Decca Bradford - No Colour.

[af024]

Actually it's a Decca in disguise - it's a Decca 30 all right, but it's badged as a Classic.

The model is Classic CC2234 and has what looks like a Decca Series 30 chassis inside. The back says, 'Made by Decca'.

I was given this old set and have managed to get everything up and running ok apart from colour.

I am consulting the Decca Service Manual for the CS2030, 2230, 2630, 2631 30 Series.

Here goes with a description of the problem.

I have shorted out TP205 and TP206 to bypass the colour killer circuit. This biases TR206 correctly, and chroma info reaches the delay line and onto pins 8 and 9 of IC2. So far so good.

• The signal at TP204 looks ok.
• The signal at the base of TR210 looks ok.
• The signals at the collector and emitter of TR211 look ok.
• Signals appear at the junction of C243 and R267 plus at the junction of C241 and R266.
• Not much sign of anything the other side of R267 or R266 and nothing on the wiper of VR269.
• The collector of TR212 is sitting at pretty much 0Vd.c.
• Disconnecting the base of TR212 from the wiper of VR269 causes the collector of TR212 to rise to approx 5V.
• 4.43MHz reference oscillator running – looks ok at TP209.
• There is no 7.8kHz at TP207 and hence no PAL SW signal to pin 11 of IC2.

As a side issue, the b-e voltage of TR205 was originally sitting at approx 1.2Vd.c.. I replaced it and the b-e voltage of the replacement fell to approx 0.6Vd.c. Strangely, the original transistor tested ok on my Peak tester. I didn’t put it back in.
The HT fuse (F1) kept blowing (intermittently). I tracked the problem down to a faulty PY500A (nasty flash-overs taking place inside). So far, all is well with the second hand replacement.
The trippler was arcing near the chassis, I cleaned the entire area up, applied silicon sealant around the cable exit and all is now ok.
Geometry, colour purity, static and dynamic convergence all came in ok after I changed a couple of pots and performed several iterations.
Focus ok (albeit right at one end of the travel at the moment).
Grey scale ok.

So, returning to the lack of colour, and bearing in mind that I’m still trying to get my head round colour de-coders, would anyone have any ideas that might be able to help me please? I’d certainly be very grateful. Along with the lack of kitchen table space, I’ve now pretty much run out of inspiration at the moment. Oh, by the way, I replaced D205 and D206 and TR212 – absolutely no change.

Thank you in anticipation,



Andy

[Mike Phelan]

Hi Andy
Don't have a circuit to hand, and others will come in, but for now:
What is TR212?
If the ref osc is running and you have overridden the killer (which would not have been open if there is no 7.8khz ident) then you should have either locked colour with possible inverted R-Y because no ident or colour "running through" because the ref osc is not locked = no burst.

If the latter is so, check the burst channel and whether there is a pulse from the LOPT to the burst gate (resistor O/C?)

If there is no colour at all, check the chroma channel.

[af024]

Thanks for your thoughts Mike. With precise tuning I can achieve some ‘running through’ colour – predominantly blue and yellow.

TR212 is described as a DC AMP and follows the phase detector after the burst amp. There’s a pulse shaper which gets its input from the LOPTx. Its output feeds the burst amp. From what I can see, it looks like this is all ok since I’m getting signals at the base of the burst amp which look pretty much like those provided on the circuit for reference. D205 and D206 form part of the phase detector circuit. There doesn’t seem to be anything from the output of that to feed the DC AMP. That's where I get stuck. Well, that along with the wrong d.c. voltage on the DC AMP TR212 transistor.

Mmmm.... interesting.

Andy

[af024]

Actually that's not strictly true, there's a DC output present on the wiper of VR269 which can be varied by adjusting VR269. I guess that's what's intended now that I've put my brain in gear. The fact remains however that TR212 remains turned fully on. What's the chances that the crystal and/or D207 is faulty? I would be surprised though since, within the resolution of my scope, I can see 4.4 ish MHz at TP209. Any ideas folks?

So near, yet so far ...

Cheers,


Andy
PS Gotta go away with work most of this week so sorry for any delays in responding.

[Mike Phelan]

Andy
If you can get colour running through and 4.43mhz, the crystal and therefore ref osc is probably OK, and the fault is in the the burst channel.

I am flying blind without a circuit, but chroma will be fed to the burst channel, and the pulse from the LOPT will "open" the burst gate to pass 4.43mhz during the back porch.
Is this pulse OK, and is the short bit of 4.43 getting through?
TR212 sounds like the problem, or something in the phase detector - caps, diodes? It acts as a reactance and appears like a variable capacitor to the reference osc. If there is a fault in the phase detector the voltages on TR212 will be wrong.

Do you want to scan this part of the circuit so we can have a look?

HTH

[af024]

Hello Mike,

Thanks again for your comments. I'll have a go with a scan - it's pretty difficult commenting without it I know.

I'd love to know what signal to expect on the wiper of VR269. Would it be DC only, or, should it be a square wave at half line frequency (7.8kHz) with a DC bias? Let's see what I can do with a scan Mike ...

Regards,


Andy

[af024]

The scans ...

[Mike Phelan]

Just printed it out, Andy - will have a look tonight.

[af024]

Thanks Mike, much appreciated - it's driving me mad now.

Regards,

Andy

[Hybrid tellies]

The decoders on these were highly reliable, the common problem we had was weak or running/unlocked colour, as you describe, nearly always cleared by replacing the 4.43Mhz xtal.
The few other problems we had were mostly due to caps or low gain transistors.
Good luck

[Mikey405]

If you're tearing your hair out and you don't mind clambering over piles of VCRs and TVs, you're welcome to bring the Decca round here if you would like some help. Solihull isn't too far from Ashby-D-L-Z I think.

From Mike.

[Mike Phelan]

Right, Andy, 'ere we go, as they say in the Earwig Song.

Looking at the circuit, this is a brief rundown on how that bit works:
The burst phase detector is the usual loop like a flywheel sync setup. The gated 4.43 mhz burst from the collector of TR211 is compared with the output from the ref osc via C247 and R270, the result being the DC level on the base of TR212 that biases the varicap D207.
VR 269 will adjust the phase and frequency of the ref osc.

The burst will also be fed to the base of TR209 which is tuned to 7.8khz and rectified by D203 for the colour killer, and to the PAL switch in the IC.

OK - enough waffle. As others have said, try the crystal first. If it has low output, there is not enough signal through R270 to lock the phase detector, so the colour "runs through" and the ident signal isn't present to open the killer.

If that is OK, I would proceed as follows. Don't take too much notice of voltages yet. You may have already done some of these checks. Override the killer, then see if there are coloured bars, and if you can adjust the ref osc phase with VR269 so there is only one bar running through slowly.
This means that the phase det DC conditions - diodes, caps and TR212 are OK.
Look at TP269 - should be a couple of volts of 4.43MHz at least. If not, or if no coloured bars, look at voltages around TR213/214.

If you cannot adjust the bars to a "null" then all the components around the phase detector and TR212 are worth a look. Has anyone been at the core?

HTH, and keep us posted!

[af024]

Thanks everyone for your comments, useful suggestions and explainations. Time to book kitchen table space again so that I can have another looksy. I'll update this thread as soon as I've got some more info.

Thanks for your very kind offer too Mike - I'll keep at it as long as my poor brain cell will allow and keep you posted. I'm very grateful for such thoughfulness.

Back soon (I hope) ...

Regards one all all,

Andy

[af024]

Well here’s the update guys ….

The signal at TP209 was sitting at something like 11Vpk-pk. The signal the other side R270 was at roughly 10Vpk-pk.

I changed the crystal anyway (just in case it’s frequency was off and it couldn’t be pulled by the varicap) – no change, still no colour. Voltages remained pretty much the same at the same points.

With ‘running through colour’, I adjusted VR269 right through its entire travel (slowly) and it did not affect the ‘running colour’ speed at all.

I moved onto the phase detector and just in case I was going mad, I tested each and every component individually out of circuit thus:-

VR269
R270
R271
R267
R266
D205
D206
C241
C243
C242
R265
L206 dc meas only (2R)
R268
R274
R272
R263
D204

All ok (some R’s slightly high, some C’s slightly low, but nothing too drastic).

TR210 and TR211 ok.

I then lost my ‘running through colour’ and the signal at TP204. TR205 had failed. I don’t have anymore BF197s, so I replaced it with a BF422. Not sure whether that was a good replacement really since it worked for a bit then failed again (b-e voltage went to something like 1.2Vd.c. just like the original BF197).

So now it looks like two faults.

Is this witchcraft or what?

Regards,


Andy

[af024]

Even with everything working as it should, I still can't see how the 7.8kHz amp get's it's reference signal. I mean, if the output from VR269 is supposed to be a DC signal only, then where's the pulse?

Perhaps I've finally gone mad.

[cr1100]

Attached is a comprehensive rundown on vintage PAL decoder servicing which may be of use. If you have any queries, drop me a line,
Alf.

[af024]

Thanks Alf, very thoughtful - I'll have a read and see if I can digest.

Regards,

Andy

[Focus Diode]

I don't know much about the Decca Bradfords but hope the following scan, taken from the excellent "The Comprehensive Colour TV Manual Vol.1" by J.McCourt will be of some help. This I presume is based on the Series 10 chassis which has some differences to the 30 Series in the decoder area.

Cheers,
Brian

[Mike Phelan]

Originally Posted by af024

Even with everything working as it should, I still can't see how the 7.8kHz amp get's it's reference signal. I mean, if the output from VR269 is supposed to be a DC signal only, then where's the pulse?

Perhaps I've finally gone mad.

Andy
This may be a little adrift of the mark - it is a few decades since I had anything to do with such things, but:

The voltage from the wiper of VR269 would be DC if there was no 7.8KHz "swinging burst". As there is, this ripple due to the phase detector goes from the base of TR212 to the ident amp. The R & C between base and collector, and the varicap mean that the collector is purely DC, so the ref osc phase is constant.

Comments, anyone?

[af024]

Thanks Brian, I'll print that one out and see if it helps.

As regards what signal I should be expecting at the wiper of VR269, all I can say is that I'm seeing the signal as descibed at the collector of TR211 and I'm also seeing 4.43MHz at the junction of diodes D205 and D206. What I don't see is anything 7.8kHz related to go to TR209.

By the way, does anyone think that a BF422 is ok as a replacement for a BF197. I'm wondering if it's all low gain transistors now - especially in the light of at least two transistors now with b-e voltages at approx 1.2Vd.c.. Do these BF197s, BC147s and BC148s suffer this sort of problem I wonder? The trouble is I can't seem to buy them from anywhere these days - so equivalents are the name of the game now. Should I just replace on mass and see where that goes? Not very scientific I know.

Regards,

Andy