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high_vacuum_house 17th Oct 2020 1:48 pm

Thorn 1590 set HT procedure
Good afternoon,
I have just picked up a near mint Ferguson Thorn 1590 chassis telly and although it seems to work ok I want to check the HT is correct before overloading the CRT heater.

I cannot find my vintage television CD or my service data CD rom My wife has been tidying up again so gawd knows where these will turn up !! So I am flying a bit blind with this one as I haven't got any service information or circuit diagram to hand.

Could someone tell me what the correct HT voltage should be and where to measure it from.

Many thanks,
Christopher Capener

Heatercathodeshort 17th Oct 2020 2:14 pm

Re: Thorn 1590 set HT procedure
Check it across C87 1000uf or the collector of VT21, the regulator transistor on the heatsink for 11.6v. Check the value of the adjust pot. It should be 220 ohms. If it has risen replace it with a good quality type such as a ceramic. J.

high_vacuum_house 17th Oct 2020 7:45 pm

Re: Thorn 1590 set HT procedure
Good evening,
That has done the trick and the HT is now spot on. I think this set may have had a replacement tube fitted at some point as it is a Mullard A31-410W quick heat CRT and not a Mazda CRT.

The quick heat function of this particular CRT was not known until I found the specifications of the tube and was the concern I had that the heater was being overrun as the picture appeared rather quickly. The tube heater is quite bright even though the heater voltage is correct. Interestingly this Mullard CRT is made in Taiwan!!

Christopher Capener

Restoration73 17th Oct 2020 11:06 pm

Re: Thorn 1590 set HT procedure
All the data in post #18;

high_vacuum_house 18th Oct 2020 6:14 pm

Re: Thorn 1590 set HT procedure
4 Attachment(s)
Good afternoon,
Many thanks for the link Restoration, That has all of the useful servicing information.

This was an Ebay purchase and only 2 miles from our house. It was pretty dusty inside which was a good thing. luckily everything has been kept dry and the vacuum cleaner and 1" paintbrush soon took care of that. A careful tweak of the set HT pot and a tweak of the height pot brought the raster to the correct size. The remaining dust on the 10 ohm dropper in the power supply made a smell but all was ok. The 4 channel tuner on these Thorn sets can be troublesome but on this example it works faultlessly. The cabinet was given a once over with a damp cloth. Even the aluminium brights on the tuner knobs which tend to fall off are all there. It does give a very good account for itself.

I connected my 625 line RF modulator to a HDMI to baseband video/audio out converter and connected this to my computer so I can play Youtube and DVD's through the computer to the television.

This particular model was the first television our family had and it remained for many years and it was eventually dumped with power supply and dried up electrolytic capacitor issues and a black plastic Sony Trinitron on a stand replaced it in the early 90's

Christopher Capener

Heatercathodeshort 18th Oct 2020 6:48 pm

Re: Thorn 1590 set HT procedure
Great news. I have never seen such a fluff and dust covered 1590/91. Always a good sign. John.

Welsh Anorak 19th Oct 2020 9:53 am

Re: Thorn 1590 set HT procedure
I'm sure the's a 1590 under there!
I'd be tempted to replace the HT preset as they had this habit of changing value and upsetting the CRT. It's a bit fiddly, but a better quality pot is a good investment. You might need a good vacuum cleaner, though!
Some did have Mullard CRTs, ISTR.

Hybrid tellies 19th Oct 2020 10:55 am

Re: Thorn 1590 set HT procedure
This is exactly the same as my 1590 with that Orange labelled Taiwanese Mullard CRT which is in very good condition. Mine is nice and complete and gives a a very good account of itself.
But recently the tuner started to play up which is down to those plastic washers on the push button mechanism. There is a fix which uses cup washers to replace the plastic washers I will have to dig out the details which I have stashed away somewhere.

Maarten 19th Oct 2020 6:23 pm

Re: Thorn 1590 set HT procedure

Originally Posted by high_vacuum_house (Post 1300632)
Interestingly this Mullard CRT is made in Taiwan!!

That sounds about right. Taiwan was one of the largest producers of small screen TV sets, so Philips built a picture tube factory to supply their own and other local setmakers. I think their Spanish and one of their Dutch factories also made some small B/W tubes at least until the early 1980's. All production was shifted to Taiwan at some point in time. It was cost effective to send small tubes around the world in bulk packaging.

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