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Ekco T344 plus another one

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PYE625
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I would expect that you should be getting in the order of 8v of video signal at the vision detector.

I think it may be safe to say that V8 video output is operating fine and the fault is prior to the vision detector diode. I believe you replaced the diode and achieved a slight improvement previously?

V3 and V5 could be checked for low gain by sustitution, and if are ok, then some voltage checks in their respective IF stages are needed. V3 is a variable gain valve so it could be interesting to check the AGC voltage action for instance.

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Posted : 06/05/2020 5:16 pm
helloekco
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Thanks both - I am wondering about the detector diode, because I replaced it with a silicon type. (The original had similar high resistance in either direction, and the germanium ones I ordered just didn't seem to work.)

I'll investigate the voltages around the IF stages first.

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Topic starter Posted : 06/05/2020 8:30 pm
Nuvistor
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A silicon diode may not be good enough for a vision detector, usually germanium, but definitely check the IF stages.

 OA70/71 OA90/91

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Posted : 06/05/2020 8:53 pm
Nuvistor
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Attached is a page from Patchett about video demodulation and importance of low inter electrode capacity which may not occur with a junction type silicon diode.

E36C81C8 4061 409B 9427 99ED7EC07711
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Posted : 06/05/2020 9:12 pm
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helloekco
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I tried the original detector diode again, as well as a modern germanium one, but the contrast was poorer with both.

I've checked the voltages around V3 and V5 and they're reasonably close to spec - V3 differs the most if I remember correctly, at the grid, which is about 70v compared to the 55v specified on the service sheet.

I was going through the AGC parts of the circuit when I discovered an error I'd made much earlier on - when replacing the first round of wax paper capacitors, I'd connected the replacement C39 to the wrong post! When I realised this I really thought I'd cracked the problem, but disappointingly when I corrected it and tried the set, although it made an improvement, things were still not right. The peak output of the detector is now about 1.5v compared to approximately 0.9v before. The contrast on screen appears virtually the same.

An interesting thing I found though was that if I connect the Hedghog via an RF signal booster I can actually get the signal strong enough to distort and invert at maximum contrast, something which the set hasn't been able to do since I rejuvenated the CRT. If I then back the contrast control off a bit I do get better contrast than I've seen before. At this very maximum signal strength, the AGC voltage is about -0.5v. I also tried shorting the AGC to chassis and it just results in the same overload of the signal; but there doesn't seem to be a point where the blacks are dark enough without the overall brightness being lowered.

I'm worried that the only way to resolve this is going to be to get into the tuner box, bearing in mind the 125v I measured at the anode of V1. Does this all sound like low input to the IF stages could be the cause?

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Topic starter Posted : 08/05/2020 8:23 pm
PYE625
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Lets go back a little.... did you get the tuner valves the wrong way round at some point?

If you have a fine pitched probe for your multimeter, or perhaps improvise one, you could pull the tuner valves out a little just enough to get to the pins and measure their voltages. Just a simple check to be sure you actually have HT getting to the anodes will be a good start, especially V1. I'm thinking along the lines of a burnt out open circuit resistor in the tuner....just a thought? Has to be worth a check.

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Posted : 08/05/2020 10:49 pm
helloekco
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@pye625 Yes, right at the start of working on the set I discovered that V1 and V2 were the wrong way around.

I haven't checked since last week, but I did have HT at the anode of V1 (checked by removing the tuner box's cover plate) but only about 125V. When I initially checked last week it was 115V, but I managed to replace the anode resistor in situ (very tricky).

The V2 anode resistor looked impossible without removing the tuner, as did almost all of the other components, so I'm really hoping that's not the problem.

I will try your suggestion to measure voltages at the other pins.

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Topic starter Posted : 09/05/2020 12:24 am
Nuvistor
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If you need to replace parts in the tuner remember to keep lead length and location the same or it could require realigning. The drum with the tuning coils can be removed, it’s held in by spring clips, giving much better access to components.

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Posted : 09/05/2020 6:35 am
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PYE625
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But if gain was low in the tuner, I would expect a grainy picture with low contrast and weak sound.

Now, if it were me doing this repair, it is highly likely I would have got the vision detector diode the wrong way round. But as I am not, this could not happen..... could it. ? 

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Posted : 09/05/2020 7:06 am
helloekco
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Posted by: @pye625

But if gain was low in the tuner, I would expect a grainy picture with low contrast and weak sound.

Now, if it were me doing this repair, it is highly likely I would have got the vision detector diode the wrong way round. But as I am not, this could not happen..... could it. ? 

That is a good point and one that's occurred to me - the sound to me seems absolutely fine.

" highly likely I would have got the vision detector diode the wrong way round." ?? Yes understood, I did check and double check this.

I've just ordered some NOS vintage OA91s to try instead (thanks for the tip @nuvistor). My original diode by the way was a GD3 (apparently the same as a OA60) but I couldn't find any of these for sale. Actually, if I get impatient waiting for them to arrive I might just borrow the detector out of my other set, seen as it's easily accessible without removing the chassis. I would / should have done this earlier I realise, I just don't like the idea of removing loads of stuff that I'll then have to remember to replace when I get around to working on it.

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Topic starter Posted : 09/05/2020 12:45 pm
Nuvistor
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I would wait for the delivery rather than take the diode out of the other receiver and risk damaging it.

Many stories, not TV, of someone having one faulty and one working pieces of equipment and soon it was two faulty pieces of equipment.

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Posted : 09/05/2020 1:41 pm
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PYE625
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You might be better off checking the vision IF coils for continuity, L15,16,19,20 and even 26 to the vision amplifier V8 grid. Not to mention all the resistors in those IF stages too. One thing worth looking for is broken ferrite cores within the IF cans. A torch will help you see if any look cracked or tampered with. I imagine they are hexagonal shaped and it dosn't take much to break them.

The fact you have strong sound and no grain in the picture might suggest the tuner is ok. Normally, I would expect the gain of the tuner to be slightly stronger at the low Band I end, and lesser at the higher Band III. You could try this out by selecting different channels and see if the vision is stronger down at Band I frequencies. It will prove that the biscuits in the tuner are OK if results seem generally similar on all available channels though.

 

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Posted : 09/05/2020 5:56 pm
Katie Bush
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Posted by: @nuvistor

I would wait for the delivery rather than take the diode out of the other receiver and risk damaging it.

Many stories, not TV, of someone having one faulty and one working pieces of equipment and soon it was two faulty pieces of equipment.

 

Erm, you weren't looking over my shoulders in the mid '70s, by any chance? ?

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Posted : 09/05/2020 6:45 pm
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Katie Bush
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@pye625

It seems to me that I recall an EKCO of some model or other, in the mid '70s, which exhibited similar symptoms to these, as this set stands today, only to find that there was just one duff EF80 in the vision strip. I didn't have any EF80s to hand, so inserted an EF184 in its stead - brilliant picture! Just to be a smart Alice, I thought I'd try an EF183 as well - that didn't work at all well.

I ended up replacing all my EF80s with EF184, and the result was a set could receive channels that it had previously been deaf/blind to, though that is by-the-bye.

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Posted : 09/05/2020 6:54 pm
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freya
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Think i mentioned this before but may not have been in this thread but its a good idea to check the HT2 feed resistor might be R52 it does burn up.

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Posted : 09/05/2020 6:57 pm
Nuvistor
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EF183/4 were the Frame grid valves that resulted in IF strips in many sets that used 2 Vision IF valves requiring only one EF184 and a EF183 common IF amp. The mutual conductance of the EF183 was 12ma/v whereas the EF85 was about 6Ma/v. The EF184 was 15ma/v whereas the EF80/was around 7ma/v

The circuits had to be designed to make use of the higher gain, probably why you got away with the normal mu of the EF184 but not the variable mu EF183.

These valve were introduced at the same time as the frame grid tuner valves such as the triode PC97 and triode pentode PCF801, these frame grid type valves greatly increased the gain while reducing costs and complexity.

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Posted : 09/05/2020 7:13 pm
PYE625
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This piece on "valves and their habits" is rather interesting, especially the last paragraph "RF pentode substitution notes".

http://www.r-type.org/articles/art-012.htm

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Posted : 09/05/2020 7:44 pm
Katie Bush
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@pye625

That figures! - It more or less says what I observed with my EKCO (I wish I could remember the model - vertical chassis, PCB timebase section, big bottle boost diode and line output valves, dates from about 1958 to 60). It was a damnably good set, and I wish I still had it now.

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Posted : 10/05/2020 10:03 pm
helloekco
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Posted by: @pye625

You might be better off checking the vision IF coils for continuity, L15,16,19,20 and even 26 to the vision amplifier V8 grid. Not to mention all the resistors in those IF stages too. One thing worth looking for is broken ferrite cores within the IF cans.

I checked the cores as you suggested, and they all look fine - except for the one on the tuner box!

Photo 10 05 2020, 17 08 57

It looks as though someone's tried to turn it with a screwdriver - fortunately none of the fragments of the core seem loose, and given that the sound and the tuning itself seem to be OK, I think I can get away with leaving it alone.

I had a scare today - I was checking the voltages around the IF valves and suddenly the line timebase went dead. There was no whistle, just the hum of the frame timebase, and the EHT vanished. I eventually figured out that there was no continuity between the top caps of the boost diode and the line output valve, and after sanity checking this with my other set I feared the worst. However, after much probing it turned out to be my dodgy soldering when I'd put the LOPT frame back onto the set, after my checks on it a couple of weeks ago... phew. Why it picked that particular moment to fail, I have no idea.

So the outcome of today was also that the voltages around V3 and V5 seem to be more or less spot on; not very helpful really! I think right now I just need to wait for the detector diodes I ordered as a next step.

An interesting sidenote is that I checked the detector diode on my other set, and it appears as though it may be connected the wrong way around. I'll get a photo another time, but it is of a type that's marked with a '+' and '-', and the '+' appears to be connected to the terminal where the cathode is connected on this set. As I mentioned before, I have checked the orientation of the diode against the service sheet multiple times(!).

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Topic starter Posted : 10/05/2020 10:59 pm
PYE625
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Might be interesting to deliberately put it the "wrong" way round on the T344 , merely to observe the effect on screen. It won't harm the set in any way.

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Posted : 11/05/2020 8:22 am
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