Decca Bradford - No Colour.

[ronbryan]

The Towers Transistor Selector suggests a BF173 as an equivalent of the BF197.

The BF422 is a high voltage (250V Vce) medium bandwidth (ft 60 min), medium current (200mA) device, whereas the BF197 is a low voltage (20V Vce), high bandwidth (ft 275 min) low current (25mA) device.

I don't think the BF422 is a very good sub for a BF197.

BF197 transistors are listed by Cricklewood Electronics

http://www.cricklewoodelectronics.co...cat=185&page=2


Ron

[ppppenguin]

Since it's only in the decoder why not just try any ordinary silicon NPN device that you have to hand. Even a BC108 stands a good chance of working though its internal capacitances will be rather higher than a purpose designed HF device. Since it's a low voltage application you certainly won't damage anything.

This sort of sub wouldn't work in an IF amp.

[Steve_P]

I wouldn't do this at this stage. They might not work, and will create more faults in the process if they don't. Always try these things in a working decoder first to to be sure - something that AF024 hasn't got right now.

AF024 - Do you have a scope?

Cheers,

Steve P

[af024]

BF173, don't think I've got any of those either. Doh. I feel a call to Cricklewood coming on ...

I guess I should try and get the right bits.

Steve, yes I've got a scope - not the best on the planet though, but just about ok for the job. With a working TR205 in place, I was getting all the signals as described on the circuit at the following points - TP204, base of TR210, collector of TR211 (and emitter).
Nothing at TP207, no PAL SW etc.
By the way, I think that the resistor marked 'R223' (just below the words 'SAT CONTROL') is wrong. I think it's actually R233 - I digress.

What remains a bit worrying is that I've had two transistors go faulty with b-e voltages sitting at roughly 1.2Vd.c. (should have been 0.6Vd.c.). Interestingly I tested them on a Peak tester and strangely 1 out of 10 tests came up ok - the majority of tests reckoned that it was a diode.

The saga continues ...

[Steve_P]

Right, I've had a look at this and I would check by CHANGING rather than substitution : TR208, TR209, D203, C232, C234. Then if there is still nowt, note the position of L205, and rock it about to see if anything happens. Of course, again look with the scope on TP207.

If there's something there, head for TR212 and fault find further. If not, check at Pin 11 of the I.C. and to the base of TR206.

Hope this helps!

Steve P

[Mike Phelan]

Hi Andy
I may have missed some of the replies, but thought it might be useful to summarise my thoughts on it.

If VR269 allows the colour to 'run through' either side of the zero point, then everything to the right of VR269 must be OK, and either:

C247 is faulty and there is no feedback path to the BPD (burst phase detector) , OR the problem is between the TR210 base burst amp and the BPD. Having no ident signal supports that.

If the 23v p-p gated burst on TR211 collector is OK, that narrows it down to C247 or the BPD components including the coil. I wonder if what is coming through the burst gate is burst, not a bit of chroma signal because the gate pulse is somehow wrong?

Disconnect C247 and apply sig gen at 4.43Mhz to jct R270/271. Set VR269 to "zero beat". You should be able to get a zero beat by swinging the sig gen around slightly.

[af024]

Thanks again guys, I'll book table space again, see what I can find out and report back. I've also got some new BF197s and BC148s to play with, so that's a good start.

Regards,


Andy

[af024]

Well, thinking that I had a rather dodgy set of semiconductors (probably as a result of the earlier PY500A flash-over), I changed the following transistors:-

TR205, TR206, TR207, TR208, TR209, TR210, TR211, TR212 and TR213

I also changed the following diodes:-

D203, D205 and D206

C232 and C234 are both ok – not leaky and values are ok.

The 25Vd.c. supply is ok.

C247 was indeed faulty – it measured 2.6k ohm. Well done Mike! C217 (same type) measured 13k. I changed both. I was very surprised about this since I don’t think I’ve ever come across this type of failure before for disk ceramics.

The following Ls, Cs and Rs are fine:-

C214, C215, L202, C216, C218, C225, C223, C228, C204, C245, C237, C246, C208, R225, R226, R221, R222, R223, R224, R226, R228.

With TP205 linked to TP206 I have ‘running-through colour’ restored. I can now achieve ‘zero beat’ by rocking VR269 – something I haven’t been able to do before. At least there’s a bit of progress.

I rocked the core of L205 in and out slightly and managed (just once) to get a proper lock and it stayed for about 30 seconds with the TP205 to TP206 link removed, then it was lost. I’ve not been able to achieve lock again since.

The waveform shown on the base of TR210 is present but it’s only sitting at approx 3Vpk-pk and not 4Vpk-pk. The chrominance part is sitting at roughly 400mV.

The signal shown at the collector of TR211 is present but only at 3.5Vpk-pk not 23Vpk-pk. The signal at TP207 is very dirty and only has a few mV. No wonder it doesn’t work.

I measured the dc conditions around TR210 and TR211 and they were at approx:-

TR210 b = 21V
TR210 e = 19V
TR210 c = 0.05V
TR211 b = 0.05V
TR211 e = 0.05V
TR211 c = 19V

The published Decca data is this:-

TR210 b = 10.2V
TR210 e = 9.6V
TR210 c = 19V
TR211 b = -0.1V
TR211 e = 0V
TR211 c = 15.5V

My measurements don’t agree of course.

What’s even more confusing is that the circuit shows the base of TR211 connected to the collector of TR210, so why should Decca claim that the base of TR211 should be at -0.1V and the collector of TR210 be at 19V? That’s simply is NOT possible. I think Kirchoff and Ohm’s Law would have to say something about that! Maybe they teach a different version at schools today? I digress.

The signal type (and amplitude) at TP204 look ok so too does that at TP200.

So, that tells me (I think) that there is something wrong with the BURST AMP and/or the PHASE DETECTOR. I keep returning to these locations but with no joy though. How strange. At some point I could have sworn that I’d seen a 23Vpk-pk signal at the collector of TR211 (a couple of weeks ago). Maybe I’m going mad (no comments please).

Stuck again.

Regards,


Andy

[af024]

Sorry guys, I just knew that there was something wrong with those reference voltages. As if chasing the problem isn’t bad enough, for some reason I picked up the Decca series 10 manual and looked at the table of voltages in there and tried to make sense of those with the Decca 30 circuit in front of me. Yes a MUST be going mad.

Here are the correct published voltages for the Decca 30:-

TR210 b = 21.7V
TR210 e = 19.8V
TR210 c = 0.15V
TR211 b = 0.15V
TR211 e = 0.09V
TR211 c = 19.5V
Looking at these, and allowing for the uncertainty in measurement, I don’t think my set is far out.

Regards,

Andy

[Mike Phelan]

I rocked the core of L205 in and out slightly and managed (just once) to get a proper lock and it stayed for about 30 seconds with the TP205 to TP206 link removed, then it was lost. I’ve not been able to achieve lock again since.

I keep thinking that there is something wrong physically with L205, Andy - worth opening it up to have a look? All the voltages and waveforms seem OK to me, and it should work.

One more thing - have you tried disconnecting D204, the ACC rectifier?</columbo>

[af024]

There’s no tin round L205. It’s the one shown in the photo. It tests ok for continuity. No nasty looking problems evident.

As regards the ACG, that was going to be my next plan of attack – you pipped me at the post. I’ve just lifted one side of D204 out of circuit and tried running it up again. Get what .. colour! So I thought let’s reconnect and see if it goes away again – it didn’t, it carried on working until about 2 mins down the line when it stopped again. So nothing conclusive there – apart from reconfirming perhaps that signals are very low – especially that at the collector of T211.

Photos (just for interest really) are:-

L205
One of the sick disk caps (this one is C247)
Sick R217

The mystery continues ....

[Focus Diode]

This reply probably doesn't relate to the Bradford sets, but a few years ago I was checking over a decoder in a GEC Hybrid CTV which suffered from intermittent loss of colour and running colour.

The overall picture wasn't upto much and on checking the boost voltage and EHT they were both found to be low. Re-setting the set EHT control to give 23.5KV gave much better results- and full colour! As a check the control was turned down slightly and colour was again lost.

I remember with the Bradfords, on reading that the top cap leads to the PL509 and PY500A have to be in good condition as charred leads can upset the operation of the rest of the set.

Most unlikely but worth checking nevertheless.
Cheers,
Brian

[Mike Phelan]

Originally Posted by af024

So nothing conclusive there – apart from reconfirming perhaps that signals are very low – especially that at the collector of T211.

Oh - must have missed that, Andy. That will be the reason that it sometimes locks or almost locks.
I think the original fault would have been C247 - the loop feedback cap that was upsetting the BPD output voltage.

If TR211 output - gated burst - is low amplitude on a scope, that is the problem. Is there pure burst - no chroma info - there?
Is the chroma on TR209 base OK? Is the gate pulse from the sync sep OK?
Bear with me as I haven't read through every post and you may have covered all this!

[Focus Diode]

Found this piece in "Your Problems Solved" in the October 1978 issue of "Television" magazine. Thought I would forward it.

Brian

[af024]

Thanks Brian and Mike.

Right at the start, I did have problems with both the PL509 and the PY500A. The top cap was just about to fall off from the PL509, and I did have that nasty flash-over in the PY500A (now replaced). I sorted all that out and checked for problems with the leads etc – all ok.

When I braved the first power-up (seems months ago now), focus and width (plus convergence) was all over the place. I changed some resistors in and around VR451 (R452 and R450 I think). They had gone very high in value. I then re-adjusted VR451 to obtain the correct width. Everything seemed ok so I proceeded to deal with focus, geometry and convergence. I did notice a gradual decrease in width over time and I’ve now ended up with the wiper of VR451 towards one end of its travel. I don’t know whether this is significant or not. Things have settled down in that regard now though and I haven’t had to adjust it again for some considerable time. I never did determine what was going on there. Regrettably too, I have no means of determining what the actual EHT is. I must confess to not having measured the 295V rail either (is this the ‘boost’ voltage?) since I thought that it must be present for me to get blue, green and red drive and I’m getting reasonable dc conditions at the likes of the base of TR210 (which seems to be connected to it via a convoluted route). I will measure it though – it does seem a reasonable idea. I suppose I should have done that earlier. Am I supposed to adjust VR415 to obtain 295V or correct width? There’s no other way to adjust the 295V or is there?

I wonder if C238 might be a bit poorly? Wouldn’t that affect the signal appearing at the base of TR211 and in turn explain the low signal at its collector?

Anyhow, let’s see if I can answer the remaining questions …

TR211 output is indeed very low – 3.5Vpk-pk compared with 23Vpk-pk. Again all dc conditions around that stage seem comparable with that quoted by the manufacturer.

I’m not sure that I can easily discriminate chroma from burst at the collector of TR211. It looks like burst as per the waveform sketch (with some noise), but how can I tell?

I’m getting the signal as sketched on the circuit for the base of TR210, so does this mean that the gate pulse is ok? Should I be looking elsewhere?

There’s pretty much nothing at the base of TR209 and never really has been. TR208 colour killer doesn’t get conducting as a consequence. There must be something present however (albeit very low) since there are signs of activity not unlike the sketch at TP207 (but at extremely low level and very noisy). I can’t display it on the scope very well.

I guess that’s all to report for this particular instalment. Tune in same time, same channel for the continuing saga …

Regards,


Andy

[af024]

Oh, thanks for the snippet too Brian - very thoughtful. I'll try to see if I've got those suggestions covered.

Andy

[Hybrid tellies]

The HT line is none variable on most Hybrid colour tv's. You should see near enough 295v at the top of the RGB load resistors, don't worry to much if this is a bit low say down to 285v, as this is an unstabilised line. On the boost line you should see just over 800v
If the you have only just got enough width then check the high value resistors in series with the width control. On the 30 chassis they are 2x 820k and a 5.6m. Always worth checking is the value of the width control which can go low or of course a worn PL509.
I would check out the line output stage first just in case it is effecting the decoder. I remember seeing many Bradfords with severly reduced width but still good colour on the rather narrow picture.

[Mike Phelan]

Andy
Comments below:

I wonder if C238 might be a bit poorly? Wouldn’t that affect the signal appearing at the base of TR211 and in turn explain the low signal at its collector?
Yes - if it was O/C. Check it is just DC at this point.
Anyhow, let’s see if I can answer the remaining questions …

TR211 output is indeed very low – 3.5Vpk-pk compared with 23Vpk-pk. Again all dc conditions around that stage seem comparable with that quoted by the manufacturer.
That, then, is the problem.

I’m not sure that I can easily discriminate chroma from burst at the collector of TR211. It looks like burst as per the waveform sketch (with some noise), but how can I tell?
You cannot really, but if the gate pulse is OK, TR210 base should show the burst sitting in the middle of the pulse, as shown in the cct.
The gate pulse comes from the sync separator, so don't worry about line OP stage.

I’m getting the signal as sketched on the circuit for the base of TR210, so does this mean that the gate pulse is ok? Should I be looking elsewhere?
Yes and yes.

There’s pretty much nothing at the base of TR209 and never really has been. TR208 colour killer doesn’t get conducting as a consequence.
That will be so unless the ref osc locks and the 7.8KHz appears at the BPD - not relevant at the moment.

There must be something present however (albeit very low) since there are signs of activity not unlike the sketch at TP207 (but at extremely low level and very noisy). I can’t display it on the scope very well.

I guess that’s all to report for this particular instalment. Tune in same time, same channel for the continuing saga …

Keep looking around TR210 - I am convinced the problem is around there if the signal at its base is exactly as shown and the burst part of the waveform is the correct amplitude and in the correct place.

[af024]

Thanks Simon and Mike. More for me to think about at the weekend. Back with more results soon I hope.

Regards,


Andy

[af024]

Sorry for the delay in update, of late it's been difficult to allocate some time. Anyhow, here goes with the latest ...

I managed to borrow a couple of spare decoders (thanks PM!). I plugged one in and everything worked fine. At least I knew that the fault was my decoder (no real surprise there I suppose). I swapped decoders again and tried once more to track the fault down.

I discovered a cracked ferrite core in L201, so I wondered if there had been a phantom twiddler in there. I therefore dug the remains out and replaced the core. In order to establish whether the panel had been got at, I decided to follow the alignment procedures as detailed in the manual – just in case things were miles out. Firstly, I discovered that I couldn’t adjust VR239 to obtain 18Vd.c. at the emitter of TR204. I traced that fault to R213 which was sitting at something like 70k instead of 56k. This improved the situation and I was able to adjust up to 18Vd.c. ok.

Due to my continuing suspicions with the TR211 stage, I also did more probing about round there. I found R268 sitting rather high, so I also replaced that.

I then moved on to adjust the chrominance bandpass filter coil L202 by injecting 3.4MHz and adjusting for maximum amplitude at TP204 (as per the service info). It was a bit out yes. I also tackled the sound rejector's L200 whilst I was about it with 6MHz and adjusting for minimum amplitude.

I also dealt with L201 and L207. Somewhere along the line, the thing burst into life albeit with displaced colour, so I had a go at adjusting L205 – all as per the manual and the results were very good indeed.

At this point, my poor old isolation transformer was rather warm, so I switched off to return a couple of hours later.

Feeling very pleased with myself, I decided to crack a bottle open. Job well done I thought.

Not so, I turned on again to continue with the rest of the procedure – my intention was to start with the subcarrier reference phase adjustment L206, but after about 10 mins and actually before I’d even got my nylon adjuster into the ferrite adjuster, the thing stopped working again. I couldn’t believe it.

Talk about being cheesed off!!

What I did notice, was that whilst it was working, the signal at the collector of TR211 was approx 23V as per that shown on the circuit. The signal at TP207 was also appreciable and well-defined. When the thing doesn’t work, the signal at the collector of TR211 falls to approx 3V and that at TP207 vastly reduces almost to noise.

No amount of tapping, board flexing, core ‘rocking’, coil pushing etc brings it back to life. Again, even in failed condition, all d.c. conditions around TR210 and TR211 are favourable. L206 meters out ok.

Defeat the colour killer and you’re back to ‘running through’ colour.

Talk about a challenge.

Oh, and just to add insult to injury, I’ve also knocked the purity rings away from where I put them several weeks ago, so I’ve now got to go there again now.

It's really bugging me now.

I wonder if it would make a nice paperweight?

Regards,


Andy