RBM A823 decoder issue

[Nuvistor]

Originally Posted by kan_turk

it's just that commercial pressures often did not permit this

That’s very true and happens a lot, the difference is they had the experience but it didn’t work as well as other manufactures sets.

The CTV25 was not a cut down design, it followed other designs very closely, it was generally very well built. It just didn’t produce the picture others did, why I don’t know. Apart from the LOPTX troubles it was a reliable TV with some stock faults.

The A823 was in some ways a revolutionary design for the chroma section, again others were innovating and producing better pictures.

After saying that about the A823 many were sold/rented and most owners were happy with them.

[rbm1973]

I do have another 22 inch A823 which i bought at the Wooton bassett auction
it had a tuner fault and a few other problems which were sorted easily.
iirc it too has a 2 chip decoder but the later type.

this is a picture of it taken in november the picture is very good certainly nothing like the 20 inch i am having the problems with.

[Heatercathodeshort]

The RBM chassis as a whole was certainly reliable. Pity about the decoder. The power supply was OK but would have benefited from better ventilation.

I sold the original 1969 models, the ones with the polystyrene 3rd harmonic .0047uf tuning caps, later replaced by green Plessey 16kc/s specials.

I think one of the mods was turning the timebase panel upside down to improve interlace? It was a long time ago!
Only problem then was that height and frame hold controls were then pointing forwards, not a real problem as the timebases were rock solid.

At least they were predictable and could be cleared off the bench in most cases in less that 20 mins. We didn't know how lucky we were. Regards, John.

[ukcol]

Originally Posted by Heatercathodeshort

I think one of the mods was turning the timebase panel upside down to improve interlace?

The early A823 had a problem with the line sync particularly if the line hold had been set accurately.

When the channel was changed either a normal picture was displayed or the line lock out of phase with a black bar down the middle of the picture. The symptom was caused by pulses from the line output stage being induced into the windings of the discriminator transformer in the flywheel sync circuit. Turning the PCB upside down improved matters.

Later RBM provided a modified discriminator transformer in which both the primary and secondary windings had their phases reversed; as far as the flywheel sync circuit was concerned the circuit phases were unchanged but the pulse from the line output stage was induced in opposite polarity and no lomger caused a problem.

[Philips210]

Hi

I also seem to remember reading in Television magazine a recommendation to turn the scan correction board upside down. That's the smallish board located above the sloping mesh screen to the line output stage. I can't recall the reason for this 'modification'.

Regards
Symon.

[Heatercathodeshort]

These problems should have been sorted out before releasing the 823 to the public.
This is the reason that the industry collapsed in favour of the receivers originating from Japan that had a far higher standard of quality control.

Toshiba took over the RBM plant at Plymouth in the late 70's. Paperwork discovered by Toshiba revealed that the last RBM chassis built, I believe the T24 [not the Toshiba based chassis] had a first switch on rate after assembly of only 23%, the balance having to be reworked. By 1982 the T. C2020 series was as high as 98%.

During the factories life it was 99.8%, better than Japan itself. It proves we could do it with good management. The Plymouth factory originally built for Bush Radio was the height of excellence! They served me with incredibly reliable receivers. RIP yet again. John.

[AC/HL]

Sadly, lions led by donkeys applies to much more than the usual subject of the quote. The basic problem is competing with much bigger markets. Many of our manufacturers had basically good products, but as world markets opened up the scale of the cost cutting needed to stay competitive gradually killed them. Once the competitors became partners (owners!) things changed rapidly.

[thermionic]

Originally Posted by Heatercathodeshort

Toshiba took over the RBM plant at Plymouth in the late 70's. Paperwork discovered by Toshiba revealed that the last RBM chassis built, I believe the T24 [not the Toshiba based chassis] had a first switch on rate after assembly of only 23%...... John.

Would the last Chassis have been the T26? I seem to remember that the T24 was the Toshiba chassis, although, it could be my memory playing tricks...

[neil29]

I think the T26 was the last but i'm not sure. I have a co-op badged one in a tatty cabinet and it does work but the sound drops out and comes back when it feels like it.


Cheers
Neil.

[flyingtech55]

We did quite a few of these sets mostly on rental. They were by and large very reliable...the only triplers I ever renewed were as a result of burn ups and that was only one or two. I only ever renewed one LOPTx again as a result of a burn up. The thermistor in the power supply used to fail (VA1024 ?) and of course the BT106 used to short. The big electrolytics in the bottom of the centre section used to go O/C (I think I might still have a couple of them). They were a lot more reliable than the Philips G8 in my opinion and certainly a lot more reliable than the Pye 205.

However, I always thought the picture lacked 'punch' and was always slightly 'murky'. The sound was always sort of muffled as well. I found the decoder was pretty reliable but was tricky to work on in situ. I made up a set of extension cables using the plugs from a scrapped set which allowed me to work on the decoder panel flat on the bench. I got fairly good at aligning the decoders. The company I worked for then were quite good in that they allowed you the time to really 'get to grips' with the sets you would meet in the field. The tricky part of the setup was that transformer thingy at the bottom edge of the decoder panel. You could set it up on the bench but had to 'tweak' it when the panel was refitted into the set in its normal position. I think it was called something like the 'swinging burst' transformer or something like that. I think I may have an original manual up in the loft if you want it.

TimR

[Nuvistor]

The Pye CT 205 (697 chassis) was in my opinion a disaster to say it was basically the same design as their successful dual standard and 691 chassis. Appreciate it was an interim while they got the 725 chassis sorted but still the 697 was very poor.
All down to costs, Pye had money problems and after the 731 and 741 chassis they used the Philips G11 design.

[Philips210]

Hi.

I thought the last RBM chassis was the T26 using the 30AX system and it wasn't a bad set though I only came across a few.
The T24 was a Toshiba design and what a truly reliable chassis that was. I reckon that chassis rates as one of the best ever produced even compared to today's modern sets.

Regards
Symon.

[Philips210]

Hi.

In relation to the OP colour problem, I've just been reading a short article by Mick Dutton in Television magazine, July 1981. It concerns overhauling the A823 chassis. He quotes, "When setting up the two-chip decoder it's quite common to find that the V reference phase coil 3L5 cannot be set for correct overlay without Hanover bars. The cause of this is the chroma delay line having gone out of tolerance, the only cure being replacement".

It may be worth trying another chroma delay line to see if there's any improvement. I wonder if a later, physically smaller chroma delay line from say a more recent TV can be utilised.

Regards
Symon.

[rbm1973]

These were my thoughts as everything else has been tried it was also suggested by another member via private message,

he quoted that if it was an ITT delay line they were known for going off tolerance.
and suggested fitting a Mullard DL20.

Does anyone have a Mullard DL20 or similar knocking around?

[Nuvistor]

I have not had a chroma delay fail but perhaps they were not old enough when I finished in the trade.
I can only think it’s the transducer or it’s fixing to the glass that fails, the glass part shouldn’t change unless cracked.

[Heatercathodeshort]

Yes you are right. The 23% related to the T20 and T22. I remember the 30AX T26. There was nothing inside the case compared with the earlier chassis.

The T24 never broke down. The first fully built UK Toshiba was the C2090/2290. It was a very good seller [1979/80] and the only problem I had with the first few were faulty tuners. The tuners had been manufactured by RBM. Toshiba soon rectified the problem and they ran for well over ten years without even adjusting the customer controls.

The chassis was also fitted in some receivers marketed by Currys and Bush. John.

[Nuvistor]

Originally Posted by Heatercathodeshort

Yes you are right. The 23% related to the T20 and T22.

That’s probably why I thought the T20 chassis was not of the quality expected. I would have seen the T22 but cannot place it now. Surprising what we later find out, I didn’t know the failure rate off the production line before this and your earlier post. RBM would certainly not have wanted those figures published.

[Heatercathodeshort]

RBM literally walked out of the factory leaving everything as it was even down to the coffee cups! I had some interesting chats with my rep.

Toshiba completely rebuilt the assembly lines etc. A lot of good equipment must have been dumped as it was at Philips and Thorn. Water under the bridge. RBM had a very good spares dept and could supply parts for some very old models, [including the later oil filled Murphys] and all the 823 bits by return of post. That helped the trade a lot. John.

[flyingtech55]

I remember the Z718 chassis as well as the T22s very well. The early T22s gave a lot of trouble. One of the most common faults on the T22 was the 910 ohm resistor on the hinge down panel between the two 'cheeks'. I think it was part of a 12V regulator. Also the line stage harmonic tuning capacitor used to fail (or change value) and cause the line output transistor to fail.

I never saw a T24 because they were just out when I left the trade. I was pretty fed up with it by then. Latterly, when I was a bench 'engineer', I used to do blanket replacements on incoming sets. In the case of the T22 it was the 910 ohm, the line tuning capacitor, the resistors in series with the A1 controls and a few other bits which I can't remember now. It saved a lot messing about.

Tim

[samjmann]

Hi,

Back in the seventies/eighties I used to work for a firm that rented out the A823 chassis. We used to have a test rig so as the decoder could be worked on a flat surface on both sides. Around 50% of the decoders we got back needed to be re-aligned. We ended up working from an abridged version of the alignment instructions. The fault I saw in the pictures was normally down to delay line phase/amplitude adjustments, or has been mentioned the chroma delay line out of spec. I'm not sure if we were re-aligning to mask drift in other components, but a tweaked decoder looked sparkling compared to how it came to us. The varicap tuner versions of this chassis always seemed low gain compared to the manually tuned versions, this along with a usually badly aligned IF from new meant that in my opinion the pictures on these sets wasn't that good.

If you put a Philips G8 with the combined IF/decoder next to a A823, there was no comparison... Why Bush used this passive regenerator circuit that was I think derived from an NTSC design I don't know! The decoder was a sod to work on in the Tv, we were spoilt with a test rig!

Congratulations on the work you've done so far.

Regards, SJM.