Strange fault with Decca 30 series

[Hunts smoothing bomb]

Hi all,

Have been playing around with an old Decca CS2030 that Tazman1966 kindly gave to me, I boosted the tube a couple of times and it responded well.
I left the set running for probably 3 hours and then the picture sudenly went off! It didn't fade, it just went straight out.

The whole set is working as it should, the only thing is no picture.
I went in the shed last night to take some voltage measurements and I have:-

25Kv on the tube final anode
around 400v on all the A1's
around 140v on the tube cathodes
around 14v on the grids
and of course the tube heaters are lit!

If I pull the cathode leads off the decoder panel and momentarily touch them to chassis I get nice red, green and blue rasters respectively.
Could it be a fault with the tube or anything else as I'm a bit stuck.

Thanks
Lee

[Steve_P]

Not going to be the tube. Not got the diagram to hand, but resolder the tube base, make sure all the wires are in securely and then head for the power supply board and check fuses. Also, are the 3 output transistors passing any current?

Cheers,

Steve P.

[Mike Phelan]

Voltages sound reasonable, Lee, albeit the A1s are a wee bit low.
Are the tube sockets all actually touching the pins of the CRT?
Try earthing the CRT grids to see if you get a picture.
Try Steve's suggestion: easiest way is to short the base and emitter of one of the output transistors and see if the collector voltage rises.

[Studio263]

The A1's do sound a little low but it depends what sort of meter you are using, the source impedence is pretty high.
When you short the cathode leads to the chassis the resulting coloured rasters should be searingly bright, possibly accompanied by "complaining" noises from the line output stage. If they are a bit weak that supports the low A1 theory.
However, in the context of the rest of the story it sounds to me as if the tube has flashed over and popped one of the IC's in the decoder. Blasting the cathodes throws muck around everywhere inside and you often get a few fireworks in the first few hours (I ruined an RBM A823AV like this back in the day, apparently you should tip the set forwards a bit when you blast it so the bits fall away harmlessly, I didn't becasue the set was a big cabinet model, black mark). A 'scope with a x10 probe will show if you have and video signal on the collector of each RGB output transistor, if not trace back from there.

[dominicbeesley]

Hi Tazman, I think Dave gave me most of the Decca bits when I bought his a while back.

Not sure I've got any spare decoder chips available though...

Hi Lee,

I'd say that if you're getting something when shorting the cathodes to ground that you're probably not getting any signal out of the drivers.

If you could hook up a scope on one of them see if there's anything coming through to the drivers. If not I'd check:

a) Try wacking up the pre-set brightness R601 I think on the PSU panel

b) Is the video getting through from the IF (black and red wires at the bottom left of the decoder panel should be about 2V p-p video coming through on the red wire)

c) Is the blanking circuitry up the spout - check to see if video signals are getting to the decoder IC but nothing coming out to the drivers.
I cant remember which pin of the IC is the blanking input but it's R287 that goes to it, you could try shorting out R287 with a jump lead and see if that has an effect?

d) Try fiddling with the beam limiter (Sorry can't remember the VR number but its on the top right of the decoder board). If this is playing up you may be overrunning the LOP valve so make sure its not getting too hot and bothered.

The beam limiter monitors the cathode voltage of the LOP valve and if it goes much above a couple of volts it attenuates the video to stop an overbright picture damaging the cathodes and LOP valve.

If you've boosted the emission a lot you may have put this out of whack? Or it could just be a crappy preset resistor, they all seem quite noisy and fidgety on my set even after plenty of contact cleaner.

Dom

[Hunts smoothing bomb]

Hi guys,

I have tried the following,

a) Adjusting preset brightness

b) fiddling with beam limiter and DC restoration? (the other pot near
the beam limiter?)

Thanks for all your info and advice chaps, it's a great help!
I probably won't have any time now to have a fiddle until
Saturday night, so until then.......

Cheers
Lee

[ALANS ANITAS]

Hi

Donberg Electronics in Ireland list the MC1327 at 4.19 Euros plus p&p. They also list LOPT's for the Philips G8 .

Regards

ALAN

[Hunts smoothing bomb]

Hi Everyone,

Glyn (Welsh Anorak) Has very kindly sent me two MC1327 IC's in the post, It's my birthday Thursday and so am treating my self to two days off work to play with tellies It doesn't annoy SWMBO then, only that she's jealous that she doesn't have the days off!! anyway so hopefully the IC's will turn up before then. I canit remember what the other IC is but one question,

If this other IC had popped then I presume there would be no sound? as I have sound I presume it to be perfectly servicable?

Thanks
Lee

[Hunts smoothing bomb]

Hi all,

Studio263 you were indeed right! It was indeed a U/S MC1327. A big thankyou to Glyn (welsh Anorak) who kindly sent me two MC1327 IC's.

I had an IC holder in my bits box so soldered this in so the new IC can simply be plugged in.

Looking at the pics makes the tube look very good indeed but I must say the camera is lying! The tube is very low it suffers with blue flaring. I'm just seeing if some use will improve it a bit.

Cheers
Lee