BRC 2000 Chassis
Model: Ferguson 3700
serial No. 1816
System: 405/625 Line
Original List Price : £362.18
Valves: None, all transistor
CRT: Mazda A63 120X
Here we have another 3700 so that makes two in the collection. This one is in superb original condition, full of house dust and undisturbed for many years. What is really quite amazing is the set has serial No 1816, my other 3700 is 1814 !! That means they are only separated by one set which has serial No 1815. What were the chance of getting two sets that are so close in manufacturing.
All the boards were removed for inspection and careful cleaning. Looks like this might be a 1967 (August to be precise) model from the delay line. Of course it could just be stock they had in 68, or it may have had a Chroma module swapped in, though my gut tells me this is all original as nothing seems disturbed. Guess I can never be totally sure on the year.
The main supply rails from this module are as follows:-
- 240VAC into bridge rectifiers (W11-w14) giving roughly a 270VDC supply. This supplies the 68V zener (W15) and provides the 135V for * CRT Htr protection, video output stages, brightness stabiliser, system solenoids and degauss.
- 55VAC into bridge rectifiers (W1-W4) giving roughly a 58VDC supply, current for EHT generator module is drawn from this supply.
- 68VAC into bridge rectifiers (W5-W8) giving roughly a 75VDC supply. This provides the collector current for the 66V emitter follower VT2. The current for the regulator circuits which in turn provide the supplies for LTB,FTB and sound modules. The aux supply to the video module and finally a 55V return to the PSU for the two 30V stabilisers.
- 6.3VAC supply for the CRT heaters
* Note: The CRT heater circuit is maintained at 135VDC above chassis to avoid cathode-heater stresses. This is obtained from a potential divider R22-R23 connected across the 270VDC supply.
The PSU module is very good condition with what appears to be very little if no prior work having been carried out.
The other modules are equally in great condition, perhaps further evidence this has rarely been touched, was found when trying to remove the modules for inspection and cleaning. On my other three 2K sets removing the modules has been fairly straight forward, this one however was the opposite.
The boards were firmly stuck ( good sign they are tight) in their pinch plug sockets, refusing to leave home. It took ages of careful PPP (persuasion, pressure and pulling) to get them to release their grip. The worst to remove were the I.F. and LTB modules, I ended up with a bloody palm attacked by the print side whilst protecting it from flexing.
I really don’t think they ever been out, inspection of the boards will reveal if this is true and if any prior work has been carried out. I don’t think so as it still has a full compliment of Callins.
After cleaning the layers of dust away I can think It looks like all the modules seem not to have had any work carried out on them whatsoever. The print side looks as pristine as the day it left the factory. I noted C17 ( in the audio amp circuit) on the Field & Sound module has leaked its electrolyte and of C89 had blown its top on the i.f module, these are the only obvious faults.
Further evidence this set is quite an early example is the clear case tripler.
Next Step: Reform the PSU Electrolytics.
The electrolytics are C3 & C6 are both 2000uF/100V, C12 & C13 in one multi can are 100+200uF/350V. These were reformed in little over an hour up to rated voltage +10% on the Hunts CRB3. This unit allows you to slowing increase the voltage whilst monitoring leakage via the neon indicator. All electrolytics reformed.
Starting with C6 below, you can see the start of the process, the neon indicator showing leakage as the voltage steadily climbs which is a good sign. Once the leakage light extinguishes and the voltage stabilises, the next higher voltage range is selected on the CRB3. The indicator strikes again and the voltage steadily climbs once again. This process is repeated until rated voltage is achieved and zero leakage indicated.
Next Step: Replace the C7 & C8
Caps C7 and C8 had vented (see below). However when ESR tested they were perfectly fine and the capacity was 403uF. When tested for leakage there was none. Despite this they were changed. This was more difficult than it looks due to the tight space and having to try and keyhole solder. All done.
Next Step: Replace the callins electrolytics on the EHT regulator board
Replaced the callins on the EHT regulator module. C2, C8 150uF/75V and C4 64uF/32V.
Next: Two physically leaking caps on the Field timebase/Sound module. C5 had a tiny hole in the top and it was weeping electrolyte. C17 had already spilled it juice. C5, C17 100uF/32V. I will also ESR test the remaining Callins on this board.
All the modules edge connectors have been cleaned up
Next Step: Replaced high voltage Callins and some that test bad.
A few replaced such as FTB/SND module C25 400uF/25V, C10 250uF/64V, Video module C22 1uF/350V, I.F. module C89 250uF/35V, PSU regulator module C6 1uF/350V, Chroma module C41 1uF/50V
That’s pretty much it for now, its almost time to reassemble and start thinking about first power on. Few more checks yet, especially the Tx, though I’m not really expecting any trouble there.
Powered up minus the Line timebase and EHT generator modules and of course no tripler. This will let me see how the supplies are behaving and avoid any EHT dramatics. No smoke or bangs and things look stable.
Despite always good practice to watch EHT, I especially need to be watchful as I believe there’s a scenario where a short on VT6 will cause EHT to rocket sky high, so I need to see and shut down quickly if that suddenly develops. Upon application of power there was a degauss thump and clattering from what I believe was the solenoids. Checked TP1 and TP3 on the power regulator, both seem to be OK but I noticed that TP3 voltage dropping away to 35V from 52V. Not sure if this is due to the unloaded state or if there’s something else happening. Fitted the LTB,EHT Reg and tripler. Powered up and had an initial EHT of 27kV, this dropped after about 30-40secs to 22kV. Now 22-23kV is where I would normally set EHT to preserve the triplers. What’s concerning me is why so initially high and why the drop off.
This turned out as I suspected when unloaded the voltage drop off occurs. With everything installed the voltage is stable at TP3 , albeit 4V down. The brightness control was cranked up, once I returned that to a more sensible level the EHT seems to be settled. Now I need to run through the PSU regulator setup and the EHT regulator setup before doing anything else.
Other problems, field scan is reduced and there’s an i.f. fault as there’s lack of snow and sound hiss, its deaf and blind!
Well I’ve proved there’s a fault in this sets i.f. module. I temporarily fitted another spare i.f. board, I then had snow and sound.
Feeding in a test signal I tuned in a rather squished and rolling TCF. Set the hold and adjusted the height pre-set ( this needs cleaning or replacing) and was rewarded with a full field scan.
So back to the bench to see what’s wrong with the original i.f. module. No colour, grey-scale is poo, convergence problems, focus needs sorting, EHT regulation, etc.
After much work things are getting better, however when turned on this morning I had virtually no sound. Upon investigation it was found the 2nd Sound amp (VT5) had died. At this point I thought, OK I’m going to raid one of the 2K IF scrapper panels for a couple of BF173’s. Replace the 3rd Vision IF amp VT3 and the 2nd sound IF amp. This was done, powered up and I could now hear lovely rich sound once again but nothing on screen!! A glance at the EHT showed why, there was none! Errr!
Picture prior to EHT failing
Powered down checked all the connections, nothing was amiss so powered on again. This time I was greeted with a nice test card as well as sound. Then the picture started getting bigger and bigger. Oh no, and a look at the EHT meter once again confirmed it, I could see EHT falling away and the screen went blank.
The fault lies within this sets EHT generator module as I swapped in the 19″ EHT module and the set once again came up fine and stayed up, so the tripler is good. Putting the original EHT module back, the set comes up briefly and then EHT dies away. My first suspects are VT6(output) VT7(EHT generator). The 2000 service manual states VT6 is a Ti /D1693 and that’s what I have fitted. The manual further lists VT7 as a R1038, however I have Ti /D1417 installed. Interestingly I have a provisional release service manual for the 2000, and a D1417 is exactly what BRC state they fitted, however the transistors are not compatible with any other board other than the (a) version. Further evidence that this is likely an early set.
The cause of no EHT is VT7 the Ti /D1417 I tried testing it on the DCA55 tester and it came up as a diode. I was not completely confident the DCA55 could test a high power transistor so reverted to testing with a multimeter. Testing the NPN D1417:
With the -ve lead to the emitter and the +ve lead to the base, I had an ohms reading of 16.3K. Now swapping the leads around +ve to emitter and -ve to base, for an NPN I shouldn’t get a reading, well I did, 42K, therefore D1417 is duff.
This presents a problem, I don’t have any of theses extremely rare D1417 transistors in stock. Also its not easy to swap in an R1038 as the pin out for base and emitter are orientated differently as are the PCB track layouts, the version (a) board is specifically printed for the D1417. I would also need to fit a different heat-sink (oc5-051) as the current existing one (oc5-048) but again has different pin spacing.
Thankfully Langrex have some in stock, now on order and awaiting delivery.
The BF173’s for the I.F. Strip arrived from Bulgaria. I tested them all and they are OK, what I have noticed is the gain on all these are way better. The one I added for the third vision IF from a scrapper board the other day was just 49. These when tested all seem to be around the 130-150
So with the replacement new EHT generator transistor VT7 ( D1417) installed a new higher gain BF173 for the third vision IF, I’m now back in business. EHT is stable and if I’m not mistaken I think the test card looks a lot cleaner. I think John ( Jayceebee) was on the right line of thought with regards to the VIF aligned to the characteristics of BF173 and not BF199’s. There’s still convergence to sort out and I’ve not done any real book setup but it is starting to look good.
Now I can hopefully get back to where I was a few days ago. To recap on where that is, the set is not running with its own Video or Chroma modules. The original video board has a fault (all red) and the original chroma works but needs setting up. They were only subbed so I could get to grips with the IF fault and not have other faults distracting me.
Now to look at the Video module and diagnose the cause of the all red fault. I checked the usual suspects for overly red picture, VT8, VT10 & VT11 all were found to be innocent. I thought I would then check the red 25K skeleton (10K on type 235 board) bias pre-set R42. Observing the current position, I rocked it back and forth, it was noted that there was a distinct improvement. The same was then actioned on the green ( R61) and blue ( R79) bias pre-sets. This cleaned the wiper tracks and I was able to roughly set a decent picture (see below). Well that was easy! That’s the last board fixed.
So what’s left to do? All the boards require their set up procedure to be followed, especially the chroma module.