As even a cursory glance at ebay will surely tell you, these iconic retro vintage mid-century portables are now worth easily in excess of £100..... ahem . Nope, I have no clue as to how the humble black & white bedroom and caravan portable has elevated itself from can't-give-'em-away-at-a-car-boot-sale to megabucks status seemingly overnight.
Anyway regardless of that nonsense, I inherited this grubby example when I moved house. There it was, along with several music centres and a rather nice Dansette, so I'm not complaining! It's pretty obviously lain dormant for years. The inside was absolutely full of dust which had to be vacuumed out to even see the pcb. Powering up revealed a full scan but just a blank raster with no snow but some of the expected hiss on sound, albeit somewhat subdued. Touching an aerial lead to the socket produces a mush on sound regardless of rotating the tuner but with nothing at all on screen. Waggling the contrast slider does produce lots of streaking consistent with a dirty control, so it would seem the video stage is probably working.
This is (I believe) the 1691 chassis so given the love for all things Thorn round these parts, this seems the perfect place to try and revive it.
The front end of the set is:
VT1 & VT2 (Both BF198) Gain controlled IF amp
VT3 & VT4 (Both BF198) IF amp
VT5 (BF254) Video Driver
VT6 (BC238) AGC amp
VT7 (BF391) Video output
A quick voltage check seems to show problems with VT1 and VT2. Actual readings are in brackets.
VT1 e 1.9v (0v) b 3.7v (0.5v) c 9.1v (10.6v)
VT2 e 1.9v (0v) b 3.1v (0.5v) c 9.5v (10.6v)
I'm still getting my head around transistors and their operation, but I guess that low (or zero) emitter and base readings plus a high collector reading must point a finger somewhere.
On the next stage of amplification VT3 looks to be rather more healthy. VT4 is buried under some metal screening, so I've not accessed that yet to check voltages.
VT3 e (to chassis) b 0.6v (0.7v) c 6.2v (5.0v)
I guess on a set of this vintage it's likely to be full of dying capacitors and dying (or dead) lockfit transistors so I'd be grateful for any guidance as to to how best to proceed...
[..] given the love for all things Thorn round these parts
Indeed and there's an entire blog section dedicated to Thorn, check out the revamped new blog boxes on the homepage, should make finding things a lot easier. https://www.radios-tv.co.uk
By the way the Thorn 1691 chassis data is in the library complete with circuit description, schematics and wave-forms. You have access.
I hope you don't mind but I deleted your last cct photo as it was a bit blurred and your readings and expected ones, a little hard to see. I replaced it with a much more clear snippet and put your readings in red. This should help folk who want to help/play along with this repair.
I'm still getting my head around transistors and their operation, but I guess that low (or zero) emitter and base readings plus a high collector reading must point a finger somewhere.
Perhaps this might help with getting your head around transistors. Not my intention to teach you to suck eggs, apologies in advance. Some transistor theory might help you to look at your readings and get a feel for what's going on.
A transistor acts as a switch, main current flows from collector to emitter and the signal current flows from base to emitter. The signal current at the base is what controls the switch. So a current flowing from the base to emitter is what opens the switch, allowing current to flow from the collector to emitter.
To have an NPN transistor turned on to allow current to flow between the collector and emitter, you must have at least .7v applied between the base and emitter, otherwise it's off.
PNP transistors are different, instead of the minimum .7V on the base, they require a minimum difference of .7V between the collector emitter voltage and the base emitter voltage. So a difference of less than .7V the transistor is off, a difference of more than .7V and the transistor is on.
Now with that in mind perhaps the readings on your NPN's may speak to you more clearly and perhaps give you clues as to where to start looking and testing, more covered here. ?
Hi Steve, If you find yourself in need of any bits or bobs (complete sets for parts ? ) then I have a couple of tvs here you can have free if you want them.
If neither VT1 and 2 have base emitter shorts I would check the AGC line, it is biased from what I can tell by a voltage from W11, but there are quite a few components from that point to the bases of the two transistors.
The BF197 VT4and 5 in other sets were prone to failing, voltage checks would confirm, not seen this set before so advice is general.
Hi Steve, all good advice above and in most cases it will be a BF197 which is the cause of the problem. I usually measure across the transistor, +VE to base and -VE to emitter, if it’s between 0.5v and 0.7v I would be looking elsewhere. If your set has BF198s fitted then the BF197s may be a BF199.
Frank is looking where I'm looking - just as a quick check, first check the potential at the junction of R30 and R33, and then see if the same voltage appears at C34 on the AGC line. If C34 is a tant...
I remember seeing many of these tellies in classrooms when I was at primary and secondary school in the '70s. Some of my friends with wealthier parents than mine had them as bedroom sets too.
Well goodness me, thank you for all the replies and interest in this little set!
Thanks Chris for going to the trouble of tidying up the relevant part of the diagram and for the transistor guidance. Much appreciated and yes it's good to dip a toe back in the waters of VRAT once again. I'm still finding my way around all the nooks and crannies of the site. It's a bit like wandering round a town and discovering interesting side streets. So much useful info here. Most of my time is divided between work, renovating the house I bought last year and caring for aged parents, so what with all that and a bit of a loss of mojo (which we all seem to go through in this hobby) tellies seem to have gone by the wayside. Having been furloughed from work back in April I should have more time on my hands, and yet....
Anyway back to the patient and I have to correct myself- VT1 and 2 are BF196 transistors, not BF198. Blame my eyesight. Makes me a bit of a 'BF' too.
I checked the voltage at the R30 and R33 junction- it's just over 1.5v. The voltage at C34 is 0.5v. And yes it is a blue tant, as in 'beware of'.
Incidentally out of interest the voltages around VT6 AGC (BC238) check out pretty much OK.
Here you go, the Fergy 3847 as it appeared in the spring edition of the 1981 Argos catalogue. Item No.5 £75.99.
I clearly remember leafing through this catalogue, it was shortly after I'd turned 18 and bought my first TV for my bedroom. I opted for the one below it, item No.7 the Fidelity, I preferred the styling and it was £12 cheaper at £62.99.
Incidentally out of interest the voltages around VT6 AGC (BC238) check out pretty much OK.
e (to chassis) b 0.6v (0.7v) c 0.07v (0.05v)
Yes, that'd be correct. The way it's set up is what's known as "forward AGC" - the initial bias for the IF transistors comes from R30/R31/R33 and if VT6 is turned off, its collector voltage will rise and eventually the collector potential will cause W10 to conduct and drive the bases of VT1/2 more positive.
Patchett volume three has a section on transistor IF amplifier AGC in TV sets and describes the reason for the use of forward bias. Worth the read, not long, a couple of pages. Available in the Data Library page 27.
The transistor design results in its gain dropping for increased collector current, there is more in the article about why it’s required i.e. to reduce distortion that would occur with reverse bias with low impedance circuits etc.
There will be changes in input resistance and capacitance which can damp the tuned circuits but the circuit should reduce this to a minimum.
I think I have the basics correct.
Save posting a photo of the page the book is worth download, in fact all four books in the series are worth downloading.
Thanks, Frank - I'll have a look but I'll also refresh my memory tonight as I have a copy of Amos by my bedside: to wit, Principles of Transistor Circuits.
By coincidence, it reminded me of the 1939 Murphy radio models that you had to provide with a fixed potential on the AGC/AVC line when aligning them as they were engineered in such a fashion that the response of the IF amplifier broadened on strong signals, giving automatic selectivity.
I’ve got a TV22 cabinet with one of these portables fitted in it! Last time I tried it out it had the symptoms you describe, never bothered looking into it though, the set just languished in a cupboard as the biggest disappointment I ever bought! One day I might revamp it, maybe add some direct video inputs and hide a freeview tuner in it.
Here you go, the Fergy 3847 as it appeared in the spring edition of the 1981 Argos catalogue. Item No.5 £75.99.
I clearly remember leafing through this catalogue, it was shortly after I'd turned 18 and bought my first TV for my bedroom. I opted for the one below it, item No.7 the Fidelity, I preferred the styling and it was £12 cheaper at £62.99.
Mmm that Argos page is very evocative and the range of portables on offer is still so familiar to me. A veritable classic line-up! As a keen 11 year old I'd have been analysing every set on there at the time. The Fidelity was and is a good looking model, also available in remote form too- one of the very few b&w portables with that option- the other being the Pye/Philips at the top of the page. I remember I also liked the look of the Sonatel too.
The Ferguson 3847 could be had in black with smart woodgrain trim and was also marketed as an Ultra, the Marconi and HMV brands having fallen by the wayside at this point. You could even buy it wearing a 'Boots Audio' badge in the days when they had electrical departments in larger stores. If you wanted a Boots-branded 12" set you had the choice of the Ferguson's baby brother or- a left-field choice - the Bush Rambler 3.
The Fidelity was and is a good looking model, also available in remote form too- one of the very few b&w portables with that option
Hi Steve,
I'm glad you mentioned that as I was convinced my one had a remote. I remembered it was a slim affair like the one at the top of the page. However after looking in the catalogue I assumed I must have been mistaken as it was not mentioned, only the non remote. After your post I then went and checked the winter catalogue for 1981 and there it was. So I wasn't mistaken or going mad after all. ?
Browsing all the catalogues makes me wish I could just pop down the local Argos click-N-collect. Anyone else yearn for all the old products displayed among the pages of the old catalogues.
That's the fella Chris! I'm sure I used to have a full range Fidelity brochure with that set in it. I'll have to see if I can find it. Meanwhile for your delectation here is part of the rather stylish Boots tv brochure from 1981- a little dog eared as I picked this up in-store when it was current! This shows the mysterious 'Boots TV12' which I only discovered last year is in fact a Bush Ranger 3 in disguise.
I'm also quite nostalgic when it comes to those old Argos catalogues. Luckily, I seem to have acquired several of those portable tellies over the years, or at least variants of them. I have the Philips version of the Pye portable with ultrasonic remote control, an unusual HMV branded version of the Fidelity portable with infrared remote control and the Sonatel MR-T750. One of my co-workers was a former engineer at Sonatel in London; he told me a bit about his time there, repairing the T750 televisions.
I also have a couple of the Thorn 1691 14" sets, the main subject of this discussion. One has a now yellowish white cabinet, the other is in a black plastic cabinet with a printed woodgrain effect. Both of them worked when last tried, so it doesn't seem like a lot goes wrong with them, though I do remember some of them being used on the C&G 224 course at technical college in the mid-80s. The lecturers would put faults on them for the students to find.
First of all, Steve mentioned a lack of 'snow' on the screen but noise appeared on the picture when the contrast control was moved. The brightness and contrast sliders are notoriously flaky. I would clean these first and check the resistance of the control is varying properly. Next step, the tuner voltage. Check the 33v is present at IC1-R42, then check the voltage at pin 5 on the tuner module. As the tuning knob is rotated, it should change smoothly from 0v at one end to somewhere between 16-33v at the other. If not, try cleaning the tuning pot.
I would check the above things first before going further.
From the observed voltages on VT1 and VT2 it looks like they are simply turned off rather than faulty. Whatever connects to the TP3 line seems to be turning them off. I don't have the rest of the circuit to advise any further. I'm also not sure if the expected voltages are measured with or without a signal applied - this would also make a difference.
