Repair Advice for Philips 385U TV Set Please?

I recently repaired a Philips 385U 9 " 405 line TV set. It's a London model. When the set arrived I replaced all of the capacitors throughout as well as any deteriorated wiring etc. When powered up the set worked well and gave good picture and sound.

I used the set a few times, on about the 4th or 5th time the set powered up as normal and was working well. After about 10 minutes the picture collapsed in to just a line across the screen. Now when the set is powered up the sound is fine but there is just one line across the screen itself. (So I presume the EHT is obviously OK).

I've had the set to pieces, I replaced the UL46 and UL44 valves with new replacements, and the UY41 with a new UY42. I also checked the scanning coils and they gave continuity readings. I then checked the Frame Output transformer (T4) and both windings were intact, next I checked the Frame Blocking transformer (T3) and again both windings were intact.

I also took voltage readings of both the Frame Output transformer and the Frame blocking transformer. The Frame Output transformer has 24V DC to one winding and 0V DC to the winding which connects to the scanning coils.

The Frame Blocking transformer has 11.5V DC to one winding and 12.5V DC to the other.

I have checked various resistors in the area of the two transformers and they all seem to be fine, and now I have come to a brick wall.

Can anyone help please by suggesting what I may be overlooking? Is there another valve that is likely to need replacement or is there a certain resistor that may need replacing or perhaps a capacitor that I may need to replace again? Is something like the Frame Output transformer likely to need replacing even though it's giving a continuity reading? Any help or advice on this would be greatly appreciated. Thanks in advance for any help.

[Ed_Dinning]

Hi, It sounds like one of those occasions where getting the scope out will be the quickest solution, so you can see exactly what is happening. Make sure you use a *10 probe (10meg) as there are often high impedance points that a normal 1meg probe will load and prevent operation. Good luck! Ed Dinning

[Duke_Nukem]

I don't have the set's data to hand but try measuring the blocking oscillators grid-cathode volts (DC meter but use quite a high voltage range) ... if it is reasonably -ve then at least the oscillator is running.

It is worth changing the blocking osc valve as a check.

What is the frame output valve's cathode voltage in this condition ?

Also give the height and vertical hold controls a wiggle in case they are playing up.

Shorted turns in a transformer could occur, a fault which won't necessarily appear using simple meter readings but it is fairly unlikely.

TTFN,
Jon

[Mike Phelan]

Memory on these is completely faded,
but if the frame osc has stopped, is there a boost HT supply to the height control through a high-value resistor? Something like 2m2? Could be o/c


Mike

[oldeurope]

Put a few volts AC (from the heater) over 0,1uF to
g1 of the VEP [video output pentode]. If something happens , the V output
stage is ok. Then you have to search in the V osc.
Darius

Thanks for this, are we referring to the UL46 Frame Output valve here?

[Ed_Dinning]

Could also be video output pentode, I've seen this dodge described before. Blocking oscillator is usually the valve with a very small tramsformer associated with its anode and grid circuits (trace the undersise wiring, often a triode)

Ed

Thanks Ed, that's a useful tip too, there is a small transformer underneath, which I had been lead to believe is the Frame Blocking transformer? I'll check the valve directly associated with this, I know the transformer is certainly intact as I checked that. On the circuit diagram on the BVWS CD ROM this is identified as T3. It goes to a UCH42 valve, so I'll try replacing that next.

[Ed_Dinning]

That sounds like it, there were often mounted under the chassis and were quite small, I havent the circuits with me so I don't know if it's T3.
The trannys don't often give trouble but the have a lot of turns of very fine wire. Ratios were typically between 4:1 and 10:1, if its dead re-winding is a real pain but you may find another small transformer will operate in this position, possible with some adjustments or the associated R's & C's.

Ed

That's great Ed, thank you. I did check both windings on the transformer and they both seem fine, so I think repacing the associated valve is a very good next step. I've just ordered one on-line so hopefully I'll have that in a day or so and can progress with tracing the fault.

Thanks again.