BRC/Thorn 8000 Chassis
Model: Ferguson 3712
Original List Price : £182.90
CRT: A44 271X
The Actual Set
I’ve been after one of these early single standard sets for some time they were quite a milestone as they were the first sub £200 colour television to be available in the UK. This enabled a vast number of folk to experience for the first time, affordable colour television.
I put a request out in the forums wanted section looking for a Fergy 3712 or Marconiphone 4712. I was happy to take on anything and that included basket cases. To my delight forum member BRC 3000 (Dave) came up trumps, with a basket case, thanks Dave.
The set has a number of challenges here are just a few, there’s bound to be more discovered as time passes :-
- Missing its back cover
- Missing its timebase
- Missing PSU and sound module
- CRT PCB cracked in half. One of those halves is also badly bent and almost in half again
- Chassis is badly corroded and components badly corroded.
- CRT is low
- Control knobs missing
- Thorn tuner will not latch on any channel (usual washer failure)
- Tuner mounting to cabinet, one mount point thread stripped
- Aerial input socket severely corroded
- Cut-out severely corroded
- C714/C715 smoothing can badly leaked electrolyte
- Dropper severely corroded
- Purity rings corroded
- Many pots and pre-sets are heavily on various parts of the modules are corroded and in some cases seized
- Signals panel mounting pivot broken loose
- Front CRT fascia has been painted matt black as well as half the tuner control panel. This should be silver.
Although the list looks daunting, all should prove to be fixable. The CRT PCB is a bit of a worry but I’ve stitched together a busted 3ooo line-timebase so chance to practice those skills now on the 8000 CRT PCB base.
The CRT has been left cooking on the B&K 467, after an hour it has shown no signs of improvement. This is the only worry, looks like it will need a bop Still at present that’s the least of the problems the set does seem to have been stored in damp conditions during its life. Certainly parts of the chassis seem to have been in water contact, there’s even a tide mark on the signals module. This set will likely present many opportunities for component replacement.
Unfortunately this set is non starter at present until I can find a missing PSU/sound module. The CRT PCB I may be able to repair but it would always be a worry due to the extreme damage. Assuming I can get it running then there will be the cabinet front to tackle, it will need to be resprayed to its original silver finish. Only time will tell how this one develops so do check back as I tackle this poor old Fergy 3712 but for now that’s it I’m afraid.
After 2 hours no improvement so I ran a clean & balance on all three guns. Red and Green responded well, blue however needed a couple of goes before life was seen. Now the CRT reads good on all three guns.
Time to remove the messy chassis and see what we’re dealing with.
Due to the many problems but mainly the missing PSU/Snd sadly here ends the repair……well for now.
CRT PCB Repair
Joined the parts together, just need to stitch the tracks now.
And the final track stitch
Whilst waiting to figure out the plan of action for the missing power supply, I turned my attention to the very sad cabinet. In the past some bright spark had taken to daubing the front fascia in black paint, this would be returned to silver. I would also need to search my stock of badges to see if I can recreate the Ferguson logo and text.
Here are the results of an afternoons work, its not perfect then neither is the TV but…. Its way better than before.
Chassis Rewired from 8000 to 8000A:
The reason for this modification is to address the missing PSU/Sound Module. I don’t have an 8000 PSU in stock but do have an 8500. The following information provided by an observant Jayceebee (John).
Below are the modifications to convert the wiring harness to an 8000A and enable the 8500 psu to be used, no modifications are required to the actual board. None of this is my work Chris, it’s all documented in the manual, I came across it by accident.
1. Trace the red/orange cable from 11/5 on the timebase board, it should go to 10/1 on the psu. Remove it from 10/1 and connect to the lower connection of the new R729 1k section of the dropper
2. Add a length of wire from the top connection of the 56ohm section on the dropper (new R727) and connect to the junction of L406/C444
3. Disconnect the lead from 11/1 which goes to 10/8 and connect to 10/1
4. Add a link from the top connection of R709 to the junction of the 56ohm and 1k (new R727/729) sections of the dropper.
An inspection of the original 8000 dropper showed the R709 section had been smoking the weed! its way high 😆
Luckily I have a new old stock 8000A dropper in stock. Interestingly the 12R section on this one also has a 6R tap. I’ll get on with the rewiring a little later and the chassis repair work will continue in this repair blog.
Work Begins On The Chassis
And now the rewire to 8000A spec to allow the 8500 PSU ( centre of chassis) to be used.
Chassis Rewire complete: Dropper connections
At last a Power Supply and Sound Output Module Installed
Next steps….. replace the leaked multi-can electrolytic. Then due to the original open circuit 47R dropper section, check the line output transistor and tuning capacitor as one or both of these may be fried. Not forgetting yet another Thorn tuner repair due to washer rot, still this will be the 6th or 7th one I’ve done now so I’m getting quick at them now.
C714/C715 2500uF 63V 500mA ripple electrolytic replacement
Do you think I should try and reform the original……. Nah I have a new one in stock, though that one I will reform so as not to stress it after years of being dormant.
New electrolytic being reformed to rated voltage on the bridge.
New electrolytic reformed and installed
With the PSU/Sound module installed, wiring harness cleaned and re-strapped where sticky tape had failed, Its starting to look a much more presentable chassis now.
The final tasks before initial power application are checks on the line transistor and remove the green meanie. Last but not least fix that seized Thorn tuner. Hopefully power by the weekend.
Having fixed the Ferguson 3734 9600 the main bench has now been freed up for the 3712.
Tackle The Tuner; BRC ‘Washer Rot’
Frequent visitors to my repair blogs or forum threads, will know about the dreaded BRC/Thorn tuner washer rot. I’ve tackled countless now and the procedure is well documented but if you’re new here and have stumbled upon the blog, then the procedure is detailed here.
The tuner in this Ferguson 3712 has the problem, therfore If I want to be able to progress this set and attempt signal acquisition, the tuner needs to be repaired.
And then it was repaired
The Fergy was missing one bright from one of the tuning buttons and missing all its control knobs. No problem I have a stock of vintage CTV knobs, here is just a small sample of some of the stock, there are a few more larger draws to call into action if required.
CRT PCB broken component replacement
Final Task- prior to power up
Line output transistor and Flyback tuning cap checks
VT401 is a BU105/02 and tested OK.
The flyback tuning capacitor C406 tested OK but these green caps although looking innocent, don’t age well. They tend to start to bubble away shortly after application of power. You have to remember the tuning cap is subjected to the high frequency switching and pulses of the line output stage. If this cap fails you can end up with sky high EHT and the consequences of that are not good.
The cap value is 1N5 @1500V. Typically I have all sorts of flyback caps in stock but sods law not these at that voltage. I’ve ordered a whole load so the first application of power may have to wait for them to arrive. I might go for power up with the drive to VT401 disconnected and see how the rails fair, we shall see……
The chassis and cabinet after some TLC
The chassis :
- De gunk chassis of filth
- Fit missing timebase
- Repair broken CRT PCB
- Fit missing power supply & sound module
- Replace Dropper
- Rewire harness from 8000 to 8000A so as to accept 8500 PSU
- Repair seized tuner and replace knob bright
- Replace C714/C715 and reform
- Reform C704 C706
- Repair CRT dag
- Repair CRT dag straps
- Clean & Balance Red, Green & blue CRT cathodes
- Fit 5 missing control knobs
- Clean & de-gunk
- Return fascia to silver by respraying
- Clean and remove black paint from control fascia
- Fit Ferguson colour logo and model logo
- Clean, touch up cabinet and polish
The fun begins as it will be power application, then the real work begins….. component level diagnosing the no doubt many faults that exist in the electronics. If the other poorly stored sets I’ve worked on are anything to go by, there should be some interesting times ahead.
Remember to check back soon…….
Power Application Day
Apologies for the lack of up date, I took delivery of a boat load of vintage colour tellies and got very side tracked. The new flyback caps arrived and one was duly fitted.
I then went over the whole chassis checking and double checking all my work and that everything was connected as it should be. I left the drive disconnected from VT401 and added a 75W dummy load across C704 and also had R725 preset dialled at minimum. With a meter monitoring the 25V rail I applied power.
There was a crackle from the speaker, the dummy load was alight and the 25V rail is present. left it running, no smoke, nothing getting hot and bothered. Its ALIVE!!!!!
Next I’ll get the scope out and check the various sections of the circuit.
Field Oscillator output and input sawtooth
Line oscillator is running if a little off frequency
VT401 connected, we have light.
R725 is still set to minimum so collector peak volts not set. EHT starts at 15kV and drops to around 10kV then goes up a little and down again. Not certain as to why its falling away like this, could be a doubler problem or perhaps due to the CPV not set.
There’s also no snow on the raster, so there’s an I.F. fault too.
The no I.F was due to an earth strap on the signals board coming loose. Now I have a very faint TCF. The raster is only getting around 9-10kV it drop and grows with the adjustment of brightness and contrast which seem to have little effect on the picture. I think I’m going to replace the rectifier tray.
I set VT401 CPV to 1280V.
The EHT starts off at about 20kV, the raster is quite a good grey defined TCF. Its occasionally spitting from the anode cap ( glad a shrouded with rubber), this maybe due to the in-line EHT meter not allowing the anode cap to set down. It drops thereafter and sits at around 14-15kV with the bluey/purple large raster you see below.
The other problem is plumes of smoke pouring off the dropper, that does not seem right.
I’ve established the original rectifier tray is not so good as the EHT would drop back to about 9kV. With a replacement the EHT behaves in the same manner EHT starts off at 20kV with a defined reasonable black and white TCF. In just under a minute the EHT starts to drop back but now sits at a better 14kV.
I don’t seem to have much brightness or contrast control either, one could almost say excessive brightness.
There’s no colour
Picture goes purplish
No sound although cracks and pops in speaker if vol control waggled
The dropper has plumes of smoke coming off it there’s some serious heat over on that side, though a voltage check would seem to show expected readings so it must be damp and dust, if not these sets in their small cabinets must have cooked!
I need to pause and think, my initial worry is the LOPT but with the condition of the set prior to this, coupled with it receiving donor boards, there could be a whole bag of other things going on. If you Switch off and back on again the raster is reasonable then degrades, this is repeatable.
Sequence : Start at 20kV dropping to 14kV, No brightness or contrast control.
Thanks to John (jayceebee) for suggesting a trace fault on my jigsaw repaired PCB, it was indeed the case of losing the bottom end voltage to the A1 pots. I check revealed a new hairline trace break that feeds the 170V. It was had to spot as it was right where the 645A screen print lettering was.
A good place to end the day
After some further work this morning a quick and dirty twiddle, things are starting too look a whole lot better. EHT is stable at about 18kV. Of course there’s a whole host of things to sort, purity, static, dynamic, then all the other setups pre-set brightness A1’s etc. Line lock is a little twitchy from time to time I need to look into that but now I think its time to figure out where the colour is hiding.
“Where’s the colour Gone?…. Little Ferguson….Far Far Away”
sung to the tune “Chirpy chirpy cheep cheep”
Checked voltage at VT114 collector its about 900mV so then checked for the burst at TP10, it was there, see below.
Next disabled the colour killer with a 56K ohm resistor from the base of VT115 to the +25V rail, this made no difference.
Next check was TP12 for the reference oscillator nothing! No ident waveforms present either. And no its not the crystal, it never is, I replaced and it was the same.
I suspected VT111 was fried, VT111 is a lockfit ( I hear the boo’s & Hisses) was indeed toast and tested as a diode. It was replaced with a BF199. I fully expected to have colour or at least unlocked colour silly me but no, its not going to be that easy.
Time to take a voltage map of decoder to see where things are at, after which I now suspect VT110 and VT111 may be the cause of my woes. That’s for tomorrow though as I’ve had enough for today.
Voltage map of decoder: Key ( ) = expected
|VT110 (F.E.T.)||D.C Amp||19.1 (9)||2.3 (2)||.127|
|VT109||Burst Amp||1.1||2.2 (2)||25.4 (25)|
|VT111||Reference Osc||17.1 (4)||17.1 (3)||18 (24)|
|VT112||7.8 Amp||2.1 (6)||6.5 (5)||24.8 (24)|
|VT113||Colour Killer||24.6 (26)||13.1 (11)||25.3 (25)|
|VT114||7.8 Squarer||25.3 (25)||25.3 (11)||.9|
|VT115||Colour ctrl driver||.9||2.7 (11)||25.3 (25)|
Update: No Colour
First thing to do was check R404 over on the timbase, it was fine. Next check with the scope showed I had burst but no reference oscillator, now I had changed VT111 a BF197 for a new BF199 but the voltages on the base and emitter were just so wrong. I replaced the newly installed BF199 with a NOS Telefunken BF197, this now restored the base and collector voltages to near book. a check with the scope showed I still didn’t have the ref osc waveform. I replaced the crystal and for once this turned out to be the right move as now the ref osc waveform was present, so now I had burst and reference oscillator but still no colour. Next I checked for the ident pulses and the 30V p-p sine wave at W109 and for 14V at tp13, they were all present.
This left just three places to check, a fault being in the chroma amp, delay line driver or IC3. Checking the voltages on VT117 the delay line driver, another BF197, showed there was no voltage at the emitter. Removed and found it to be faulty, installed another Telefunken BF197, this restored the voltage and now I have instead of “First Light”, “First Colour”, albeit incorrect phase colour.
Now to find out why and fully set up the decoder as it may have been twiddled with. So out with the colour bar generator and checks in the PAL switch.
Just shows, I thought I had set everything back as original as I marked everything when removed. That was flawed logic as it assumed it had never been disturbed in its chequered past, clearly it had. Decoder is working but I have a weird fault where there is shading to the lower half of the test card. I’ve marked it on the third photo but it can also be seen on the previous two.
Anyway that’s enough for today plenty of progress made, I’ll pick this up again tomorrow and try and suss this shading fault. I seem to remember a similar fault on the 3000 but cannot for the life of me remember what that was caused by or even if it would apply to this chassis.
Shading appears to have gone now after a power cycle, perhaps an intermittent fault or decoder madness setting in. Swampy is starting to look really good now, I’m becoming quite fond of it, in fact I think it will end up sitting with my Baird 8724, another set rescued from the brink, they can sit and chat about their ordeal.
What to do about the missing rear cover?
something had to be done.
I had a 20″ Philips KT3 back cover in stock (no TV) but although when cut to the correct height and width it fitted perfectly, I was scuppered on depth. The chassis on the 8000 and its tube neck/crt pcb protruding too much for the shallow KT3 back. I pondered a solution to this for a while then had a light bulb moment. I just recently scrapped an old Grundig that had a duff tube, had salvaged the chassis and other bits for stock including another 20″ rear cover.
Hmmmmm….. how about a splice.
Happy to say its worked a treat, the two halves glued together, reinforced with microporous tape soaked in glue. Once set the whole rear cover will be sprayed to blend as one.
It’s not perfect, It’s certainly not original but it is functional. The best I can do under the circumstance as the chances of finding an original rear cover is minimal.
I had to come up with another solution for the new back.
The original would have been correctly formed so as to provide a recess at the bottom to give access to the aerial input, contrast control and cut-out reset. These are right at the very bottom centre and my newly made back is about 2″ deeper at the bottom.
I could have made a cut out so as to gain access but it would have been a right fiddle, weakened the structure and it would never have been easy to connect and disconnect the aerial, let alone fingers fumbling up in there even if there is isolating card. Therefore I decided to fit an extension socket to the rear for the aerial, I will at a later date also bypass and extend the contrast control to a similar position and perhaps the cut-out.
Should an original back become available, these changes are simple reversals.
Final task to this TV is a full setup and spray the new back.
Component count thus far to get it working 1x LTB module, 1x PSU/snd module (including harness rewire), 1x Rectifier tray, 1x dropper, 2x resistors on the CRT PCB, 1x Smoother, 1x across the line filter cap, 1x flyback tuning cap, 2x transistor in decoder circuit, 1x PAL crystal. Other than those the set is running on its original components.
The Fergy keeps shutting EHT down, if I wait about a minute, power up again, it will stay on for a while then die, repeat Ad infinitum.
I replaced the rectifier tray with a NOS one from the stores, this seemed to improve the picture but I still have EHT shutdown. I hoped this might be the cause (W401/W402) in the old tray but the problem came back. I’m wondering now if its the LOPT, given the poor condition of the set the LOPT may well be toast.
Then finally there was no EHT whatsoever.
checked the rails :-
- 240V present
- 45V is there if a little high at 50V
- 25V rail spot on
- The 180V rail is present at the cathode of W704, This feeds the regulator Thyristor anode W703, I would expect to see 180V on the +/ve terminal of C704 I had 1.7V !!!
I powered off had a think, powered on to recheck and this time the voltage at C704 started up at 130V dropping to 108V and the isolation transformer starts humming like crazy real deep van Helsing hum. Oh and I’m not producing any EHT now whatsoever.
PSU fault? LOPT? I’m wondering if its the thyristor W703 or some component in that circuit?
Turns out VT401 had expired
Replaced with a BU206. I also got to the bottom of the line hold problem. It wasn’t not going wildly off frequency, just enough that the picture floats left or right sometimes a little quicker than others. When I got it locked, the test card occasionally juddered and wanted to pull left or right then you have this balancing act to do on L405. This was traced to a cracked trace between R401 & R402. Line now stable.
I’ve done my best with the convergence, its Ok-ish, but many of the pots are iffy or need cleaning. Every now and then its gets upset and a tap to the panel or pot 5 corrects red misalignment. I’m going to remove the board, clean and or replace pots that need it, hopefully will get some stability.
Rear Cover finished.
As a finishing touch and to reflect where the set came from, I attached a label to the back. This card was found inside the set, the TV hailing from Metervision in Bournemouth.