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Old 17th Jan 2018, 2:05 am   #1
rbm1973
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Default RBM A823 decoder issue

just restored this 20 inch A823 one fault was no colour,
this was cured by replacing the sl917 ic (note 2 ic decoder panel fitted)

but now have colour but it does not look right although locked it looks like watching a 405 line set and the reds appear to be incorrect,

see attached images these are the A823 and my 55 inch LG oled displaying the same image you can clearly see the difference .
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Old 17th Jan 2018, 8:11 am   #2
Mikey405
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Default Re: RBM A823 decoder issue

Hi RBM.

It looks to me like the PAL switch has stopped. I don't have the circuit to hand and I can't remember the last time I looked at an A823 decoder but this is where I'd turn my attention. I'll have a look at the manual when I return home in the vague hope I might be able to provide some enlightenment.

Kind regards.

From Mike.
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Old 17th Jan 2018, 8:43 am   #3
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Default Re: RBM A823 decoder issue

Long time ago for me also but I never thought the colour was great on these sets. Always a little noisy though a lot better than yours at the moment. Wasn't there a bistable phase pot in the centre of the decoder that used to make the faces turn green if incorrectly set? I wonder what the effect would be if slightly out?

Excellent article in Nov/Dec 1977 Television Mag on these sets by R W Thomson (Search on the web and you'll find a copy)
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Last edited by pichacker; 17th Jan 2018 at 8:59 am.
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Old 17th Jan 2018, 8:55 am   #4
rbm1973
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Default Re: RBM A823 decoder issue

My service info i no longer have as all the stuff we repair now is all the flat panel junk!
i will try and get a copy of the service info from an ex colleague and photo copy it,
i will be looking around the bistable circuit.
its been over 30 years since i ever had a fault with an A823 decoder!
Like the set itself my brain is getting old lol, so it too needs refreshing now i am back repairing 70's stuff.
Although i must say i rarely ever experienced any faults with the A823 decoder,
i found it to be very reliable,although this particular set has been messed about with by the previous owner!
it looks like the sl917 demodulator matrixing ic has also been replaced(not by me).
i do have some of these ics NOS knocking around,
Just to make it a little clearer the set had intermittent colour most of the time no colour!
this was indeed cured by replacement of the sl917 ic this restored colour but the bistable looks like it isn't operating correctly.
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Old 17th Jan 2018, 10:54 am   #5
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Default Re: RBM A823 decoder issue

Quote:
Originally Posted by pichacker View Post
Excellent article in Nov/Dec 1977 Television Mag on these sets by R W Thomson
http://www.americanradiohistory.com/...n_Magazine.htm

The original 2 IC decoder is covered by Part 1 of this article (Nov 1977 page 154).
Parts 2 & 3 are in Dec 1977 on page 88 and Jan 1978 p154 respectively.
The penultimate paragraph in part 3 may be of particular interest.


"Decoder Tip
This brings us to the end of our faults survey on the
original version of the A823 chassis, but here's a final
decoder one. The phase of the reference signal fed to the
R -Y demodulator in the i.c. on the decoder panel is set by
transformer 3T3, whose earthy side is decoupled by the
100μF electrolytic 3C21. If 3C21 goes open -circuit, the
phasing will be affected and the demodulation wrong. There
will be a colour shift therefore, with red changing to muddy
brown and green weak, along with poor definition. These
symptoms can also be due to an ageing tube however, so
when they're met it pays to check 3C21
"
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Old 17th Jan 2018, 11:51 am   #6
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Default Re: RBM A823 decoder issue

Just to clear things up, I was the previous owner and I did not mess about with it .
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Old 17th Jan 2018, 12:24 pm   #7
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Default Re: RBM A823 decoder issue

I am not sure which type of two chip decoder is fitted to your set, but you will need a good quality pattern generator, displaying standard colour bars to set the decoder up correctly.
Please find attached the decoder section of the RBM 823 manual, which shows the first two chip decoder type Z180 and the later modifications. Also an article by John Coombes in the March 1976 issue of Television magazine , on the Z584, Z971 versions of the decoder.
I hope this information is of some use.
Regards Stan.
Attached Files
File Type: pdf Bush A823 Manual {Section D-Z180}.pdf (1.69 MB, 169 views)
File Type: pdf Rank Z584 Decoder Panel[TEL Mar76}.pdf (261.1 KB, 91 views)
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Old 17th Jan 2018, 1:46 pm   #8
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Default Re: RBM A823 decoder issue

Hi

That looks like it's displaying Hanover Blinds. It would be interesting to see the display if fed with the Philips PM5544 test pattern or alternatively Test Card G, the one that has the all important brackets for decoder assessment.

Regards
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Old 17th Jan 2018, 7:05 pm   #9
rbm1973
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Default Re: RBM A823 decoder issue

Quote:
Originally Posted by neil29 View Post
Just to clear things up, I was the previous owner and I did not mess about with it .
A previous owner i know it wasn't you as you said it was not touched by you
and i believe every word you have told me Neil, upon meeting you i found you a very pleasant and honest person to deal with this being the second lot of tv's, I have collected from you.

but the decoder had been worked on before and i know this wasn't you !
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Old 17th Jan 2018, 11:57 pm   #10
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Default Re: RBM A823 decoder issue

Although I have not touched one of these sets for many years, I was once advised by RBMs technical dept at Boreham Wood that if changing the SL917 chip did not cure the fault, many chroma faults ranging from no colour to incorrect colours, intermittent or permanent could be cured by changing the following electrolytic caps on the lower half of the decoder panel. 3C2 3C17 3C23 and 3C16. These are all 50mfd. May be worth a try for your fault as we all know how suspect electrolytics can be. Good luck.

Alan
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Old 18th Jan 2018, 2:51 am   #11
rbm1973
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Default Re: RBM A823 decoder issue

here is the set displaying colour bars the reds still don't look right and the fine lines in the picture you may notice.
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Old 18th Jan 2018, 9:35 am   #12
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Default Re: RBM A823 decoder issue

I remember that PCB had many tantalum capacitors, these were not reliable.
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Old 18th Jan 2018, 10:07 am   #13
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Default Re: RBM A823 decoder issue

I hated the dual chip decoder. Large numbers of these models were sold by the local discount houses that eventually landed on my workbench. All of them produced a rotten colour picture from new, typical of your display.

The tuning was also odd looking as if the IF strip was off alignment. The colour was also noisy. Adjustment to the delay line balance pot improved matters but they were never good. I seem to remember that all the vari cap tuned models for some explainable reason were particularly bad.

I had a long chat with RBM technical and he agreed about the poor colour quality but stated that an extensive modification involving both the IF and decoder boards was needed to clear the matter up. The time required was extensive and he did not recommend the mod!

Strangely I don't remember any customer complaints [sigh of relief] but the colour quality was shocking compared with say the Thorn 3000.

The single chip decoder produced excellent results. Regards, John.
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Old 18th Jan 2018, 10:26 am   #14
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Default Re: RBM A823 decoder issue

The famous Hanover Bars, something to do with R-Y switching not working IIRC.
I hated this chassis. The pictures always looked rather glum - a kind of dismal lack of punch & sparkle compared to other sets of the time. I remember fiddling with one on the bench for ages, adjusting A1, drive levels etc to try to put some life into it. No joy!
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Old 18th Jan 2018, 10:29 am   #15
AJSmith625
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Default Re: RBM A823 decoder issue

John (HCS) has just beat me to what I was going to suggest. Try a touch either way on 3RV3 direct path gain pot other wise known as delay line balance. Make a note of the pots position first and set back at this if no improvement. Looking at the red bar on your colour bars one line is more saturated red than the next, known as Hanover Blinds and usually points to an imbalance between the delayed chroma path through the delay line and the direct path. If it was a PAL switch fault the red bar would be showing red green red green on alternate lines.

Alan
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Old 18th Jan 2018, 10:53 am   #16
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Default Re: RBM A823 decoder issue

Interesting comments about the picture quality, I never thought they were a good picture, even the single chip one was not as good as some other sets but the two chip on was much worse.
I had not heard of the mods referenced by John, we stocked them but other sets sold better when viewed side by side.

We were a small shop, taken over by another small business, they were exclusive to RBM with many of these on rental. The two engineers there loved them, mind they had no experience of anything else and give them their due they knew the chassis inside out.

Looking at that photo, it should be better than it is, something not right with it.
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Old 18th Jan 2018, 11:01 am   #17
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Default Re: RBM A823 decoder issue

I've replaced a couple of tantalums on at least two if/sound boards which were across the power supply line. as they literally burnt out the resistor on the power board which obviously resulted in no sound.




Cheers
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Old 18th Jan 2018, 3:14 pm   #18
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Default Re: RBM A823 decoder issue

Hi.

I agree that the pictures displayed by the RBM A823 series were'nt brilliant. Not in the same league as a Philips G8 or BRC 3000/3500 or the great GEC 2110.
It was quite a fair set overall though. All panels were reasonably easy to change but access for checking voltages/waveforms especially the decoder/RGB board was limited. The line output transformer and tripler were'nt easy to get at but then these two components especially the LOPTx was very reliable. The Power supply board tended to cook a bit after a few years service.
These sets used a huge amount of those PCB type carbon composition resistors which were'nt all that good during the service life of the sets. I reckon the vast majority of these must need replacing now due to drifting high. If the A823 had a Mullard type decoder then I think they'd have been a very good set.

Regards
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Old 18th Jan 2018, 3:26 pm   #19
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Default Re: RBM A823 decoder issue

With all the experience Bush had with colour tv supplying monitors etc for experimental work with the BBC, they then produced for the public the CTV25 followed by the A823 chassis, not the best colour pictures.
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Old 18th Jan 2018, 5:38 pm   #20
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Default Re: RBM A823 decoder issue

There is a world of difference between producing low volume, essentially no expenses spared, product for the broadcast/professional market and high volume, cut to the bone, product for the domestic - sometimes one cut too many was made or insufficient soak testing was performed - in some cases sets that got a bad name in the trade could, with very little extra attention, have been very reliable - it's just that commercial pressures often did not permit this
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