I have recently been working on this set, which is one of those that my parents are likely to have watched at some point, as my dad threw very little away and this is one of 3 405 line TVs that I always used to find fascinating to look at in the loft when I was a child. I did a little work on this set about 25 years ago, when I replaced the tube after the neck got broken on the original when my dad was bringing it down through the loft hatch. Back then I got as far as getting a raster on it, but had a shadow on part of the screen.
I decided to have a proper go at restoring it a few months back, and at present have a picture that has compression at the bottom and I am unable to completely fill the screen with an acceptable image in terms of linearity. This set has had an awful lot of work done to it (mostly before my limited tinkering of 25 years ago) and has had the metal rectifier removed and replaced by a diode in parallel with a capacitor (the HT measures spot on at 205v on an Avo 8, so I’ve left this alone so far) and the sound and frame output transformers have been replaced. The wiring on the underside of the chassis was/is a bit of a mess with quite a few modifications and I am trying to put everything back to how it was originally.
One modification is a 680k resistor across one half of the volume control, which had clearly been added and does not appear in the circuit diagram. I tried removing this, but get a strange ‘beating’ hum effect at higher volume which is also reflected in the size of the raster, so this has been put back in and normal operation is restored – I am guessing this mod was made when the replacement output transformer was put in, which is an RS standard sound output one.
Another modification which has been made, is that the lead from the line hold pot which in the circuit diagram goes to a point between the anode of the PCF80 line sync discriminator valve and the anode load resistors (see snip of circuit diagram below with X at this point) has been moved and placed between added resistors between HT and chassis (8.9k to HT and 100k to chassis). Clearly I need to remove these resistors and replace the wire in its original location, but I’m curious as to why such a modification would have been made in the first place. Presumably this was done as a work-around in response to a fault.
I have replaced all waxies/Hunts/grey RS plastic capacitors, the PCL83 frame output valve, the line output, efficiency diode and EHT rectifier valves and all out of tolerance resistors I’ve been able to find in the frame output stage.
I initially had no contrast control, but found the wire wound pot to be open circuit at one end, so a small folded square of tin foil inserted between the wire and the case to short out the defective part of the control has remedied this.
The point has been made as to whether the frame output transformer is appropriate, and the replacement tx does look a bit modern. I have another frame output transformer from a Bush T36 chassis which has continuity, but am not sure whether this would be appropriate for this set, as I understand that the T36 had high impedance scan coils.
Any thoughts anyone has on all this would be very welcome. Thanks.
Laurence
Does the PCL83 frame output valve have an electrolytic as the pentode cathode bypass? This could be partly due to the non linearity of the frame if faulty, but my gut feeling is that the correct transformer (or one from a similar design of frame stage) would be the ultimate cure.
To understand the black art of electronics is to understand witchcraft. Andrew.
@pye625 Yes it has a 100uF electrolytic in that position. This had been replaced in the past and wired to the tag strip where the metal rectifier used to be, but I've put a smaller one in that fits in the hole where the original one used to be located. This has had no effect at all on the linearity, so I think you're right about that transformer.
Laurence
I have this evening undone the modification that had been made involving the connection of the line hold pot to some added resistors between chassis and HT that I referred to in my first post, connecting it back to where the circuit diagram shows it to the anode of the PCF80 line sync discriminator valve. The service sheet quotes a voltage of 53v at the PCF80 anode and I previously had about 48v dc here, but after connecting the line hold pot back to this point I'm getting 87v and the set is operating normally (apart from my existing frame output problem of course).
Is this higher voltage reading something I should be concerned about or is it not likely to cause any issues? I will of course check the values of the anode load resistors as they could be causing this higher reading. The voltage readings were taken with an Avo model 8.
Laurence
Is this higher voltage reading something I should be concerned about or is it not likely to cause any issues?
I very much doubt it will be a problem, the voltage will depend largely upon the condition of the valve and various factors such as HT level. If the stage is working as you would normally expect, then no need to be concerned.
The mod. could have been done for almost any reason, perhaps for a failing previous PCF80 that caused drift out of hold lock range and they didn't have one to hand for replacement.
To understand the black art of electronics is to understand witchcraft. Andrew.
From the shape of the line spacing it does look a lot like a load mismatch. You've tackled all the usual suspects.
Do you have a scope? Be careful with it around the vertical anode circuit. For an initial check put 1m connected to the anode and measure the other end of that. But it would be good to be able to confirm if you are under or over on the matching.
I have a dual trace Hameg 'scope that an ex-television engineer gave to me some years back, but this will be another restoration project in itself at some point as only one of the traces works. I will have a go at doing what you've suggested once I've sorted the 'scope out.
Laurence
Ah, the Hameg! I had one, it was my 1st digital scope.
Lots of 6116 static RAM and 4000 series logic!
Do you have the schematics for it?
Thinking about your problem - measure the DC voltage on G1 of the pentode and tell me if it goes negative when you turn the height up. And by how much. That indicates the valve is being driven into grid current and implies that the load impedance is too low.
Thanks @irob2345, I will try this when I next work on this set and let you know the result.
The Hameg is a HM312-8 that I don't have the schematic for.
Laurence
The last time I worked on an RGD The 17 was in 1966, that long ago. Along with the electrically identical Regentone TT7 not many were sold. These were high performance receivers designed for fringe area reception. Design of these receivers is a follow-on from the RGD 1756 series. The frame timebase of the TT7 and The 17 is very similar to Ekco models such as the T311 and T330.
Till Eulenspiegel.
