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Old 13th Aug 2008, 12:11 am   #1
stuie319
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Default RBM 823 query

Hello folks

I'm trying to breath life into an old A823. About 15 years ago I rescued an ex rental one from work, because i was taken with the aesthetics (white murphy version) Unfortunatly it was pinched from storage before i even got to have a look inside. Anyway i've aquired another, and its as poorly as expected. The decoder doesnt work for a start, but theres a more fundamental problem.
The focus is wildly out, and i'm wondering if the CRT is u/s although I hope not.
I've no circuit but the arrangement seems straightforward, fed from the tripler through the carbon rod control to the CRT via a 100K resistor. All seems well there and there is a modicum of adjustment.
Here's where it gets a bit odd. I presumed the focus supply to be too high for my DVM to measure, but i had a go, purely to see if there was anything there. Measuring it results in perfect focus which lasts from seconds to minutes before it reverts to none. I've checked the supply path and the tube base connections, all seem ok.
Does it sound like a CRT problem? I'm a bit puzzled as to what i'm influencing with the high impedance digi meter that persuades it to behave temporarily.
The attached pic shows how bad it is.
Any thoughts will be greatfully recieved

Regards
Stu
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Old 13th Aug 2008, 12:37 am   #2
Steve_P
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Default Re: RBM 823 query

Where did you measure it?

Anyway : Make sure the spark gap is clean. And the connections to the boards. Also make sure there are good contacts on the tube base.

Try a tripler first, then the resistors on the way to the tube.

I wouldn't try and measure the focus volts with a standard meter. The fact that you are able to suggests a fault somewhere. Not seen these tubes do this though!

Cheers,

Steve P.
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Old 13th Aug 2008, 1:07 am   #3
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Default Re: RBM 823 query

Hi Steve,

Thanks for the quick reply.
The spark gap is fine, it was my first port of call, the crt connections seem clean and sound as well.

I realise its not sensible to measure focus volts with a normal meter , but i was trying to establish whether i had any or not - at least if the meter read over level there was something there (I used works meter not my own ) I wish I had an EHT meter have you seen where the tripler lives

I do hope its not the tube though, i've had a tap around on the neck and it doesnt seem to influence the fault

Regards,
Stu
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Old 13th Aug 2008, 8:48 am   #4
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Default Re: RBM 823 query

Focus problems were a common fault with the RBM chassis that followed (Z718, T20 series) and in the case of those the problem was with the tube socket. For some odd reason both the focus pin on the tube and the contacts in the socket would corrode quite badly, leading to a poor contact.

I'd suspect the focus control itself first though, the rod is not carbon, it's an odd mix of stuff that makes it voltage dependent (e.g. it's a VDR) and the characteristics of these things can (and do) change. If loading the focus pin with a DVM improves the focus it sounds as if the VDR could be open-circuit at the earthy end, you have too much voltage rather than not enough.

This sort of fault brings to mind a Les Lawry-Johns story in the September 1981 issue of Television. In his case removing the focus lead completely improved the focus, the set was a BRC 8500 and in that instance the EHT rectifier was the cause of the trouble.
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Old 13th Aug 2008, 5:27 pm   #5
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Default Re: RBM 823 query

Hi
Wonder if that was a set I was bidding on? If so I'm glad it's gone to 'one of us...'

Never had many problems there except the spark gap (disconnect it for now) and the resistors. My manual says there is a 4M7 at the top and bottom (earthy) end of the VDR. It sounds like you have too many volts at the CRT (pointing to the lower resistor), hence the meter affecting things, but I should have thought it would have exploded with around 8kV at its input!
This is a delta-gun CRT and is unlikely to be faulty - ditto the socket. If it's not the resistor then I'd go for the rod itself or the tripler - and I do happen to have a NOS one in the box if you get stuck!
The fun starts when you find there's perfect focus but no colour.....
Glyn
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Old 13th Aug 2008, 5:47 pm   #6
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Default Re: RBM 823 query

I'm also surprised the DVM survived. Typical DVM has 10M input resistance which will heavily load the focus resistor chain and drag down the focus voltage dramatically. Welsh Anorak's diagnosis sounds very plausible. At least the resistors are easy to test.
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Old 13th Aug 2008, 7:10 pm   #7
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Default Re: RBM 823 query

I would check both focus resisters first, they are on either end of the Focus VDR both with a value of 4.7 Mohms. This was a common fault on tv's of this era including the Decca Bradford and ITT/CVC5/9 chassis.
If I remember correctly they were quite tricky to change on the A823 chassis but I would check/change these before accusing the tripler.
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Old 13th Aug 2008, 8:16 pm   #8
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Default Re: RBM 823 query

I guess, as the others have already said, that the resistor to go will be the 4.7M at the bottom end of the VDR. At least this is mounted on the "easy" side of the metal can. (6R11 on the diagram).

If, after you've fixed the focus problem, you have a picture which always seems just a little bit too dark, and where the brightness seems to change depending on the scene, change the 820K resistor between the LOPTY and the tripler. It invariably drifts up in value and can cause this brightness fault, presumably affecting the beam-limiter circuit. This resistor can also cause relatively poor EHT regulation, but because of the brightness problem it often isn't all that obvious. Unfortunately this resistor is the most difficult component in the world to get to, and you need to dismantle a lot of the metal-work to get at it. (It's soldered directly onto the back of the LOPTY.)

Hope this is of some help Stu.

Kind regards.

From Mike.
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Old 13th Aug 2008, 9:06 pm   #9
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Default Re: RBM 823 query

Good evening chaps,

Firstly, wow, thanks for all the quick replies!
Ahh so the focus 'pot' is actually a VDR is it? I did wonder at the arrangement

Wonder if that was a set I was bidding on? If so I'm glad it's gone to 'one of us...

Oops, sorry Glyn, I'll have to make sure I do a good job then! If its any consolation, cosmetically its worse than it looked on the pic, and yes there are decoder problems for me to worry about later.

My inclination was that the focus supply was low rather than high, since lifting the wiper on the VDR worsened rather than improved matters. I can see the logic behind it being too high and the meter loading it down though. I'd seen the 4M7 on the earthy side, and it had measured ok, I hadn't however spotted the one on the tripler side (it was hidden in the sleeving from the tripler to the VDR), and its value varies when measured, from around 18M to completely o/c. Having looked again the tripler doesnt look that bad to replace should it ever be required, unlike the 820K Mikey refers to!

I'll hunt a replacement 4M7, and report back the outcome, but its looking likely is it not.
Chroma problems then

Thank you all once again for your prompt response and assistance

Regards,
Stu
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Old 13th Aug 2008, 9:34 pm   #10
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Default Re: RBM 823 query

Hi Stu

The triplers were very reliable in the A823s. I remember very rarely renewing them. In fact I can only remember renewing one or two in all the years I used to be a 'telly engineer'.

TimR
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Old 14th Aug 2008, 4:34 pm   #11
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Default Re: RBM 823 query

just to add my 4d's worth I had this problem with mine
both the resistors on the ends of the focus control were high open circuit in fact.
replacing them brought everything back into focus

Rich
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Old 16th Aug 2008, 8:00 pm   #12
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Default Re: RBM 823 query

Hello Gents,

Quick update - the resistor on the 'hot' side of the VDR was indeed the culprit, I had to temporarily sub a 5M6 value in because I couldnt (and still can't) find a 4M7 resistor rated over .25W.

Then disaster - the focus problem returned, because the connection had parted company with the tripler I shall endeavour to repair this, I don't want to scrap a tripler thats functional. Assuming this is successful the decoder problems look varied and scary, but i'd rather try a scientific approach first before ploughing through fault crib sheets should i fail.
Thus 2 questions - does anyone know the correct rating of the resistor in the focus chain and where one can be purchased, and does anybody have a copy of the service manual that they would be prepared to sell or lend?

Thanks in advance,

Regards
Stu
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Old 16th Aug 2008, 8:18 pm   #13
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Default Re: RBM 823 query

from what I remember the resistor solders onto a small plug that fits onto the tripler it was a right B*****d to get it back onto the tripler!
I've got some 2w 4m7 resistors kicking about here somewhere from when I did mine I'll see if I can find them
as I recall I had trouble finding some too I think they came from Farnells that was a couple of years ago though

I've got a photo copy of the manual you're welcome to copy of that

Rich
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Old 16th Aug 2008, 8:24 pm   #14
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Default Re: RBM 823 query

Hi Molochkartovski

The resistor was soldered straight on to a pin on the tripler, which has become detatched, however its not impossible of course that the sets seen attention before in its life. Looking at it I'd say that the one on the cold end of the VDR has been replaced previously

Regards
Stu
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Old 16th Aug 2008, 8:26 pm   #15
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Default Re: RBM 823 query

must have been a common problem from the look of it
my one the hot end was high and cold end was open circuit

Rich
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Old 18th Aug 2008, 1:19 pm   #16
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Default Re: RBM 823 query

Hi
As said above, the triplers were very reliable - but I'm sure there's one here if you need it - though changing it is not that easy! I think I've only ever changed one because of a damaged EHT lead.
I've got a number of RS resistors that did seem to work in this type of circuit, although the voltage drop is not in spec. If you need anything, feel free to send me a pm!
I'll have a search for a presentable manual - mine is in tatters, especially on the decoder page....wonder why??
Glyn
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Old 20th Aug 2008, 9:01 pm   #17
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Default Re: RBM 823 query

Hello Folks,

Glyn,

Thanks for the kind offer, I've got this daft notion that i might be able to fix the extant tripler if i can get enough purchase on the focus termination; it seems a pity to use up a NOS one to replace one thats essentially working, i'll attempt it at first at least.
I think there may even be a scrap chassis at my folks somewhere.
I'll send you a pm re: resistors and manual

Thanks again

Regards
Stu
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Old 21st Aug 2008, 5:39 pm   #18
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Default Re: RBM 823 query

Hello,

The original 4M7 resistors used in this circuit were made by Erie and had a ceramic body enclosing the carbon composition resistive element. The reason these were used is that they could withstand - up to a point - the high voltage developed across them. Carbon composition resistors are also good at coping with very short term current surges - such as when there is a flashover in the CRT. Even so, they eventually drift off value or go open circuit, as you have found.

In order to avoid the replacments failing prematurely, you need to use a modern equivalent to the carbon composition type. I have seen such resistors in either the Farnell or RS Components catalogues/websites. You may not be able to find a 1W version in the value you want, in which case you could use two 2M2 0.5W in series - but make sure the soldered joints are rounded and smooth, to avoid corona discharge.

Take a look at Farnell part number 135-7957. They are metal glaze, 1W, 3500V dc working, 5% tolerance and are good at pulse/surge handling.

Regards,

Dazzlevision
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Old 21st Aug 2008, 11:26 pm   #19
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Default Re: RBM 823 query

Quote:
Originally Posted by dazzlevision View Post
Take a look at Farnell part number 135-7957. They are metal glaze, 1W, 3500V dc working, 5% tolerance and are good at pulse/surge handling.
I must have made a good choice that's the same ones I used on mine

I thought they were 2w maybe they're only 1w then

Rich
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