GEC 9 Inch TV

[Techman]

You must use a signal from a proper standards converter or the output from an RF signal generator to create bars on the screen.

[Radio Tech]

Hello Edward,

I have now been loaned a standards converter and the set will produce a picture but at the current time lacks height and I have got to work through this section and decide which component is at fault. Without a substitute KT81 or its equivalent I cant be certain if the frame time base valve is u/s, there are some waxies around this area and are odd value ones at that, like C98 0.05uf, the set over all seems to employ values like 0.1uf, 0.05uf, 0.005uf, 0.001uf, and 0.5uf of which I haven't got, I am not too sure where I can source these values at the moment, perhaps a member has a stock of these, I must admit though, this set is certainly a challenge to repair and something to keep my brains in gear.

Regards

Ken

[Freya]

Ken, the capacitors are now
0.5uF is now 0.47uF,
0.05uF is 0.047uF
0.005uF is 0.0047uF
0.25uF is 0.22uF
0.025uF is 0.022uF

etc, hope it helps.
Voltage wise replace with 630 volts in power, line and frame stages. Use 400v in RF/IF mainly because of physical size.

[Radio Tech]

Many thanks Stephen for that info, much appreciated.

Ken

[Radio Tech]

Just a little update on the TV, I have changed the capacitors that relate to the vertical scan, but still no luck in getting the vertical time base operating to give a full picture. so any help in this area greatly appreciated.

Ken

[Freya]

Can you post a picture of what you're getting please

[Radio Tech]

Hi Stephen,

Here is a picture of the frame time base fault, I have replaced C33 an 8uF electrolytic capacitor which is on the HT line of the Frame Time Base. Although I checked the Frame Output Valve, a KT81 and this seemed to have a reasonably emission, I haven't yet replaced it.

Regards

Ken

[Hunts smoothing bomb]

When you said you hadn't got full height I didn't expect to see that.
you have no vertical scan at all.
Check continuity of scan coils plus the primary and secondary of the FOPT (frame output transformer)
after this change all of the Bakelite encapsulated type capacitors in the frame stage. This will more than likely restore operation if the transformer and scan coil Windings check out.
Even if the frame output valve is low I would expect to see some vertical scan on the screen.

[Freya]

Hi Ken,
I have had a look at the diagram now, sorry for delay.

Assuming oscillator is running then Suspects would be C97 32uF on the cathode of KT81, R119 Height pot 1k. If you are unsure about KT81 then swap with audio output KT81
With an oscilloscope and isolation transformer this type of fault could be solved in a short space of time, not sure if you have these pieces of equipment.

If the oscillator is not running (V23 L63) the capacitors around this are likely.
C93 0.0047uF, C95 0.01uF,C96 0.047uF,C98 0.047uF

The other possibilities are T3 and T4 as already mentioned by HSB

Hope this helps

[Radio Tech]

Thanks Lee and Stephen,

As far as I know the Frame Time Base transformer windings are ok, the scan coils are ok, I have changed most caps around the KT81 but not around the L63, I will have a check over the weekend and report back, I only wish I could get hold of some 8uF @500V electrolytic capacitors new ones though, the ones I had were u/s and very leaky.

Best wishes

Ken

[Freya]

They are available from justradio but you can use 10uF instead which is much easier to source

[high_vacuum_house]

The BVWS do 8uF at 500 volt capacitors. I have a couple and they are very good. If you have a bulletin, look inside the front cover for values and ordering information. There is a good range of high voltage electrolytic capacitors and 600V axial polyester capacitors.
Christopher Capener

[Radio Tech]

Thanks for info on caps, Christopher and Stephen, I managed to re-stuff C31, not that it made any difference though. I checked at the grid of the KT81 and there is no input from the oscillator V23 (L63) according to the manual it's a free running oscillator when no signal is being received at the aerial, I checked both resistors at the grid of V24 (KT81) and both read a bit high. So now I think its time to find those two valves to substitute the ones mentioned.

Regards

Ken

[Radio Tech]

Hi folks and those following this thread,

I have succeeded in getting a picture on the screen, after looking at the input to the grid of the KT81 frame output valve for the sawtooth waveform that should be there if the oscillator was running which it wasn't, I dug a bit further and removed the 220K resistor, this was reading 330k so replaced this with a 680k and 330k in parallel to obtain the near value 223k, I also replaced C96 an 0.05 which read 1.5uF with two 100nF in series to get the value of 50nF (0.05uF) and after a few adjustments of some of the preset controls I got the picture I expected to see. Just a bit more to do to though, got to get the radio working and a few little other jobs, so more to follow. Many thanks to those who have given some help in tracing the problem.

Regards

Ken

PS sorry for the quality of pictures, I did not aim the camera properly

[Freya]

Well done Ken, that's great progress.
Its looking a bit dark still, do you have further scope on the brightness ?
It may pay to check C43, C79and any associated resistors R125,R124,R54,R56.

[Radio Tech]

Hello Stephen

I think there is further scope on the brightness, but will check those parts anyway. Got to replace C97 and one or two others that look suspect but also got to check the radio out and get this feature working, this particular set is one of those that are extremely heavy and the only way I have found to work on the set underneath is to get it on one end, that's the power supply end and put a roll of tape under the frame of the mains transformer so the whole chassis does not topple. Will update everyone as I progress further.

Best wishes

Ken

[Freya]

There are only a few capacitors in the radio section that might stop it working, you will need a fairly long aerial to pull in anything.

[Radio Tech]

Hi folks

Here's a little more on the progress of this GEC BT7092 set, I checked around V14 and found the local oscillator working, with an aerial in the socket I then followed the signal source up to V16 a DL82 double diode triode, sure enough the signal appeared there but as there were some waxie caps around this valve I am now in the process of replacing those especially C70, C71, and C67 and then see what is at grid 1 of V18 a Z77 and follow through to the output of this valve replacing C103 if required.. More to come on this post.

Regards

Ken

[Radio Tech]

The radio is now working, and the two culprits were C67 and C70, these were easy to replace but the other two that I intend to replace C71 and C103 are located on a tag board held within the power supply section and not at all easy to get at, a bit more logic thinking for this I think folks. .... More to follow

Ken

[Tazman1966]

Keep plodding through and it'll come good