1970 Ferguson 3703 - BRC 3000 Chassis

If you happen to have any of there bi-polar capacitors just put them in the naughty drawer and leave them there

My stock of non polarised caps turned up this afternoon so whilst the Baird is on soak I thought seeing how I was in BRC 3000 chassis mode, I should continue with the Fergy.

Installed the missing C214 on the video board and this has improved things (see below). Still much to do bringing this one up to scratch, hardly any colour control or contrast, the picture is shifted over to the right. I tried swapping the plug around on the LTB but that did nothing. I will run through the set-up as I did for the Baird i.e. EHT, Grid, Porch etc.

Anyway plenty to get stuck into as the Fergy's boards are full of rough looking components and no doubt tracing the voltages and waveforms will reveal the likely suspects.

A-ha Chris. Your very good photo gives us a clue. You can see some foldover on the right hand side of the screen. You'll find that the line hold needs adjustment. You may also find that the colour improves when the line hold is adjusted...

Cheers Tas, the colour didn't improve but the line hold certainly sorted the picture shift issue, I must admit I had not spotted the fold-over you do have eagle eye.

It seemed really down on reds so I raised the red gun ever so slightly and the colours started to look better. I need to go through the set up but first I'm going to service all the boards.

If it's not a camera effect, there is some colour shading on the picture...definitely a pinkish hue on the right. I normally found that this was caused by decoupling caps in the RGB drive supply. Not being a Thorn expert this may be completely wrong but probably worth changing any decoupling caps anyway.

Had the set on for a while, I watched what is most likely a cap/s or resistor/s failing ( leakage or over heating). The picture went steadily darker and darker fading finally to nothing. Switching off an waiting to cool, when put back on the picture recovers somewhat but then goes through the fade out process.

I think it is succumbing to usage without repair and is in need of recapping/resistor checks on a fair few sections, I will fetch some pics.

Picture re-established,

I suspected the 3 decouplers as shown above C707, C708 & C709 1000pF these were replaced. I also suspected the three beam switches S701, S702 & S703 as when operated the only one appearing to do anything, was green. So these were replaced with straps until I can find suitable replacements. That's two convergence boards needing A1 switches now.

there is some colour shading on the picture...definitely a pinkish hue on the right. I normally found that this was caused by decoupling caps in the RGB drive supply. Not being a Thorn expert this may be completely wrong but probably worth changing any decoupling caps anyway.

Yes Rich I see this too and will look into it, as I mentioned above the caps in this set are looking well rough. I wonder if the confined cabinet of a 19" tends to give them a harder life, less ventilation.

Yes Rich I see this too and will look into it, as I mentioned above the caps in this set are looking well rough. I wonder if the confined cabinet of a 19" tends to give them a harder life, less ventilation.

I often wondered that too, especially where the chassis has been 'folded-up' to fit within the tighter confines.

Since the last picture the Fergy suffered a few issues, one with the I/F gain which was down to C177 O/C, the other was the CRT was low on Blue at 20% compared to Red and Green on 70 %. Tickled it on the tester, it responded well with Blue now on 70 %.

So this afternoon I have it giving a good B&W picture ( still have left and right shaded differently as mentioned before). I want to look at the lack of colour, can I confirm that to override the colour killer you just connect an 82K at the end of R339, rear most point, the other end to chassis?

I've looked in my manual and that's all I can find except it does go on to mention opening link A&B etc but that looks like its for setting up the OSC, I just want to stop the colour killer.

Hello Chris
I think I replaced a pot on the decoder as the original had fallen to pieces. Have a look and see what pot has been replaced and that may be the only thing that needs adjusting.
Regards Stan.

Hi Stan,

I take it you mean R308 the set ACC pre-set? if so I had already checked that and adjusted through its range, no colour. Hence wanting to stop the colour killer to see if the fault might then reveal itself.

Just mapped the voltages on the Fergy chroma board I think this lot explains the loss of colour.

VT301 Burst gate amp - emitter is high
VT302 looks ok
VT303 DC Amp - collector way too high
VT304 Reference OSC - emitter high
VT305 looks ok
VT306 looks ok
VT307 Switched Amp - collector volts missing
VT308 Polarity Splitter - looks ok emitter marginally low
VT309 Chrominance Amp - collector and emitter out of min max
VT310 Delay Line Drive - collector high, no emitter volts

Adding an 82K to R339 and ground to stop the colour killer still results in a B&W picture

After having mapped the voltages of the Chroma transistors which revealed many issues, I started to check the waveforms.

Starting with the Burst gate & Amp (VT301/2), the voltage on the emitter of VT301 is almost double I'm guess that may be down to C355 or C302. I did a check for the gated burst waveform and it is present albeit a couple of volts low, could this be related to the high voltage of the emitter of VT301?

Next VT303 the DC Amp, the voltage checks yesterday have this way way out, with the collector at 28.9V when it should be 4V and the emitter low at .5V when it should be 1.1V. As a result the ½-line frequency component is missing from the emitter when checked with the scope.

R315 isn't open circuit slider?
Also R309 slider ought to be maybe 1.5V approx.

5 & 6 link looks like normal and 6 &4 linked will pull down base and turn off VT303.

If W303 was short it might create this effect too.

Does DC at VT303 base change with R315 and/or R309 adjustment?

Wow Chris, that CRT has come up a treat. With the killer overriden and the colour control at maximum are there any signs of even the slightest chroma noise at all (confetti as we called it)?

No signs of any confetti would lead me to 'scope a colour bar input through from the IF to the matrix, particularly VT309. It might also be a good idea to go through the refence oscilator set up in the manual and see if you can get it to free run at around 4.43Mhz. At the moment with the high volts on VT303 collector it will be way off frequency. Beware that no colour can also be caused by the VT110 or C173 on the IF panel.

The chroma panel was the most reliable board on a 3000, there weren't many stock faults but you know your way around a Pye 697 decoder so I'm sure you'll crack this in no time.

John.

Hi John,

No signs of confetti.

I had eliminated the Fergy IF (VT110 and C173)by putting in the Baird Chroma panel, and got a nice bright colour test card, so the fault definitely lies within the Fergy chroma board.

Funny how the Baird is now helping to serve as a test jig when a few days ago it was the other way around. I've replaced R308 with a new 50K skeleton as one leg fell off the old one.

17V is missing from TP2 as there is no chroma
VT308 scope points 22 & 23 blanking pulses present
VT306 scope point 19 ½ line frequency sine missing
VT305 scope point 18, Ref osc is running but only at 2V p-p instead of 4V p-p

The hunt for the culprit continues

Further checks on the chroma board. Errors found on the waveform for the 7.8kHz amp, Reference Oscillator 2v down, no waveform present on VT307 emitter and the R-Y demodulator.

In circuit diode tests seem to suggest they are OK.

Diode/Type/Forward voltage drop
W301/OA91/.320
W302/BA155/.580
W303/BA155/.570
W304/BA102/.515
W305/OA91/.312
W306/BA155/.570
W307/BA155/.560
W308/OA91/.297
W309/OA47/.250
W310/OA47/.250
W311/AA143/.250
W312/AA143/.250
W313/AA143/.253
W314/AA143/.250
W315/OA91/.312
W316/OA91.312
W317/OA91/.312
W318/AA143/.250
W319/AA143/.250
W320/AA143/.250
W321/AA143/.301
W322/OA91/.301
W323/OA91/.320

Concentrate on getting the subcarrier osc amplitude and frequency correct, set it up to free run as per the procedure on the model cards, the crystal is of course the most likely culprit. There are also several high value resistors around VT301/2 and those skeleton presets as you know can be troublesome.

You do seem to have some chroma in waveform 24 but it will be low if you're using a ÷10 probe. To measure it correctly you need a 1:1 probe terminated with a 67pF cap and 1M ohm resistor, see the notes on P26 of the manual.

Hi Chris.

I dug out my own Fergie 3000 and have taken a few waveforms on the decoder. As others have said these decoders are usually very reliable and give a cracking picture. The only faults I've ever really had have been the delay line driver transistor and chroma amp.

Anyway, as you have no colour at all I think I'd be inclined to have a quick look at the waveform on the base and the emitter of the delay line driver transistor. With the colour control set to 50% and the set tuned to colour bars, you should have something roughly resembling the attached pictures.

They're basically of the same waveform, both from V310 - The first being the base and an amplitude of 800mV P-P and the second from the collector and an amplitude of about 2.8V P-P. Don't worry if they're not exactly the same - as long as they're of that kind of order. (you need to use a x10 probe BTW).

I'll leave the set on its side and the scope downstairs so if you want any more waveforms then just ask.

Thanks Chris.

Kind regards.

From Mike.
PS. These decoders have rather a lot of those lockfit BC147s / BF194s etc. so it might be just as quick (if a little bit non-scientific) just to go through and test them and make sure they're all okay.

Hi Mike,

the chroma is barely there as can be seen in to photo, I too am beginning to suspect transistors

When I install the Bairds 3000 board, chroma is as expected.