Rather grim Thorn 1400 - Marconiphone 4649

[1100 man]

I could have bought a perfectly respectable example of this set at Harpenden 5 years ago - I think the fact that it said 'good working order' was the reason I didn't: after all, where's the fun in that!?

Instead, I bid on a tatty one in the auction, hoping to get it for a fiver or so, but ended up in a bidding war and had to pay £23 quid!

I got it back home and gave it a quick look over. It had obviously sat somewhere very damp as the plywood bottom had rotted and was very soft. The metal trim on the front was covered in corrosion spots and the VHF tuner knob was broken. The cabinet was tatty and there were a few scratches on the screen. All in all, it looked rather sorry for itself.

Removing the back at least confirmed that there was actually a chassis present! Both tuners were also in residence and apart from the 6F28 video amp valve having a white top, the rest of the chassis looked fairly unmolested from what I could see of it under the grime and damp cobwebs!

I hooked up the tube tester and was pleasantly surprised that the Mazda Goldstar had plenty of emission. Actually, people knock Mazda tubes, but I've never found them to be any worse than Mullard ones - I guess by the mid '60's they had become much more reliable.

I replaced the back, made a mental note to get a 6F28 and poked the set away in a corner and forgot all about it for five years....

....Until last night! I suddenly had an urge to drag it out and do something with it. So I did!

Removing the back showed that three more valves had leaked all their vacuum away and now had white tops. I rifled through my meagre stock of second hand valves and found a PCL85 frame valve, a 30PL1 Audio O/P valve and an EF184 IF valve. The ones I removed all had green pins due to corrosion.

I stripped back the very hard mains lead to reveal completely black conductors - another sure sign that it's been very damp, and connected it up to some mains.

Turning the on/off switch produced a bright flash from the 1.5A fuse and that was the end of that! A quick check with the meter showed that the HT rectifier diode, W11, was short, so a BY133 was fitted in its place. A new fuse and a second power up and the results were less dramatic. The valve heaters started to glow and shortly a loud buzz came from the speaker.

So far so good, Ah, where's that smoke coming from? Looks like R97, 470R feed to the audio stage. Check the circuit and find it's decoupled by a 12mfd capacitor - check the board and oh look: it's a Callins! This was hot to the touch confirming its guilt and was quickly replaced with a 22mfd temporarily.

At least with Thorn sets that use the 'jellypot' LOPT's, you know the line stage is always going to work. Powering up again proved the point and a raster appeared. Applying some UHF signals and giving it a quick tune, produced a quite decent, contrasty picture although with a very distorted frame scan.

Changing the system switch manually to 405 and applying some test card 'C', proved that the signal side was working on 405 as well.

So I am happy with initial progress and so far the tube looks to be pretty good. The next step is to sort out the frame stage. There are plenty of capacitors there which will inevitably be leaky, but none of them are of the notorious 'change on sight' types, so we shall see who the culprits are later on this evening!

I shall add some pictures later on as well once I've had a further play.

All the best
Nick

[1100 man]

Pictures...

[McMurdo]

It's always 'fun' isn't it when something's been that damp! I'm assuming its corrosion at the valve pins that breaks the seal. It does look like it'll give a good pic, I wonder if any more damp related faults will emerge in time. You'll have to make it your daily watcher to see

[Heatercathodeshort]

What a cracking tube? The 1400 is one of the most reliable chassis ever produced. Very unusual to see a failed mains dropper unlike the 1500.
A few caps in the frame stage together with maybe some new presets and it will soldier on for years.
As Kevin mentioned, no LOPT saturation problems, maybe the odd 5 stick EHT tray and away you go.
I have a big liking for Thorn and Ekco TVs. No matter how basket case they appear to be you can always get them working usually at low cost. The original capacitors had a very long life but of course after all these years tend to produce linearity faults. They will probably test OK but are usually leaky or changed capacity. 100% linearity should be easily obtained John.

[Philips210]

Nick's set looks to be a good project and is coming along very well.

I also like the 1400 it's a good reliable chassis that was easy to work on. Thorn usually came up with a design that was easily serviceable and thus well accepted. Their jelly pot LOPTs were great and a shame other manufacturers didn't take a leaf out of Thorn's book.
The only potential safety issue with the 1400 was the position of the line output and efficiency diode valves when the chassis was swung open, a possible zap hazard! I've had one of two zaps from LOPT valve top caps but not from a 1400. ISTR one was a Pye hybrid and the other a Decca Bradford 10 series. They learned their lesson when the 1500 chassis was released with those valves on the opposite side of the chassis.

That Goldstar tube looks healthy. I actually put one in a Philips 300 where the original Mullard tube developed an internal short that couldn't be removed. The GS tube gave a lovely sharp picture and lasted many years.

Regards,
Symon

[1100 man]

Originally Posted by McMurdo

It's always 'fun' isn't it when something's been that damp! I'm assuming its corrosion at the valve pins that breaks the seal. It does look like it'll give a good pic, I wonder if any more damp related faults will emerge in time. You'll have to make it your daily watcher to see

Hi Kevin,
I have a soft spot for TV's at the 'basket case' end of the spectrum - they are always good fun to get going! I'm sure you are right about the corrosion around the valve pins. The PCL85 actually has a crack starting from one of the green crusty pins.

My feeling is that this set had not seen heavy use during its life - all the degradation has happened due to poor storage. All the valves were Mazda ones and the only work that has been done is a new width control has been soldered to the print side of the board. The board itself is in good order with only slight discolouration around the efficiency diode.

Hi John,
There was a 16R 'polo mint' soldered across the mains dropper which was very crusty and gently expired last night. Interestingly, there doesn't seem to be any adjustment on the HT side to cater for different mains voltages? It's specified as 200 - 250V. I assume that as both the line & frame timebases are stabilised, the actual HT voltage is not very critical?

Before diving in and changing caps in the frame stage, I stepped back and thought about it. The scan in the picture is pretty linear - it just stops half way through. I thought it was more likely to be a low emission valve rather than leaky caps. Sure enough another PCL85 restored a perfectly linear scan!

Cheers
Nick

[Hybrid tellies]

This looks very promising especially with the condition of that tube.

We had quite a lot of UHF 625 blackspots around Bath and would use either RBM A640's or the BRC 1400 for our customers living in these areas.

We found the 1400's worked very well on 405 better than they did on 625.

[Colourstar]

Originally Posted by 1100 man

Before diving in and changing caps in the frame stage, I stepped back and thought about it. The scan in the picture is pretty linear - it just stops half way through. I thought it was more likely to be a low emission valve rather than leaky caps. Sure enough another PCL85 restored a perfectly linear scan!

That's a very good point- well worth remembering!

Really good to see one of these revived Nick. I'm pretty sure I have (or had) a brochure somewhere featuring your model, which dates from 1968/9. It would have been one of the first to feature the new 'square screen' 20" crts.

BRC certainly got their value out of those control knobs over the years. I have an 1968 Ultra (1400) and a 1974 Alba (1500) here which both use them!

Steve

[1100 man]

I've been doing a bit more to this set over the last week or so. The board is very sensitive to movement or taps with the blunt end of a screwdriver. Some of this is due to filthy valve pins & bases and some due to the systems switch.

Very noticeable though was the amount of hum distorting the raster. A large band would pass up or down the screen pulling the verticals sideways. The main smoothing block seemed a good place to start. This has two 150mfd and three 100mfd caps in the same can. I removed it from the board and two of its legs fell off!

I would normally, before applying power for the first time, remove the main smoothing cap and reform each section using the variac (the only use I've ever found for it!), rectifier, series resistor and AVO to monitor the leakage current. I do one section per evening, gradually increasing the voltage over several hours. A leakage current of 200 - 400 microamps seems fairly typical for a 100mfd cap running at 300V.

So rather belatedly, as I'd already been running the set for a short while, I went through the reforming process anyway. The end result was quite good with all sections reading close to their stated capacitance on my cheap tester and leakage currents below 400 microamps.

After repairing the failed legs and reinstalling it in the board, the hum problem was no better. I assume it is caused by either mains hum or 50Hz from the frame stage, or a combination of both, getting into the line oscillator/ sync separator causing a change in phase of the line pulses? I will have to investigate further. Does anyone have any good ideas as to what might be causing it?

Another issue is slightly low width, with the control at maximum. I'm guessing, like the 1500, the 1400 had its component values changed during production? The circuit in the 'red books' shows the width control to be 1Meg with 330K in series. My copy of the 'schools' version (which would have been a large screen), shows the control as a 2.2Meg with 680K in series. Does anyone have the correct information?

I'll have another play with it tonight and see what I can find.

Cheers
Nick

[SunSPARC]

C52 12uF (4uF in early sets) which decouples HT4 often gets overlooked. It lurks down near the 30PL1 in the sound section. It will probably be a Callins horror.

2.2M + 680K was the later recommendation, I believe.

[Hybrid tellies]

Quite a challenging project you have got here.
On 1400's and early 1500's there was a must do modification on the width chain to prevent a fire.
The width control changed to 2.2Meg with the feed resistor increased to 680K.
many years ago I had a really good 1400 go up in flames because of this.

[1100 man]

Originally Posted by SunSPARC

C52 12uF (4uF in early sets) which decouples HT4 often gets overlooked. It lurks down near the 30PL1 in the sound section. It will probably be a Callins horror.

Ah ha! SunSparc, you are spot on - well done! I looked more closely at the circuit last night and discovered they had put some effort into keeping 50Hz out of the line oscillator & sync DC amp valve. It's got its own HT supply, HT4 which is decoupled by C52. It was a 4mfd in my set and a rather odd orange TCC made thing with a green plastic base. I've not seen one of those before. Replacing that with a 4.7mfd got rid of almost all of the ripple on the scan. A 22mfd made things even better!

On 1400's and early 1500's there was a must do modification on the width chain to prevent a fire.
The width control changed to 2.2Meg with the feed resistor increased to 680K.
many years ago I had a really good 1400 go up in flames because of this.

That's interesting, Simon. I've often wondered why the original 1 meg with 330K in series would cause any problems. After all, even though it's fed from the boost HT rail of 700V give or take, the current through the resistor & preset would only be 0.526ma. That equates to a power of 0.23W in the preset & 0.091W in the 330K resistor, assuming I've done that right!

The only thing I can think of is that presets & pots in general almost always seem to go low in value, whereas fixed resistors always go high. I wonder if the 1 meg preset gradually went lower in value causing the current through it to increase?

Anyway, my set had a 330K fitted, so I assume originally it would have had a 1 meg pot. Someone in the past had removed the 2.2 meg 'local distant' preset and fitted this in place of the width control - quite inventive I thought! That 2.2 meg has gone down to 1.5 meg!

Hopefully I can find a 2.2 meg that actually measures 2.2 meg ONO!

Thanks for the replies,
Cheers
Nick

[Maarten]

Could it be a problem with the rated working voltage of the pot and/or the resistor?

Also, the rated dissipation of the pot might still be exceeded dependent on its setting as it actually varies with the setting.

Both fixed and variable resistors do have a SOA just as semiconductors.

[1100 man]

Originally Posted by Maarten

Could it be a problem with the rated working voltage of the pot and/or the resistor?

Also, the rated dissipation of the pot might still be exceeded dependent on its setting as it actually varies with the setting.

Both fixed and variable resistors do have a SOA just as semiconductors.

Hi Maarten,

Whatever the failure mode is, it obviously isn't one that the designers foresaw. They would have looked at the component ratings supplied by the manufacturer and been happy that the 1 meg pot and 330K resistor were well within their ratings when new. What they would not have known was how the parameters of each component would change once they had been in service for a while. I wonder how long after manufacture the problem took to appear?

While I was investigating the not-quite-enough-width problem, I found that the screen voltage on the PL504 line O/P valve was rather low at 160V. This is fed from the inevitable 2K2 wirewound resistor from the 225V HT rail. The HT rail was about right, so something was drawing too much current. The screen grid supply is decoupled by C115, another orange TCC 1MFD thing. I replaced this and the measured voltage actually went down to 125V!

I dug out another PL504 and switched on again. I was greeted by much hissing from the tripler & eht cap and a superwide picture. I grabbed a screwdriver and wound the width pot down and all became calm again. With the width set correctly, the screen voltage was now a much more respectable 207V. So the poor old Mazda PL504, which may have been the original one, as all the other valves are Mazda, has come to the end of its life.

So things are getting better but the picture is still very 'twitchy'. The frame scan keeps contracting in the middle and if you give the frame O/P transformer a whack, it either corrects it or the scan collapses to a line momentarily. There are no obvious dry joints on the transformer or anywhere else, but I resoldered it anyway without much hope of any improvement. Sure enough there wasn't any!

I think I'll remove the transformer and examine its leadout wires where they solder to the pins. Also on the list is a good clean of the valve pins & bases and the system switch - all of which seem sensitive to being tapped.

Also the sound has yet to put in an appearance - all I've got so far is a hum from the speaker and an almighty crackling if I wiggle the 30PL1 sound O/P valve - I've not investigated this yet.

Still, it's all good fun!
Cheers
Nick

[Andy - G8MNM]

Having read this thread with interest , I dragged out my Ferguson 3645 14" transportable from its 7-8 year resting place, still wrapped up in bubble wrap from when we moved to France 7 years ago!

I had previously replaced quite a few out of spec resistors, a large brown Hunts capacitor and a decoupling cap C53 4uF - which had been causing a horizontal ripple on the display.

I have now replaced the 330K resistor with a 680 K (I had previously just replaced it with another 330K. I will have to find a replacement width pot though.

At the time I didn't know about the dreaded Callins caps and so didn't replace them on sight.

Fortunately alerted by this post I have just replaced all 3? Callins caps. The most important one being C81. I tested it on my ESR meter and it read 18 Ohm!

Had I powered up the set then bang and smoke!

I also noticed that the mains fuse had been replaced by a mains 3 Amp! Swapped it out for the correct 1.5 Amp.

Just wondering about the 'Blue & White' capacitors? Are they suspect too?

Thanks again for the info on the Callins!

Andy

[Welsh Anorak]

In a word - yes!

[Andy - G8MNM]

Thanks Glyn!

[dazzlevision]

Originally Posted by Andy - G8MNM

Just wondering about the 'Blue & White' capacitors? Are they suspect too?
Andy

Take a look at this thread: https://www.vintage-radio.net/forum/...ad.php?t=98964

If you mean the third capacitor from the top in the RHS photo, then yes indeed. They were made by Dubilier UK and also sold (rebranded) by Radiospares/RS Components (originally in the Radiospares/RS grey encapsulation).

During their long production period, they could have grey, white, blue and yellow or blue and white plastic encapsulation.

[Neil Purling]

Had one of those in a Thorn 8500 CTV explode on me. The shock of the 'bang' was so great both feet literally left the ground.

[1100 man]

Originally Posted by Andy - G8MNM

Having read this thread with interest , I dragged out my Ferguson 3645 14" transportable from its 7-8 year resting place, still wrapped up in bubble wrap from when we moved to France 7 years ago!

Just wondering about the 'Blue & White' capacitors? Are they suspect too?

Hi Andy,
I'm surprised Thorn used the 1400 in a 14" set, after all it's a pretty huge board and not really suited for small screen use. Presumably, the board is still mounted vertically as it would be in a larger screen model?

I've not actually had too much trouble with the 'blue /white' range of capacitors over the years. There are a whole bunch of blue/yellow ones in my set which so far have not caused any problems. Of course they could all just be totally open circuit....

A valve TV is really not a pleasant place to be for an electrolytic capacitor - there is just way too much heat to dry them out. C115 (1mfd in my set) decouples the screen grid supply for the PL504. It's mounted right below the valve socket so didn't stand a chance!

To be fair to all the British component manufacturers of the era, mass producing components down to a cost and achieving a certain level of reliability was a very difficult challenge. If money was no object, I'm sure they could have made excellent capacitors!

It must have been a nightmare manufacturing complete TV sets. Selecting each component that would do the job for the lowest cost and not knowing how well that component would stand up during the first year of the set's life...

Anyway, I've discovered why there is no sound. Firstly, the system switch was not making good contact, so the sound detector was not actually connected to the output valve. It is also the case that you can have sound or you can have picture, but not both at the same time!

The alignment is miles out. If you turn the fine tuner until the picture completely disappears, you tune in the sound. It's no better on 625. Whether someone has had a twiddle, or whether it's due to the damp storage, I don't know, but it's another thing on the 'to do' list.

Cheers
Nick