Toshiba 32HFX73 Help

[brodek]

Ok here is the link to the full schematic... getting with the times...

https://drive.google.com/file/d/1W6k...ew?usp=sharing

[brodek]

Link to the full schematic

https://drive.google.com/file/d/1W6k...ew?usp=sharing

[brodek]

Hi All. I've continued to probe around the set and I'm 99% sure the micro is my issue. I can't find any external reason for it not to function so I can only assume the chip itself is bad. I guess anything can fail but I thought these parts were typically pretty reliable. In any event I can't find any replacements or data online. There is a spec sheet but has pretty much nothing beyond the pinout. Anyone have or know where I can score an MN102H85KFL OR the whole signal board Toshiba P/N 23788843 SIGNAL BOARD, PD1174B

[brodek]

Hi All. I was about to give up on this set. I was about to button her up with a tear in my eye after 19 years of service...she decided to bounce back from the grave!! It also hinted at what the problem might be. (I need to find some time to verify) After being off AND unplugged for over a week I plugged her in and hit the power button on the remote. She sprang right back to life again! I left her to warm up a bit and that is when I noticed that the volume down was "stuck" on. IT was at zero and constantly on the screen. The set won't respond to the remote at all. I shut her down and she was back to not turning on again. After it cooled off for a minute I could power it back up but it acted as if random buttons on the front were being pressed. I started to poke around the Key A and B signals/voltages but everything is functioning as expected. Now the problem in it's entirety has gone away or I cleared whatever the "short" was causing the issue when I started looking around. My theory on this set is that something was bridging the front buttons making the micro think a button was being pressed and it apparently then ignores all other inputs. I have it running now and probed up to hopefully catch it in action but something tells me the fault is gone. Anyway she's back up and running for now. I'll post again if I can actually find the fault. Thanks to everyone who offered some help and insight on this set.

[brodek]

Ok so the fault isn't gone entirely. What I'm finding now is that after a bit of run time the Key B voltage is dropping from 3.2 at the micro to 2.5. I have eliminated the switch board as it is a separate board and you can just pull the connector. No change to the voltage. So the issue appears to be isolated to the signal board. I did check the 3.3V supply and it is rock solid. When you watch it on the scope it seems to jump from the 3.2 down to the 2.5 vs a slow drift. Not a lot of parts in the circuit so I should be able to narrow it down hopefully pretty quickly. I did confirm it stops responding to the remote when in this fault condition.

[brodek]

Not sure if anyone is still following this or even cares but just in case here is the latest update. :-) Looking at the schematic for the keys it's really just a 3.3V supply (which is solid) goes through two 33K (one for Key A and one for B) resisters and 1K to the micro, after the 33K it also goes to the various keys that have different resistors to ground. So I have think there is a A to D converter or a set of comparitors in the micro to let it know what buttons are being pushed. I eliminated everything else by pulling the connectors and Key B still drops to 2.5 volts or so. It's REALLY weird as it doesn't drift down it toggles as if it's TTL logic vs some drifting analog component. (see attached scope shot, I probed Key and and B at the connector on the signal board) Just to be 100% sure I pulled the two .01 uF decouple/bounce caps near the micro on the off chance they were starting to short (these are surface mount chip caps which can crack and get flaky) didn't change the behavior. So I'm back to a bad micro that seems to act up more significantly with temperature. I need to pick up some freeze spray just for the fun of making the fault come and go. Interested to see any feedback, comments or suggestions?

[old_tv_nut]

Give us a hint - what schematic page is this on?

[old_tv_nut]

OK, found the page.
Looks like 3.2 V is supposed to be the supply voltage for the keys, correct?

And then the key press drops the voltage by putting a resistor from Key B to ground making a voltage divider from 3.2 volts through 33K to the Key B line and through the switched resistor to ground.

If that's correct, then a drop to 2.5 volts would be equivalent to a resistance to ground of 118k ohms. There appears to be no component near this value that could be shorting between Key B and ground unless it's the IC.

To be sure, though, I would check whether the 3.2 volt supply itself is dropping to 2.5 volts intermittently.

[zeno]

May have missed it but have you tried pulling the key inputs
& using the remote. If its an SMD no easy way. Any TAC switch
is suspect for leakage. On older sets we would unhook most of the micro
& just keep the ON/OFF path. Then hook things up til it stops working.
Almost always a TAC switch or little disc.
Also check & be sure you are getting a good RST, you can also
force it to test ( RST or NOT RST ). Lytic cxaps & RST transistors
the usual perps here.
BTW you are covering things well but did you pull the batteries from the
remote ?

73 Zeno
LFOD !

[brodek]

Thanks for the follow up questions and support. I think you found them, but the pages in the schematic are 14 for the micro and 3.3V supply. Key inputs are control circuit 1 page 76 and control circuit 2 pg74 (really just a pass through for key and remote signals). In my scope capture I posted I’m monitoring the Key A and B. Top trace is A which you can see is stable when B drops. So I know the 3.3V is good. Zeno to answer your question. I have pulled the connection for the keys and the low voltage condition stays on Key B. (until I power off and let the set cool for a few minutes. This has to be unplugged or the fault doesn’t clear. I did try pulling just the key connections and the remote still doesn’t work… to be 100% clear it works at initial power on it stops when I see the Key B voltage drop to 2.5. This is really an odd failure. Initially it would not come on at all. I could force the system on with the picture I posted earlier. (but no video, vertical was messed up, retrace lines) Now the set comes on as soon as you plug it in and everything works. I can turn it on and off remote works the whole bit. After a few minutes of run time the remote stops working. (corresponds 100% to Key B dropping to 2.5V). So I ordered some freeze spray today and I’ll try that to see if I can make the problem come and go. Says it will be a week before it gets here. Thanks again for the support. I started life as a tech, then test engineering, failure analysis engineering now I’m a manager. My experience is in industrial electronics so I’m a little less experienced with TVs (though I work on consumer stuff as a hobby in my spare time). My wife wants this set gone BTW. I however love this old set which I’ve had for almost 20 years now. Was the first new TV I got for my first house… lots of sentimental value for me. Speaking of which I am looking for a Motorola Quasar Works in a Drawer hybrid set. I think it was a 71 or 72. It’s what got me into electronics. I remember “helping” my dad fix the TV from time to time. I was hooked as soon as his slid the drawer out….

[brodek]

I wanted to follow up with what I think is the final fix for this issue on this set. I was not able to "clear" the fault using freeze spray on the micro. So oddly it didn't seem to be temperature related. It did seem to be power on time related. When the set is plugged in I would see the voltage drop after a period of time causing the fault condition. I had already removed the two "debounce" caps so they were eliminated as a possible cause. I then went back and did a much better job of cleaning up around the micro pins and these caps. After doing that the fault was cleared permanently. So either I had some tin whisker growth that I couldn't see with the naked eye or some type of dendritic growth that took some extra cleaning to clear completely. It's been a few months and the set is still working perfectly. Glad to have the old girl working again.

[zeno]

Wow, nice sticktoitness. Screen shot shows multiple probs ( vert, retrace, hoz,
pin & no vid ++ ) so it must have been data related ( no coms).
This is the type of problem that will send most techs to the local bar.
Well done !!!

73 Zeno
LFOD !

[brodek]

Thanks Zeno. The screen shot was from me forcing the set on by disconnecting the power off connections to the micro. I had initially thought that one of the protection circuits was keeping the set off. What I discovered was that the key controls were going to "invalid" levels and making the micro basically lock up an stop responding IE you can't turn the set on via remote or pushbutton. I spend a few nights at the local bar crying in my beer over this set but I hate not getting to 100% root cause, keeps me up at night.