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Bush WS6674 grey raster with f/back lines - no sound
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LesWoollam
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PostPosted: Tue Sep 19, 2006 8:46 pm    Post subject: Bush WS6674 grey raster with f/back lines - no sound Reply with quote

When turning-on with the remote, the led turns green and a 'thump' is heard from the speakers. This is accompanied by a bright grey raster with f/back lines, which, after about 1 sec. changes to a dull grey raster with f/back lines. Remote then seems to do nothing apart from being able to put the set in standby mode.
Help!
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PostPosted: Sat Sep 23, 2006 9:02 pm    Post subject: Reply with quote

I guess no-one has come accross this fault with a WS6674. Could anyone please give me any 'generic' tips for tracking-down this fault?
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PostPosted: Mon Sep 25, 2006 7:29 pm    Post subject: Reply with quote

First thing to check would be that the B+ was within specification and without excessive ripple. Then check the hold down capacitor and the waveshape at the coupling transformer for the HOT. Look for failed solderjoints, especially around the larger components.
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Les Woollam
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PostPosted: Tue Sep 26, 2006 5:14 am    Post subject: Reply with quote

OK, this seems to have turned into a bit of an intermittent fault.
Turned the set on today and got picture and sound. . . but volume and programme change buttons on set did not respond, nor did many buttons on remote. After a couple of minutes set went back to original fault (raster, no picture/no sound) - help Sad
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PostPosted: Tue Sep 26, 2006 7:17 pm    Post subject: Reply with quote

Test all of the powersupplies are present and in spec while the unit is faulting. Look for failed solder joints and test all filter capacitors as these can go open circuit intermittently. I am not familiar with your particular unit, but you may need the schematic diagram to trace all of the supplies. Good luck!
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Les Woollam
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PostPosted: Wed Sep 27, 2006 1:40 am    Post subject: Reply with quote

OK, maybe I'm getting somewhere now.
I'm now able to get sound (ocassionally).
I've checked the B+ voltage and it's sitting at 99V (according to the schematic it should be 145V). Any ideas??
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PostPosted: Wed Sep 27, 2006 10:03 pm    Post subject: Reply with quote

It is hard to know where to start. Does it make a sound like it is working hard or a light undervoltage sound? If it is working hard then you may have a short on one of the secondaries, which should be isolated before you power it up too often.
If you measure the mains filter capacitor you should get about 320Vdc, or a bit less depending on how much it is loaded. You can measure the ac voltage across this capacitor as it is only 100Hz, which should be no more than a couple of volts. If it is more than this then it may need replacement. Next to test is the B+ filter capacitor, you will need a CRO or ESR meter as this one travels much faster, maybe 15KHz. If you replace it, use a high temp (105 deg C) low ESR replacement. Then test the rest of the electrolytics in the power supply, usually there are two smaller ones sometimes in a heatsink where they overheat. Test all of the large low value resistors and high value resistors, you may have to remove them to get accurate results.
Remove the HOT and solder a light globe from B+ to B+ground, make sure you get it right or you will blow the powersupply if you energize it unloaded. If it is still low on volts, there is sometimes a regulator/switching multi-legged chip that switches the primary of the chopper transformer to regulate the B+.
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Les Woollam
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PostPosted: Thu Sep 28, 2006 7:17 pm    Post subject: Reply with quote

Many thanks for all the detailed pointers. . . certainly plenty to go on there!
I've since tested all PSU voltages, and they all seem to be sitting at around two thirds of the correct voltage - would this suggest maybe one of the rails being under too great a load thereby preventing the PSU from fully powering-up.?
Would it be OK to disconnect the rails one by one to test for this, or is there a risk of damaging any of the circuitry?
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PostPosted: Thu Sep 28, 2006 7:41 pm    Post subject: Reply with quote

You could test them all to ground with your ohm meter...

Or remove the HOT, connect a 100 Watt globe from B+ to B+ ground, this will test the primary P.S.
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Les Woollam
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PostPosted: Thu Sep 28, 2006 8:33 pm    Post subject: Reply with quote

Thanks for all your help.
I've since discovered this is a PT92 chassis which, apparently, is used in many sets.
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