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CTV New addition, Baird M708

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Katie Bush
(@katie-bush)
Illustrious V-Ratter Moderator

Those rogue wires - You don't suppose that they're the remaining vestiges of someone's attempt at a 'poke and hope' to find points to extract, or inject, the signals they needed, or thought they needed, to do whatever they were trying to do?

I'm just curious because I have a Peto Scott mono 405 set which has a couple of rogue wires hanging out of the back. I haven't investigated it yet, but the seller said it was his set when he was a youngster, and he was trying to "adapt it to record from".  Of course, that was about forty years ago and he couldn't remember quite what he'd done, or if it actually worked.  I'm just leaning toward the idea that your Baird may have fallen upon the same kind of 'best guess' or 'suck it and see' approach.

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Posted : 17/10/2021 3:59 pm
colourmaster
(@colourmaster)
Reputable V-Ratter Registered

Hi Katie , 

I have checked the system switches and they seem to be working as they should ,  it looks to me to be a loss of the luminance signal and the sync . I will trace it through the stages , but not now as I'm bleary eyed . 

Regards Gary.

IMG 20211017 171407
IMG 20211017 170844 1

 

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Topic starter Posted : 17/10/2021 6:33 pm
crustytv
(@crustytv)
Vrat Founder Admin
Posted by: @colourmaster

t looks to me to be a loss of the luminance signal and the sync

Hi Gary, as I'm sure you've read and are aware, but it's worth mentioning in the thread for completeness and those looking in that are not, so they can follow.

The PFL200(b) luminance half fail and can cause problems for; R359, C303, R345, D24, R355, R347 are all worth checking. One other fault I've read about for luminance is L230 and finally TR32, luminance delay line driver transistor.

lum1

For sync D31/D32 are worth checking and as mentioned in servicing by E.Trundle replace if they are OA81 replace with BA144 or 1N4148. Also check R461,R469, R475 & C442.

sync

As I say apologies had you already planned all that, but as I say above worth just mentioning for others.

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Posted : 17/10/2021 8:54 pm
colourmaster
(@colourmaster)
Reputable V-Ratter Registered

Hi Chris

Many thanks for that info , I'm not sure if the two faults are connected but those tips you've posts will be invaluable .

Regards Gary.

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Topic starter Posted : 17/10/2021 9:13 pm
Nuvistor
(@nuvistor)
Famed V-Ratter Registered

Not in this model but in others I have had the luminance delay line go open circuit.

 

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Posted : 17/10/2021 9:45 pm
Jayceebee
(@jayceebee)
Noble V-Ratter Registered

Hi Gary, sync take off is from the resistor that feeds the red cathode of the CRT so luminance needs to be present all the way up to this point to get any syncs.

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Posted : 17/10/2021 11:31 pm
crustytv liked
colourmaster
(@colourmaster)
Reputable V-Ratter Registered

Hi John 

Many thanks for your info , I'm feeling my way around this set as I have no experience with this chassis . I'll keep everyone updated when I resume .

Regards Gary .

 

 

 

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Topic starter Posted : 18/10/2021 6:49 pm
Katie Bush
(@katie-bush)
Illustrious V-Ratter Moderator

@nuvistor One engineer of about 45 years past once told me a cheap and dirty trick was to simply bypass the delay line. The luma and chroma will be out of register, but the sync will be present. If the syncs lock up, the picture will be clearly visible, albeit with luma and chroma somewhat displaced from each other, indicating an O/C delay line.

I once bought a G6 where the luminance delay line had a thin strand of what looked like telephone wire running from one end to the other. The picture wasn't hideous, but it definitely wasn't right. The previous owners had been happy enough to use it that way for about a year and a half! - A spare delay line from a scrap set soon put things to rights.

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Posted : 18/10/2021 7:19 pm
Nuvistor
(@nuvistor)
Famed V-Ratter Registered

@katie-bush 

I had to short them out to get the set working until I went back with a replacement. I always told the customer what I had done and it’s effect on the picture, when I returned they usually said it wasn’t a big problem. Luckily it wasn’t a common problem so it only occurred a couple of times or I would have had replacements on the van.

 

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Posted : 18/10/2021 11:42 pm
Till Eulenspiegel
(@till)
Famed V-Ratter Registered

The sync separator is conventional except for a differentiated line pulse from pulse shaper triode V8b cathode being applied to the grid of the sync separator valve V15b via C427 (5.6pF). 

The source of the pulse comes from the centre tap of deflector coil balance coil L415

Baird 700 Series Luma Amp
Baird Luma 2
Baird 700 Series CRT cct
Baird Luma LTB

 Till Eulenspiegel.

 

 

 

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Posted : 20/10/2021 3:21 pm
PYE625
(@pye625)
Famed V-Ratter Registered

I know that on my set V5 had to be in good condition. For instance, a new Mullard PFL200 was found to be needed as a couple of other used valves slowly went into thermal runaway resulting in a red hot anode (V5b). On- screen symptom was steadily increasing brightness. R359 (cathode resistor of 68 ohms) will bear the brunt if any problems do, or have, arisen here.

This may not be of any relevance to the current fault condition, but is something to watch for.

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Posted : 20/10/2021 5:34 pm
colourmaster
(@colourmaster)
Reputable V-Ratter Registered

Hi Everyone

I finally managed to get back to the Baird this afternoon , I think that I may have found the problem . A lead with a resistor has come adrift from the back of the IF panel . I can't seem to be able to trace where it should go .

Regards Gary.

IMG 20211030 145343
IMG 20211030 145505
IMG 20211030 145505

 

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Topic starter Posted : 30/10/2021 3:24 pm
crustytv
(@crustytv)
Vrat Founder Admin

Where does the other end of the lead that is loose, go? The resistor on the end of the lead, looks to be Red, Red, Orange, Gold, which as a 22k 5%. If I've read that right, and from the position of the lead in the first photo before you moved it, it looks like that may well be R500, which is a 22K 5%. In which case it goes to the top left outer, most trace.

20211030 154142
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Posted : 30/10/2021 3:28 pm
colourmaster
(@colourmaster)
Reputable V-Ratter Registered

Hi Chris 

Thanks for your help , I can't seem to be able to trace where the other end goes . It disappears down into the loom . I'll try and trace it . Do you think it could have something to do with the luminance ? .

Regards Gary

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Topic starter Posted : 30/10/2021 3:48 pm
crustytv
(@crustytv)
Vrat Founder Admin

I might be wrong, but I believe the resistor is R500 the lead it's attached to will go over to R94/R95 at the base of TR10, try continuity check to confirm yay or nay. I wouldn't trust me though, as I'm no expert, and I'm more often wrong than right, but it's probably worth a check just in case I'm not.

20211030 155535
20211030 155552
20211030 154142

 

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Posted : 30/10/2021 3:58 pm
colourmaster
(@colourmaster)
Reputable V-Ratter Registered

Hi Chris, thanks for the info . I've tried it where you suggested but still no difference . I'll plod on . 

Regards Gary.

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Topic starter Posted : 30/10/2021 5:34 pm
colourmaster
(@colourmaster)
Reputable V-Ratter Registered

Hi again 

Forgot to mention that I've been enjoying some test card and music through my trusty old G6 while looking at the Baird.

IMG 20211030 161823 2

 

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Topic starter Posted : 30/10/2021 5:39 pm
ntscuser and PYE625 liked
crustytv
(@crustytv)
Vrat Founder Admin
Posted by: @colourmaster

Hi Chris, thanks for the info . I've tried it where you suggested but still no difference . I'll plod on .

Hi Gary, I had a feeling it wouldn't fix the underlying lum fault, but I'm fairly sure that's where your loose wire should be fitted. It looks to be associated with signal strength, as TR10 is the agc peak-level detector.

Keep plugging away, I'm sure you'll narrow the lum fault down to a few suspects.

p.s.

Loving the G6, hoping to get mine displaying a picture this winter when I fit the new lopt and maybe finally get past line stage troubles, to start tackling the many decoder faults which are bound to be present.

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Posted : 30/10/2021 6:23 pm
colourmaster
(@colourmaster)
Reputable V-Ratter Registered

Hi Everyone

Today I finally got back to the Baird, I thought I would have a good look at the delay line. I noticed that the tape that is wrapped around it had been disturbed.

On closer inspection the top connection was broken, I managed to solder a link in. On switch on, I was rewarded with a none linear picture. After a few adjustments, the results are pretty good. I don't know whether the broken wire was man made or not, but I'm very pleased with the progress. Now the next job is repairing the mask.

IMG 20211113 150405
IMG 20211113 152823 2
IMG 20211113 155801 2
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Topic starter Posted : 13/11/2021 8:43 pm
Lloyd, Till Eulenspiegel, PYE625 and 1 people liked
PYE625
(@pye625)
Famed V-Ratter Registered

In a word, Superb ! 👍 

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Posted : 13/11/2021 9:12 pm
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