Personally I always want to know what the EHT is running right at first switch on, this is why I ended up getting an in-line EHT meter like the BrandenBerg. Right from the word go you safely ascertain if your EHT is too high or low and if too high switch off before any problems. I've learnt on colour TV's to avoid the wand type meters, they have caused more problems with cracks spits and flashes especially if the EHT is way high. One thing I would have also done before switch on is have the EHT preset turned right down, you can always go up once you know things are ok and these sets run fine on 19-20kV. You might want to consider doing that. In fact I almost always run my sets down from the 24-25kV settling on around 21-22kV, it gives the triplers an easier time and we do need to start preserving these now.
None of the following is based on experience, it is however based on reading the service documents in the library. Hopefully it might be of use to you or jog others into debate and comment.
One thing I've read is you should check 6R11
, if this fails the full unattenuated
output of the tripler gets to the CRT and will cause the S.G to eventually explode and or burst into flames. Probably not related to your CRT PCB trouble but certainly something I would want to check to at least eliminate for peace of mind.
What is more likely and requires checking is there are 9 other spark gaps on the PCB other than the obvious biggy SG3
(rated 6-8kv) that need to be checked. Your violent cracking could be emanating from one or more of these. SG1,2,4,5,6,7,8,9
All are rated at 1.5kV. They should all be checked for condition and the gaps cleaned of any deposits.
Also there are two transistors on the CRT PCB 4VT1
, debatable whether they've survived the violence you describe in that area and are often discussed as being toast due to flash-overs. Failure of these results in dull picture and flyback lines.
The following also seems relevant to some of your initial "first light" symptoms and worth a check:-
goes open the beam limiter diode 4D2 is left without any forward bias and there will be a loss of brightness
. For uncontrollable brightness also check the clamp/brightness pulse feed cap 8C11
. Check 4R5/6/7
which can go open removing the appropriate first anode supply and subsequently colour loss
. This is also true if 7C7/8/9
on the convergence panel go short or the first anode presets 7RV11/12/13
go open so check those too.
As you have EHT we know the tripler is OK so perhaps you should remove and test the BD131 line driver transistor. Again reading suggests this is also prone to going short which also blows the l.t fuse 8F1
has it? Sometimes it goes open, if that is the case you should check 5C25
Don't forget the other checks I mentioned 7 posts back, I'll repeat here so you have them all in one place.The few checks from my earlier post eliminate from suspicion for a "No Raster" scenario. Forget No.2 as we now know you have EHT.
- Is 6R6 burning up? then check 6C6 .0047uF.
Is 6R9 is burning up? Check Tripler as it may be toast.
- You have EHT but no raster so check 8R1 36R and 6R7 2.7K.
- Check 6C13 for s/c ? .01uF, 6R7 will overheat as no A1 voltages.
- Check 8R1 for o/c - this will give no voltage to the RGB emitter followers 3VT12, 3VT13, 3VT14
- Check the demodulator and matrix I.C. it could be faulty, destroyed by the arcing? (see voltage table below)
A good few things to get on with and all easy checks there without needing to power up and soiling your undergarments
Brian said the HT was high and he adjusted it but did not state what all the other rails ( 18V,20V,25V, 200V) were like, are they OK. the 18V supply to the decoder and I/F especially interesting as the no colour could be related.
Anyway Steve had it worked out of the box so to speak what fun would that have been and what would you have learned? The greatest fun and education lies in a set that fights you and in the process so much is learned.