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Old 12th Sep 2018, 8:44 pm   #1
andrewn
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Default LOPT substitution

Due to the unavailability of the LOPT for my TV I was sold an "equivalent". I have been provided with an HR Diemen HR7291 instead of the correct HR7018. It won't fit the pcb due to it having an extra pin coming out of the focus/screen pot part of the transformer.
Comparing the diagrams I was wondering if its simply a matter of either linking pin 7 to 11 or connecting 11 to ground. I am unsure if the difference of layout on the diagram and slightly different voltages matter. Pin 6 (heater) is shown separated on the HR7018 but with pin 4 (ground) shown twice.

http://www.hrdiemen.com/reparation/flyback/scheme/7291
http://www.hrdiemen.com/reparation/flyback/scheme/7018
I've attached a scan of the LOPT section of the circuit diagram (Zanussi 20ZA374GB - Seleco BS700 chassis). Its a poor quality photocopy.
Can anyone advise if I can use the transformer? Regards, Andrew.
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Old 12th Sep 2018, 10:13 pm   #2
1100 man
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Default Re: LOPT substitution

Hmm, it looks like it could work but would need some further study of the diagrams. I don't know what effect the different heater winding arrangements would make.
Could you take a picture of the pins of both transformers?
Do you know for sure that the old LOPT is dead?
Who came up with the substitution- was it HR themselves?

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Nick
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Old 13th Sep 2018, 8:27 pm   #3
andrewn
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Default Re: LOPT substitution

I've attached photos of the new and old LOPT. Donberg Electronics made the substitution. The old LOPT failed in use. It had been playing up for a while making a strange hissing/whistling noise for the first few seconds at switch on. Eventually it went pop and the picture shrank very small. There's a small crack in the side where it spat out the smoke.

I've looked at a circuit diagram of another seleco set that is listed on the HR site for the HR7291. The windings are laid out different again with pin 6 separate so it all adds to the confusion but it shows the extra pin 11 connected to ground with pin 7 going to ground via a capacitor like my set.
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Old 14th Sep 2018, 11:33 pm   #4
McMurdo
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Default Re: LOPT substitution

on the pcb diagram it suggests both the bottom of the focus /a1 assy and eht stack connected to the same point, then to chassis via the capacitor arrangement, so I'd be tempted to join pin 11 to the existing pin 7.
If you get a decent picture I'd be tempted to check the EHT voltage to make sure its not wildly out.
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Old 16th Sep 2018, 11:01 am   #5
andrewn
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Default Re: LOPT substitution

I'm pleased to report success with this. I linked pin 7 & 11 and it works fine. I don't have an EHT meter but the data says it has a lower output of 19.9kv instead of 21.7kv. The set actually starts up better with this new LOPT. Before when the picture appeared it would have flyback lines and colours would shift about and then settle to a correct picture. Now the picture appears in a normal way like any other tv. Nice to have this (rare?) set working again!
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