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What to do with a Philips T-vette...

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Lloyd
(@lloyd)
Noble V-Ratter Registered

Ok, I've found an AC188K in a jar of junk which measures sensibly on the tester, a quick google shows its similar to the AC128, just slightly lesser ratings, so Ive bunged it in! Back on the 12V bench PSU, and I've set the 11V line to 10.8ishV, its currently 10.87on 625, and switching to 405 it goes to 10.91V. The transistor is getting warm, but not hot, but it does have it's own heatsink block which may be keeping it just right. I'll run it a bit longer and see how it goes, if all is ok then it'll be a new AC128!

Out of interest, I stuck the AC128 on the component tester again, this time it says it's a PNP, and looks sensible, but if you warm it up it then says 'no, unknown or damaged part' and stays like that until it's cooled down again, where it reads normally.

Regards,

Lloyd

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Topic starter Posted : 23/10/2021 8:07 pm
Lloyd
(@lloyd)
Noble V-Ratter Registered

Well, it's still running over 20 mins later, and the 11V line is currently 10.88V, so it's not gone up much! It's on 625 lines, the 405 whistle was deafening! I've plugged the speaker in, and I've got good strong sound, I think it's on BBC4 presently.

Regards,

Lloyd

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Topic starter Posted : 23/10/2021 8:32 pm
PYE625 liked
sideband
(@sideband)
Famed V-Ratter Moderator

I have found that germanium transistors can become heat- sensitive. I found this out when tackling the Perdio Portarama with excessive height. Just holding between thumb and finger was enough to bring the height down. A new OC81 cured the problem.

Some ACxxx also have the tin whisker problem. I had to replace the driver (AC128) in my Roberts as it had intermittent shorts between the electrodes....if you whacked it hard enough it would read normally but I decided that wasn't best practice so replaced it!

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Posted : 23/10/2021 9:45 pm
Lloyd liked
Lloyd
(@lloyd)
Noble V-Ratter Registered

That’s interesting! I wonder what causes it? Maybe I’ll make a thermometer out of it!

I was reading your thread on your T-vette earlier, you certainly had a few problems with it!

I’ve got a few photos of the earlier shenanigans, first up, the dodgy solder joint on the 10 ohm resistor

95DC6F04 F61D 4FA5 A389 B31FA209D0B5

a few shots of the roasted system switch contacts

48E3D9A7 7085 4FAB BC7F 75020C34C268
BDDA5031 DBA1 4332 B63F 4A5E927269C1
443EC735 B597 477E A05A 95FE29AD2033

the interesting bench setup…

CFF3EA60 1C99 410A A4C9 D855BE35A28F

and the temporary replacement for the AC128

6013358D E5B0 4B12 9D2C 96219237BD0D

maybe tomorrow, if the set behaves itself I might actually get the crowbar in place then we can put the tube in and see what we get!

 Regards 

Lloyd 

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Topic starter Posted : 23/10/2021 10:17 pm
PYE625 liked
PYE625
(@pye625)
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Tranny-sisters are funny things, but when they are made from geraniums, you enter an Alice-in-wonderland world. Anything can happen 😳 

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Posted : 23/10/2021 10:46 pm
Lloyd liked
Lloyd
(@lloyd)
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I'm currently having a play with the crowbar (!), just wondering what rating of fuse to give it? I was thinking along the lines of around 100mA, since the tube heater only needs around 60mA, Also wondering whether to go for fast blow or slow blow, I was thinking fast would be better, not just for cutting power to the CRT heater quickly, but also the zener diode, as I'd like it to still be useable after a fault!

I did find some AC128's, but they turned out to be faulty, one looked totally unused, still has it's 2" long legs! but the component tester showed it as 2 diodes, and in the set it clearly wasn't working at all, but the AC188K that I bunged in has held up well. I'm going to try the tube in a bit and power it up carefully just to see what I get on screen.

Regards,

Lloyd.

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Topic starter Posted : 25/10/2021 7:25 pm
PYE625
(@pye625)
Famed V-Ratter Registered

Hi Lloyd, don't forget that the current taken by the heater will be more when switched on from cold. You may be better off with an anti-surge fuse, but in any case it would be worth monitoring the current with a meter upon switch-on to get an idea of cold and then hot values.

A switch-on surge of current could be quite a bit higher than 100mA, especially with capacitor charging etc.

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Posted : 25/10/2021 8:07 pm
Lloyd liked
Lloyd
(@lloyd)
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We have life!!!

The tube is in, the LT is set to 10.8V, the zener is fitted to the tube base straight across the heater, a fuse has been added, but it’s 500mA slow blow, the smallest fuse I have in stock.

There were new faults to sort out on switch on, I was greeted with frame collapse, I spent a good long while looking for duff solder joints and pinged off wiring, and found none.

IMG 8649

I checked around the resistors in the frame stage and also found nothing amiss! During these tests I made a guess that the output stage was working, as the frame would begin to expand when probing around the oscillator transistor. I checked the transformer that contains L4086, L4087 and L4088, this also checked out fine. There are only 2 caps in the oscillator area of the circuit, one a mullard mustard which was most likely fine, and a yellow electrolytic 4uF 16V, which I decided looked suspect, so I swapped it for a new 4.7uF 63V part, I have a bag full of them. This time I got full frame scan! It’s great to see a picture on this set again after all this time!

IMG 8653
IMG 8654

There is still more to do, there is a sync fault where the frame jitters and sometimes rolls depending on picture content, even though there is a good range of lock on the hold control. It’s looking good so far though!

 Regards 

Lloyd 

 

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Topic starter Posted : 27/10/2021 7:31 pm
PYE625
(@pye625)
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Excellent !

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Posted : 27/10/2021 7:39 pm
Katie Bush
(@katie-bush)
Illustrious V-Ratter Moderator

That's brilliant - if you see what I mean!

Aside from the frame fault you describe, everything else looks okay.

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Posted : 27/10/2021 8:01 pm
Lloyd
(@lloyd)
Noble V-Ratter Registered

This TV is doing my head in! Just when you think it's OK, it decides it's not!!

I've discovered the frame sync fault is only present on 625 lines, 405 is perfect! So when a bright scene comes along the frame looses sync just slightly, so the picture drops to the bottom of the screen and jiggles about down there, occasionally rolling upwards too, once the bright scene is gone everything reverts to normal. There is a slight bit of pulling on the line too. As you can imagine, a scene with flashing lights sends the poor thing into a frenzy of vertical bouncing! Top of the pops would have been painful to watch on this set...

It's really annoying as the picture is excellent apart from this. I'm no expert when it comes to transistorised TV's, but I've tried looking at the waveforms in the manual, and had a stab round with the scope, waveform '3' seems to vary with picture content, it looses it's defined pulse shapes when a bright scene is encountered, but I've no idea why!

Test card looks bloomin awful on it too..

IMG 8658

Regards,

Lloyd

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Topic starter Posted : 30/10/2021 8:10 pm
Katie Bush
(@katie-bush)
Illustrious V-Ratter Moderator

Ooh, a bit of nasty 'cogging' - decoupling caps? Or maybe a need for some attenuation in the RF feed?

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Posted : 30/10/2021 8:32 pm
Lloyd liked
PYE625
(@pye625)
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Posted by: @lloyd

Top of the pops would have been painful to watch on this set...

Some might say on any set! 😋 

Have you tried reducing the RF signal input, it could be overloading.

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Posted : 30/10/2021 8:44 pm
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Lloyd
(@lloyd)
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Didn’t think of that! I’ll give it a shot, I’ve got some attenuators somewhere 🙂

 Regards 

Lloyd

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Topic starter Posted : 30/10/2021 11:14 pm
Katie Bush
(@katie-bush)
Illustrious V-Ratter Moderator
Posted by: @lloyd

Didn’t think of that! I’ll give it a shot, I’ve got some attenuators somewhere 🙂

 Regards 

Lloyd

Seems like a logical step. If it only affects 625 lines, the issue must lie in the IF or vision detection sections. The frame TB is the same for both standards, so unlikely to be at fault - But I'm happy to be proved wrong.

The easiest to implement, and will show itself up if it is the case, plus, the only real difference in the entire set, is where it gets its RF from. It's very unlikely that the either source will replicate the original broadcast levels, but it's also unlikely that they'd be equivalent to each other.

Be ready for a lot of attenuators! - I've had three or more in series before today, and still needed more. A quick, but very unscientific method would be to withdraw the AE plug from its socket, and very slowly and carefully draw it away from the socket. If you're lucky, you'll find a spot (an air-gap) just right enough to allow a signal to pass, but also to be attenuated.

 

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Posted : 30/10/2021 11:23 pm
Lloyd
(@lloyd)
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I thought we'd gotten to the bottom of this problem earlier! I found an attenuator, plugged it in, tuned in the set, and we had a perfect picture! No frame jitter, so I left it playing on the bench whilst I did a few other things, and when the video with a few flashing lights came on sure enough, it decided to start it's old tricks again... Strangely, mis-tuning would fix the frame issue, but then of course the sound was just noise, and the picture was covered in patterning.

Another interesting thing I noticed was that when I first turned the set on the left side of the picture had a bit of a horizontal linearity issue, and as the set warmed it would begin jiggling until eventually it just stayed without the linearity issue! I did manage to replicate the issue by blasting inside the set with a big fan to cool it, once the linearity issue had gone away again there was what looked like ringing on the picture, more noticeable on a blank raster as light and dark areas, with a very noticeable light band at the right side of the raster, not sure what's going on there!

I don't think this set likes me....

Lloyd.

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Topic starter Posted : 31/10/2021 9:37 pm
Jayceebee
(@jayceebee)
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My T-Vette was not particularly stable on 625 either but 405 was rock solid, also had a bit of a soot and whitewash look about.  Changing a couple of sub 1uf electrolytics in the AGC/video stage I think improved matters.

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Posted : 31/10/2021 10:27 pm
Katie Bush and Lloyd liked
Lloyd
(@lloyd)
Noble V-Ratter Registered

Cheers John, I'm just looking at the caps now, there's some right odd values in there! What would be best to replace a 1.6uF cap with? I've got some 1uF caps, or I can order some 2.2uF's, I'm going to have to order some 47uF's to replace the 40uF that I've just found! As for the 0.64uF cap, I've replaced it with a 0.68uF film cap since I have a bag full that I got to build the MiniMod.

Regards,

Lloyd

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Topic starter Posted : 01/11/2021 9:19 pm
sideband
(@sideband)
Famed V-Ratter Moderator

You could try scoping the supply to the sync stage. Those yellow electrolytics seem to hold up quite well but they are getting on for nearly 60 years old now. Some of those old Philips values do seem a bit odd by todays standards but you can easily use 'nearest' conventional modern values. I never understood why they used .47uF and .68uF electrolytics. They weren't very good and we used to replace them with polyprops. You could find 64uF, 640uF, 4uF etc in there. Just use the nearest modern value and it will be fine.

 

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Posted : 01/11/2021 10:39 pm
Lloyd liked
Cathovisor
(@cathovisor)
Illustrious V-Ratter Registered

To me, Lloyd's instances of losing sync on bright scenes implies there's rather more video getting through the sync sep than there should be, or that AC coupling isn't as good as it could be. That says either bias or degraded electrolytics.

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Posted : 02/11/2021 12:16 pm
Lloyd and Katie Bush liked
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