Help with resistor in television

Discussion in 'General Electronics Chat' started by Chris Ronayne, Mar 11, 2013.

  1. Chris Ronayne

    Thread Starter New Member

    Mar 11, 2013
    10
    0
    Hello all,

    I'm Chris Ronayne, a 17y/o Australian CRT television collector and restorer. I have worked on a few different sets, but have finally run across an issue that requires more thought than usual.

    I have a 12" TEMPEST TE-12M B&W television set from the 1980's, which runs on the NSW standard 240VAC 50Hz mains power. It's a relatively cheaply made set, with the usual poor quality components found in lower end Chinese made products from the period.

    The set starts up fine (despite the broken power switch/volume pot) and the CRT is showing proper geometry and focus. However, after a few minutes of operation, the vertical deflection starts to decrease, leaving me with a vertically compressed image. This is also accompanied by an unpleasant burning electronic smell.

    I am planning on following the usual routine, replacing higher voltage/capacitance electrolytic capacitors, but have also found a cooked resistor, R306 on the circuit. It appears to be in the vertical deflection circuit, thus explaining the decrease in deflection.

    My problem is, that I don't know the value of the resistor, I don't have any similar circuits to compare it to, and I don't have the luxury of a schematic/circuit diagram. The resistor has been fried, and the resistance identification stripes have been rendered unreadable.

    I have thought up two possible solutions:

    1) Measure the current resistance, and then replace with a suitable trimmer pot. Then set the pot to the measured resistance and adjust accordingly. From there, I might proceed to locate a resistor of appropriate value to replace the pot.

    2) Measure the resistance before use (and subsequent heat gain/resistance loss), and replace with a similar value resistor.

    I don't know if either of these methods would work satisfactorily, but I don't appear to have any other options available.

    In the meantime, I will proceed to replace the caps and attempt to locate the fault that doomed the resistor. I will also attempt to post some photos, provided this forum allows for that. Otherwise, I will link them to my Flickr account.

    Cheers,

    Chris Ronayne
     
  2. Chris Ronayne

    Thread Starter New Member

    Mar 11, 2013
    10
    0
    Hi all,

    Here's the photos of the set. The reverse seemd to happen when I turned it on today. It started off with bad vertical deflection and then corrected itself a few minutes later. Regardless, I'd still like to replace the components (especially the resistor) to eliminate some issues that may pop up in the future.

    Here's my TEMPEST TE-12M 12" telly from the 1980s.
    [​IMG]

    Here's how it looks when you turn it on (usually), and how it should always look.
    [​IMG]

    And here's how it usually looks after several minutes of operation.
    [​IMG]

    This is the PCB, filled with cheaply made components typical of el cheapo cr*ppo Chinese sets of the period.
    [​IMG]

    And here's the burnt resistor in question.
    [​IMG]

    Cheers,

    Chris
     
  3. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    These blue caps are bad quality. Replace these first and see if it is changing anything.

    Resistors don't change value normally they burn out or they still work 1:1.

    I have fairly blackened resistors here looking much worse, but still working 100%. The complete engraving is visible.
     
  4. Chris Ronayne

    Thread Starter New Member

    Mar 11, 2013
    10
    0
    Thanks mate for the reply.

    I've been gradually going through and replacing the electrolytic caps. Unfortunately, the set has decided to stop working. Possibly a blown fuse? I'll have to check it out tomorrow...

    Chris
     
  5. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    Resistors can change their value, their value can increase. I found resistors with a few hundred kΩ in SMPS , several times. They were supposed to be low resistance. I guess they partially burnt, but nothing was visible.
     
  6. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    Yes but rather from high voltage. I have read about that somewhere, and it would be affecting carbon resistors.

    In the picture these are metal film resistors, still working normally. For BJT driving.

    Any discoloring = outside of spec.
     
  7. JayArr

    New Member

    Mar 11, 2013
    14
    0
    Hi Chris

    I have one of those TVs in my store room and it still works so I can take the back off and get the resistor value for you but not right now, I'm at work.

    Give me a day and I'll get back to you.

    Jeff
     
  8. Chris Ronayne

    Thread Starter New Member

    Mar 11, 2013
    10
    0
    Hi Jeff,

    Thank you! I'd like to replace the resistor to avoid future problems.

    I got this telly from a dumpster at my local TAFE (Mid North Coast, Port Macquarie campus), which was one of about a dozen of these sets. I went back a day later to get a better working one (and one for a friend), but they'd all disappeared. I hope they went to a good home...

    This set is starting to show more issues than I care to count.

    Some of the problems so far:

    - Bad vertical deflection
    - Broken & dirty power & volume potentiometer.
    - Possible issue with the speaker (can't be confirmed until volume pot is replaced)
    - Burnt resistor
    - Case missing screws (should have some somewhere)
    - Electrolytic caps in need of replacement
    - Issue with antenna/related circuitry (may be fixed with replacement of caps)
    - Television now refusing to turn on (possible blown fuse, need to diagnose first)

    These are, for the most part easy repairs, but it's more of an issue finding adequate time amongst my school work, and being able to get the correct parts for the job. Luckily, I have a lot (and I mean a lot) of scavenged components from old solid state televisions to get this set working.

    Other sets (such as my late 1960's 12" AWA P1) require more sophisticated and harder-to-locate parts. Especially the caps and some of the valves.

    Chris
     
  9. absf

    Senior Member

    Dec 29, 2010
    1,493
    372
    I dont see any parts that are difficult to get besides the flyback and the horizontal coupling transformers. Are you able to get the Philips chip in the middle of the board locally and what's the smaller DIP chip behind the Philips chip.

    Looks like your TV uses 2 tuners. One for VHF and one for UHF? You have to open up the tuner and clean the selector contacts as that can get dirty after long period of idling in storage. You can also start tracing both sides of the burnt resistors and see where they are going to. If one side go to a transistor and the other goes to the dc supply, then most probably the transistor is leaky or the components surrounding that transistor is having problems.

    I wish you best of luck in repairing and if you need any parts that you can't get locally or eBay, I think it could be solved easily as many here do stock up some old parts....

    Allen
     
  10. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    The vertical transistor probably has issues.

    Resistors don't up and decide to burn up, look for a shorted semiconductor in the path.
     
  11. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    The adjustable resistors are of very cheap making. Typically silver coated, look for discoloring or blackened.
     
  12. debe

    Well-Known Member

    Sep 21, 2010
    947
    184
    Hi Chris, I used to repair TVs & have a few circuit diag of small port B&W tvs left (have thrown a lot out). What is the number on the philips chip TDA ????.
     
  13. Chris Ronayne

    Thread Starter New Member

    Mar 11, 2013
    10
    0
    Hi all,

    Thanks for the replies! I'll have to check out the ICs this afternoon when I get home.

    Chris
     
  14. JayArr

    New Member

    Mar 11, 2013
    14
    0
    Hi Chris

    I pulled my TV today to get that value but the circuit board in mine is different, I guess Canada and Austrailia don't use the same TV system, ours is NTSC I'm guessing yours is PAL?

    Jeff
     
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