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Old 15th Sep 2006, 6:54 pm   #1
neil29
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Default Decca Bradford 2230 - dead then green raster

2230 is completely dead, fuses intact etc.
Can anybody point me in the right direction as to what to look for?
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Old 15th Sep 2006, 7:10 pm   #2
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Default Re: decca bradford

The thermal cutout?

The H.T. might well be present, but the L.T. may be down. Check all the valves for heater continuity - and don't be caught out - there's one behind the main chassis as well! An O/C heater will cause this symptom!

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Old 15th Sep 2006, 7:52 pm   #3
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Default Re: decca bradford

Hello Neil,
As Steve mentions it may well be an O/C valve filament usually the PY500 or PL509. Its more likely to be something else as you say it is completely dead. If the tube heaters are unlit, you will have to check the mains input with a meter. You may find the on/off switch is faulty especially as you say the mains fuse is ok. Have you any basic test gear i.e. a very cheap meter? Regards John.
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Old 16th Sep 2006, 4:32 pm   #4
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Default Re: decca bradford

Steve, I did check all valves for continuity as well as the wiring/plugs etc.

John I've already replaced the mains switch as it was totally useless.

After resoldering the line output valves etc, I put the chassis back in and switched it on, and the heaters lit up, the line output stage buzzed into life and I've also got sound trouble.

Picture's a bit dark and its green, no other colours at all.

Any suggestions?
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Old 16th Sep 2006, 5:19 pm   #5
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Default Re: decca bradford

Hello again Neil,
At least you have got something on the screen. I suspect that the first anode pots have been 'twiddled'.
They are on the convergence board and should be set to give a good black and white picture with the colour turned down and the brightness turned about two thirds on.
That's a start anyway.

Another thread regarding a faulty MC1327 I/C in the Bradford surprised me. I have never replaced one of these but it could cause your symptoms but very unlikely.

If you fancy a trip along the M4 I would be more than willing to sort it out, FOC of course!

Regards John.
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Old 16th Sep 2006, 7:36 pm   #6
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Default Re: decca bradford

Hello John

Even when the first anode pots are turned up fully, all I get is a very dull green picture, with virtually no contrast, with every control turned to its limit.

I also appreciate the tube is probably well worn by now, so do you still think the IC is at fault or could there be something else as well?
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Old 16th Sep 2006, 8:46 pm   #7
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Default Re: decca bradford

Hi
This is probrably the tube but you need to check the base voltages first.
Do the A1 pots alter anything - i.e. make the picture even darker?
If they do then I would suspect the tube at this stage and you need to check it next.

Danny

PS: I have never known an IC to fail, but these are a few years older now.
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Old 16th Sep 2006, 9:07 pm   #8
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Default Re: decca bradford

If it's green with flyback lines could be the green output transistor o/c; pull the green output lead off, does the green go?
Can you see any other colours now? If so swap the green and red over: has it gone peak red now?
If so check the output transistor these can also go dry. Also if it's not that pull the aerial out and compare A1 and cathode voltages on all 3 guns; these should be similar. If not that's where the problem lies, also check the spark gaps for S/C

Hope this helps
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Old 16th Sep 2006, 9:42 pm   #9
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Post Re: decca bradford

Hi Neil
Glad you got the set going. I cannot add much to what has already been said. As has been mentioned you need to check the cathode and A1 voltages on the CRT base, other things to check are those video drive presets located just underneath the RGB output transistors on the decoder board, which use to cause lots of problems then of course the MC1327 IC.
Mick's advice on swapping the output leads to check the other guns is highly recommended and a quick way of localising RGB output faults and checking the state of the tube.
As I live just down the M4 there is a possibility I could come and take a look if you get stuck.
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Old 17th Sep 2006, 3:59 pm   #10
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Default Re: decca bradford

Mick, I did swap over the leads and and it made no difference at all.
The green was the strongest each time whether it was connected to the red/blue output.
Do you/anyone else think it is the IC, (which is the texas one) or output transistor etc?
I don't have a circuit diagram so can someone let me know the voltages to expect?
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Old 17th Sep 2006, 4:49 pm   #11
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Default Re: decca bradford

Neil, it can't be the ic if you've swapped the leads you've proved the fault is elsewhere, have you checked the A1 voltages? if they are ok and the spark gaps are ok then I'm afraid the CRT is suspect.
Mick
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Old 17th Sep 2006, 5:55 pm   #12
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Default Re: decca bradford

I agree with Mick about the CRT but I would still like to see what the A1 voltages are! The Bradford chassis was very 'kind' to crt's and I can only remember replacing a few. I cannot find my Bradford manuals but I seem to remember a preset brightness control somewhere on the chassis. It may be on the long board that carries the audio stage and psu bits. Oh well we tried.
Regards John.
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Old 17th Sep 2006, 8:51 pm   #13
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Default Re: decca bradford

Tube pins:cathodes, 2- red, 6- green, 11- blue, around 200v plugging aerial in and out should cause meter to swing if all is ok
A1's 4- red, 5- green, 13-blue, around 400v
preset brightness is VR601 on power/sound panel
Hope this helps
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Old 18th Sep 2006, 12:04 am   #14
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Default Re: decca bradford

Hi Neil
It looks like you have proved the fault away from decoder board (and RGB output stages) towards the tube or A1 fault. If you check the A1 voltages on the pins that Mick mentioned they should be varied by the A1 pots from between aprox 400-700v as the A1 network on these sets lies between the HT line and boost line.

As this set uses RGB drive all the grids are strapped together so you should see somethink betwen -38 to +25v on pins 3,7 and 12 depending on the setting of the preset brightness control which is located on the power supply sound output board.

Unfortunately it does look like you could have a flat CRT here, but check the voltages on the CRT base first. If it is the tube they do occasionally show up on ebay and/or it might be possible to boost it with a tube reactivator.
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Old 18th Sep 2006, 2:01 pm   #15
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Default Re: decca bradford

Hi
Some Bradfords were fitted with Sylvania tubes (22", yellow label) which were hopeless compared with the Mullards, and ofter gave these symptoms. Boosting doesn't work with them!
Glyn
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Old 18th Sep 2006, 4:03 pm   #16
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Default Re: decca bradford

The voltages are:
A1 = 536 to 662
Grid = 23
Cathodes = 115 to 125
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Old 19th Sep 2006, 6:56 pm   #17
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Default Re: decca bradford

Whoa
23v on the grid, that tube really is being driven into the ground. Our company had hundreds of these wonderful sets out on rental as far as the 22" models were concerned they either had Mullard or Mazda tubes in them.
Never heard of Sylvania tubes before.
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Old 19th Sep 2006, 7:13 pm   #18
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Default Re: decca bradford

Due to a shortage of A56-120X tubes in the70's Mullard rebadged some Sylvania and Japanese tubes and yes they had yellow labels. They were always marked with country of origin. Sylvania were the first company to make a 'bonded' 23" black and white tube in 1961, the 23SP4 and that was hopeless in the long life department. Looks like you have a rotten one in your Bradford. Regards John.
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Old 19th Sep 2006, 8:10 pm   #19
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Default Re: decca bradford

Hi
Keep an eye open on ebay - there were some NOS Mullards a few weeks ago.
The Sylvania CRTs were mainly fitted to sets badged Telefunken (the one with the odd 7-flap varicap tuner) - probably that's why you didn't come across them, Simon - lucky you! This was towards the end of the chassis production run but before the final 'blackfronts'.
I thought the tubes were Hungarian, but I might be wrong. American suggests RCA, not noted for longevity.
Not sure if I should admit this, but I used to find disconnecting each cathode and grounding it via a medium value resistor briefly would result in a brilliant colour on screen, each of the same intensity on a good tube. A bit brutal, I admit, but it served as a pretty good way of determining a duff tube, especially in the field!
Glyn
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