Hitachi 27AX3B no power after attempting to fix vertical deflection


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Hitachi 27AX3B no power after attempting to fix vertical deflection

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NT7000
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Jan 18, 2018, 7:42 PM

Post #1 of 35 (2474 views)
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Hitachi 27AX3B no power after attempting to fix vertical deflection	Can't Post

Hi folks, I have an old CRT TV I'd like to fix for retro gaming, And I could use some tech advice. Here's the story so far: I got this tv out of storage, where it was somewhat damp (I know, red flags already) , got it home and left it to dry for a few days. gave it a quick sanity check, plugged it in, and turned it on. No vertical deflection. I found a service manual, and found the vertical deflection IC, and starting checking voltages. The 25V rail was low, and there was one pin with a bad solder joint. I replaced a diode in between the vert. def. IC and the FBT, and fixed the bad solder joint. I turned the set on, and I had vertical deflection, but the image was 2" smaller than the screen, and the top and bottom appeared to be curled on top of themselves. I went to change the channel so I could get the OSD to come up, then nothing. I lost everything save the main relay clicking. Any advice (besides the usual "get a new tv" line :) ?

(This post was edited by NT7000 on Jan 19, 2018, 6:51 PM)

jts1957
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Jan 18, 2018, 10:11 PM

Post #2 of 35 (2470 views)
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Re: [NT7000] Hitachi 27AX3B no power after attempting to fix vertical deflection [In reply to]	Can't Post

Q951 & Q952 voltages? C / B / E

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Location: Far, Far Away

NT7000
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Jan 19, 2018, 2:16 AM

Post #3 of 35 (2464 views)
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Re: [jts1957] Hitachi 27AX3B no power after attempting to fix vertical deflection [In reply to]	Can't Post

OK, after some fiddling, I got low voltage back, but the set is still dead. I'm getting power at the HOT, so I assume startup circuit?

UPDATE: I checked the 25V rail going to T701, and I have no power. I checked Q953, and I have 0V at the collector with the tv "on" . Is it safe to jump this transistor to test?

(This post was edited by NT7000 on Jan 19, 2018, 3:40 AM)

jts1957
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Jan 19, 2018, 2:48 AM

Post #4 of 35 (2462 views)
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Re: [NT7000] Hitachi 27AX3B no power after attempting to fix vertical deflection [In reply to]	Can't Post

Just B-Plus on H Output collector or H. Drive on base too?
So presumably Standby and 130V are there. What about Run B-pluses? Scan derived B-Pluses?

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Location: Far, Far Away

NT7000
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Jan 19, 2018, 4:31 AM

Post #5 of 35 (2458 views)
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Re: [jts1957] Hitachi 27AX3B no power after attempting to fix vertical deflection [In reply to]	Can't Post

Collector to emitter on HOT is ~150V. Q953 has 30V at the emitter, 29V at the base, and 0V at the collector. Is it safe to bridge E and C to see if the TV starts? Y'all have access to this TV's service manual, right? I'd paste a copy on here, but I don't see a place to attach files. I'm new to servicing CRT televisions, but not new to electronics, so describing the exact circuit I'm describing is somewhat difficult. It appears to my inexperienced eyes to be the HOT is not receiving the initial startup pulse, therefore the feedback loop never starts, and the TV remains dead. That initial power to the 25V rail comes from the collector of Q953. Are my assumptions correct? BTW, is it normal for a service manual to have parts mislabeled? Some parts are tagged with numbers that differ with what's written on the PCB.

jts1957
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Jan 19, 2018, 6:41 AM

Post #6 of 35 (2444 views)
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Re: [NT7000] Hitachi 27AX3B no power after attempting to fix vertical deflection [In reply to]	Can't Post

No. 0025 manual.

Turn on starts, Pin 45 of IC0101 to ??109 Power On-Off switching transistor then to coil of RL901.

HOut needs H Drive.

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Location: Far, Far Away

(This post was edited by jts1957 on Jan 19, 2018, 7:29 PM)

NT7000
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Jan 19, 2018, 6:49 PM

Post #7 of 35 (2436 views)
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Re: [jts1957] Hitachi 27AX3B no power after attempting to fix vertical deflection [In reply to]	Can't Post

Oops, neglected to mention I got RL901 back. Either a bad connection to mains, or a corroded power switch, but I got that relay to function again. Anyway, the problem seems to be that the 25V rail is down. The H.Drive waveform appears to come from the jungle IC, where it goes to transistor Q710, which pulls down a transformer, which in turn drives the HOT. T701 needs to have power at it's windings, and it gets that from the 25V rail. For startup purposes, 25V is derived from Q953, which ultimately gets it's power from T951. I followed the power with a multimeter from the power cord to the point where I lost power, which happened at Q953. My question was is it safe to jump Q953 to see if the set will start, or do I risk breaking something? I need to check to see if the jungle is generating an H.Drive signal, but even if it does, with no power to T701, I don't think the set will start.

EDIT: Updated OP for clarification, it was the power cord I was using that prevented the main relay clicking, but I fixed that :)

(This post was edited by NT7000 on Jan 19, 2018, 6:55 PM)

jts1957
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Jan 19, 2018, 7:46 PM

Post #8 of 35 (2432 views)
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Re: [NT7000] Hitachi 27AX3B no power after attempting to fix vertical deflection [In reply to]	Can't Post

Can you control relay (turn TV On-Off)?
Check jungle for H. Drive waveform. Even for a moment?
How does Q953 Test? D792? R792?
Voltage chart says 22.3V on collector of Q953, you say you have zero. Approx. 36V on other two terminals?

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Location: Far, Far Away

(This post was edited by jts1957 on Jan 19, 2018, 7:51 PM)

NT7000
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Jan 20, 2018, 1:01 AM

Post #9 of 35 (2424 views)
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Re: [jts1957] Hitachi 27AX3B no power after attempting to fix vertical deflection [In reply to]	Can't Post

Yes, I can control the main power relay.

I can't check for H.Drive since I don't have access to an oscilliscope, unless there is a way to do it with a basic multimeter.

Q953 to GND :B=29.7vC=0vE=32.5v

R792= 0.5 ohms
D792= not a diode anymore :( showing 1.143 on diode test in the normal direction, .568 reverse. What's so bad is I replaced that part to get vertical deflection back. Time to replace (again).

jts1957
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Jan 20, 2018, 1:40 AM

Post #10 of 35 (2421 views)
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Re: [NT7000] Hitachi 27AX3B no power after attempting to fix vertical deflection [In reply to]	Can't Post

We can come back to H. Drive.

Testing parts IN or OUT of circuit? (I prefer OUT).
With a multimeter set to 'Diode Check.'
Diode= OL then reverse leads and will give you a 'reading.' https://4donline.ihs.com/...2B6ED9A73243A3159BB0
Transistor= Six possible lead connections - On a standard transistor you will get four OL and two 'readings.'

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Location: Far, Far Away

(This post was edited by jts1957 on Jan 20, 2018, 1:50 AM)

NT7000
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Jan 20, 2018, 2:02 AM

Post #11 of 35 (2417 views)
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Re: [jts1957] Hitachi 27AX3B no power after attempting to fix vertical deflection [In reply to]	Can't Post

Testing in circuit, since I cannot be sure what is broke, and don't wan to desolder every part on the board until I run into the one at fault.

jts1957
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Jan 20, 2018, 3:37 AM

Post #12 of 35 (2412 views)
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Re: [NT7000] Hitachi 27AX3B no power after attempting to fix vertical deflection [In reply to]	Can't Post

In general, you only need to 'float' or lift a component's leads/pins to accurately test them. Semiconductors. A diode is easy. Only two leads, either one can be floated. Three lead transistors only two out of the three need to be unsoldered. Taking snapshots will help when PCB is not fully labeled. Resistors can be tested in circuit - take a reading with meter on ohms, reverse leads, if same reading as first and within components value/tolerence specs, it's good.
Series/parallel components/connections can affect in circuit readings.

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Location: Far, Far Away

NT7000
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Jan 28, 2018, 10:31 PM

Post #13 of 35 (2369 views)
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Re: [jts1957] Hitachi 27AX3B no power after attempting to fix vertical deflection [In reply to]	Can't Post

OK, I picked up an oscilloscope on the cheap, HP 1220A, waiting for probes to arrive in the mail. I'll update when I get the probes, and learn how to properly use this thing.

jts1957
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Jan 28, 2018, 11:14 PM

Post #14 of 35 (2368 views)
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Re: [NT7000] Hitachi 27AX3B no power after attempting to fix vertical deflection [In reply to]	Can't Post

https://literature.cdn.keysight.com/...90001.pdf?id=1857183

http://www.electronicrepairguide.com/...voltage-circuit.html

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Location: Far, Far Away

NT7000
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Jan 28, 2018, 11:50 PM

Post #15 of 35 (2366 views)
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Re: [jts1957] Hitachi 27AX3B no power after attempting to fix vertical deflection [In reply to]	Can't Post

Yeah, I've got that particular copy of the manual, wish I could find a better quality copy.

jts1957
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Jan 29, 2018, 2:37 AM

Post #16 of 35 (2365 views)
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Re: [NT7000] Hitachi 27AX3B no power after attempting to fix vertical deflection [In reply to]	Can't Post

Sample: https://elektrotanya.com/...sm.pdf/download.html

Purchase: https://www.qservice.tv/...p/shopexd.asp?id=824

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NT7000
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Jan 29, 2018, 5:39 PM

Post #17 of 35 (2355 views)
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Re: [jts1957] Hitachi 27AX3B no power after attempting to fix vertical deflection [In reply to]	Can't Post

OK, so last night I decided to play with the TV again, just to see if I could glean any info from it, and I found something interesting. I turned the TV on (with the board outside the chassis, upside down so I could get to some test points) and of course I heard the main relay click.I also heard the FBT whine for a split second, then a light clicking noise near the FBT. I turned the board right side up to investigate and turned it on again, and noticed the tiniest spark on R792A near the FBT (diagram has it listed as R792), so that resistor has burned, which helps explain the absence of the 25V rail, but I don't know what could have burned it in the first place. I did reflow a few pins on the vertical deflection IC when I was trying to get vertical deflection working, could I have a burned vert. def. IC? That would explain a lot. At least those are cheap, and readily attainable. Dumb question about R792A: Fusible resistors are the same thing as flameproof, right? All I need to do is order one with the same resistance and wattage, or is there some other property to be aware of? I also noticed one resistor missing completely, R730. All that remains of it are it's legs. This resistor seems to be part of the cathode heater circuit, which doesn't appear to be related. Any thoughts on this?

jts1957
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Jan 29, 2018, 8:33 PM

Post #18 of 35 (2351 views)
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Re: [NT7000] Hitachi 27AX3B no power after attempting to fix vertical deflection [In reply to]	Can't Post

http://www.yageo.com/...eo%20LR_FKN_2013.pdf

Quote

How does Q953 Test? D792? R792?

R730 is present on all chassis variants.

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Location: Far, Far Away

NT7000
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Feb 1, 2018, 5:07 AM

Post #19 of 35 (2337 views)
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Re: [jts1957] Hitachi 27AX3B no power after attempting to fix vertical deflection [In reply to]	Can't Post

Alright, I removed the remains of R730, and R792. I tested the the 25V rail to GND, and the resistance is high enough to assure nothing is shorted out. I removed and tested D721, Q710, and Q953, all checked good according to the multimeter. When I was initially trying to get vertical deflection going, I saw that the 25V rail was at 18V, could it be that R792 was weak, and the extra current draw of vert. def. IC finished it off? Likewise, Q953 checked out OK when removed from the board, and that makes me think that it is trying to apply power to the 25V rail, but the current draw is enough to keep the voltage low. Any other parts I should check, since I'm trying to get a grocery list together?

jts1957
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Feb 1, 2018, 5:35 AM

Post #20 of 35 (2336 views)
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Re: [NT7000] Hitachi 27AX3B no power after attempting to fix vertical deflection [In reply to]	Can't Post

R730 82 ohms 1 watt
R792 .5 ohm 1/4 watt
D792 High Speed Switching?
C627 100mF 35 volt
C794 470mF 35 volt

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Location: Far, Far Away

(This post was edited by jts1957 on Feb 1, 2018, 6:58 AM)

NT7000
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Feb 1, 2018, 5:58 AM

Post #21 of 35 (2334 views)
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Re: [jts1957] Hitachi 27AX3B no power after attempting to fix vertical deflection [In reply to]	Can't Post

I assume replace C627 and C793 to be safe, right? Are you sure you meant C793, not C794? C793 is on the 5V rail. D792 tested fine, replace anyway?

jts1957
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Feb 1, 2018, 7:16 AM

Post #22 of 35 (2331 views)
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Re: [NT7000] Hitachi 27AX3B no power after attempting to fix vertical deflection [In reply to]	Can't Post

Quote

I replaced a diode in between the vert. def. IC and the FBT

?NOT D792?

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Location: Far, Far Away

(This post was edited by jts1957 on Feb 1, 2018, 7:31 AM)

NT7000
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Feb 1, 2018, 4:16 PM

Post #23 of 35 (2325 views)
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Re: [jts1957] Hitachi 27AX3B no power after attempting to fix vertical deflection [In reply to]	Can't Post

Yes, it was D792. I checked it again anyway to make sure it wasn't caught in the carnage. Is it possible R792 was weak, and the act of restoring power to the vertical deflection IC killed it?

jts1957
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Feb 1, 2018, 8:35 PM

Post #24 of 35 (2316 views)
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Re: [NT7000] Hitachi 27AX3B no power after attempting to fix vertical deflection [In reply to]	Can't Post

A resistor is either good or bad, there is no in between. (decreased or increased in value, arcing ... all bad)

I wanted to be sure you replaced diode with scan derived fast switching - NOT a 60 Hz power supply one.

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Location: Far, Far Away

NT7000
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Feb 1, 2018, 8:42 PM

Post #25 of 35 (2314 views)
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Re: [jts1957] Hitachi 27AX3B no power after attempting to fix vertical deflection [In reply to]	Can't Post

I just wanted to find out why the resistor burned in the first place... I don't want to replace it to have it burn again. As for the diode, I pulled up the datasheet for it and it's replacement and made sure every value met or exceeded the original specs. I think they were both good for 150+ MHz.

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