Decca Bradford - No Colour.

[ronbryan]

Andy

Have you considered the possibility of a shorted turn in L206? This might not show up with a continuity or resistance test. Or maybe C242 is intermittant open or short.

Ron

[Mike Phelan]

Hi Andy
I think you can then concentrate exclusively on TR211 stage. I would be very tempted to swap L206 before going much further.

[af024]

Not sure I could swap L206 to be honest - I have no such spares. C242 - yes I'll certainly take a look at that and probably replace it anyway (anything I can eliminate is good). What's interesting is that the last few times I've switched on (from cold), the thing will work for a bit (10 mins or so), then it will simply stop.

It's a little monkey for sure.

Back with an update soon ...

Regards,

Andy

[Steve_P]

Open it up before switch on, and put your scope on TP207 and a meter on TR211. Then power up and watch....

Cheers,

Steve P

[af024]

On the basis that the d.c. conditions around TR211 seems to remain pretty much the same working/not working, I'm just wondering what to meter.

Interestingly too, L206 is the coil I opened up a while back. Nothing seemed untoward then, although I accept that shorted turns are difficult to get a visual on.

Regards,


Andy

[Mike Phelan]

Worth a squirt of freezer on the coil, Andy? Ignore me if you have already tried it.

[af024]

Yes I think I've squirted just about everything in the vacinity to be honest. Thanks for the thought though.

As it happens, I've got some excellent news.

With the help of the working decoder, I managed to sketch some waveform traces for the intermediate points not detailed in the service manual.

This, together with very close inspection of the signal at the collector of TR205 showed me that there was very little burst evident when the set decided to go into sulk mode. The signal at the input of TR205 always looked ok.

Of course that didn’t necessarily mean that the AGC stage was playing up, since by its very nature it’s a closed loop feedback system, but temporarily lifting one leg of D204 brought colour back during a sick period. I’d done this before of course with no conclusive outcome. I suppose it was masked by the other faults (now resolved).

I removed the diode and tested it on a standard analogue multimeter set to 1 x ohm scale. It read just like a diode should – a reading one way and nothing the other. I then changed range to 1k x and measured in both directions again. Ah ha, there was some leakage in the other direction evident now – something like 20k. I replaced it for a new 1N4148 and tried again.

Colour at last AND it didn’t pack up after 10 minutes. It was pretty poor though, so I decided to go right through the alignment process from start to finish. One ferrite core fell right through its former. The threads were shot it. I used a bit of cord to jam things up a bit.

Then, having messed up my purity, static and dynamic convergence during all the fun, I decided to go through all that again too (plus another geometry check).

The width control still has to be set right at the end of its travel and I can’t find any more high value resistors around the line oscillator stage, so I’ve put it down either to a lazy PCF802 or PL509. If the width tries to pull in again, I’ll go in search of two new valves and see where that takes me.

A couple of convergence pots are also at the end of their travel, but the adjustment is ok.

Some presets are VERY noisy – especially the three gun drive controls just below the output transistors. At one point I lost the blue gun altogether. Just a little rock either side brought it back. I hope they hold out.

There’s a bit of sound buzz when sharp lines are in the picture (not sure what to do about that) and there’s some (assumed) Teletext noise at the top, but it does now display a great colour picture from a DVD testcard (see photos) and it has been doing so for well over 2 hours now, so I think it’s a case of job done!

Many thanks to everyone for their assistance – I am very grateful.

Regards,


Andy

[Focus Diode]

Well done Andy! That's a lovely looking set. I've not seen a Bradford like that before. Always great to hear of successes following a tricky problem. I've yet to succeed with my CVC5 but will get back to it soon.

For the width problem the excellent McCourt book gives these items to check:

R540 (5M6), R452 (1M5)- oddly both these can also be responsible for excessive width! C431 (220pF), C435 (150pF) if o/c and VDR401 (E298ZZ/05). The control itself could have changed value too.

Speaking of no colour problems in general I should mention a problem one can get with a DVD recorder. Sometimes they're not quite 100% compatible with conventional colour sets thus this may give a B&W display or running colour when normal off air is full colour. I've had this with two LiteOn machines on both my CVC5 and G6 (and the s/s 22" model I fitted a new LOPT to). Careful re-setting of the reference oscillator coil or pre-set witll give reliable results on both DVD and off air.

I forgot to re check this with the s/s G6 on fitting a new LOPT and carrying out other work. It went back to our member System A with no colour! I then realised what had happened!

Cheers,
Brian

[af024]

Many thanks one and all. Yes it's been a bit of a struggle to be honest, but with everyone's help it's made the journey much more pleasurable and less frustrating than it might have been. I've also learnt quite a bit along the way. I've never been anywhere near a colour decoder before. I've always thought that they were black magic devices.

I think my specimen was an ex-rental set - probably this explains the different cabinet layout. I quite like it though. It spent the last 20 years in a drafty old out building. Amazing it survived really.

Thanks too for the tips regarding width. I think that the back should come off once more.

Many thanks again everyone - it's been very interesting for sure.

Best regards,


Andy

[af024]

I still haven't looked at the set again with the width issue in mind, I will of course, but what I'm now wondering about are those Teletext lines at the top. Does anyone think that there could be something wrong with frame blanking at all? Maybe flyback is taking too long to happen and it's causing a trace on the way back up at say line number 6 or something? The 2 lines in question are quite a way down from the top.

Regards,

Andy

[Focus Diode]

Found this in "Your Problems Solved" Television November 1978.

As for the Teletext lines the good news is this is fast becoming an obsolete problem! Freeview not only gives text line free pictures but allows one to zoom the image to fill the entire screen-wonderful not to have those black bars at the top and bottom! Macrovision DVDs (thankfully not so common now) gives two lines at the top which fade on and off.

Cheers,
Brian

[af024]

Thanks Brian,

That's a useful extract for my width investigations - your searching about is much appreciated.

You are right about Freeview though - there are no Teletext lines displayed at all. The trouble is that my Freeview box doesn't have an in-built modulator (isn't that dandy!!), so I had to lash in an external one to give my set a try. I must say though, the picture looks a little 'soft' via the Freeview box - it's much better via a proper genunie analogue broadcast.

Regards,

Andy

[Zelandeth]

"The picture looks a little 'soft' via the Freeview box..."

I noted a similar effect recently when my parents seven year old Grundig packed in, and we ended up using an old Pye StudioCOLOUR portable. The Pye has no AV input, so I had to use the RF output from the Sky decoder.

Initially, I just assumed the picture to be horrible on the Pye (we have no terrestrial signal where we stay, it's Sky or Sky), with apparently huge problems (major colour bleed for one) with pretty much every aspect of the image.

...then, when setting the VCR up a week or so later (which DOES have an AV input), first the I turned the VCR on (it defaults to the AV input on switch on)...I was greeted with an absolutely perfect picture. Turns out that the RF encoder in the digibox is just useless - using the one in the VCR bypassed that problem.

Maybe worth a shot in your case?

[af024]

Thanks, certainly worth a try. Having said this, I have seen similar soft pictures via a SCART connector - straight to the back of other sets.

I wonder if the Television mag did any articles on removing Teletext lines on Bradfords?

Cheers,

Andy

[Simon.P]

Originally Posted by af024

I still haven't looked at the set again with the width issue in mind, I will of course, but what I'm now wondering about are those Teletext lines at the top. Does anyone think that there could be something wrong with frame blanking at all? Maybe flyback is taking too long to happen and it's causing a trace on the way back up at say line number 6 or something? The 2 lines in question are quite a way down from the top.

Regards,

Andy

I had a 2230 up to about 10 years ago, and I had a problem with Teletext interference. Look at TELEVISION, September '90 p866 - this is a mod I devised to deal with it. You might need a capacitor (try 10nF) across the 820 ohm resistor in the diagram to prevent 'shadowing' caused by pickup of line-frequency pulses - I subsequently found this out.

BTW, these sets tend to sound as if they have a bunged-up nose - I found this annoying, and rewired the volume pot to remove the negative feedback from the PCL82.