Hitachi C1714TE stuck in standby

[3Phase615V]

Hi,

I have owned and used, almost daily, a very reliable Hitachi C1714TE 17" colour TV for 22 years. However, for the first time ever it's failed to switch on fully and won't budge out of standby. With this particular TV, the red power-on/standby LED lights brightly when in standby then dimly when the set is turned on, so as not to be a distraction during TV viewing I guess. Anyway, on switching on the TV power button, I can hear for about a second, as it normally does, the tube static crackle, but then nothing. At the same time the TV is powered on, the standby LED lights dimly during the static charge but then glows brightly to indicate the TV has switched to standby. In this state neither the TV remote nor the built-in channel up/down buttons bring the TV on as they should.

So with that, I found the TV's service manual online (C1714TE Service Manual) and took a few voltage measurements. As expected with there being standby power, on the primary side, I found just over 300V-AC at the collector of Q903 (ON4584/BUT12AF) but that the base of Q903 doesn't connect via R918 (not installed) to the collector of Q902 but instead goes directly to the emitter of Q901. This can be seen by comparing the schematic to the track side image (see attached). Also the transistors stated for Q901 (2SA1390C/D) and Q902 (BC368) in the schematic don't match the ones installed in my TV, with Q901 being a JC558B and Q902 being a 2SC368. I check the stated SM voltages on Q901/2 and found they did not really match the stated values, but as I have standby power I've assumed they must be okay.

So moving on to the secondary side, I checked the output voltages on T901 and found the cathode side of D951 (S1) to have 117VDC and the cathode side of D852 (S3) to have 14.1VDC. I guess with these voltages I can at least assume I have a good power supply but I am not sure of how the standby circuit works here so I am in need of some guidance and pointers on what to test next as clearly something is blocking the power-up sequence. Any advice gratefully received.

Here are my voltage test results.

Q903
B: 0 V
C: 318 V
E: -2.14 V

Q902
B: -2.7 V
C: -2.38 V
E: -2.0 V

Q901
B: -0.6 V
C: -0.645 V
E: 0 V

D951 cathode: 117 V
D952 cathode: 14.1 V

Cheers!

[McMurdo]

Q953 looks like the standby switch, controlled from the micro, I can't see any other control for the psu other than that.

[3Phase615V]

Hi Kevin,

Many thanks for replying to my post.

Looking at what you've suggested I guess I should have followed that track to the microprocessor before now. I tested the voltages on Q953 and got the results below. So I pulled Q953 out of the board and tested it in my component tester and it appears to be okay.

Q953
E: 14.1 V
C: 0.8 V
B: 13.65 V

So then based on the collector voltage being lower on Q953 than stated in the SM (12V) I worked back to see if Q952 was functioning:

Q952
E: 0 V
C: 13.4 V
B: 0 V

Interestingly, the base is dead so I checked the voltage on the track leading from pin 20 (S/BY) on the microcontroller and it was 0V. I powered off the TV and connected up pin 20 to an analogue multimeter to monitor it on switch on. Immediately after powering the TV on the meter needle jumps to about 1.6V then immediately falls back to 0V. Just in case I checked the microcontroller was receiving sufficient power and it appears to be okay:

SAA5290 microcontroller
VDDM microcontroller power supply (pin 20) - 4.9V
+5 V analog/text power supply (pins 38/39) - 1.16V ( I assume these are not 5V as the TV is in standby)

So at this point I'm wondering if I need to pull out Q952 to test it's not the cause of the failure to switch on Q953?

Does that make sense?

[dsergeant]

Gut feeling is that you have shorted turns in the line output transformer. Or quite possibly a shorted field output chip (very common on Hitachi's). The fact that you get a brief static charge on the tube probably confirms this, it tries to start than trips. Try looking around the field chip first.

Dave

[3Phase615V]

Hi Dave,

Thanks very much for replying to my post and your insight.

So would that be the TA8427K IC (7-pin power amp chip for driving a deflection circuit of a colour TV) that you're referring to as the field output chip? Also, how does the protection work when, like you suggest, the LOPT or field output chip has a short? I mean is it the microcontroller sensing the short after sending it's initial signal to turn off the standby circuit, which explains the S/BY voltage falling back to zero?

Thanks for your help.

[ben]

I would start by checking for dry joints around the line, RGB o/p and frame stages, an the yoke connector and its terminals. The fact the EHT comes up is reassuring.

You may also get away with lifting the supply line to the frame stage, see if you then get a horizontal line. Not familiar with this set, so no idea if it will let you!

[ms660]

It might be worth checking that the line stage EHT over volt hasn't triggered the comparator, the base of Q705 should be 0v.

Note: In the schematic from the elektrotanya download there's no connection shown from the collector of Q705 to the standby output of IC001, I think there should be.....

Lawrence.

[3Phase615V]

Originally Posted by ben

I would start by checking for dry joints around the line, RGB o/p and frame stages, an the yoke connector and its terminals. The fact the EHT comes up is reassuring.

You may also get away with lifting the supply line to the frame stage, see if you then get a horizontal line. Not familiar with this set, so no idea if it will let you!

Hi Ben,

Many thanks for your suggestions.

To try to make things easier I've stripped out all but the most useful pages of the service manual, including schematic, in case anyone has trouble downloading the full service manual from the original link I posted. I've attached it to this message.

I should have mentioned that I checked the whole main board PCB and yoke PCB at the start of my checks for visible dry joints and found a few but none seemed to have actually broken the connection between solder pad and component leg. I re-soldered all of them anyway and also joints where heat stress or high power is drawn through the joint. That said, however, I took another look at the LOPT solder connections (is that what you mean my "line"?), RGB o/p (that's the yoke PCB right?), the yoke connector and its terminals and all look good. I found no breakages testing the continuity between PCB-track-to-component-leg connections of the LOPT or yoke PCB to terminal legs.

I have to admit I am not yet sure what you mean by "frame stages". Could you give me pointer?

Also, where would I lift the supply line to the frame stage?

Thanks!

[3Phase615V]

Originally Posted by ms660

It might be worth checking that the line stage EHT over volt hasn't triggered the comparator, the base of Q705 should be 0v.

Note: In the schematic from the elektrotanya download there's no connection shown from the collector of Q705 to the standby output of IC001, I think there should be.....

Lawrence.

Thanks Lawrence. I'll check both of those things shortly and report back.

[dsergeant]

Frame=Field=Vertical Deflection, all pretty interchangeable words for the same thing. Yes, the TA8427K chip, try isolating its supply rail. These chips fail all the time.

[3Phase615V]

Heheh! Jackpot I think, Dave!!! There is indeed a short or at least some causal fault in TA8427K (IC601). It's killing the 27V supply rail.

I lifted one side of the supply rail wire link across the back of the chip, powered on, heard the CRT static crackle and now the standby LED flickers on and off as it should in response to the remote control. I then checked the voltage on the supply rail to the TA8427K and it's back up at a very healthy 29V. When I powered off I got a white flash from the screen, which seems to indicate it was energised. Additionally, since C605 (100F, 35V) connects the pump-up power supply to the the pump-up output on pin 7, I tested it out of circuit and its ESR was way off at 31R, so that needs replacing as well.

I've just taken the TA8427K out of the PCB and tested the resistance of all the pins to ground. Pin 2 (Vertical Output) to ground shows 1.1R both ways. , so I guess that's where the fault lies.

So I just checked Cricklewood Electronics and they've got the TA8427K for a mere £1.80. Hopefully that'll be all that's faulty with the TV but I'll report back.

Thanks Dave and everyone. I'll update once I've replaced the chip and cap.

Lawrence, I think the link from the base of Q705 to the S/BY output of IC001 may be through the 5V(D) connection?

[ms660]

Originally Posted by 3Phase615V

Lawrence, I think the link from the base of Q705 to the S/BY output of IC001 may be through the 5V(D) connection?

It's Q705's collector connection to pin 20 of IC001 that's missing on the schematic, the short on the 27 volt rail is what triggered Q705's base to go +ve and pull down the collector and hence the standby output of IC001 (pin 20)

Basically the 27 volt supply is derived from a secondary winding of the line output transformer, any excess loading of that rail will be reflected in the transformers primary, the primary has a sensing resistor in series with it, the increased current (due to the fault) flowing through it causes an increase in the voltage dropped across it (Ohms law) once that voltage equals Q204's turn on voltage Q204 turns on, that in turn turns on Q705 which in turn grounds the standby rail of IC001, which in effect grounds the base of Q952 which then turns of and thus turns off Q953 and so shuts off the 9 volt supply that powers the IC that contains the time base generators, thus shutting down the line output stage.

Lawrence.

[3Phase615V]

Thanks very much Lawrence for the detailed explanation - appreciated. I was very much looking to understand the "mechanism" of how the protection worked. It's really fascinating to me how much goes into the design of these circuits - really clever and brilliant thinking.

And, yes, sorry I traced the S/BY to the the base of Q705, not the collector. I'll check if the collector to S/BY exists on the actual PCB but presumably it must based on what you've described.

[3Phase615V]

Hi Lawrence, I can confirm that there is a 0 resistance connection from the collector of Q705 to pin 20 OF IC001. Disappointingly there appear to be a few discrepancies between the schematic and the track side circuitry image, with the track side image being accurate. Unfortunately the photocopy quality of the schematic lacks a decent resolution and is missing patches here and then so perhaps the connection didn't show up well compared the the original printed version of the service manual.

[dsergeant]

Glad you found it. When I repaired loads of CRT sets some years ago there were various faults that you just knew about. Field chip in Hitachi's (and dry joints on eyelets in some of the later models), line transistor shorts, LOPT shorted turns, 1uF 400V caps in Panasonic VCRs. Didn't waste time, went straight to them. Experience works wonders.
Good luck with the repair.

Dave

[Welsh Anorak]

Although the cause of the IC's failure is usually due to dry joints on its pins, now it's a few years old I'd replace the 100uF capacitor beside it as well.

[3Phase615V]

Cheers Glyn. I covered that cap (C605, 100uF, 35V) in post 11 of this thread (2nd paragraph). I've replaced it from my spares and picked up the TA8427K earlier today. Hopefully I'll have a working TV when I get to swap out the bad chip later.

[3Phase615V]

Good news - fitted the TA8427K and the TV is working again. Yay! Many thanks to everyone who contributed!

[ben]

Great news! Another one saved from the tip.

One thing I meant to point out earlier, will put it here for future reference. It is always wise to discharge the caps in the supply line (from the line o/p trfmr or wherever) to the frame chip before fitting a new one.

[3Phase615V]

Ah! Thank you, Ben. I must confess I didn't do that! The chip didn't seem to mind though, fortunately.

Thanks again for your advice isolating the fault - you were right on point as well.

Btw, I've noticed the brightness gain seems to have increased quite markedly and I've turned it down a few notches via the settings menu. Is that expected after a field chip change? Is there any setting up of the focus or anything else that should be done post a repair like this? Not necessarily anything to do with the field chip failure/replacement but I've wondered if I can improve the focus by adjusting the control on the LOPT. Reading ticker tape items, clocks, TV channel items, Up Next, etc at the bottom and corners of the screen on channels isn't great on CRTs of this size these days but I think TV channels in general assume everyone has an LCD, LED or plasma TV now so they expect the inherent sharpness in the newer technologies. I don't have the Phillips circle test pattern but I think I saw somewhere on here it can be download it onto DVD. Wondering if it's worth setting the TV up again according to the service manual?