UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Television and Video

Notices

Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

Reply
 
Thread Tools
Old 2nd Sep 2017, 11:50 pm   #1
Rick62008
Pentode
 
Join Date: Jan 2011
Location: Braga, Portugal.
Posts: 231
Default S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chassis

Hi all!

I'm trying to figure what keeps burning the horizontal transistor on this F19 crt tv chassis. They are quite cheap but i'm already on my second transistor and i wouln't like to have another one blown.

Long story short, this tv suddently died making that kown clicking sound indicating something was shorting the switched mode power supply.
Removing the shorted S2055N transistor that drives the horizontal deflection yoke revealed to be what's causing the short and without it the TV turns on, but with no picture obviously.

Immediately after turning on the TV with a new S2055N it shorted right away again

So i added another one but left the whole deflection yoke disconnected from the board and by doing this the TV turns on without burning the SN2055N and i get the typical dot in the center of the screen which is logical since the yoke is disconnected.
So turning the TV on with the yoke disconnected the TV works, but if i plug the horizontal yoke to the board and then turn the TV on the horizontal transistor fails.

The vertical deflection coil measures about 8,2 ohms and the horizontal deflection coil measures 1,7 ohms. Could this be it? I was expecting a lower value on the horizontal deflection coil but not that low.

Schematics provided in PDF format.

Notice that in the schematics provided the transistor used is a BU508, but the rest is pretty much spot on!

Click image for larger version

Name:	2.jpg
Views:	181
Size:	137.9 KB
ID:	148811

FORMENTI+F19.pdf
Rick62008 is offline   Reply With Quote
Old 3rd Sep 2017, 4:14 am   #2
6.3volts
Tetrode
 
6.3volts's Avatar
 
Join Date: Nov 2009
Location: Welshpool, Powys, UK.
Posts: 80
Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

Check for a short between the two sets of coils. Otherwise look for shorted turns in the line coils. You can 'ring test' them. Lots of information about this on google.
6.3volts is offline   Reply With Quote
Old 3rd Sep 2017, 7:27 am   #3
Argus25
Heptode
 
Argus25's Avatar
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 608
Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

Hi,

The main reason why a horizontal output transistor fails is that its peak collector voltage is exceeded. They generally do not fail due to average high current and heat dissipation issues unless the the heat sinking is grossly inadequate. Therefore, something like shorted turns in the H yoke or line output transformer is an unlikely culprit for transistor failure, but not impossible.

When the transistor is driven into conduction by the drive voltage (and current supplied to the transistors' base) the current builds up in the output transformer and yoke.This scans the right hand side of the raster.

Often the yoke has the higher inductance and the higher stored magnetic energy at the end of scan. When the transistor switches off, this magnetic energy is returned in an ocillatory fashion (for half a cycle called the flyback time) to the yoke and transformer's self capacity and specifically the tuning capacitors on the transistor's collector circuit, in this case according to your schematic , C68 and C69. (In oscillatory circuits the magnetic energy of the inductance gets exchanged to electric field energy in the capacitance. In the case of a H output stage only one half cycle of oscillation appears due to the damper diode, so you see a high voltage pulse on the transistor's collector)

The voltage across a capacitor increases as the capacity drops, so if for any reason either of these capacitors had lost capacity or were open circuit, then the peak voltage on the transistors collector would increase drastically, destroying the transistor, specifically when the yoke was connected, as it is the yoke stores most of the magnetic field energy. (shorted turns in a yoke on the other hand would decrease the stored energy)

So check those capacitors and their connections.

Even if those tuning capacitors referred to above are ok, there is yet another way the transistor's peak collector voltage can get exceeded:

This occurs if there is a timing defect in the drive signal to the H output transistor and it is switched on for too long a time period. In this case the peak yoke currents are higher and the energy stored at the end of scan higher and therefore the peak voltages on the tuning capacitors (and transistor's collector) are higher.

So check the transistors drive signal, generally the transistor is switched on for about 2/3 of the line scanning period. (The damper diode current scans the left side of the raster, not the transistor).

Last edited by Argus25; 3rd Sep 2017 at 7:40 am. Reason: typos
Argus25 is offline   Reply With Quote
Old 3rd Sep 2017, 10:19 am   #4
FERNSEH
Dekatron
 
FERNSEH's Avatar
 
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 4,556
Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

Check for dry soldering joints on flyback tuning capacitors C68 and C69, both have common enough values but must be up to the task for flyback tuning.

DFWB.
FERNSEH is offline   Reply With Quote
Old 3rd Sep 2017, 12:48 pm   #5
Maarten
Octode
 
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 1,548
Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

Quite cheap, but what brand? HOTs are known to be faked sometimes, could you post a picture?

Of course, there could very well (also) be a problem somewhere else in the circuit so all other suggestions should be evaluated as well.
Maarten is offline   Reply With Quote
Old 3rd Sep 2017, 1:09 pm   #6
Argus25
Heptode
 
Argus25's Avatar
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 608
Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

Quote:
Originally Posted by Maarten View Post
HOTs are known to be faked sometimes
I agree with this. It is interesting with fakes, a lot of the time the fakers get away with it because the device is not pushed near its max ratings in a "threatening environment". But a horizontal output stage often takes the device close to its limits, especially the peak collector-base voltage. In some cases the peak collector voltage only needs to go 15% to 20% higher than what it is running at normally to destroy the transistor. So increases in the supply voltage to the H output stage, a drop in the tuning capacity, or the transistor turned on for a longer period will all result in destruction of the transistor.
Its also important that if the H output transistor has an integral damper diode (collector to emitter) that it is up to spec, the fakers might get that fouled up too.
Argus25 is offline   Reply With Quote
Old 3rd Sep 2017, 2:18 pm   #7
steve1010uk
Tetrode
 
Join Date: Nov 2006
Location: East Yorkshire, UK.
Posts: 85
Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

Check carefully around the line output stage and power supply for dry joints especially around the line drive transformer T2 and the line output transformer, also especially anything connected to the collector of the line output transistor often a joint can look good but still be dry,
As others have said make sure the replacement transistor is sourced from a reputable supplier as there are many fakes, you could also try using a BU508D or similar if cheaper ,

With the line output transistor removed measure the HT volts sometimes it goes high.

It's not likely to be the scan coils (Yoke) but could be a dry joint on the scan coil plug

As with most tv's a blown output transistor is usually caused by either high HT, problems around the base drive circuit or most often dry joints

If all else fails check C65 C68


Steve
steve1010uk is offline   Reply With Quote
Old 3rd Sep 2017, 6:00 pm   #8
Rick62008
Pentode
 
Join Date: Jan 2011
Location: Braga, Portugal.
Posts: 231
Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

Thank you all for you responses and really helpful explanations, they made sense and they were spot on!

C68 had a bad joint and was in a pretty sorry state as you can see by the picture bellow:

Click image for larger version

Name:	z.jpg
Views:	201
Size:	94.8 KB
ID:	148886

The schematic says that this is 10nF capacitor but mine had printed 15nF and only measured 3nF.
In my junk of spare parts i was able to find a BU508A and a really, really old 7nF capacitor. I've tried them in circuit and i was able to get a nearly full horizontal deflection without blowing the transistor.
Eventually the yoke started arcing with the CRT after a few 2 minutes and the transistor failed, but this time i guess it happened because of the "not so up to spec" parts.

The thing is, i was only getting horizontal deflection, not vertical deflection.
The IC responsible for vertical deflection is a TDA8351. On pin 7 i get a short to ground. I'm not really acquainted with these ICs but pin 7 measuring only 2 ohms to ground didn't made sense to me. Could this be also bad?
If that's the case, I didn't know that a horizontal deflection failure could cause also a problem in the vertical deflection area.

Also here is a picture of the S2055N as you required:

Click image for larger version

Name:	z2.jpg
Views:	155
Size:	181.4 KB
ID:	148887
Rick62008 is offline   Reply With Quote
Old 3rd Sep 2017, 6:12 pm   #9
McMurdo
Nonode
 
McMurdo's Avatar
 
Join Date: Jun 2009
Location: Staffordshire Moorlands, UK.
Posts: 2,721
Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

the wrong value tuning capacitor may have allowed the lop stage to work but with rather high EHT, a potentially dangerous situation as regards the other circuits in the set.
ISTR the philips 2A for example used to get routine dry joints on the tuning caps which would turn the whole thing into some sort of van de graff generator.
__________________
Kevin
McMurdo is offline   Reply With Quote
Old 3rd Sep 2017, 6:40 pm   #10
ukcol
Dekatron
 
ukcol's Avatar
 
Join Date: May 2003
Location: Harlaxton, Lincolnshire, UK.
Posts: 3,149
Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

Just reinforcing what Kevin has said, the flyback tuning capacitor's value is critical and halving its value would have sent the EHT sky high. The value often varies in a give chassis and is usually CRT size dependant. Fit the same value as the one you removed (15nF) don't fit the value given on the circuit you have.

Pin 7 is the output pin of the TDA8351, this IC was probably destroyed by the original fault. This IC fault has probably taken out a fusible resistor in the HT feed to pin 6 as well.

When you had the arcing at the scan coils it may have punched a hole in the neck of the CRT check for this before you start spending money on replacement parts.
__________________
Best Regards, Colin McLaughlin
BVWS Member
ukcol is offline   Reply With Quote
Old 3rd Sep 2017, 7:25 pm   #11
Rick62008
Pentode
 
Join Date: Jan 2011
Location: Braga, Portugal.
Posts: 231
Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

Quote:
Originally Posted by ukcol View Post
When you had the arcing at the scan coils it may have punched a hole in the neck of the CRT check for this before you start spending money on replacement parts.
I really hope that isn't the case :\
This TV belongs to a relative and he said today that before the TV died he heard a loud arcing noise, then the image went from a horizontal line to a dot, and then it shorted. All this in a really short amount of time. Only if i was told about all this information from the get-go
Probably he got the arcing because the degrading capacitor started to drop it's capacitance ability.

Tomorrow i'm going to try to source the right 15nF capacitor and a new S2055N transistor. I believe i'm only going to get a BU508D instead of a S2055N at my local store but it should work at least for testing purposes as someone advised.

This only to check if i get horizontal deflection without arcing.
If so then everything worked and the CRT is ok. Next step should be getting a new vertical deflection chip and check for those fusible resistors.
Rick62008 is offline   Reply With Quote
Old 3rd Sep 2017, 7:33 pm   #12
Boater Sam
Octode
 
Boater Sam's Avatar
 
Join Date: Dec 2013
Location: Middlewich, Cheshire, UK. & Winter in the Philippines.
Posts: 1,986
Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

I have had a similar failure when the solder around the LOPT connection pins has crystallized in the oversize holes in the pcb, causing a resistive connection and destroying the OP transistor.
__________________
Boater Sam.
BVWS Member
Boater Sam is online now   Reply With Quote
Old 3rd Sep 2017, 8:38 pm   #13
hans
Heptode
 
hans's Avatar
 
Join Date: Jan 2008
Location: Oslo, Norway.
Posts: 532
Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

Quote:
Originally Posted by Rick62008 View Post
I've tried them in circuit and i was able to get a nearly full horizontal deflection
A typical sign of excessive EHT. Not a good sign at all.
hans is offline   Reply With Quote
Old 3rd Sep 2017, 9:27 pm   #14
Rick62008
Pentode
 
Join Date: Jan 2011
Location: Braga, Portugal.
Posts: 231
Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

Quote:
Originally Posted by hans View Post
Quote:
Originally Posted by Rick62008 View Post
I've tried them in circuit and i was able to get a nearly full horizontal deflection
A typical sign of excessive EHT. Not a good sign at all.
Because of the 7nF capacitor i added right?
The original capacitor in circuit was only 3nF instead of 15nF due to it's degradation which is even a worse case scenario. Could this have damaged the flyback transformer?

Eitherway i'll try to get tomorrow the right capacitor and either a S2055N or a BU508D and post the results.
Also since the vertical deflection IC is bad, would you recommend removing it before testing the TV with the new capacitor and horizontal deflection transistor?
Even if partially damaged i wonder if the TV powers on without it, leading to a confusing conclusion.
Rick62008 is offline   Reply With Quote
Old 3rd Sep 2017, 10:30 pm   #15
Argus25
Heptode
 
Argus25's Avatar
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 608
Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

Quote:
Originally Posted by Rick62008 View Post
C68 had a bad joint and was in a pretty sorry state as you can see by the picture
Yes, it is interesting how these capacitors fail, and they are supposed to be high quality and a safety rated part because if they lose capacity and the EHT climbs as a result, there is more opportunity for Xrays.

Of these types of capacitors I have disassembled for failure analysis, they have lost capacitance because the metal film has been eaten away and there is a variable amount of damage to the insulating layers. It appears that nearly all metalized film capacitors can do this. ( The worst cases are seen in X2 rated caps, It occurs when the peak currents are high. In those ones the paper can turn to carbon too and they start drawing current and burn up). Sometimes resistance develops at the points were the lead in wires and film are bonded.Ideally the capacitor would have a solid foil and this would be welded to the lead in wires, but many are not like this.

Film capacitors were thought to be a good idea as they are compact, but they have this vulnerability relating to the fact that the metal film can be too thin to support the peak currents in the external circuit, but it takes time for them to degrade.

On the topic raised by Ukcol, it is possible for a spark or plasma to puncture glass, even microscopically. I once saw an arc puncture one end of an Argon tube connected to a 15kV supply. Atmospheric gases slowly leaked in and its color in that area changed from blue to a yellow pink look. Over about 15min the whole tube had changed color, then it failed, it was quite a sight to watch.

Last edited by Argus25; 3rd Sep 2017 at 10:47 pm.
Argus25 is offline   Reply With Quote
Old 3rd Sep 2017, 10:48 pm   #16
Rick62008
Pentode
 
Join Date: Jan 2011
Location: Braga, Portugal.
Posts: 231
Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

Don't modern EHTs have built in safety measures to avoid Xray emisions? I've noticed that the hotizontal line was shrunk and quite dim.
Rick62008 is offline   Reply With Quote
Old 3rd Sep 2017, 10:55 pm   #17
Argus25
Heptode
 
Argus25's Avatar
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 608
Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

Yes, I think as far as the CRT design goes, the Xray shielding in more modern color CRTs is better. The worst ones probably were the large sized older monochrome crt's with high EHT's where there was little done to reduce Xray emissions from the actual tube. Some sets have circuits that monitor the EHT indirectly by measuring one of the line output transformer's voltages and shut if off if its too high. This is not in all sets though.

Some TV's & monitors have scan failure detect circuits, that cut off the EHT say if the vertical deflection fails. I have one monitor where there are separate output transistors for the scan (yoke) and the EHT. In that one they shut off the EHT if either the H or V scan fails. Normally, of course, in a common design with a single HOT, if the line scan fails, so does the EHT, so a protection circuit there is not needed.

Last edited by Argus25; 3rd Sep 2017 at 11:03 pm.
Argus25 is offline   Reply With Quote
Old 4th Sep 2017, 2:12 am   #18
Maarten
Octode
 
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 1,548
Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

I'm not surprised the TDA8351 failed. They are run very near their maximum ratings (in some sets from Vestel and Grundig, they even failed routinely. The cure was to lower its supply a few volts by adding a small coil in series). Also it is normally fed from the line output transformer so if the tuning cap is suddenly low or open, the tension rises above the maximum value even before the protection kicks in.

As to the BU508D, it depends on the design of the set whether it will work reliably without modifications or at all (Sony sets are quite critical, I don't know about Formenti). If you get the TV going again, switch off after 10 minutes and touch the heat sink. If you can't comfortably hold it, chances are you need a S2055, BU2508D or even a BU2520D.
Maarten is offline   Reply With Quote
Old 4th Sep 2017, 3:41 am   #19
Argus25
Heptode
 
Argus25's Avatar
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 608
Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

Quote:
Originally Posted by Maarten View Post
I'm not surprised the TDA8351 failed. They are run very near their maximum ratings (in some sets from Vestel and Grundig, they even failed routinely. .
Many might not share my opinion on this whinge, but I think one of the worst things to happen to CRT based TV's and monitors was the incorporation of IC based vertical deflection systems. They were so much better when the output amplifier was based on discrete components. The heat was not localized in one place. They were easier to repair and adjust and often had better linearity control options.

One good example would be the design of IBM's 5153 color monitor, that had a beautiful vertical output system based on a discrete transistor amplifier. In the 5154 EGA monitor (the new superior dual standard CGA/EGA monitor) they changed that for a compact vertical deflection IC. This IC totally roasts the phenolic pcb around it to carbon and dries out the electrolytic caps that were placed near it.

I decided in the end that the only way to get the quality was to make the early and better CGA monitor scan convert from 15.75 Khz to 21.85Khz, so I managed that, there is an article here :

http://www.worldphaco.com/uploads/IN...OR_TO_AUTO.pdf

In NZ in the late 70's, color TV's were introduced using Philips IC's, including a vertical scan output IC, they were the leading cause of vertical scan failure at that time, it was always without exception a thermal/dissipation issue. So I guess that could be blamed on inadequate heat sinking.

I'm sure the audio enthusiasts felt the same when many audio amps went away from discrete transistors and became IC based with a similar set of issues.
Argus25 is offline   Reply With Quote
Old 4th Sep 2017, 12:11 pm   #20
steve1010uk
Tetrode
 
Join Date: Nov 2006
Location: East Yorkshire, UK.
Posts: 85
Default Re: S2055N horizontal deflection transistor keeps failing on CRT TV with the F19 chas

As others have said it's important to use the correct value cap, exactly the same value and voltage as you removed,
If you can obtain a good quality S2055D try that but if the original transistor is the totally plastic type without a metal tab then if trying a BU208D it should be the plastic
( BU508DF ? ) version,

Get the line output stage working correctly then we can worry about the frame output !

Steve
steve1010uk is offline   Reply With Quote
Reply

Thread Tools



All times are GMT. The time now is 6:34 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Copyright ©2002 - 2017, Paul Stenning.