Marconi 703 Mastergram Fault - request for advice please.

[DoctorWho]

We've been having some work done here and so I haven't been able to use any of the sets for about 2 months.

I powered up the Marconi 703 Mastergram to-day and the television has developed a fault, the picture is extremely bright, and the brightness control has no effect, the sensitivity and contrast controls do have effect, and you can see what is going on, but it's also hard to get decent frame hold, and, the fact that the brightness control does nothing suggests to me that it is related to that area, but I don't know where to look.

When a picture is present it also kind of pulsates, and flyback lines are present, I presume this is due to not being able to turn bown the brightness? I removed the signal altogether and the tube was still fully bright, I tried additional attenuation and even a different aerial lead, but whilst other sets were running perfectly (confirming that the converter is sending out a good signal) the Mastergram has this problem.

It seems to be related to brightness, as you can definitely see that the contrast and sensitivity controls are doing what they should do, but I really don't know where to look on such an eary set as this. Is this likely to be a failed component, a valve which needs replacing, or something else?

The television sound is fine, so no problems there, but you cannot reduce the brightness at all.

Any help in solving this would be greatly appreciated. I'm very nervous to remove the sync chassis, and so would like to know exactly where to look, and anything that I can try before having to go to that measure.

Thank you.

Peter.

[Steve_P]

Is the brighness control itself OK? If it is, does turning it vary the voltages around it and on the tube?

Cheers,

Steve P

[DoctorWho]

Hello Steve,

Thanks for your reply. I'm not sure, as, to be totally honest, I'm not sure which connection to check for this, any chance anyone would know before I attempt to take a voltage reading?

Thankd again,

Peter.

[FERNSEH]

Hi Peter,
The brightness control should be OK. The most likely reason for the uncontrollable brightness fault could be the resistor that is connected to one of the brightness control outer tags and the HT line has gone high value or OC.
The sync and timebase unit is similar to that of the 702 and 901 models and the brightness control is fitted on this unit.
The brightness control supplies the cathode of the CRT, therefore an increase of voltage will deduce the picture brightness. The positive going video from the high level vision demodulator supplies the CRT grid (wehnelt).

Trace the brown lead from the CRT connector to the timebase unit, confirm that voltage varies when the brightness control is adjusted. The voltage should vary between 10 to 35 volts. No or low volts will be caused by R33 which is 75000 ohms. The brightness control pot. is 5000ohms.

The CRT grid connection is the green lead which goes to the TRF unit.

I hope these notes proves useful. Of course remember that the set employs a maims transformer for the 5KV EHT supply. Be careful. DFWB.

[DoctorWho]

Many thanks for this very helpful information David, I have not looked at the Mastergram at all yet until I heard back from you, now that you have pointed me in the right direction I feel as though I have the confidence to tackle this, or, at the very least, give it my best attempt. I hope to do this either later to-day or tomorrow, I will report back my findings.

Sorry to sound dim, but which brown lead should I be taking voltage readings from? Is there only one from the CRT to the sync chassis, and is it easily recognised?

Thanks again for the help. As you restored the set I knew you'd be the most likely person to know where to look.

All the best,

Peter.

[KEITHW]

Hi Peter,

sorry to hear about your problem with the Mastergram. Hope you get it fixed soon.

Best wishes,
Keith

[DoctorWho]

Thanks Keith, I hope to soo.

Thanks David for the guidelines of where to locate this fault, unfortunately though the fault has not yet been found, when I initially tested the set the CRT Cathode was receiving 8.8 V DC with the Brightness at minimum and 2.9 V DC with the Brightness control at maximum. I only found one resistor high and so replaced that, the following are my findings:

R33 Measured 65.1 K Ohms

R34 Measured 1.758 K Ohms

R15 Measured 2.21 K Ohms

R14 was a bit high and measured 83.3 K Ohms and so was replaced by 71.2 K Ohms.

R12 had already been replaced with a slightly higher value than original and measured 9.88 K Ohms. The set had been working fine with this.

C13 had also previously been replaced with a 0.047uf, again the set had been working fine with this.

The voltage measurement at the HT Red Lead for the Sync Chassis is 302 V DC.

After R14 had been replaced the CRT Cathode measured as follows:
Brightness Control At Minimum: 10.77 V DC
Brightness Control At Maximum: 4.83 V DC

I checked that the Brightness potentiometer is working, and it certainly appears to be doing so, similarly I check continuity from the Brightness Control to R33, from R33 to HT, from Brightness Control to R34, from R34 to Chassis, from Brightness Control Slider to R15, and all showed continuity.

The Brightness Potentiometer measured 4.82 K Ohms and the meter readings changed with the slider.

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

I'm not now sure if the minus voltage readings were due to me having the leads crossed?

The picture can still be seen, but is very unstable and pulsates, and, as it appears that the voltage being received at the CRT Cathode is much lower than is should be then does this seem to indicate the fault is here? I'm not sur what the pulsating picture or lack of hold/lock indicates?

The Sensitivity and Contrast Controls seem to function as they should, as far as I can tell, but the picture isn't locking well, especially on Frame Hold, but also on Line Hold.

I'm guessing that these voltages indicate a problem, but I have no idea what. I have made sure that both CRT plugs are securely connected, and they seem fine. So now I'm really stuck.

Any further suggestions/advice would be very much appreciated. I have absolutely no idea of what to test/look for next, so I am very grateful of any pointers/suggestions.

The last time the set was turned on before yesterday it was working perfectly, and it was untouched during the last 2 months, but when turned on yesterday this fault was present.

Many thanks for the help,

All the best,

Peter.

[Steve_P]

Has anyone got the circuit and can post the relevant parts for this job? Then we can all post our collective eyes over it and have a look?

Cheers,

Steve P

[Chris55000]

Hi!

Is the CRT cathode-modulated? I'm afraid from the readings/symptoms you've posted its pointing to a partial heater-cathode s/c in the CRT!!!

I've not got gen to hand but what I'd try is to see if the CRT heaters are at chassis-potential and if they are, isolate the heater winding of the mains tranny from chassis and connect a 100K resistor from either heater pin to the CRT cathode. (An EHT mains supply was mentioned, so I'm assuming the heater'll be transformer fed, 2V @ 2A very probably!)

If the CRT is cathode-modulated & got a h/k leak in it then the video amplitude & frequency-response'd go west as well as the brightness becoming uncontrollable, hence the associated difficulties with the line and frame-hold you're experiencing!

If the CRT is 4.0 or 6.3V run off a winding feeding other valves then you'll need to get a low-capacity isolating transformer, and I'm not sure where from these days! Either way you really need to chat to someone about getting your CRT tested h/k-wise before getting too bogged down!

Chris Williams

[DoctorWho]

Here is the circuit if the sync unit, thanks in advance for any help.

[DoctorWho]

Hello Chris,

Thanks for your reply, I do hope it's not the CRT, but I'm not able to answer your questions for sure, maybe others here who know more about these early sets will be able to answer this?

Thank you,

Peter.

[Chris55000]

Hi!

Can you post the Power-pack Circuit? The 10-35V CRT cathode-volts range quoted by Memner "Frenesh" is exactly what I'd be expecting to find, assuming the HT supply to the Sync (Timebase) Chassis is 350V, very likely to be with 4V valves!

Chris Williams

[DoctorWho]

Here is the power supply circuitry.

It should be noted that the mains EHT transformer, which, I believe, also combines the heater winding for the CRT, was replaced during restoration, and is therefore a seperate transformer from the one which supplies the other circuits in the set.

If this is a CRT issue what can I do?

[Steve_P]

These sets aren't really my strong area either, Peter and Chris, but we can only do our best!

Fernesh is, and he tells us that the Brightness Control goes to the Cathode, and the Modulation to the grid.

Assuming the Power Supply is OK, and all other components linked with the Brightness Control are OK, then it is looking like a Heater Cathode Short. I have one or two ways of clearing this, all a bit dangerous! The safest is to obtain a tube reactivator with an H-K Short clearing function. Sometimes works.

Don't want to do much else because it's an old set. Of course you could send the tube to Philiipe in France.

Cheers,

Steve P

[DoctorWho]

Thanks Steve, I understand your concern here, mine is the same. Basically I want to preserve the set, but also be able to use it.

I'd be interested to hear of ways to clear a heater-cathode short, not sure how practical it would be to send this CRT to France, or even how I'd go about it, as it is so huge, and it is also Pyrex, and extremely fragile.

Out of interest, could a CV1085 CRT be made to work in this set? I'm told that it has a base like the VCR97 (whatever that is) and is electrostaic deflection.

Has anyone got a suitable CRT reactivator which would clear this fault which I could borrow, together with instructions of how to use it.

If this is a heater-cathoder short then is this likely to have caused any other damage inside the set?

Any further thoughts/suggestions appreciated.

Peter.

[G8KBG Tony]

It looks like the CRT cathode is connected to the heaters at the tube base and that the heater circuit is floating in respect to chassis. Therefore I don't think a heater/cathode short would cause this problem. (Unless, of course, the circuit was modified during restoration.)

Might be worth changing C13 in case this has gone leaky.

As a matter of interest, what is the voltage on the CRT grid?

[Chris55000]

Hi!

Just looked at the Power Supply Circuit & from what I can make out of it there already *is* a direct connection between CRT cathode and one side of the heater so a h/k short wouldn't (or shouldn't!) affect the CRT cathode volts but a *grid/cathode short* would, and so would the CRT heater-winding going down to earth, but you mentioned it's had a replacement tranny so I'm afraid it's looking like Tester Time!

Sorry to be the bearer of bad news but from the 2 circuits/info already posted I can't see any other solution!

Chris Williams

[DoctorWho]

Hello G8KBG, well, so far as I am aware, the circuit wasn't modified during restoration, all that was done was that the faulty mains EHT transformer was replaced with a new one, but to the same spec, and, as mentioned previously, the CRT heater windings also come from that new transformer.

I'm no expert here at all, but I believe that the CRT cathode is indeed connected to the heaters at the tube vase, as the set employs different chassis for each part of the set, and they come together in the plug connecting to the CRT.

Thanks for the suggestion re C13, this was replaced during restoration, but I can easily replace it again if it is thought that this may be the cause.

I haven't measured the voltage of the CRT grid, I will do so and post it here.

Chris - Thanks for your further reply, does that mean it is unlikely to be the CRT? That's a relief if so. Sorry to sound dense, but it's no problem getting out the test meter, but what voltages do I need to measure? If you could give me some pointers as to where to start I'd be very grateful, also, how can I tell if there is a short within the CRT?

Thanks again for the advice.

Peter.

[G8KBG Tony]

Well, a leaky C13 is certainly a suspect. Just disconnecting one end and seeing if the fault clears would be a quick test.

The voltage reading on the grid would give an indication if there is a cathode/grid short within the CRT.

Cheers,

Tony

[Steve_P]

Measure that and come back. Chris and I are going down the same path here I think.

Try this - Take the tube base off, and give the tube neck a couple of good cracks with the handle of a screwdriver. Not enough to damage, break or gas the tube obviously, but it might just cure it!

If you don't want to do this, I quite understand!

Cheers,

Steve P