 views)
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Re: [Ron.M] CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted [In reply to]	Can't Post

Need a stronger eyeglass prescription, Ron?

Quote

BU2508AF

-------------------------
Location: Far, Far Away

(This post was edited by jts1957 on May 18, 2016, 6:31 PM)

t0nito
User

May 18, 2016, 8:18 PM

Post #15 of 24 (3852 views)
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Re: [Ron.M] CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted [In reply to]	Can't Post

Actually I think I need a prescription, the transistors actually do have the same form factor, just different lettering.

Is there any way I can test the set with another HOT, maybe with a bulb in series with the HOT itself so it won't pop instantly in case the flyback is indeed bad?

(This post was edited by t0nito on May 18, 2016, 8:22 PM)

jts1957
Veteran

May 18, 2016, 9:41 PM

Post #16 of 24 (3845 views)
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Re: [t0nito] CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted [In reply to]	Can't Post

Putting a bulb in the B-Plus path usually suffices. Float B-Plus pin of flyback and attach one side of the bulb to the floating pin and the other side of bulb to some convenient point on the foil that used to connect to the flyback.

Another idea - without HOT transistor in set, see if base drive is present.

-------------------------
Location: Far, Far Away

t0nito
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May 18, 2016, 9:50 PM

Post #17 of 24 (3843 views)
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Re: [jts1957] CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted [In reply to]	Can't Post

The bulb thing I can do, to see if base drive is present I would need an oscilloscope right? Too bad I don't have one :(

My DMM has a frequency setting though, do you think that would work or would the pulses fry it?

I believe I do have horizontal drive though, as right before the transistor "popped" I heard the HV fire for a second.

What bulb wattage would you recommend? B+ is 150V I only have 230V ones as that's the mains voltage here.

(This post was edited by t0nito on May 18, 2016, 9:52 PM)

jts1957
Veteran

May 18, 2016, 10:30 PM

Post #18 of 24 (3840 views)
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Re: [t0nito] CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted [In reply to]	Can't Post

I personally use my fancy Sencore test equipment, so I've forgotten most of the particulars. Most seem to be addressed: http://www.repairfaq.org/...ew/tvfaq/tvtslbt.htm

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Location: Far, Far Away

t0nito
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May 18, 2016, 11:03 PM

Post #19 of 24 (3838 views)
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Re: [jts1957] CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted [In reply to]	Can't Post

Thanks, I wouldn't mind some fancy equipment too, but I'm just a hobbyist who does the occasional repair now and then, and with the help from you guys when I get stuck :D, every repair is a learning process. Too bad CRTs are rarer and rarer, they're the ones I find most interesting.

Ron.M
Veteran

May 19, 2016, 1:41 AM

Post #20 of 24 (3829 views)
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Re: [t0nito] CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted [In reply to]	Can't Post

jts1957:...

Quote

Need a stronger eyeglass prescription, Ron?

Actually , the schematic was not that clear...
Thanks for the correction...

t0nito;....
THANKS for that clear pix... Smile

..
In the pix you posted I'm guessing that the transistor on the right side is the original as the name BU2508AF can be clearly seen..
However , what is on the transistor on the LEFT side ???...
Perhaps jts1957 can update me on that one too ???.. Wink

..

Later...Ron.M... Cool

..
Veteran & Retired TV technician

jts1957
Veteran

May 19, 2016, 2:22 AM

Post #21 of 24 (3818 views)
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Re: [Ron.M] CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted [In reply to]	Can't Post

I'll take a stab at it ...

BU
2508AF
PJ ??? (a date/mfg code)

-------------------------
Location: Far, Far Away

t0nito
User

May 19, 2016, 9:36 AM

Post #22 of 24 (3813 views)
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Re: [Ron.M] CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted [In reply to]	Can't Post

The transistor that was on the set was the left one, the right one was the one I ordered.

But I think this transistor was replace before, the solder wasn't original and the owner said it had been serviced a few years ago.

Ron.M
Veteran

May 19, 2016, 4:52 PM

Post #23 of 24 (3805 views)
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Re: [t0nito] CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted [In reply to]	Can't Post

t0nito;....

The replacement HOT is not the cause of it's failure..
The problem lies elsewhere..
Try the bulb test if you are so inclined ...
You might get a handle on what's going on...

One thing to try....
You'll need a good HOT for this..
UNPLUG the CRT neck panel & the yoke connector..
Turn the TV on for 3 seconds & watch for "smoke"..
Unplug the TV & with a moistened thumb , check the temp of the HOT..
IF it is very hot , the Flyback IS bad..
IF the TV starts to "chirp" again , the flyback IS bad...
If not , something else IS the cause...

Later...Ron.M... Cool

...
Veteran & Retired TV technician

t0nito
User

May 19, 2016, 6:22 PM

Post #24 of 24 (3797 views)
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Re: [Ron.M] CRT Vestel 11AK37-8 Shorted HOT replaced, still shorted [In reply to]	Can't Post

Hey Ron.

I only read your reply after I actually fixed the set.

I received a new HOT today I installed it and desoldered the B+ from the flyback and connected a 150W bulb in series, the HV fired up and the bulb lit dimly, I put a 60W bulb, HV fired up and the bulb was dim, though brighter, I put in a 40W bulb, HV fired up again and the bulb was dim as well. I took a shot and resoldered the flyback pin. TV turned on as normal.

I left the TV on for a few seconds and touched the HOT, it was completely cold.

I then left the TV on for about 20 min, the HOT was barely warm.

It's been on for over an hour now, so far everything's fine.

My conclusion, the original HOT shorted due to a cold solder joint on the capacitor. The replacement HOT was a dodgy dud as it shorted in a split second.

I'm gonna leave it on for a while before returning it, it has an awesome picture!

Thanks guys!

(This post was edited by t0nito on May 19, 2016, 6:35 PM)

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