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Forum 141

TX9 PC1040

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Rebel Rafter
(@rebel-rafter)
Posts: 184
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Topic starter
 

Hi, folks, RR here. Here's something to get folk thinking, can anyone think of an alternative use for the big choke ( L65 ) on the old TX9 thyristor power supply? I've got one of these old main boards which worked when last used about 20 years ago but is now a source of spares for my own 20" TX9 which I'm currently fitting with a scart connector, using the home-made video preamp that was previously in my TX10. Whether it will work remains to be seen. Another thing I could never understand with the Tx9 thyristor power supply is why I could never get it to work off an isolation transformer, every time I tried it always tripped the crowbar and popped the main fuse, the transformer was more than big enough at 1000VA, anyone got any ideas? RR.

 
Posted : 28/07/2015 10:39 pm
Jayceebee
(@jayceebee)
Posts: 2027
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Hi RR, sorry but I no longer have any circuit diagrams of the TX9 but if the L65 is the huge choke at the rear edge of the chassis then I'm afraid its a special component and needs to be replaced with the exact type as it had a tap on the winding. There were two versions of the TX9 that used the thyristor PSU, the original PC1001 fitted with a UPC1365 video/chroma processor and later PC1040 with the better known TDA3560. Can't say I ever had any problems getting them to run on an isolation transformer but Thorn never managed to get the design quite right, intermittent/random fuse blowing being a major problem.

The choke was modified later in production with a change in the position of the tap and some component changes ISTR but the problem never really went away.

The last and best PC1044 version used a chopper PSU design using a TDA4600, the performance was excellent.

John.

John.

 
Posted : 28/07/2015 11:43 pm
Rebel Rafter
(@rebel-rafter)
Posts: 184
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Topic starter
 

Hi, Jayceebee, I quite agree about the later and final TX9, with the TDA4600, I've got one from 1983 that I've had for years and it still works fine, the only problem I've had is dry joints so I thoroughly resoldered every joint on the main board and put that problem to bed. It's even still got the original tube! That one works fine from an isolating transformer and mine even has one built in. One thing I could never understand is why didn't thorn make the power supply isolating on the PC1044 version, the transformer has separate windings and if my memory serves me right I think some other setmakers did actually use the same PSU design with isolation. I was wondering what possible other uses could there be for the big choke as I've got one surplus to requirements. I hope the loptx on the older chassis is the same as the one on the PC1044 as it's now the only spare one I've got should the one in my TX9 ever fail. RR.

 
Posted : 29/07/2015 12:19 am
Jayceebee
(@jayceebee)
Posts: 2027
Prominent Member Registered
 

Hi RR, The TX100 PSU was fully isolated, the primary being very similar to that used in the TX9 PC1044. I suspect the bean counters were at work and decided on a much cheaper transformer at the expense of isolation. With some judicious print cutting it is possible to isolate the primary and secondary of the PSU on the PC1044, I tried this many years ago feeding RGB in signals from a BBC Model B. It performed reasonably well but there was a possible safety issue because of the low spec of the transformer :ccf , just as well the Beeb was earthed. I wonder if it would have worked OK with todays ELCB's.

The LOPT is universal across all versions and screen sizes, I must have repaired many hundreds of TX9's but it was extremely reliable and can only recall a couple of failures.

John.

John.

 
Posted : 29/07/2015 10:48 pm
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