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Old 8th Oct 2021, 12:14 pm   #1
ElectroRelics
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Default Marconi 2955R Shrunken Text/Linearity Woes

Hi All,

Long time lurker I'm afraid, but what a fantastic forum and wealth of information you have here.

I recently purchased a Marconi 2995R Test Set, this is the same as the 2955A but with the Off air receiver.

I was aware of a number of faults when I purchased it and am slowly working through everything.

There is a strange issue with the CRT, where a portion of it has shrunken text, it looks like a linearity issue but making adjustments on the linearity magnet makes no difference.

If I adjust the V position this shrunken part of the picture remains in exactly the same place on the CRT and moving the image up and down shows that.

First port of call was to take the AC1 board out and check the capacitors, I replaced
2x 470uF/16v
1x 1000uF/35v
1x 2200uF/16v
1x 100uF/50v
1x 3.3uF/50v

There are a few other electrolytics there I didn't replace as I don't have that value or they are axial and it would be quite a messy stretch to fit a radial in it's place due to the already long lead on the axials.

All those capacitors that I had removed tested fine with my EVB ESR Meter and my DE-5000 LCR meter.

I checked all the other capacitors and they all seemed fine, even C20 and C16 which are on the one end of the deflection coil, but I will replace these when I order some parts as I don't have them here.

Looking at the waveform from the output of the BU806 and before and after L1, between the pulses there is another small pulse and it's also not flat, it does look like where this extra small pulse is would be in that part of the CRT i'm seeing this issue. But I do not know what a pulse looks like on a perfectly working 2955A/R

I flipped the wires around on the deflection coil and the image flipped upside down as expected, now I was suspecting the deflection coil on the yoke as being the issue, BUT this issue now flips with the image to the bottom of the CRT. So I think I can rule out the CRT/Yoke?

I've attached a picture of the pulse, but this has been shown with the current yoke connected as I do not have any other way of loading this circuit up.
Please ignore the voltage reading on my scope, it just a digital overlay on an analogue scope. It is not indicative of the actual voltage.

As most people who are familiar with this model of test equipment know, the Vertical on these is actually what would be the Horizontal drive on a traditional CRT, Marconi rotated the CRT 90 Degrees.

I'm a bit baffled with this and don't know where to go next, I'm not sure if something is creating a dodgy pulse like the 74LS211, transistors or a passive like a capacitor is acting up at a certain frequency.

Hopefully someone would have come across similar or can guide me in the right direction.

Here is a Youtube link to a video I made of the problem while adjusting the V position to show it is not in the video signal but part of the scan system.
https://youtu.be/EF3Ke5V2aJg

Thank you for reading this
Dale
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Old 8th Oct 2021, 9:16 pm   #2
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Default Re: Marconi 2955R Shrunken Text/Linearity Woes

Just taken some measurements using the oscilloscope on the AC1 Board.

******************************9Zg3SJDJb4zrtgxb6
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Old 9th Oct 2021, 1:47 pm   #3
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Default Re: Marconi 2955R Shrunken Text/Linearity Woes

That link requires signing into Google and some of our members might not be keen on doing that.

It would be better to post the pictures as attachments.
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Old 9th Oct 2021, 2:43 pm   #4
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Default Re: Marconi 2955R Shrunken Text/Linearity Woes

Thank you for your reply, I've attached the images below.

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Old 9th Oct 2021, 2:44 pm   #5
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Default Re: Marconi 2955R Shrunken Text/Linearity Woes

More pictures:
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Old 9th Oct 2021, 8:10 pm   #6
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Default Re: Marconi 2955R Shrunken Text/Linearity Woes

Just looked at the Base and Collector of TR3 (BU806)
I can't help but think the collector waveform should be taller, thinner and a bit higher voltage. And the base should be more of a sawtooth?
Picture attached.
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Old 10th Oct 2021, 12:17 pm   #7
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Default Re: Marconi 2955R Shrunken Text/Linearity Woes

I think some of these measurements may be leading me up the garden path.
Looking at the cold side of the H Scan coil, the current waveform is perfectly linear with no distortion.
So I'm starting to suspect there being an inductive issue somewhere within the H scan coil winding.
A very kind person from the Marconi group on groups.io is posting me out a scan coil to test which will be with me in a few days.
And I will report back, if that doesn't cure it I'm going to suspect the either the BU806 or the 74LS221 next.
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Old 13th Oct 2021, 11:22 am   #8
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Default Re: Marconi 2955R Shrunken Text/Linearity Woes

Replaced the scan coils/yoke today, the fault remains so that's one thing ruled out.
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Old 13th Oct 2021, 12:43 pm   #9
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Default Re: Marconi 2955R Shrunken Text/Linearity Woes

It look like some non-linearity/instability in the drive signal, from your prior description in the line out rather than the frame out (rotated by 90 degrees ?).

I can't really see the discontinuity in detail but it looks like there might be a bit of oscillation in there ...

if that's the case, I would check any compensation components (e.g small ceramic caps) and that all the semiconductors are originals in the region where that discontinuity first appears (as subs can introduce stability issues).

dc
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Old 13th Oct 2021, 12:58 pm   #10
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Default Re: Marconi 2955R Shrunken Text/Linearity Woes

Hi Dave,

Many thanks for your reply.
Yes I can see the oscillation but wasn't sure if that was enough to cause this issue. But by the time it gets to the BU806 there is quite a prominent peak.

The IC that controls the H drive and LOPT is a 74LS221, that is fairly easy to remove as it is not behind the metal screen that covers the top half of the board. It is definitely an original TI device date coded 1988.

I think I'll swap this out first, as the person I bought this off was adamant this fault was there from new and they just got used to it.

Now I'm always the cynic and thing well is that actually the case or has this developed. As it's 31 years old and I can't remember that far back.

But it may well be that IC, I'll see if I can find a replacement somewhere and change the decoupling capactitors around it.
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Old 13th Oct 2021, 2:16 pm   #11
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Default Re: Marconi 2955R Shrunken Text/Linearity Woes

I'd be surprised if its that, I don't see how a 'dual multivibrators' would cause it. But I am quite often surprised

I'll see if I can find a service manual !

dc
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Old 13th Oct 2021, 2:22 pm   #12
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Default Re: Marconi 2955R Shrunken Text/Linearity Woes

Hi Dave,

Page 397 and 398 here:
https://bama.edebris.com/download/ma...e%20Manual.pdf
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Old 13th Oct 2021, 2:31 pm   #13
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Default Re: Marconi 2955R Shrunken Text/Linearity Woes

I hope, the relevant portion of the schematic

dc

EDIT
If I read this right, TR3's base should be a square wave, it is being switched on and off by the Q output.
The collector current should be a ramp as its driving the scan coil inductor ...
But its more complex than that, as its also got that transformer to deal with.
Maybe something on the transformers load is misbehaving or there is a lack of decoupling to some other signal.

dc
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Old 13th Oct 2021, 4:06 pm   #14
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Default Re: Marconi 2955R Shrunken Text/Linearity Woes

Quote:
Originally Posted by dave cox View Post
I hope, the relevant portion of the schematic

dc

EDIT
If I read this right, TR3's base should be a square wave, it is being switched on and off by the Q output.
The collector current should be a ramp as its driving the scan coil inductor ...
But its more complex than that, as its also got that transformer to deal with.
Maybe something on the transformers load is misbehaving or there is a lack of decoupling to some other signal.

dc
Yes thats correct,
The schematic you have there is slightly older with some different capacitor values. The updated schematic has a 3.3uF instead of a 10uF and there is also a decoupling capacitor on the 5V rail to the 74LS221 but mine has both of those updates.

I was expecting to see a ramp/sawtooth on the base of TR3 but maybe a square wave is correct. The little blip on the collector is odd but I seem to be able to trace that back to the 74LS221 which is why I susspected that.

Not sure what to check or replace.
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Old 13th Oct 2021, 5:27 pm   #15
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Default Re: Marconi 2955R Shrunken Text/Linearity Woes

If you haven't already checked ...

The 12V feed to L4 -> C30 -> L3 -> C31 & D11 cathode.
I would expect / guess this is supposed to be a smoothed +12V feed to the Tx, with reverse blocking by D11.

The 24V feed from R34 / C18 to the transformer.

C11, Zener D14 and R41.

pins 5 & 12 of the LS221 are going in anti-phase (they almost certainly are)

All the voltages on the other side of that Tx, 475,150 & 50.
The Hv if you are brave and have the right probe!

The TV guys may be more familiar with this type of line / flyback system ...

Happy hunting, dc
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Old 14th Oct 2021, 2:07 pm   #16
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Default Re: Marconi 2955R Shrunken Text/Linearity Woes

Hi All,

Todays update.

I was about to do an order on Farnell today and swap all the poly and ceramic capacitors, including the S correction capacitors.

Before I did that I thought I'd swap out some with values that I already have.
I thought I'd replace the decoupling capacitors on IC2, C12 and C13, which made no difference.
I added another capacitor parallel to C27 (5V decoupling on IC2, only on some AC1 Boards) and noticed no change.

Put the original capacitors back in C12/C13 and C27 as they tested perfectly.

Then got out the Freezer spray and a heat gun, still couldn't find anything.

Then I grabbed a plastic anti-static brush instead of my usual wooden one by mistake, just to check for dry joints.
As soon as I got near the LOPT the picture would start to collapse, in EXACTLY the same place as this linearity issue is.

So something is not right somewhere with the LOPT I think!

Video here on YouTube:
https://youtu.be/TWW2ebBtDIY
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Old 15th Oct 2021, 9:13 pm   #17
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Default Re: Marconi 2955R Shrunken Text/Linearity Woes

Any of that old glue they used to anchor things, it has been known to cause all sorts of issues.
Just a thought.
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Old 15th Oct 2021, 9:24 pm   #18
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Default Re: Marconi 2955R Shrunken Text/Linearity Woes

Quote:
Originally Posted by Freya View Post
Any of that old glue they used to anchor things, it has been known to cause all sorts of issues.
Just a thought.

Thanks for your reply, there is no glue on that board.
One of my thoughts when I did look in that area I noticed residue from where the label was on the LOPT but I cleaned that off and no change.

Glue can be a real pain, I've repaired a lot of electronic items where I have found the glue holding components in place has turned conductive or corrosive!
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Old 1st Nov 2021, 10:49 pm   #19
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Default Re: Marconi 2955R Shrunken Text/Linearity Woes

A little update, I haven't had chance to play with this the past week so decided to put this on the bench today with a fresh mind,
I changed the diodes and it made no difference, I guess I was clutching at straws with those.

I got the HV probe out to check the EHT and it seems quite low, the service manual states 15KV but I'm seeing 8.25KV so it really is starting to look like the LOPT is faulty.

Voltages are as follows:
15KV = 8.25KV
475V = 445V
150V = 150V
50V = 44.7V
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Old 2nd Nov 2021, 8:41 pm   #20
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Default Re: Marconi 2955R Shrunken Text/Linearity Woes

I think if the LOPT was faulty, EHT would be zero. Lopt failure is usually caused by a shorted turn on the overwind, and that causes shutdown with no EHT generation.
i have no idea what this one is like, but years ago I had some mains Grundig B/W tvs with failed lopts. They were no longer available, so I dismantled the lopt, removed the overwind and fitted a tripler (may have been a doubler) and that fixed it. The width was either too narrow or too wide, and either a cap or resistor change fixed it. I no longer have my original notes to refer to, so that is why I am a bit vague.
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