Oh no - another V4!

[Steve_P]

That sounds to me as though the EHT is low. Could be for a number of reasons...

Are the valves OK? is the line whistle of the correct frequency? Capacitor change - the one to the first grid of the Line Output Valve is a favourite, as is the boost capacitor.

Take out the LOPT and put it somewhere warm for a couple of weeks, to get rid of any moisture. And maybe change the EY51?

Cheers,

Steve P.

[PJL]

Originally Posted by dominicbeesley

This just doesn't seem right to me why is K going down when brightness goes down!?!

Then I knocked the ion trap again and got a raster of sorts - this time with brightness at minimum! fiddling about with ion trap and focus I got blurry but distinct lines then it went again when fiddling with ion-trap.

Then I noticed the CRT base is loose!

My conclusion: broken K wire in CRT base which is occassionally making contact...but what to do? Is it as simple as solder sucking the pins and sliding off or will that knacker the job?

Any advice gratefully accepted if no better ideas while I have my dinner out comes that sucker!

Dom

Having spent far too much time looking at the 'other' thread, here are some answers.
- A1 reads low due to 10M feed resistor so x2 when using DMM
- Is it grid current keepiing the voltages the same? G1 should be < K
- Plenty of measurements in the other thread. Boost should be >450V

[dominicbeesley]

Thanks Steve P

It certainly is - just not sure why it is so intermittent and fussy - it has come to life briefly a few times. I've been listening to the whistle and it sounds pretty close I was listening carefully and it didn't sound to change when the picture died.

I've already changed the V10 (LOP) grid coupler before I started and boost cap afterwards...still not much change

PJL,

I've been using a valve voltmeter as well for the high impedance measurements and boost (before the 10M resistor - which I've swapped) is only 260V.

I've just got a Pye manual from Jonz Valve Pages (he very kindly scanned it last night) this has a lot more info about the line stage and what to expect voltage wise so will have a poke round with a scope later.

For a while I thought that it was grid current and a gassy valve but when it came to life yesterday for the last time there was a half decent raster...well there was lines - about twice as thick as they should be and spaced out but the tube looked like it wasn't entirely dead.

Here's a few measurements:


I made a few measurements last night before I went to bed...

Numbers in brackets are those quoted in Trader sheet - all taken with Valve voltmeter
V12[A] 177V {198}

V10[S] 163 {153}
V10[K] 6 {4.8}
V10[G] -13.5

V9a[A] 83.7 {110}
V9a[K] 3.8 {3.6}

V9b[A] 132 {85}
V9b[S] 115 {96}


Tonight I think I'll try grounding the line flywheel discriminator output and messing with line hold to get a reasonable frequency and see if that helps any.

Scope through the line stage and try and get boost voltage up to 400+V - I'm still not sure what it should be!

Measure all resistors in line circuit and check the scope points in the V4 technical manual.

It seems the other thread has damped everyone's enthusiasm for thinking about V4s! I should have bought that Fergy instead...Thanks for any help or advice!

Thanks

Dom

[oldticktock]

460V on one side of the boost cap 199V on the other.

Not that you probably need much help but I would imagine most of the advice is in my V4 thread anyway, escpecially at the stage of power up your at I had similiar issues, not your CRT issues though (My crt short came a lot later on)

I had the same issues with the EY51 though, I just replaced it and was able to draw a 12mm EHT spark. My PY81 was duff as well, eventually failing.

Meet you in the V4 therapy group soon

[dominicbeesley]

Thanks Chris,

I found another EY51 which I swapped out of my TV24 before realising that the ion-trap was on backwards! So hopefully there was nothing wrong with it!

I'll try the PL&PY81 in the valve tester but have been a bit loath to disturb them as the top caps connectors are more or less rusted on. Most things I touch seem to be rusted solid or fall off....or both!

Is great fun - I would have been really disappointed if it had worked first time. I reckon if it keeps me from boozing all week it will have paid for itself!

Dom

[Boom]

Originally Posted by dominicbeesley

I've just got a Pye manual from Jonz Valve Pages

I couldn't resist posting this from the site. It's a manufacturers help guide for a Sobell set. They sure knew what they were doing in those days.

Dave

[ppppenguin]

Which reminds me of a definition that would be worthy of Mr Parabola of this parish:

Horizontal hold: When a heavy TV has fallen on you and you're pinned to the ground.

If this goes any further we'd better start a new thread.

[dominicbeesley]

Progress of sorts...

Strapped junction R46, R47, R48 to chassis (line flywheel discriminator) and turned line hold up full.

Noted discriminator diodes look to have been replaced with tiny modern ones

Got a bright but blurred raster - brightness control does little

EY51 lit up brighter but still not full brightness

Strapped grid to chassis via 100k resistor and got a dimmer more focussed raster with focus collar fully clockwise. Grid is now just 20V below Cathode

Boost is up to 381V

Tried giving it a sniff of Aurora for laughs - sound on channel 1 but no picture.

Line frequency beat is picked up on nearby radio - notice that it changes when hand near line hold control. Also noticed it warbles quite a bit, raster quite ragged - instability?

Odd behaviour when grid held low with 10k resistor picture gradually fades out and won't come back, repeatable.

Odd interaction between brightness, focus collar and ion-trap with a change in any one affecting the others!

Does this sound like I've got air in the tube?

There's no getter that I can see to check but on the plus side I can almost get a finely focussed raster and I can't see any odd blue/purple glows in the tube neck.

Picture shows a dimmed raster.

Dom

[murphyv310]

Originally Posted by Boom

I couldn't resist posting this from the site. It's a manufacturers help guide for a Sobell set. They sure knew what they were doing in those days.

Dave

Brilliant I just love these!!

[murphyv310]

Hi Dom
As Steve_P correctly says it could be a damp LOPT, I would put it somewhere really warm and while waiting for it to "cook" go through all the waxies and check those 20% resistors.
I cooked mine for six months on the top of the boiler and Wow what a difference, I did another in the oven and WD 40 and all sorts of other things and it worked fine too.
Good luck with the set, they are crackers when ok, I wont get too involved though with this thread due to work commitments this week.

[Heatercathodeshort]

Tube looks OK. Base is often loose due to glue drying out. You can resolder pins just to make sure. It is possible to slip base off after unsoldering all the pins but it's difficult to line them all back up again. The wires were over 4 inches long when Mullard originally did it! Use Araldite to reglue when you are satisfied all is well. DO NOT USE SUPERGLUE. It will fracture the glass within weeks. This looks a good restoration. All the very best with it! Regards, John.

[dominicbeesley]

Thanks both,

Glad you think the tube's ok - I wasn't sure that the grid isn't drawing an armfull of current! If you all think it's ok I'll continue in the same vein.

Next up I'll see if I can get the line output stage working close to normal i.e. something where 10kHz is in the middle of the line hold control and then try and get any sort of signal through to the tube.

This is my first go at diagnosing from scratch the TV24 worked ok (after I'd worked out the ion-trap). But on that one I cheated and just replaced everything!

As to the LOPT and drying out I'll give it the backside over the radiator treatment....that's worked on the TV24. Still would like to get a bit further first...is fun!

As to the tube base - I ended up remving it completely, then soldering on extension wires and refitting. Probably a waste of time but when it was loose it was definitely shorting A2 to K when wiggled too far in one direction. For now fixed with tape....

Dom

[DangerMan]

Hi Dom,
it might not be perfect, but I have used silicone rubber to attach loose bases. It's best if you can get the stuff that is not acetic cured, but in my experience even the acetic disperses reasonably quickly if you don't use excessive amounts of the stuff and keep the component well ventilated as it cures. So far I've never had any corrosion trouble and it's always been strong enough.
You can get high a temperature silicone compound which even works reasonably well on hot valve bases and top caps... and this produces no acetic acid as it sets.
Almost guaranteed to remain flexible but sticks like... er... silicone rubber !
Pete

[dominicbeesley]

Thanks Pete,

I've got a tube of black sticky stuff somewhere which I beleive is some sort of rubber compound (like silicone but supposed to be low acid) I may well try that after I've worked out whether the black is conductive.

I had a bit of a poke round with a scope last night. Something resembling an unhappy video signal is getting as far as the video detector but not much further so I'll try and replace the recitifier tonight.

Also did some basic measurements round the line stage with a scope. It looks to be about 10kHz with the line control turned all the way up.

Also the drive levels look close but not quite about 30% down, HT is about 10% down. Drive from the bistable looks a bit flattened off at the top so will check for any out of tolerance resistors tonight....

When dosing off last night I started to wonder if the funny grid behaviour is down to leakage in the CRT socket. This is a rather flimsy pair of paxolin boards with pin-sockets and a few rivets there's a lot of crap on there and probably more between the two bits of paxolin. Not sure if cold measurements will give me a sensible answer or not but will give it a go. If it looks duff I'll try and make something up from fibre-glass and re-use the pin-sockets.

Dom

[Steve_P]

If the power supply is down then everything will be down. It's fairly simple on these, two PY82's in parallel and a few resistors to check. Make sure the PY82's are OK and if not, change them both.

Then get the horizontal hold within range. There are a few 0.1 caps here and an electrolytic, 12uF. Also R121 and R122. There's a trimmer cap too marked 'pre-set horizontal hold' that may need a tweak.

Put the Horizontal Hold in the middle, tweak the capacitor and pull it in. And use a scope for checking frequencies. Not your ears. If it's off, then the line output stage will not work properly.

Cheers,

Steve P.

[dominicbeesley]

Thanks Steve,

I wasn't sure how close I need to get with the HT - I'd been aiming for within 10% as a starting point. How close do I need to get it?

I'll have a look round and see what looks to be pulling things down. I'll try the PY's in the tester too.

I suspect theres a few screen decouplers etc pulling it down a bit, theres definitely a boiling cap somewhere but I've not spotted it yet but can hear something bubbling away every now and then!

Dom

[Steve_P]

Sort that out first. Could be anywhere in the set, use 4 out of the 5 senses and find it. You don't know what is boiling and you don't know what damage it is doing or will do.

Cheers,

Steve P.

[dominicbeesley]

Been down the workshop and got a bit further:

Tested PY82s PL81 and PY81 all good as new except a slight g-k leak on PL81
Checked resistors in line bistable, a few are out of tolerance and am going through them and checking results in between:

Changed R51 (bistable HT feed) C42, C43, C44 (cathode bias decouplers/feedback and flywheel damper). Turned back on and get improved bistable line waveform - now 130ish Vp-p (no real way of accurately measuring). Picture brigher and sharper.

Decided to do more investigation of CRT grid circuit: cleaned and resistance checked CRT socket - all ok.

Then spotted C35 - vision interference coupling cap - not spotted it before due to badly printed circuit! Snipped it out and brightness control works as advertised! Schoolboy error - I really do deserve kicking up the workshop if Trevor's got his boots on!

Fed in Aurora via attenuator and could get something like a test card after a bit of fiddling with line controls - even looks as if the APC is sort of working!

Not sure what I'll tackle next probaby more line-drive resistors...But am relieved that tube looks pretty good!

Dom

[murphyv310]

That's a good un!!
I don't think you will have too many problems with the set. I assume that the one you snipped was the culptit that was getting hot! Dom dont worry I'm sitting here with my slippers on, and have just done a 12 hour shift and driven 420 miles so not booting to-night my man

[dominicbeesley]

Thanks Trevor,

A few more caps and resistors changed and I've now got a line and frame locked and steady-ish picture.

Still got very low (7kVish) EHT and (350V) boost so the picture's a bit soft and the focus still needs to be fully clockwise...I've measured all the voltages and waveforms around the line stage (as per Pye manual) and tested the valves and can't see what's up so can only assume the LOPT isn't too healthy.

Hopefully a break tonight and I'll spot another deliberate mistake!

Dom