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Service Data Substitute a Plessey SL917 with SL918 IC document

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Jayceebee
(@jayceebee)
Posts: 2004
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Found this advert quite by chance when looking for something totally unrelated. It appears there was some documentation available to replace the Plessey chroma processing IC SL917 in the Rank A823 chassis with an SL918 which was used in the 110° Z179. Has anyone here done this and can they remember what's involved? Although I have two A823s and both the SL917 ICs are fine I would like to try an SL918 to see if there is any noticeable improvement.

I only ever saw a single Z179 model before jumping ship to Thorn but have heard comments that the picture and colour quality was much better. From what little I remember about the Z179 I think the signal stages were quite similar to the A823, the major changes were in the power and timebases to drive a 110° CRT.

sl918

John.

 
Posted : 24/11/2023 8:44 pm
Nuvistor
(@nuvistor)
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@jayceebee I didn’t know about the mod, the IC’s in the A823 Chroma didn’t give me a lot of problems. Though other parts of the set did.

Frank

 
Posted : 24/11/2023 9:35 pm
Jayceebee
(@jayceebee)
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@nuvistor Can't say if this was an official mod or not, I'd never heard of this until tonight and any IC's that I did have to replace were usually the result of some arcing issues from the PSU.

John.

 
Posted : 24/11/2023 9:50 pm
malcscott
(@malcscott)
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The Z179 chassis gave brilliant colour. The decoder was very heavy populated with components. Never heard of the mod before.

 
Posted : 04/12/2023 6:39 pm
Jayceebee
(@jayceebee)
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@malcscott I'd not heard of this either but as I said earlier I parted company with my RBM dealer just as the Z179 was introduced. I'm pretty sure though that the decoder was just slightly reworked from the A823AV with the change from an SL917 to SL918 which carried out much of the chroma processing.

John.

 
Posted : 04/12/2023 9:48 pm
Jayceebee
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@Crustytv kindly scanned and sent me a copy of the Z179 manual, this confirmed the Z179 decoder was similar to the A823AV version and the changes are in fact very small. The demodulation and matrixing circuitry SL901 circuitry is virtually the same, only major changes are to accommodate the smaller chroma delay lines and a redesign of the RGB output stage clamp pulse generator. In the chrominance section around the SL918 IC changes appear to be the addition of 3R66 which feeds a clamp pulse into the chroma signal path and two 1000pF capacitors 3C60/1 on pins 12 and 13 of the IC. There is no mention of these pins on the block diagram of the SL918 so what the caps are for is unknown.

The rest of the Z179 is a mixture of old and new and. The power supply was lifted straight from the A823 and now includes soft start. The IF is very similar to that used in the A816 mono and the Z718 with the Toshiba in-line CRT. The timebases are completely redesigned of course to cope with the demands of the 110° delta CRT. It rather reminds me of the Philps G9 as it too carried over the signals stages and PSU from the G8.

Now then, would the SL918 bring a noticeable improvement to an A823AV, to me it doesn't look a huge task to get it to work? Famous last words 😆 .

Thanks Chris for the scan.

Z179

John.

 
Posted : 07/12/2023 12:30 pm
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