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Old 20th Feb 2021, 1:31 pm   #1
technotel
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Default Thorn 1690 series. model 3845 12" B/W

Hi all
I picked up a mint clean looking 12" Thorn model 3845 B/W portable. Fully working condition. Some height/linearity adj needed and very slight and barely visible hum bar.
Can anyone give me any tips on what stock faults these sets might have and what components I should replace as a precaution? Its never been worked on looking at the amount of dust inside.
I heard that the LOPT's in these are a failure ??
Thanks guys
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Old 20th Feb 2021, 1:57 pm   #2
ortek_service
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Default Re: Thorn 1690 series. model 3845 12" B/W

I recall fixing a few of these back in the 80's.
And it was often the PSU in overload, due to a short in the line-output stage
- Unfortunately, it was usually the LOPT, which I guess would now be very difficult to obtain. But could be some NOS pattern parts somewhere.

Someone has commented about these, amongst posts here:
https://www.vintage-radio.net/forum/...d.php?t=130782

And I guess it was the potted EHT-side that most-likely failed, otherwise there may be a chance of rewinding the primary-side, that's not potted.
(I did once do that on an old Redifusion, where I had to fill the hole in the bobbin where it had arced through to the ferrite core)
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Old 20th Feb 2021, 3:08 pm   #3
technotel
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Default Re: Thorn 1690 series. model 3845 12" B/W

Quote:
Originally Posted by ortek_service View Post
I recall fixing a few of these back in the 80's.
And it was often the PSU in overload, due to a short in the line-output stage
- Unfortunately, it was usually the LOPT, which I guess would now be very difficult to obtain. But could be some NOS pattern parts somewhere.

Someone has commented about these, amongst posts here:
https://www.vintage-radio.net/forum/...d.php?t=130782

And I guess it was the potted EHT-side that most-likely failed, otherwise there may be a chance of rewinding the primary-side, that's not potted.
(I did once do that on an old Redifusion, where I had to fill the hole in the bobbin where it had arced through to the ferrite core)
Many thanks ortek_services. Yes I knew id heard somewhere of these being a weak spot on this chassis. I will try finding a nos just in case it ever fails? Thanks again
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Old 20th Feb 2021, 7:38 pm   #4
neil29
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Default Re: Thorn 1690 series. model 3845 12" B/W

I've got a couple of tatty sets here if you ever wanted any parts for it.

Cheers
Neil.
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Old 20th Feb 2021, 10:17 pm   #5
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Arrow Re: Thorn 1690 series. model 3845 12" B/W

Approx. during 1980/85, I had three of these sets come my way that needed repairs. They all had same fault: loudspeaker failure.

Al.
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Old 21st Feb 2021, 1:03 pm   #6
technotel
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Default Re: Thorn 1690 series. model 3845 12" B/W

Quote:
Originally Posted by neil29 View Post
I've got a couple of tatty sets here if you ever wanted any parts for it.

Cheers
Neil.
Hi Neil
I would be very interested in these tatty sets for spares. And I note another comment here about the speakers. Mines buzzing, deffo distorting at the lower frequencies.
all the best Terry
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Old 21st Feb 2021, 1:06 pm   #7
technotel
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Default Re: Thorn 1690 series. model 3845 12" B/W

Quote:
Originally Posted by technotel View Post
Quote:
Originally Posted by neil29 View Post
I've got a couple of tatty sets here if you ever wanted any parts for it.

Cheers
Neil.
Hi Neil
I would be very interested in these tatty sets for spares. And I note another comment here about the speakers. Mines buzzing, deffo distorting at the lower frequencies.
all the best Terry
I will upload some pics when I put it back together. I stripped it because it stank of cigs. Cabinet front and back been thoughly washed. Just waiting for it to dry.
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Old 22nd Feb 2021, 4:10 pm   #8
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Default Re: Thorn 1690 series. model 3845 12" B/W

Back in the day we used to get replacement anode caps that had an EHT rectifier in them. The theory was that the potted diode would have shorted and even if it recovered it wouldn't cause too much of an EHT increase. Not ideal, of course, but it kept hundreds of these nice sets running for reasonable money.
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Old 23rd Feb 2021, 2:30 pm   #9
technotel
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Default Re: Thorn 1690 series. model 3845 12" B/W

Quote:
Originally Posted by technotel View Post
Quote:
Originally Posted by technotel View Post
Quote:
Originally Posted by neil29 View Post
I've got a couple of tatty sets here if you ever wanted any parts for it.

Cheers
Neil.
Hi Neil
I would be very interested in these tatty sets for spares. And I note another comment here about the speakers. Mines buzzing, deffo distorting at the lower frequencies.
all the best Terry
I will upload some pics when I put it back together. I stripped it because it stank of cigs. Cabinet front and back been thoughly washed. Just waiting for it to dry.
As promised. A few pics of this very clean set. Only had to change a 10meg resistor that had gone high (23meg) to improve focus. emptied guts and removed tube to give cabinet a good wash of cigs stink. Lovely set. speaker slightly buzzy but I can live with that until another comes along.
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Old 23rd Feb 2021, 2:37 pm   #10
technotel
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Default Re: Thorn 1690 series. model 3845 12" B/W

Quote:
Originally Posted by technotel View Post
Quote:
Originally Posted by technotel View Post
Quote:
Originally Posted by technotel View Post
Hi Neil
I would be very interested in these tatty sets for spares. And I note another comment here about the speakers. Mines buzzing, deffo distorting at the lower frequencies.
all the best Terry
I will upload some pics when I put it back together. I stripped it because it stank of cigs. Cabinet front and back been thoughly washed. Just waiting for it to dry.
As promised. A few pics of this very clean set. Only had to change a 10meg resistor that had gone high (23meg) to improve focus. emptied guts and removed tube to give cabinet a good wash of cigs stink. Lovely set. speaker slightly buzzy but I can live with that until another comes along.
Sorry I forgot to add images
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Old 23rd Feb 2021, 8:53 pm   #11
Lloyd 1985
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Default Re: Thorn 1690 series. model 3845 12" B/W

Ah, one of those sets! I have one, the LOPT failed, well, the rectifier diode potted in it did, so I decided to have a go at fixing it, took a hammer and chisel to the EHT part of the LOPT, managed to liberate the faulty diode and find the end of the over wind, and replaced the diode with one from a colour set, re-potted it all and it lives again!

I did experiment with fitting a valve EHT rectifier, which also worked, but it was rather lively, fizzing and arcing a fair bit. All good fun though...

Regards
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Old 23rd Feb 2021, 11:33 pm   #12
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Default Re: Thorn 1690 series. model 3845 12" B/W

We have the Ultra-badged version bought new from the cash n carry, went through a period of 1/4w resistors going open in random areas, possibly as it was in a kitchen and exposed to cooking fat/humidity.
The usual symptom of the eht rectifier failing is a loud hum from the speaker and no picture as it tries to pull the supply down, though ours hasn't succumbed.
Lovely neat little sets I always thought.

70 in 1978
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Old 24th Feb 2021, 10:10 am   #13
Lloyd 1985
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Default Re: Thorn 1690 series. model 3845 12" B/W

I remember the failure of mine, it started with what looked like bad smoothing causing a hum bar on the picture, then eventually no picture at all and a bit of smoke from the LOPT itself. There was a noticeable lump on the side of the LOPT where the diode was, and you could draw an arc through the plastic casting where it was. Never thought I could get hold of a new LOPT so decided the hammer was the way to go!

Regards
Lloyd
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Old 24th Feb 2021, 5:15 pm   #14
technotel
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Default Re: Thorn 1690 series. model 3845 12" B/W

Thanks for the stories guys.
Lloyd. What diode did you use? Would a BY184? Its something I would attempt if needs be? Get the old Dremel out to cut it open. And the slight hum bar I have (barely visible) now concerns me after your ref to it.
cheers all
Terry
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Old 24th Feb 2021, 6:36 pm   #15
Lloyd 1985
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Default Re: Thorn 1690 series. model 3845 12" B/W

Sadly I've no idea what the replacement diode actually was, it was liberated from another potted LOPT from a small 10" colour TV which had burned one of it's windings. I can tell you what the valve was that I experimented with though! It was a DY86!

I wouldn't worry about the barely visible hum bar, the hum on the picture when my set failed was really bad, the picture was dim and had a distinct 'S' shaped verticals.

I've just found a thread over on VRAT that I wrote about the set! Extreme TV Bodgery First picture shows the hum on the picture.

Regards,
Lloyd
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Old 25th Feb 2021, 9:15 am   #16
Welsh Anorak
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Default Re: Thorn 1690 series. model 3845 12" B/W

As the diodes are so small you could do as we used to and install it in an oversized anode cap. Saves getting the hammer out!
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