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Old 11th Sep 2011, 3:37 pm   #81
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Default Re: Restorers dream part 2 the chassis.

Quote:
Originally Posted by Retired View Post
Hi,
I'm wary of taking voltage readings near the LOPT but both 30P4 and U191 were lit up is it OK to take voltage readings on both these valves with my DMM?
It's a while since I worked on a (Valve) TV.
Two quick tests are for a healthy negative voltage on the 30P4 grid, which means that the line oscillator is running, and a voltage on the screen grid. The feed resistor was a common problem here.
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Old 11th Sep 2011, 5:11 pm   #82
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Default Re: Restorers dream part 2 the chassis.

You can also get an indication of whether the LOP is running by holding a neon screwdriver close to the LOPT (no connection needed).
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Old 11th Sep 2011, 5:53 pm   #83
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Default Re: Restorers dream part 2 the chassis.

Hi,

Thanks Dom; I've taken your advice and converted an old screwdriver with a well insulated handle into a LOPT earthing tool but had to use a 4.6M 2W resistor as can be seen in the picture I also insulated the blade with rubber tubing for good measure. I found an old mirror and have now set this up so that I can watch the screen. It's amazing how much time these simple tasks take to accomplish.

I did the test as suggested but the U25 is dead. I also spent a lot of time trying to find the 500mA fuse shown in the circuit diagram to no avail; I went over the whole chassis top and underneath; I checked everywhere; I even removed the LOPT cover but the only two fuses I could find were on the outer back rated at 2A and both tested OK. This is now bugging me especially after being so easily caught out with the mains cap. Whilst searching for the fuse I noticed the two pin mains connector at the back of the set but at the moment power is fed directly to the on/off switch; would something so simple cause a problem; all the valves with the exception of U25 are lit up. I'm not taking anything for granted now. I should have checked for voltage on these pins and will do so when I go back into the workshop as one is most likely live and exposed.

Thanks Bill, I've just been back into the workshop to check voltages on 30P4 and think there is a clue here as I can't find a negative voltage on any of the pins. I haven't had time to check pin outs but reading directly from the valve the readings obtained are; Pin 1 93V; Pin 2 0V; Pin 3 No tag; Pin 4 108V; Pin 5 4V; pin 6 111V; Pin 7 0V and Pin 8 6V.

Out of curiosity I also replaced U25 as Jeffrey very kindly supplied a new one with the set; like a fool I managed to break the single wire right at the envelope; I was surprised as it broke so easily; I used a needle file and trimmed the glass back and soldered a new lead on but the valve like the previous U25 remains dead.

I also noticed a weeping 0.1 cap across the end connector of the tube so I replaced this with a new 0.1uF 630V item.

Thanks also Dave; I'll find my mains neon screwdriver tester; I've not used it for years but I'll try everything suggested.

Kind regards, Col.
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Old 11th Sep 2011, 6:30 pm   #84
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Default Re: Restorers dream part 2 the chassis.

For there to be line whistle, which I guess there isn't, there has to be a negative voltage (-1) at Pin 5. The signal here comes from the line oscillator valve. This is V18.

This valve is a 6/30L2. If you haven't done so already, change C105. Other places to look are R110, R115, C107 and of course the valve itself. It might be worth giving the line hold control a clean, make sure it works OK and giving it a tweak.

Cheers,

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Old 11th Sep 2011, 7:04 pm   #85
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Default Re: Restorers dream part 2 the chassis.

Hi Col,

You could do worse than measure the voltages as Steve suggest. You should be getting something like HT volts at the LOPT valve's screen. To find the fuse look for the 6k8 resistor connected to the LOPT valve and then trace where the other end is connected to. I wouldn't be surprised if it is one of the 2A fuses and someone in the past has "upgraded" it!

If the voltage at one end of the 6k8 resistor feeding the screen of the LOPT valve is very low the 0.003 decouplin cap might be a bit leaky and pulling it low?

I'd measure voltage of all the under-chassis pins of V12b and V14. I'm not sure about your DMM but mine are all rated at 750V and I've not broken one yet.

Have you got an oscilloscope handy if so A poke around V12b might be in order to see if it is doing anything.

Listening for line-whistle can be tricky, I know you worked in an engineering firm and whenever I've been doing any grinding on metal I can barely hear line whistles for a few days afterwards!

Your discharger looks good. But beware that it may take a while to completely discharge the CRT and that CRT's are sneaky beggars and magically charge themselves back up on their own so once you've discharged it you might want to leave your discharger connected via a jump leads...but remember to take it back off before you switch on!

I wish I wasn't so busy at work at the moment - I'd pop round and have a look!

Dom
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Old 12th Sep 2011, 8:48 am   #86
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Default Re: Restorers dream part 2 the chassis.

I was out most of yesterday so I'm catchng up with the news on the T311. Sounds like there's quite a bit of the set now potentially working. Dom has covered much of the ground so it'll look as if I'm repeating some of his wisdom.

Just a reminder for those not experienced with TVs. Never connect a meter or scope to the top caps of the LOP valves. This will simply fry your test equipment. Actually there are specialist probes that can be used but they are rare and I don't have one. Everyone I know uses a spark test as described by others in the thread. You can't do much harm to an all valve set, even if you do it wrong.

Need to establish that the line oscillator is working. Line whistle, especially as you get older and your hearing goes at the HF end, is an increasingly unreliable guide. First and foremost look at the 30P4. If anything inside is glowing red hot apart from the heater/cathode then switch off because line drive is almost certainly missing. The standard test is to check for a healthy -ve voltage at the control grid of the LOP. I would have a tendency to get out the isolating transformer and scope. To the TV techs of old this is gross overkill but it's natural for me.

After you've established that line drive is present at the LOP there should be some evidence of sparks at the top caps. Again look at the 30P4 and U191. Any signs of distress? If no distress then the LOP screen resistor and decoupler are possibles. The boost capcaitor (big 0.5uf paper jobbie) and is there HT on the U191 anode. Remember that the U191 top cap is the CATHODE, the anode is pin 5.

Just realised there's a handy 22R resistor in the 30P4 cathode circuit. Not sure why it's there and what good it does but it makes a wonderful test point. Must stick a scope on it one day and see the current waveform in the 30P4.
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Old 12th Sep 2011, 11:08 am   #87
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Default Re: Restorers dream part 2 the chassis.

U25 will not light up unless line timebase is running.

David

PS Dont think you are too far away from sucess
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Old 12th Sep 2011, 1:23 pm   #88
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Default Re: Restorers dream part 2 the chassis.

Hi,

Many thanks Steve; Dom & Jeffrey for all this additional information and suggestions; I've just printed it all off ready for an afternoon session in the workshop. I'm pleased I didn't try taking meter readings on the top caps of the LOPT valves; thank you for the previous warnings.

I'll have another go at locating the 500mA fuse just in case it is hiding somewhere laughing at me; I would have thought such a fuse would be easy to spot but then the mains cap was buried and really taught me a lasting lesson.

If I end up struggling Jeffrey I'll take your advice and carry one of the scopes back into the workshop; I had to remove them to make room on the bench for the TV.

Last night I spent a while trying to find the valve data for this set as I have an AVO VCM Mk1. I have lots of data for radio valves having bought the Mauriton discs but am I missing something as I couldn't find reference to the two valves I was looking for neither are they listed in the AVO manual? The first thing I do with a radio chassis is test the valves but with this chassis it would be time consuming. I've worked on radios where wrong valves have been installed and also valves in the wrong holders.

I bet I was the only guy on the planet at 7.45 this morning looking through repair notes whilst waiting for the supermarket to open. I had taken along the comprehensive notes kindly supplied by John to study which I had printed off.

Thanks David; I'm sure you are right about it not being far off before this chassis is working; I've been out all morning but can put some workshop time in this afternoon. Thanks again everyone.

Kind regards, Col.
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Old 12th Sep 2011, 1:52 pm   #89
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Default Re: Restorers dream part 2 the chassis.

Can't remember if you've replaced the waxies yet. If not, it's probably time to do at least a few of them. The annotated schematic I think I sent you lacks component numbers. I've attached the schematic I'm using here.

Around the line scan:
  • The big 0.5uf boost cap from the LOPT to HT+. Use 470n 400V as a replacement. The new part will look ridiculously small.
  • The 0.01uf from V12b anode to the LOP grid stopper resistor. They used a 500V part originally so I used 10n 630V but I can't see why a 400V part shouldn't be OK.
  • The 0.003uf LOP screen grid decoupler. I used 3n3 400V. Value not critical so 4n7 or even 10n should be OK.
  • The 0.1uf from the U191 anode to 0V. Use 100n 400V.
While you are at it you might as well replace "that" cap in the audio output. It will save V11 and the audio output tranny from any punishment they might be taking. It's 0.02uf so 22n 400V. 47n or even 100n won't hurt if you don't have 22n.
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Old 12th Sep 2011, 2:58 pm   #90
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Default Re: Restorers dream part 2 the chassis.

Hi,

Thank you so much Jeffrey; I've just printed off the diagram and notes and am about to have a session in the workshop.

Whilst waiting for dinner I couldn't resist popping into the workshop. I replaced both 0.003uF decoupling caps on the LOPT valves and switched the set on to check for sparks in the LOPT area.

Am I correct in thinking I've reached a major milestone because U25 is glowing? I've said many times I'm colour blind and I thought U25 was glowing a lovely blue colour but to check I asked Bron to have a look and it is fact glowing purple. Both LOPT valves are glowing but appear unstressed.

I also checked the two mains pins on the back of the chassis and sure enough the thinner one is live at mains so I need to render this safe; I'm not sure if I have a suitable plug but a piece of rubber tube will prevent me touching it.

This chassis must be very near first light; all the valves glow and there is plenty of noise through the speaker if I touch the volume control.

Would U25 glow if the 500mA fuse had blown? I'll have another look for the fuse as it is shown in both circuit diagrams but I couldn't find it yesterday and I spent a long while looking for it.

Kind regards, Col.
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Old 12th Sep 2011, 3:07 pm   #91
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Default Re: Restorers dream part 2 the chassis.

If the U25 is glowing a nice purple colour it is a sign that the line output stage is operating and producing AC EHT. The U25 is either faulty [gassy] or the EHT smoothing capacitor, the brown Visconol .001uf is faulty. It can be disconnected if this has not already been done and will have little if any effect on the raster. Good progress. J.
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Old 12th Sep 2011, 3:23 pm   #92
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Default Re: Restorers dream part 2 the chassis.

Sounds good. The LOP is definitely running and trying to make EHT. Whether blue or purple, the glow in the U25 is not healthy. There should just be a dull red glow from the heater.

Quote:
Originally Posted by Heatercathodeshort View Post
....or the EHT smoothing capacitor, the brown Visconol .001uf is faulty....
Don't remember a visconol or other EHT smoother on my T311. Can't see on on the circuit either. The CRT has a graphite inside and out to make an EHT smoothing cap. I don't know if metrosils can go s/c. The one on my T311 had gone before I owned the set and it really isn't needed.
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Old 12th Sep 2011, 3:27 pm   #93
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Default Re: Restorers dream part 2 the chassis.

I'd say as Col had to grind round the end pip to solder on a new wire to the U25 that it is definitely suspect.

Col, do you have the old one you swapped it for still?

Dom
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Old 12th Sep 2011, 5:48 pm   #94
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Default Re: Restorers dream part 2 the chassis.

Hi,

Thanks John and Jeffrey. I thought it was too good to be true but at least good progress is being made and there is signs of life at the LOPT. I can't even believe I've got this far so quickly.

I'll remove the Metrosil Jeffrey then it will be eliminated; I can always re-fit it later as it is only a minutes job.

Yes Dom I still have the old U25 and will pop it back in to see if it makes any difference. I couldn't believe my luck when the wire broke from the new LOPT kindly given to me by Jeffrey.

I'm still making slow but steady progress; it's taking me a lot of time to locate components on the chassis and also to find replacements in the workshop as it is ages since I did any chassis work.

I've now replaced the big 0.5 uF boost cap replacing it with a 470n 1KV item. Also I've changed the 0.02 uF audio cap and used a 0.022uF 630V item.

My home made miniature coil winder is proving its worth; it was incredibly fiddly whilst replacing the audio cap until I wondered if I could wind a pair of small coils so gave it a go. I snipped the old cap out leaving short tails which I cleaned by scraping then made a few coils from tinned copper wire and they worked like a dream; the picture taken in macro shows one of the caps with a pair of coils added loosely plus spare coils.

I've just knocked off as I wanted to wash my hands as they were very sticky from the old wax caps. I tested the new caps before fitting them and also tested the old caps which seriously needed changing.

Next session I'll re-fit the old U25 and as these valves are so important I'll buy a couple of spares.

This is fun.

Kind regards, Col.
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Old 12th Sep 2011, 6:34 pm   #95
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Default Re: Restorers dream part 2 the chassis.

The visconol is only fitted when uncoated tubes are employed. Some versions were fitted with CRM143/CRM171 tubes that required the evil capacitor. Your version has a coated tube and will not require one. Metrosils never go S/C as they are a passive component. I think a new U25 and a twiddle on the ion trap will produce 'first light'.
The ion trap adjustment is not so critical with Mazda tubes as it is with Mullard. Count down to ecstatic enjoyment is not far away..
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Old 12th Sep 2011, 7:16 pm   #96
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Default Re: Restorers dream part 2 the chassis.

Hi,

Many thanks John.

I've now removed the Metrosil for the time being and re-installed the original U25 which came with the LOPT.

Lo and behold a result; I can now see sparks on both LOPT valves and also on the single wire into U25 (Anode?) but not the other two wires on U25. The sparks are just visible on the top caps with a nice trail on U25.

Ion trap; is this the little magnet at the end of the CRT if so my friend Mike Phelan told me to note which way it was fitted when I first started work on this set and I made a note of it? I'm pretty sure I've got all the coils on correctly and the magnet fits on bare tube between the coil and the end cap; there is room to slide it and it can be fully rotated. I might need to be careful here?

Kind regards, Col.
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Old 12th Sep 2011, 7:38 pm   #97
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Default Re: Restorers dream part 2 the chassis.

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Lo and behold a result; I can now see sparks on both LOPT valves and also on the single wire into U25 (Anode?) but not the other two wires on U25. The sparks are just visible on the top caps with a nice trail on U25.
There's a lot more volts on the U25 anode than on the LOP anode or boost diode cathode. Hence the bigger spark. The spark on the DC side of the EHT rectifier is a lot weaker than on the AC side. Typically the AC spark is hot, vigorous and blue; the DC spark is weak, spindly and yellow. You may need to earth the tool that's drawing the spark to get a good spark from the EHT rect cathode. If the U25 heater is glowing there should be EHT present.

Before fiddling with the ion trap you may want to check the CRT operating voltages. Check that the brightness control varies the grid voltage, that there's some volts (few hundred?) on the 1st anode and that the cathode voltage is fairly close to the grid voltage. It's OK to short the grid to cathode when you're aiming for first light. This at least ensures that the CRT isn't cut off by the grid/cathode voltage.

You are tantalisingly close to first light.
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Old 12th Sep 2011, 7:58 pm   #98
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Default Re: Restorers dream part 2 the chassis.

Excellent advice Jeffrey. Hello Colin, you should be able to see an orange glow from the heater within the bell of the U25. You can spark one wire of the twin wire end of the U25 to chassis with a well insulated screwdriver. The crack might startle you but it won't do any harm to receivers of this design. If you have EHT you will probably get something on the screen if the brightness is turned up and the ion trap is about right. These Ekco receivers are very hard to kill off and are very willing to produce something on the screen at an early stage. [I wish I was in your workshop, I can't stand the tension!] J.
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Old 12th Sep 2011, 8:09 pm   #99
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Default Re: Restorers dream part 2 the chassis.

There are folk down south of the Watford Gap who are on tenterhooks

Col, PM me your address again and I'll bung a couple more U25 in the post. My stock list says I have more than 20 new boxed.
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Old 12th Sep 2011, 8:56 pm   #100
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Default Re: Restorers dream part 2 the chassis.

Hi,

Thanks Jeffrey and John; there's also a guy here in Yorkshire holding his breath for first light but unfortunately as I've been on the go since six this morning I'm running out of steam.

I've just had a quick trip into the workshop though whilst it is like a black hole to have a look at the U25; it doesn't appear to be lit up but the sparks are there with the prod direct to earth.

I'll follow the instructions tomorrow. I twiddled the controls on the side of the chassis and yes I can hear a distinct whistle; I don't know which control is which but one of them increased the whistle as it was rotated. I've dropped myself in at the deep end with this chassis even before getting used to the controls but what a way to learn; I'm like a kid with a new toy.

I've promised to spend a bit of quality time with Bron so I'm about to watch the new Doc Martin series.

Thanks ever so much for your kind offer Jeffrey; I've sent you my details.

Kind regards, Col.
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