COSSOR CT1910A TV
Hello all. could someone shed some light on a fault i have with a cossor CT1910A that i recently acquired. basically theres a problem with the sound in that when you turn it up it howls quite badly and as usual im lost 😕 . cheers neil.
It sounds like it's going unstable. If you have an audio signal generator you could disconnect the audio stages after the detector or possibly at the volume control wiper. Then inject a signal to check whether the problem is in the audio stages rather than in the RF/IF. (Given that it's volume dependent it's most likely in the audio stages.)
Once you've established which stages are being driven into oscillation suspect any decoupling capacitors (those connected to ground). Try connecting something like a 0.1uF across any decoupler. The likelihood is that your set currently lacks proper decoupling. You might also find that sticking your finger on the lower voltage parts of the signal path will help to track down the problem.
Hopefully some of the more experienced set repairers with also pitch in with suggestions.
I know this may sound strange (no pun intended) the anode of the audio triode is fed from the boost rail via R214 (2M7), with C214 (1500pF) limiter correction; if the value of this resistor is high it can cause severe distortion with a strong signal. Any faults in the boost supply can cause sound faults as well as the PCL83 its self. The circuit is in the 1962-63 volume of Radio and Television Servicing, page 357 if you would like me to scan it for you let me know and I will put it on the forum.
Without looking at the circuit, I wonder if this set is on a PCB. If it is, has it gone conductive?
I have attached the circuit from Radio and Television Servicing.
C107, C212, C210, R215.
Ok thanks Stan,Steve. will check the parts mentioned and let you know the results. cheers neil.
i know its obvious but have you tried tapping the pcl83 they are known to become microphonic , also resolder the the valve holder pins the printed circuit board is likly to of suffered from heat and from what i remember the valve holders fit into round holes and can be prone to dry joints
Well i did actually tap the valve and it did affect the noise. all ive got to do now is find a PCL83. as all ive got are PCL84s etc but no 83s. cheers neil.
OK then. ive tried a valve as you suggested Trevor but its still doing it 🙁 . so its back to changing components again and seeing,or rather listening to what happens. cheers neil.
Well ive changed C212 and its cured it 😀 . thanks again neil.
Hi Neil so you bought it, I was tempted but held back.
Hi Chris. yes i bought it alright 😯 . i did not really want to pay what they were asking but i cant resist pressing buttons 🙄 . but luckily its turned out to be better than i expected condition wise and it seems to work ok. even the motorised tuning works 🙂 cheers neil.
All being well ill have an aurora soon 😀 . and then i can see just how bad the tube might be 😯 . cheers neil.
Hello all. Well i now have my aurora and its been connected to the tv. but it shows up the sets problems, in as much as the frame hold is right down the end of its travel (not much adjustment) and the picture is rather touchy and cant be set up that well. can someone suggest what components to check/replace so i can get the tv up to scratch. thanks neil.
Can you tell us what you have already replaced? No doubt the set is full of the usual waxies which you will need to work through and replace. This would be standard procedure and how you do it is up to you. Some people prefer to replace one capacitor at a time and check the set at each component change. I would personally find this rather tedious but as you are a beginner, a lot can be learned by the 'one capacitor at a time' method. How about a picture of the set with it's faults?
From the sort of faults you describe, it's a classic case of leaky caps and possibly the odd resistor changed value.
We could of course say replace cap xxx followed by cap xxx and resistor xxx I'm sure that is not what you want. You have already identified an area you need to do some work so lets focus on that part of the circuit for starters.
This is where you should start your diagnostics. I would start looking at components around the vertical hold, Frame height and Frame linearity. As Richard rightly points out most if not all these waxy or hunts caps will be tired causing all sorts of issues even if they look innocent they will be far from that.
Unlike Richard Trevor and other more experienced engineers one at a time replacement is a must, you need to see and observe cause and effect that way you lock and learn.
Good luck and enjoy, as Richard said a picture of Test Card C will also help show any other faults.