Marconi 703 Mastergram Fault - request for advice please.

[DoctorWho]

Tony - Thanks for this advice, I'll try this and see what happens, I'll also measure the Grid voltage and let you know.

Steve - Well, if the above doesn't work then I'll try and muster up the courage to try this, after all, it's not much good as it is.

I take these readings and come back.

Thanks again,

Peter.

[Chris55000]

Hi!

The CRT in your set is Magnetic Deflection so a CV1085 Electrostatic CRT is no use! As the CRT heaters are floating its unlikely any other damage has been done!

To be absolutely sure, disconnect the Brown lead from the CRT, & reconnect the Cathode to the mid point of two resistors, an 82K 2W from Cathode-pin to HT & a 10K 2W from Cathode Pin to chassis, and switch on, looking at the brightness of the plain raster you'll get.

If it's still excessively bright your CRT is suspect *or its grid is excessively high due to a fault in the TRF chassis, e.g., last video amp low emission*, if very dim or blanked off you'll need to look at your Brightness Control cct, R33/R34/VR1 & C13 carefully again!

Chris Williams

[PJL]

DON'T HIT IT YET!!! As G8KBG says it's definitely not a h/c short as they are already hard wired together.

Another possibility is leakage in the CRT heater winding of the mains transformer.

[DoctorWho]

Chris - Thanks for clarifying about the CV1085, I had been offered one, and wasn't sure if it was suitable, at least I can now put that idea aside. Thanks also for the reassurance that no other damage is likely to have been caused.

Thanks also for the further explanation of what to look for/test, I'll print this off and read through it a couple of times before I give it a go. I'll work through your suggestions and let you know what happens.

PJL - Thanks for this, I was somewhat nervous at hitting it, so I'll try all other avenues first. The CRT heater winding is on the same transformer as the mains EHT, it has no other windings, is there likely to be leakage there, if so, how do I check for this?

Many thanks,

Peter.

[PJL]

I know nothing about tellies but I think I might be right on this one - an insulation failure on the CRT heater winding seems about the only thing likely to be pulling the cathode low (or even -ve?).

However, I am definitely not competent enough to suggest how to test for it!...Peter

[Sean Williams]

Why not try connecting a seperate heater transformer...

[Chris55000]

Hi!

If you're in any doubt about the CRT heater winding on the tranny you fitted, simply disconnect the link between CRT cathode and the heater, there should'nt be any change in Cathode Voltage.

To be sure of this, disconnect the winding from the heaters, and connect *one side only of it* to HT 280V via a 100K 2-3W resistor and a 0-10mA meter - there should be *no current flowing* if the winding insulation is OK - the insulation should withstand the test voltage for a short while!

Chris Williams

[PJL]

Originally Posted by DoctorWho

I took voltage readings either side of the following components to chassis, and the voltages found both sides of each component are given below, for example R12 294/105 which means that the voltage between each side of the component and chassis was 294 volts on one side and 105 on the other.

R12: 294/105 V DC
R14: -5/-2 V DC
R15: -6/-8 V DC
C13: -5/-12 V DC
Peter.

My last post as I really don't know enough to be commenting on this stuff but...

One side of C13 is supposed to be to chassis so why does it read -12V? Could it be a dodgy earth connection?

If it was really C11 then I am concerned that your meter may not be giving accurate DC results due to the AC component.

Lastly, I have no idea what the modulation is but it's marked low sync (is it flyback suppression?) Anyway it doesn't look like video to me so we may be looking in the wrong area and it's the grid that's off.

[FERNSEH]

Hi Peter,
I've checked the cathode voltage of the CRT of my HMV 901.
These are my findings: For max brightness the voltage reading at the connector terminal of the sync and timebase unit the figure is 8volts. For visual cut-off the voltage is 24 volts and highest reading is 50 volts.
I doubt if the CRT is faulty but it is worthwhile checking for a grid cathode short. According to CRT circuit of the 901, 702 and 703 the cathode and the cathode screen are connected together inside the tube base connector.
The attached picture shows the timebase terminal strip. The CRT cathode connector is the first on the left side.
A timebase HT supply voltage of 280 to 310 volts is correct for these sets.

In certain timebase units the brightness control can be found in the sync separator valve screen grid HT divider circuit.

I'll check the voltages in my other EMI TVs.

DFWB.

[FERNSEH]

I've checked the cathode volts of the 6/6 CRT in my HMV 900.
The readings are similar to those in the HMV 901. Max brilliance is 9 volts, visual cut-off 25 volts and the max volts are 40, brightness is control fully counter clockwise. It follows that the brightness control voltages are too low in the Mastergram.
Try removing the CRT base connectors and see if there are any changes to the cathode supply volts.
All these early EMI TVs employ a high level contrast control. The video from the vision demodulator is varied by a potentiometer. The slider goes to the CRT grid.

DFWB.

[DoctorWho]

Thanks guys for the assistance here, it's very much appreciated, and special thanks David for checking your other sets.

Things have now taken a turn for the better, we're not there yet, but I think David is definitely right when he says the CRT is alright.

I did give the CRT a **very** gentle tap, and nothing happened, everything remained the same, I then measured the CRT Grid Voltage, which showed as -15 V DC. Then I decided to just check components along the tag strip above the resistors and condensers I had been checking, when I got to R3 I put the probe on to get a voltage reading and I could see from the rear of the CRT that the picture stabilised, by pushing on the rear tag it was possible to get a stable picture, however, when I let go then the fault re-appeared. Initially I re-soldered the tags and replaced R3, but the fault was still the same. Tapping the Sync chassis in this area also made the picture stabilise momentarily.

I then freed the smallwiring loom from the rear of these components and pushed it upwards in to the area of the Sync chassis which is inside the cabinet, the picture stabilised and remained that way.

I went around to the front of the set and the Brightness control now worked, as did the Contrast and Sensitivity controls, the only thing is that the focus is not as good as it was before.

Whether this problem was due to a wire in the loom breaking down, or a breakdown in insulation which was touching on something else I do not know.

The brightness control now allows you to turn the brightness right down to nothing, or up to very bright, the full range of brightness is now available.

I now have a reasonable picture, the focus is not as good as before, and the focus control has to be fully clockwise, you can read larger credits/titles, but not the smaller ones, as they are outside the range of the focus control.

This having been established, I wondered if this throws any light on exactly what went wrong? Should I remove the Sync chassis and completely replace the loom, or should I leave well alone and use the set as it is. It is perfectly watchable, it's only on distant items and smaller writing/weather charts that the focus is not that wonderful.

Having got to this stage I'd be very grateful of any thoughts/advice as to whether I should just leave it alone, or if I should do something else? Does this suggest that the problem was related to the Focus control? Is there anything I can do to improve the focus and get it back as it was before?

At least I now have a picture again, I really am very grateful for the help I have received here, without the pointers and advice I've been given here these last two days I don't think I would have known quite where to start with this one.

I'll see how the set is in the morning, hopefully it will be working alright, in the mean time, if anyone has any other thoughts/advice I'd welcome it.

Thanks again,

All the best,

Peter.

[Ian B]

Really glad to learn you've been able to locate the fault Peter. My heart was in my mouth as I read through the thread (is it the tube or isn't it?), I hate to think how you felt!
Where would we be without the Forum, such a wealth of knowledge and experience, advice given with the only motive a desire to help. And it gives us the confidence to poke around for ourselves, which is the best way to learn!

Regards,

Ian Blackbourn

[FERNSEH]

Those early Emiscope CRTs were of a very advanced design. When you think that magetic deflection electrostatic focus CRTs did not come back in favour until the mid 1950s, it clearly shows that the pre-war EMI research team must have been one of the cleverest anywhere.

The 6/4,6/5 and 6/6 CRTs require only 15volts P-P video drive to produce a fully contrasted picture. This means that a video amplifier was not required.
In most 702 and 703 receivers the video demodulator load resistor is the 5000 ohm contrast control. The video from the slider of the contrast control is capacitor coupled to the CRT grid, or give it the old name, the Wehnelt electrode. Unfortunately this AC coupling does mean that the video DC component is lost so the black level wanders about according to picture content.
The fact that the CRT has a high slope results in the reason why the brightness control voltage requires a variation of only 15 volts to set the CRT from beam current cut-off and to full brightness.

DFWB.

[DoctorWho]

Ian - Thanks for your encouragement here, I was gutted to think that it was the CRT, I'm so grateful to know that this is not the case! I am extremely grateful for the forum, there's many a restoration I would not have been able to complete without the help obtained here.

David - Thanks for the futher information, much appreciated. It does seem that the early EMiscope CRT's were indeed very advanced! I can cope with the varying black level, that's no problem. Any idea what I need to do to get the focus back as it was, as it now no longer focusses as sharply as before.

Thanks again for the help,

Peter.

[DoctorWho]

Well I hadn't had the opportunity to turn the set back on until yesterday, and when I did the brightness, contrast and sensitivity controls work as they should, but the focus is way out, and the brighter you turn up the picture the less focussed it becomes.

From the procedure I described above it does seem obvious that the fault must be on the Sync chassis, now, when you turn the Focus control fully clockwise it is still not possible to get good focus, and the further you go anti-clockwise the worse it gets.

It does seem that the CRT is fine, but I'm not experienced enough to make an educated guess at the likely faulty connection/resistor etc causing this fault.

I can get the Sync chassis out, but if anyone can suggest where exactly I ought to look I'd be really grateful, whatever fault was causing the uncontrolled brightness, which was rectified by moving the wiring loom upwards, may perhaps be related to this very poor focus now? Before this fault appeared the picture was superb, so, hopefully, if the fault can be traced, then the set will be back as it was before.

I also wanted to ask which pin of the CRT should I take a voltge reading from for the Focus, and what sort of voltage should be present there? Perhaps if I can obtain this information then it may be easier to see what's going on there.

Thanks again for the help.

Peter.

[FERNSEH]

Hi Peter,
The EHT voltage at the CRT final anode should be 5000 volts. A drop of say, 500 volts will cause a significant loss of picture quality and focus.
Also, the result of a drop of the EHT voltage will result in the picture appearing larger.
In the 703 power unit ten series resistors are connected across the EHT reservoir capacitor. Starting from the high voltage end resistors R12, R1, R2, R3, R4, R5 and R6 have the same value and are 0.5 megohms. resistors R7,R8 and R9 are 1.5 megohm. The focus voltage is tapped off at the junction of R6 and R7 and connected to one of the outer tags of the focus control which is situated on the sync/timebase unit. The focus control pot. has a value of 375Kohms and has an 1megohm resistor connected across it. The low voltage end of the focus pot. has two series resistors R30 and R31 which are returned to chassis.
The focus control is effectively connected in parallelle with resistors R7,8 and 9 so it can be calculated that the resistance is about 1 megohm.
It follows that assuming that the EHT voltage is correct the voltage at the hot end of the focus control should be 1100 volts. The current through the bleeder resistors is 1.1mA
Before checking the values of the resistors in the EHT and focus control circuits release the EHT shorting bar to discharge the high voltage capacitors.

I can check the focus voltages in my EMI sets and report back my findings.

DFWB.

[DoctorWho]

Thanks David, that is extremely helpful and exactly what I needed to know, I decided to wait to hear from you before I did any investigating as you know the set so well.

How can I safely check that the EHT voltage is correct?

If you could check the focus voltages on one of your sets that would be appreciasted, thank you. Is it safe to measure the focus voltage with a multi-meter, as, seeing that it comes from the mains EHT circuitry, presumably it's in the same kind of danger zone as the mains EHT connection itself?

I can certainly measure the resistors you have highlighted, so long as I can physically find them

Thanks for the tip about releasing the EHT shorting bar, I'll be sure to do that before working on that part of the circuit.

I'll wait to hear back from you regarding voltages before I go any further.

Thanks again,

Peter.

[FERNSEH]

Hello Peter,
You will need an EHT meter to check the voltage at the CRT anode cap.
As for measuring the focus voltage, most multi-meters go up to 1000 volts so it could be unwise to measure the high voltage end of the focus potentiometer. But I believe that the AVO 8 has 3KV terminals. However, it is most likely one of those resistors in the EHT bleeder chain, R12, R1 to R6 has gone high value which would mean that the voltage at the high volts end of the focus control could be below 1100 volts.
But bear in mind, if one of the resistors associated with the focus control goes high value, then the voltage at the focus potentiometer will be higher than the correct figure.

DFWB.

[DoctorWho]

Thanks David, I don't actually have an EHT meter, so I'll pass on trying to measure the EHT at the anode cap. Thanks for the pointers re the focus, I'll follow through the areas you have highlighted and see what I can find.

I'll let you know how the resistors measure. Would you say this is most likely to be on the power chassis and not related to the fault previously discussed on the Sync chassis? Do you think I need to remove the Sync chassis to check over the area in which the original fault developed?

Thanks again for the help,

Peter.

[Danny]

Hi R13 will have had a shock when the crt was at maximum glow and will be worth checking. dont forget it may go high under working conditions and confuse things but dont use cheapo meters on this set as thay flash over inside. i cant quite make out the number of the other resitor in series with it but this neads checking also.Danny