I made a start on the companion to my Ekco T344 tonight, the T344F.
The chassis came out of the cabinet very easily and the front screen surround shows no signs of ever being stuck to the rubber gasket around the CRT. It's much better than the one on my other set, so if this repair doesn't work out, I think it will be finding a new home.
As you might see from pictures, the LOPT shroud on this one is original and has suffered in the usual way, so this will be something to tackle very soon - it looks good enough for now, to get the set running though.
I went through the usual variac / series lamp gentle startup sequence, EHT rectifier disconnected, eventually dispensing with the lamp, and when I got to about 200VAC I could hear an encouraging whistle from the timebase. Eventually I got it right up to full voltage without incident - still with the original-looking wax and electrolytic caps!
The timebase controls seem to be working but are a little flaky, judging by the sounds as I turned them - the pitch of the timebase sounded a little high and not very loud. Also, I couldn't draw a spark from the EHT top connector, which made me worry about the state of the LOPT.
I went ahead and reconnected the EHT rectifier and tried again; but no spark and no static on the front of the CRT. Also, the heater glow that I thought I could see in the EHT rectifier before when the top cap was off couldn't be seen. I was worried now about the LOPT, but when I powered off the TV for the evening I used a screwdriver to discharge anything in the CRT by shorting the socket of the EHT rectifier to chassis, and to my relief there was a spark, albeit quite a weak one.
I think therefore the timebases need attention first. The frame output valve looks like it's lost its vacuum somewhat, but I don't expect that could affect the line timebase?
All in all, it's encouraging though; I'll be pleased if I can get this one going without changing so many components as I did in the T344.
Does you lopt still have its sticker SA number, i managed to purchase several NOS LOPT`S last year if you need an option.
Try changing the boost cap and trying again.
@freya Thanks, that's great to know - but no, I'm afraid I can't see a sticker on it. However, I think the LOPT is probably fine and if you mean the cap C85 (between the HT and boost line) then I think you're probably correct - because...
Tonight, with the brightness and contrast right up, I managed to get a few random white streaks all over the screen by adjusting the hold controls, and a disappearing spot in the centre after power down. So it looks as though the timebases may be operating correctly but still with low EHT. However, after a few minutes of the set being powered on there was a hiss / crackle starting in the speaker which I knew to be the sign of something imminent... I caught it in time, it looks like C85 (a wax cap) was about to go into meltdown. I haven't yet been able to change it, as I'm out of suitable replacements right now. In fact, rather than have to wait for some to arrive from China I've ordered some 220nf ones instead of 150nf. From my understanding of its purpose I think that should be OK.
@nuvistor No, I'm afraid it doesn't; that one had been refitted already with a new shroud so I imagine that would have been a good opportunity for it to lose its identity.
I'm toying with the idea of buying a 3D printer to make a LOPT shroud for this set (not just for this of course!).
I changed that capacitor and now have very strong EHT. The heater on the EHT rectifier seems to glow a lot more brightly than the one on my other set, too.
I hooked up a signal to the TV and I now even have a picture! It's fairly bright, but the contrast isn't great. The main issues to resolve are with the timebases though; it won't hold vertically and the picture only comes 2/3 way down the screen, being cramped at the bottom and spread out beyond the top of the screen. I've a suspicion that this could be due in part to the gassy frame output valve, which I'll try replacing. Also, the picture does not fill the width of the screen. I did notice that the width selector was already on the maximum setting which made me suspect that there was an issue there when the TV was last used. So I think the line timebase will need some attention too.
There is sound present also, but it is weak and distorts and dies if the volume is turned up, so possibly a power issue somewhere in the audio section.
All in all though, very pleasing considering all I've changed so far is that cap!
The odd thing I've noticed with this set is that if there is no signal, the screen remains totally black with no raster visible at all. Is this anything to do with flywheel sync?
Check HT before delving too much into the time bases, may be low emission PY32/33. I don’t think the flywheel sync has anything to do with the dark screen, but watch me proved wrong. 😀
Cathode Bypass capacitor on the frame output valve was a very common fault.
Frank
Thanks @nuvistor - HT is not too far off, about 182v I think it was.
My T344 also had poor hold when I started on it, trouble is I ended up replacing almost all passive components in the timebase trying to fix the boost HT issue, and foolishy forgot to note which ones made the improvement to the hold! (As a side note, I don't think there actually was an issue with the boost HT, this one measures similarly low - I'm convinced it's an issue with both meters I have drawing too much current).
The odd thing I've noticed with this set is that if there is no signal, the screen remains totally black with no raster visible at all. Is this anything to do with flywheel sync?
405 sets don't show any 'snow' with no signal which is what I suspect you are used to seeing. I would think that if you turn the brightness up though, you should see a blank (white/grey) raster. If not, this could be something to do with your contrast issue. One stage at a time though....great results so far.
As suggested, check the cathode bypass electrolytic in the frame output stage. Common problem for the effect you have. It might still need a frame output valve as well but changing the cap might make a great improvement.
I'm convinced it's an issue with both meters I have drawing too much current).
??? they would have to be pretty basic to make a huge difference. Back in the analogue meter days, I had two. One of 20,000 opv and the other (cheaper) one of 10,000 opv. With boost circuits, I don't recall much difference in readings (I used the cheaper meter for field service).
Hi @sideband,
Thanks. No, there's no raster with the brightness right up and no signal. It had me fooled into thinking the set wasn't working for a while.
As for my meters and the boost HT reading, I've tried a mid-range DMM and a cheap analogue meter, the DMM was actually slightly better. But I think that's what's happening, as I spent ages chasing a problem with low boost that I couldn't find on my last set, even though everything was working as it ought to. Whenever I touched the probe to the contact, the meter would register over 600V for a split second before settling at about 430V: both meters did the same. Also, the CRT would noticeably dim when the meter probe was applied.
Common problem on this set and others of this vintage is the resistor from HT to the cathode of the video output valve. It goes low in value, cuts off the video amp raising the cathode voltage on the CRT. Think I have that explanation correct, it’s 27k and you may as well check the cathode bias resistor, forgotten the value of that.
This is nothing to do with the boost voltage problem.
Frank
I've got a bit further with this, but I've had to change a few caps that have almost gone into meltdown. One of them was the small electrolytic in the sound section which developed a leak, which surprised me as the larger electrolytics appear to be fine. I did also change the frame output valve as I managed to get a few NOS 30PL14s at a good price.
Currently, the picture is stable and contrasty but the sync is way off. The output from the sync separator looks OK (I think) on a scope, with a sharp leading edge but with some noise on the trailing edge.
I'm wondering whether I'm overloading the set with signal, being a fringe model, but wouldn't turning the contrast control down low resolve that? Turning the control down doesn't improve matters, the sync is lost altogether when turned low enough.
I haven't yet gone through checking much else in detail, such as the suggestions above. I'm determined to replace less on this set than my previous one, and that means understanding each fault in detail before addressing it!
Aside from the frame issue, that is a mighty fine sharply focused picture there !
Excellent results thus far. 👍
No, you are not overloading the set with signal, but for the poor frame hold it might be worth checking C111 is not leaky (a 2uf electrolytic I believe). If it is leaky, it could affect the range of the frame hold control (VR7) by dragging down the voltage too much.... along with C88 and 92, which are probably both wax capacitors. This bit of circuitry to the grid of the triode of V13B is of fairly high resistance and any DC leakage here will upset things, not to mention putting DC on the pentode section grid of V13A. You really don't want this as a new V13 could soon suffer and end up like the old one. (References from the ERT service data).
PS, C77 a 0.01uf from V13A anode, could introduce unwanted DC at the pentode grid if leaky too.
PPS, if you have already replaced the frame stage wax capacitors, then ignore all this waffle. 😉
To understand the black art of electronics is to understand witchcraft. Andrew.
We must of course not forget V17 and associated circuitry, including MR1..... the list can go on.
To understand the black art of electronics is to understand witchcraft. Andrew.
Thanks @pye625 - it's certainly looking like I picked the wrong set of the two to repair first! Looking at this one, you would have thought it was a junker that had been acquired to supply parts for the other one. So far, the CRT certainly looks better (I'm fairly sure this one has been replaced, as there is a label attached to the back containing handling instructions - no brand or type markings that I can see though).
if you have already replaced the frame stage wax capacitors, then ignore all this waffle. 😉
No, I haven't had to replace any of those components, so that's certainly helpful in knowing where to start, thanks! Looking at my annotated diagram it's actually only C87 and C78 that I've replaced so far in the timebase - C87 started dribbling wax, and C78 had been replaced previously with the incorrect value and was also showing signs of electrolyte leakage - my DMM measured it as about 20nF!
Frame stages in TV's are very critical of resistor and capacitor values. The slightest leakage in any one of the frame shaping/feedback capacitors will result in all sorts of weird shaped 'circles' on the testcard. If you haven't changed any of the caps in the frame output stage yet, then that would be the next step. If they are waxy paper, ditch them!! Don't waste time trying to measure the values or thinking that a few megohms leak won't make much difference....it will, believe me.
The above is very true.
Wax capacitors will increase in leakage as the set warms to make matters worse.
To understand the black art of electronics is to understand witchcraft. Andrew.
But of course, if you just simply changed the lot in one go, what would you learn?
Naturally there are the "must replace" ones for your and the set's safety like the mains RF bypass for instance, but some can of course await further fault-finding and investigation.
To understand the black art of electronics is to understand witchcraft. Andrew.
I finally got back onto this TV, now that the weather has turned too cold to be doing things outside.
After a couple of days at it, I now have a fairly nice stable picture:
(The test card looks bright because I had to overexpose in order to stop the camera shutter truncating the scan.)
Apart from a handful of capacitors that I had to change because they were melting, electrically leaky, or physically leaking, there were two main offenders - R45 which had dropped to a mere 10% of its nominal value and was smoldering slowly(!), causing a very dim and fading picture, and C92 (good call, @pye625!) which was causing all the bottom cramping and limiting the range of the frame hold control severely.
All in all, apart from valves, I've only changed 9 components so far.
You might be able to tell from the test card picture that there is an issue remaining, which I don't know how to describe best, other than as a light smearing of the picture from the left edge of the frame. I think I remember reading about this very symptom in the Spreadbury books, but it might be the issue with a lack of damping in the flyback / boost HT circuitry I'm thinking of, which should be OK in my set as I've changed the two likely problem caps around there. So, that's my next task, to figure that out.
Also, the audio is a little quite (but very clear) so I'll check whether R40 has gone high as it did in my other set, causing the same problem.
