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Old 12th Mar 2017, 12:20 am   #1
bionicmerlin
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Default BAIRD M628 series 620

Can anyone give me some help please.
I thought i would get on with the next set as part of my self learning.
I got this set up and running after a valve and a few components replaced.
The set has been working for about a week with a reasonable picture apart from poor hight and linearity .The controls are at max and in the past someone added extra resistors infact making the problem worse .I have now removed them. I cant find any out of spec components in these areas.
Next problem is after about a week the contrast has gone very poor .I tested the pot and resistors in this area but all ok.
One other thing is when the set is switched on often but not always it takes longer than it should to show a picture with a fast clucking noise possibly from the line output transformer.I have tried the set in the dark and cant see anything breaking down.
The set has been stored in the dry indoors for about a year.
Thanks Andy
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Old 12th Mar 2017, 12:29 am   #2
MALC SCOTT
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Default Re: BAIRD M628 series 620

Check the HT voltage, also there is a cap next to the PFL200, 10mfd if i remember gives video problems.
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Old 13th Mar 2017, 12:47 am   #3
bionicmerlin
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Default Re: BAIRD M628 series 620

Malc hi . There is no PFL 200 in this set. I have replaced about 65%of the electrolytic caps and tested the rest .
I have a diagram but no voltage test points shown on it so I'm stuck not knowing what the HT voltage should be.Measuring after the main filter cap it's about 210 volts .Can anyone advise me on what it should be. Thanks anyway Andy
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Old 13th Mar 2017, 1:08 pm   #4
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Default Re: BAIRD M628 series 620

Hi Andy,


Have any of the old capacitors been replaced in this set yet?
Because of their age and the materials used for their construction these will nearly always be leaking electrically and passing volts onto parts of the circuit that they shouldn't and upsetting things, this will more than likely account for the faults with your contrast, height and linearity, even the "lazy" line stage.


I have a Baird M648 console version, pretty much the same chassis as yours but with a 23" CRT and I had all of the faults you describe before I replaced the capacitors, a very good performer though after restoration.


All the best with the set, they are good IMHO.


Cheers
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Old 13th Mar 2017, 9:53 pm   #5
bionicmerlin
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Default Re: BAIRD M628 series 620

Lee
Apart from the main filter cap and a couple electrolitics on the if panel I have replaced them all apart from the mustard ones . After having a couple old ones blow up i thought it was the way to go.is there anything I'm missing here.
I did have a great picture to start with although a night isue Andy
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Old 14th Mar 2017, 10:54 am   #6
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Default Re: BAIRD M628 series 620

Even though not often, it has been known for the mustard type caps to fail, I had one go S/C on a Stella D/S set I had about 10 years ago, it was a heater decoupler and it shorted the valve heaters out and lit the wirewound dropper up like an electric fire element!


Your linearity issues could well be caused by an aging thermistor in the scan coils, I had to bypass the one in my set to get an acceptable picture.


Cheers
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Old 18th Mar 2017, 11:46 am   #7
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Default Re: BAIRD M628 series 620

Hi Andy,

The M in the model number could indicate that it was a set that was sold, possibly through a dealer rather than rented, not a certainty as sometimes we ran short of sets for the Radio Rentals branches.

Lack of height can be a low A1 voltage which, if severe, could also dim the picture. The usual cause was a preset resistor going low in value which I think was the focus adjustment, it is situated at the bottom of the timebase panel on the left. There was a modification, which from memory, was to increase the value of the preset. In practice the earth end was usually disconnected with a pair of wire cutters! Well we did have fourteen or more calls a day to go to......

I am away from home at present so cannot refer to service manuals or notes and the above is from memories of forty five years ago.

Best wishes, Geoffrey.
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Old 19th Mar 2017, 11:42 pm   #8
bionicmerlin
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Default Re: BAIRD M628 series 620

Sorry for the late reply guys. I did have a very good picture apart from the height now i have a disaster i think .i was thinking about putting the set to one side for a few weeks as i have been on it for months .Limited knowledge but common sense is not working this time.Can you help on this the thermistor that Lee talked about was falling apart i shorted it out and as you can see this is what happened unless some other fault has cropped up.

Im interested dont know if you guys know the answer but the VHF tuning knob has no channel markings . There is 2 Waffers inside to tuner.There is a vhf rf socket on the rear but bypasing that is a twin feeder cable thats been cut off on the back of the set going directly to the tuner. Andy
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Old 20th Mar 2017, 12:01 pm   #9
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Default Re: BAIRD M628 series 620

Hi Andy,

All I did was short the thermistor out in my set, not really sure what's happening there but it looks like you have trouble in the frame output section, I think C's and R's need checking here.

Cheers
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Old 20th Mar 2017, 10:57 pm   #10
bionicmerlin
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Default Re: BAIRD M628 series 620

Thanks Lee I will have another look Andy
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