First thing to do when working on TV's - UK Vintage Radio Repair and Restoration Discussion Forum

29th Apr 2003, 1:24 am

I always work on Radios and Tube Amps and when I work on them I discharge the filter caps once I unplug them. What are the steps in working on TV's?? I am interested in starting to work on them. I understand the hi voltage ultor has to be discharged could someone please describe the proper way to work on a TV and also do you ever trouble shoot it when on? Just curious where all the hi voltage places I have to worry about are.. Thanx BHedberg. I have my electronics diploma so I'm pretty well educated just never took the consumer electronics part ie TV's VCR's. Thanx Brian

Duke_Nukem · 29th Apr 2003, 12:46 pm

Just to start the ball rolling, the very first thing you should do is invest in an isolation transformer. Well, highly recommended for radio work too, but the much larger chassis in these sets are even easier to accidently touch and they are invariable connected to one side of the mains.

If you can, its nice to initially disable the line-output section. Usually, the (-ve) DC bias for the output tube relies on the line osc to be actually running, so if it stops be prepared for LOTS of current :-( Also it is invariable DC coupled (anode of osc -> grid of LOP valve) so there is one capacitor to change for a start !! If you are lucky there will be a fuse in the section which you can simply remove, otherwise maybe removing the connection to the LOP valve's top cap (remember to apply tons of insulation to the now floating lead!).

Now get the audio working, safe in the knowledge that all the really bad volts are disabled.

Before reconnecting LOP stage, give it a good clean as EHT is very good at finding leakage paths to where it ought not go.

Otherwise most if it is no different to radio e.g. initial visual inspection, THAT audio coupling capacitor, dodgy decoupling caps, high value resistors going open circuit, etc.

A number of faults can only be diagnosed properly with the set switched on. Most voltages are the same order as a typical valve radio, its just the LOP stage that gets a bit scarey. Best approach is to fix the -ve end of your meter on the chassis so you can proble volts one handed i.e. keeping other hand in your pocket so that if your hand does accidently touch a high voltage then at least there shouldn't be an electrical path across the chest which is the real killer.

TTFN,
Jon

30th Apr 2003, 2:39 am

LOP is that the usually red wire, ultra high voltage that goes to the rubber plunger and attaches to the picture tube? Or Ultar I seem to remember hearing in school

I thought you had to discharge that with a big probe first when shut off before unhooking? Assuming I'm correct...

Duke_Nukem · 30th Apr 2003, 12:39 pm

Ah, sorry, by LOP I meant the line output stage. This is a combination of bits, usually a line output valve (biggest valve usually), a multi-winding transformer, a boost diode and EHT rectifier. Its where a lot of the power and all the really high voltages occur, hence disabling it whilst sorting out the main HT and RF stuff seems a good idea to me.

If the timebase isn't running or the coupling cap is leaking (for example) then the LOP valve passes tons of current through the transformer, damaging the valve (dead easy to replace), maybe the boost valve (dead easy to replace) ... and the LOP transformer (you may get lucky, but don't bank on it!).

For an excellent overview of the sections within theses old tellys visit :-

http://www.penders.cwc.net/otindex.html

You should discharge the EHT before working in that area, however if you are starting up an old telly after 20+ years of idleness then there shouldn't be any stored volts.

Discharge EHT through a high impeadance, don't use a screwdriver ! The screwdriver method causes surge currents in the rest of the circuit; proper electronics (i.e. valves) won't be effected but big girls blouses (aka transistors) can be destroyed!

TTFN,
Jon

thermionic · 17th May 2003, 7:35 pm

Bryan. Another top tip. If you are used to repairing old radios, And have ( perhaps) got into the habit of touching the top cap connection of radio valves. (usually the Grid Connection) Don't get caught out with tellys !!!!
The top cap connections of the 2 valves in the line output stage ( the efficiency diode & line output ) carry SERIOUS voltage!!!!! Beware.

Simon.

Duke_Nukem · 29th Sep 2003, 9:08 pm

Hi John,

Yes, an isolation transformer would allow earthing of the chassis. This shouldn't lull you into a false sense of safety but it minimises the danger of touching something earthed and the chassis at the same time. It also means you can connect normally earthed test equipment such as oscilloscopes which you absolutely couldn't do without either an isolating tranny or removing the scope's earcth (NOT recommended).

Being a chicken, I won't touch any set - be it live chassis or supposedly isolated by its own mains transformer - without an isolation transformer.

TTFN,
Jon

mikelect · 19th Dec 2003, 12:58 pm

Isolation Trannies are available from Maplins or CPC at reasonable prices

Mike

evingar · 19th Dec 2003, 6:20 pm

Quote:

Thanks Mike
Did a search for isolation transformers on the Maplin site and brought up a list of products, did you know which transformer to go for ! ?
Cheers

Hi SJM2003

I'm not Mike

The main thing that governs what you buy (assuming they all have the relevant safety levels!) is the current they can handle. What you require is dependant on what you envisage servicing. My isolation transformer is 500 VA; so far I haven't found a TV or radio it can't " handle " .

As a guideline, my large 60s Dynatron TV is rated at 150 Watts.

Just to confuse matters further, you could also try RS components and Farnell

http://rswww.com

http://uk.farnell.com/

Regards

Chris

29th Apr 2003, 1:24 am	#1
BHedberg Guest Posts: n/a	First thing to do when working on TV's I always work on Radios and Tube Amps and when I work on them I discharge the filter caps once I unplug them. What are the steps in working on TV's?? I am interested in starting to work on them. I understand the hi voltage ultor has to be discharged could someone please describe the proper way to work on a TV and also do you ever trouble shoot it when on? Just curious where all the hi voltage places I have to worry about are.. Thanx BHedberg. I have my electronics diploma so I'm pretty well educated just never took the consumer electronics part ie TV's VCR's. Thanx Brian

29th Apr 2003, 12:46 pm	#2
Duke_Nukem Octode Join Date: Jan 2003 Location: Birmingham, West Midlands, UK. Posts: 1,268	Re: First thing to do when working on TV's Just to start the ball rolling, the very first thing you should do is invest in an isolation transformer. Well, highly recommended for radio work too, but the much larger chassis in these sets are even easier to accidently touch and they are invariable connected to one side of the mains. If you can, its nice to initially disable the line-output section. Usually, the (-ve) DC bias for the output tube relies on the line osc to be actually running, so if it stops be prepared for LOTS of current :-( Also it is invariable DC coupled (anode of osc -> grid of LOP valve) so there is one capacitor to change for a start !! If you are lucky there will be a fuse in the section which you can simply remove, otherwise maybe removing the connection to the LOP valve's top cap (remember to apply tons of insulation to the now floating lead!). Now get the audio working, safe in the knowledge that all the really bad volts are disabled. Before reconnecting LOP stage, give it a good clean as EHT is very good at finding leakage paths to where it ought not go. Otherwise most if it is no different to radio e.g. initial visual inspection, THAT audio coupling capacitor, dodgy decoupling caps, high value resistors going open circuit, etc. A number of faults can only be diagnosed properly with the set switched on. Most voltages are the same order as a typical valve radio, its just the LOP stage that gets a bit scarey. Best approach is to fix the -ve end of your meter on the chassis so you can proble volts one handed i.e. keeping other hand in your pocket so that if your hand does accidently touch a high voltage then at least there shouldn't be an electrical path across the chest which is the real killer. TTFN, Jon

30th Apr 2003, 2:39 am	#3
BHedberg Guest Posts: n/a	Re: First thing to do when working on TV's LOP is that the usually red wire, ultra high voltage that goes to the rubber plunger and attaches to the picture tube? Or Ultar I seem to remember hearing in school I thought you had to discharge that with a big probe first when shut off before unhooking? Assuming I'm correct...

30th Apr 2003, 12:39 pm	#4
Duke_Nukem Octode Join Date: Jan 2003 Location: Birmingham, West Midlands, UK. Posts: 1,268	Re: First thing to do when working on TV's Ah, sorry, by LOP I meant the line output stage. This is a combination of bits, usually a line output valve (biggest valve usually), a multi-winding transformer, a boost diode and EHT rectifier. Its where a lot of the power and all the really high voltages occur, hence disabling it whilst sorting out the main HT and RF stuff seems a good idea to me. If the timebase isn't running or the coupling cap is leaking (for example) then the LOP valve passes tons of current through the transformer, damaging the valve (dead easy to replace), maybe the boost valve (dead easy to replace) ... and the LOP transformer (you may get lucky, but don't bank on it!). For an excellent overview of the sections within theses old tellys visit :- http://www.penders.cwc.net/otindex.html You should discharge the EHT before working in that area, however if you are starting up an old telly after 20+ years of idleness then there shouldn't be any stored volts. Discharge EHT through a high impeadance, don't use a screwdriver ! The screwdriver method causes surge currents in the rest of the circuit; proper electronics (i.e. valves) won't be effected but big girls blouses (aka transistors) can be destroyed! TTFN, Jon

17th May 2003, 7:35 pm	#5
thermionic Octode Join Date: Apr 2003 Location: Cornwall, UK. Posts: 1,007	Re: First thing to do when working on TV's Bryan. Another top tip. If you are used to repairing old radios, And have ( perhaps) got into the habit of touching the top cap connection of radio valves. (usually the Grid Connection) Don't get caught out with tellys !!!! The top cap connections of the 2 valves in the line output stage ( the efficiency diode & line output ) carry SERIOUS voltage!!!!! Beware. Simon. __________________ The honesty of imperfection..........

29th Sep 2003, 9:08 pm	#6
Duke_Nukem Octode Join Date: Jan 2003 Location: Birmingham, West Midlands, UK. Posts: 1,268	Re: First thing to do when working on TV's Hi John, Yes, an isolation transformer would allow earthing of the chassis. This shouldn't lull you into a false sense of safety but it minimises the danger of touching something earthed and the chassis at the same time. It also means you can connect normally earthed test equipment such as oscilloscopes which you absolutely couldn't do without either an isolating tranny or removing the scope's earcth (NOT recommended). Being a chicken, I won't touch any set - be it live chassis or supposedly isolated by its own mains transformer - without an isolation transformer. TTFN, Jon

19th Dec 2003, 12:58 pm	#7
mikelect Retired Dormant Member Join Date: Sep 2003 Location: Wrexham, North Wales, UK. Posts: 799	Re: First thing to do when working on TV's Isolation Trannies are available from Maplins or CPC at reasonable prices Mike