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 Post subject: 1994 Zenith SM3247MK8 (Troubleshooting Questions)
PostPosted: May Thu 23, 2019 1:42 am 
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Joined: Sep Sat 01, 2018 7:03 am
Posts: 12
Hello everyone,

So I have a few issues with this TV that are real head scratchers for me. I found it in a barn and was able to get it for free. When I first powered it on, I got deflection and a raster but it was really erratic and would slightly collapse vertically towards the center on the screen a lot. After my first initial tests, it no longer does this. Now when I press the power button I hear the high voltage kick on and a static sound come from inside the CRT before the High Voltage quickly shuts off. After the HV shuts off the signal static sound starts to come out of the speakers normally. I have also noticed that the filaments don't appear to be lighting up in the CRT. I just spent an hour or so re-soldering any dry-solder joints on both circuit boards but have seen no change in the issues previously mentioned. I know this is a newer set and the 1990s Zeniths were plagued with CRT failures. I have also heard that the 32 inch CRTs had a different OEM then the sets plagued with the catastrophic failures and should be more reliable, how ever I don't how true this is. I just don't have enough experience with these sets yet to really know where to start.

Any help would be greatly appreciated!

Thanks,

- J.


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 Post subject: Re: 1994 Zenith SM3247MK8 (Troubleshooting Questions)
PostPosted: May Thu 23, 2019 3:21 pm 
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Joined: Apr Sat 06, 2013 2:18 pm
Posts: 468
Location: New Hampshire
Unless you just change the chassis you will need a manual. Almost every voltage
to run the set comes off the HV. Even the filament. Its a matter of finding the bad
stage. One thing you can do is cycle the set a few times then see if the horz output
is getting hot. If so its usually the flyback. There were also a few small electrolytics
in the power supply that did this. Best bet is post a few chassis pics, there are 2
major types this set could use.

73 Zeno 8)
LFOD !


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 Post subject: Re: 1994 Zenith SM3247MK8 (Troubleshooting Questions)
PostPosted: May Thu 23, 2019 9:01 pm 
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Joined: Sep Sat 01, 2018 7:03 am
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First off, thank you very much for the response! I really appreciate the help.

I cycled the set a few times and the Horz Output (and all the transistors on the power supply board) were cool to the touch. I have also noticed that the set makes a high pitch squick even when it is off but plugged in. I would assume that it is just a transformer somewhere in the power supply but wasn't sure if that was important? Whats really interesting is the singing transformer squeals loudly right after I unplug it. Another thing I noticed is that a massive amount of static comes off the front of the CRT (before quickly dissipating) when I press the power button. Not sure how important that is as I know static and CRTs come hand and hand but the amount coming off the thing felt a bit excessive to me. Anyway, I have attached a photo of both circuit boards and the chassis CAD drawing in the cabinet. I hope this helps figure out what capacitors may be in question or where else I should look on the board.

Thanks again for your help,

- J.


Last edited by TheFanOfFans on Jun Wed 05, 2019 5:13 pm, edited 1 time in total.

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 Post subject: Re: 1994 Zenith SM3247MK8 (Troubleshooting Questions)
PostPosted: May Thu 23, 2019 9:32 pm 
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Joined: Apr Sat 06, 2013 2:18 pm
Posts: 468
Location: New Hampshire
Sounds like HV is too high causing the strong static then HV shutdown triggers.
Squeal probably coming from the power supply transformer near front of HV
module.
Flying blind you can change the small electrolytics around that area.
Also you can look for a test point marked between 120 & 135 VDC. Hang a meter
there & if it goes higher its the power supply for sure. Go over that board REAL
good for cold joints also.

73 Zeno 8)
LFOD !


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 Post subject: Re: 1994 Zenith SM3247MK8 (Troubleshooting Questions)
PostPosted: May Fri 24, 2019 12:45 am 
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Joined: Sep Sat 01, 2018 7:03 am
Posts: 12
I just finished re-soldering most of the board and the TV still has the same issues. I am definitely going to just go ahead and replace the caps you mentioned and see if that does the trick. I consider recapping the entire power supply board but am not sure if that's a bit overkill? Also, when I was moving the board to re-solder it's connections, a weird silver disc popped off it's legs that are still soldered to the board. I honestly have no idea what it is and was curios if you did? It kinda looks like a cross between a button cell battery and a disk capacitor. I took a few photos of it.

Thanks again,

- J.


Last edited by TheFanOfFans on Jun Wed 05, 2019 5:12 pm, edited 1 time in total.

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 Post subject: Re: 1994 Zenith SM3247MK8 (Troubleshooting Questions)
PostPosted: May Fri 24, 2019 1:39 am 
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Posts: 4971
Location: Ann Arbor, MI
That is the inrush current limiter. When it is cold it has a fairly high resistance. As it heats up the resistance drops fairly low.
These do fall apart quite often. With that out of the circuit there will be no power to the set. A lot of sets use a limiter with a cold resistance of around 120 ohms. Perhaps someone who is familiar with Zenith sets can give you the correct value. You can get inrush current limiters from places like Digi-Key.

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 Post subject: Re: 1994 Zenith SM3247MK8 (Troubleshooting Questions)
PostPosted: May Fri 24, 2019 3:23 am 
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Joined: Sep Tue 30, 2014 6:08 am
Posts: 4119
Location: Norfolk, VA
That's the degaussing thermistor. Most Zenith System 3 sets and later used 25 ohm cold units - 63-10710 or 63-10710A replace most of them.

That's a PTC unit - the exact opposite of an inrush limiter. Cold, it supplies current to the degaussing coil, and as it warms up, the resistance increases, tapering the current through the degaussing coil to a low value.

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Brian
"Capacitor Cosmetologist since 1979"
USN Retired 1984-2006 (Avionics/Cal)


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 Post subject: Re: 1994 Zenith SM3247MK8 (Troubleshooting Questions)
PostPosted: May Fri 24, 2019 3:38 am 
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Location: Ann Arbor, MI
I am most familiar with the use of an inrush current limiter along with a voltage dependent resistor for degaussing. But I was away from TV servicing by 1994.
If that is a degaussing thermistor, you can just ignore it for awhile.

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Tom


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 Post subject: Re: 1994 Zenith SM3247MK8 (Troubleshooting Questions)
PostPosted: May Fri 24, 2019 12:19 pm 
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Joined: Apr Sat 06, 2013 2:18 pm
Posts: 468
Location: New Hampshire
You can resolder that thermistor to get you by. Its common to fall apart. We used to
buy then 20 at a time.
Yes I would just do the power supply caps. Sorry I cant remember more but since
this chassis was only in 27" - 35" sets almost all were fixed board level in home & I was
a bench tech.

73 Zeno 8)
LFOD !


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 Post subject: Re: 1994 Zenith SM3247MK8 (Troubleshooting Questions)
PostPosted: May Fri 24, 2019 3:27 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 272
I always just soldered the wires back onto those degaussing thermistors. Be gentle since the tend to fall apart if you apply too much heat.

Since the set more or less worked when it was first tested, but had intermittent vertical deflection, I would start from there. It's possible that the vertical IC shorted due to bad soldering, and now it's overloading the flyback secondary that powers it. I would start by taking a resistance measurement across each of the filter caps for the flyback secondaries to see if anything is shorted. It's also possible that a rectifier diode is shorted, or a fusible resistor has opened (or both).

Was the TV was sitting in a very hot location for a while before you tried it for the first time? Bad electrolytics can sometimes work well enough when warm, but not when they're room temperature.


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 Post subject: Re: 1994 Zenith SM3247MK8 (Troubleshooting Questions)
PostPosted: May Fri 24, 2019 4:43 pm 
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Joined: Jan Mon 04, 2010 2:52 am
Posts: 2911
If it were me, I would definitely start by checking the electrolytic capacitors in the HV section. In fact, some of those electrolytics may be some sort of European odd ball shaped (square/rectangular, etc.) capacitors so keep an eye out for those. I would also suggest picking up a schematic as it would be a very helpful aid. In addition, look for cold solder joints in the HV section. Sometimes those cold solder joints aren't easy to spot so you may have to wiggle the components a tad to see if any of them are loose on the foil.


Last edited by Blustar1 on May Sat 25, 2019 6:21 pm, edited 1 time in total.

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 Post subject: Re: 1994 Zenith SM3247MK8 (Troubleshooting Questions)
PostPosted: May Fri 24, 2019 7:51 pm 
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Joined: May Sun 07, 2017 11:35 am
Posts: 566
Location: Belrose, NSW, Australia
I think, rather than electrolytics, you are thinking of the HV tuning capacitor that is between Collector and Emitter of the hor OP transistor. If this is O/C, it will trip the HV protection circuit. This may be your problem.

This cap works very hard and MUST be replaced with the correct type of capacitor. There is a reason why it is so big. The wrong type of cap will fail very quickly and possibly destroy something else in the process.

But the best way to tell is to scope the output of the power supply and capture the exact voltage before it shuts down. If it goes over the correct voltage, the small electros in the power supply are the culprit. A very common fault this one, esp. for mid 90s products (google "capacitor plague" for the full story).

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 Post subject: Re: 1994 Zenith SM3247MK8 (Troubleshooting Questions)
PostPosted: May Fri 24, 2019 9:06 pm 
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Joined: Apr Sat 06, 2013 2:18 pm
Posts: 468
Location: New Hampshire
I must be getting old. I missed the vertical problem ! Anyhows on this era Zenith
90% of vert problems were just cold joints on the IC. It was common & the same on
most other brands. We called them gravy jobs & it was what put steaks on the table &
beer in the fridge :D
Most sets will have at least 2 shutdown modes.
1) over voltage, to much HV, too high B+ from power supply or "untuned" horz out.
2) over current, load on HV such as too much beam current, load on a scan derived B+
such as shorted vert, bad FBT
Sometimes add to that ........
1) no vert sweep ( usually on projo sets)
2) low audio out B+ line ( on high end sets with big amps )
For now be sure the power supply is in spec.

73 Zeno 8)
LFOD !


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 Post subject: Re: 1994 Zenith SM3247MK8 (Troubleshooting Questions)
PostPosted: May Fri 24, 2019 10:34 pm 
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Joined: Sep Sat 01, 2018 7:03 am
Posts: 12
Thank you again for all your replies! They have been very informative.

Now, I have found a few replacements for the Thermister on eBay but wasn't sure if they have to be the exact value of the original? I found a lot of ten 120 Volt 12 ohm thermistors and was wondering if they would suffice? I read a around and supposedly this component has some leniency when it comes to the value of replacement parts but just want to know for sure before I buy replacements.

I am going to make a list of capacitors on the power supply board tonight and probably just order replacements for all of them. If there was a known defect with those parts I figure I might as well just get rid of them while I am working on the set and start fresh.

My other question is if someone has an image of what the HV tuning capacitor looks like? I know what radio tuning capacitors look like but wasn't sure if they look similar?

I will also double check the solder joints again tonight to make sure I reflow the Transistor joints and that the components are secure.

Thank you again,

- J.


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 Post subject: Re: 1994 Zenith SM3247MK8 (Troubleshooting Questions)
PostPosted: May Fri 24, 2019 11:29 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 272
TheFanOfFans wrote:
Thank you again for all your replies! They have been very informative.

Now, I have found a few replacements for the Thermister on eBay but wasn't sure if they have to be the exact value of the original? I found a lot of ten 120 Volt 12 ohm thermistors and was wondering if they would suffice? I read a around and supposedly this component has some leniency when it comes to the value of replacement parts but just want to know for sure before I buy replacements.

- J.


Make sure you buy a PTC (positive temperature coefficient) thermistor! An NTC inrush current limiting thermistor will either blow the fuse, or melt the degaussing coil.


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 Post subject: Re: 1994 Zenith SM3247MK8 (Troubleshooting Questions)
PostPosted: May Sat 25, 2019 7:18 am 
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Joined: Sep Sat 01, 2018 7:03 am
Posts: 12
ac wrote:
TheFanOfFans wrote:
Thank you again for all your replies! They have been very informative.

Now, I have found a few replacements for the Thermister on eBay but wasn't sure if they have to be the exact value of the original? I found a lot of ten 120 Volt 12 ohm thermistors and was wondering if they would suffice? I read a around and supposedly this component has some leniency when it comes to the value of replacement parts but just want to know for sure before I buy replacements.

- J.


Make sure you buy a PTC (positive temperature coefficient) thermistor! An NTC inrush current limiting thermistor will either blow the fuse, or melt the degaussing coil.


Thank you for the heads up! The ones I found said that they are specifically for a degaussing circuit so I think they should be safe.

Also, I just ordered replacements for all the cheap capacitors on the power supply board. I will post an update on here as soon as I finish installing them and see if they were the issue.

Thanks again everyone for all of your help and suggestions!

- J.


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 Post subject: Re: 1994 Zenith SM3247MK8 (Troubleshooting Questions)
PostPosted: Jun Sat 01, 2019 3:06 am 
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Joined: Sep Sat 01, 2018 7:03 am
Posts: 12
Hello everyone,

So the caps arrived today and I finished recapping the power supply board a little while ago. When I first powered it on after the recapping I heard a high pitch squeal and a few arcing sounds come from within the set. I was not able to pinpoint exactly what was arcing. After it made arcing sounds a few times, the squealing sound went away. I then powered off the set and double checked my connections. I also re-soldered all of the transistors on the power supply board but saw no changes. Now the set no longer arcs and does just as it did before; High voltage turns on for less then a second before switching off and the static "no signal" sound comes from the speakers.

Does anyone have any idea what could be wrong? I honestly am very lost on where to go from here.

Any pointers would be greatly appreciated,

- J.


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 Post subject: Re: 1994 Zenith SM3247MK8 (Troubleshooting Questions)
PostPosted: Jun Sat 01, 2019 4:19 pm 
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Joined: May Sun 07, 2017 11:35 am
Posts: 566
Location: Belrose, NSW, Australia
There is a physically large capacitor connected from the hor OP transistor to ground. Probably about .01uF or thereabouts. That is the hor tuning capacitor I spoke of. It might have two wires on each end, to better handle the current and make a more reliable connection.

Look at the ratings on your cap. Replacement must be same or better, this is a critical part.

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 Post subject: Re: 1994 Zenith SM3247MK8 (Troubleshooting Questions)
PostPosted: Jun Wed 05, 2019 6:30 am 
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Joined: Sep Sat 01, 2018 7:03 am
Posts: 12
I just wanted to post a quick update here, I ended up deciding to part the set in question out. The set turned out to have A LOT of hours on it and I just couldn't justify putting anymore money into it. I have to thank all of you for your help and advice for troubleshooting this set. I will now have a better idea on where to start whenever I need to troubleshoot another TV of this era!

Thanks again for your time and all the help,

- J.


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