Ekco A239/1: A Sorry Tale.

[Joe]

This now seems to be working very well apart from the magic eye.

Before shelling out for a new EM80 I thought it wise to check that the valve itself was at fault. I don't have a spare but have got an EM81 so I stuck that in. This was only marginally brighter and the EM80 was quite usable in the Pye whence came the EM81.

Changing a few resistors improved things slightly but neither valve is as bright as they should be. The relevant bit of circuit is here so any tips would be appreciated.

   

The set also still has two Hunt's capacitors in. These are a 0.0001μF and a 0.0005μF. I don't have anything that small in stock and about the only things I can find on sale are ceramic discs. Would these be OK? I'm currently minded to leave them alone.

Cheers

Joe

[Steve_P]

The ceramics should be OK.

Which resistors did you change? Are they the correct values (Pilot!) and check the others too,

What's the HT like?

Cheers,

Steve P.

[pwdrive]

Hi Joe, If the anode voltages on the magic eye is ok and the heater voltage is up to scratch then it is usually down to the magic eye valve itself, the sensitivity of the eye ie: how far it opens and shuts would normally in a good valve depend on how far the grid voltage swings between off signal and tuned into signal, all other things being ok that is. Also bearing in mind that to get an accurate idea you would have to monitor the grid with a relatively high impedance meter or scope set to dc.
As regards the 100pf & 500pf capacitors then I would say that disc ceramics would be ok providing the voltage rating on them is ok, failing that silver mica would do the job.
Hope this is of some use.
Good luck which ever way.

Lawrence.

[Joe]

Touché, Steve. I nearly replaced R32 with a 10 meg one which had been mis-filed. I measured the one I did fit to make sure that it was right and have just checked it again.

The resistors and capacitors marked in red on the diagram in post 51 have been changed. The ones marked in green are within ten per cent.

The HT is 256V of which 185V is making it to pin 9 and 70V to pin 7, which I think is the anode. This seems darned low but I haven't yet been able to find a decent data sheet for it so don't really know.

It opens and closes OK; it's just very dim overall. As I said, if it were only the EM80 that was like it I'd be happy to conclude that it was just past it but the EM81 is hardly any better though is very bright in its own set.

- Joe

[pwdrive]

Hi Joe, that 70 volts wouldn't be that far off, looking at the thumbnail diag. you posted it looks as though the anode supply is taken from a potential divider between HT+ and HT - so that would put the mid point voltage at roughly half HT if I am reading it right (my eysight's not that good) before that section of the valve even begins to pass current. Making the grid of that section slightly -ve should more or less cut the valve off and the mid point voltage of the potential divider should then read (with a high impedance meter) approx half full HT if the resistors are ok.

Lawrence.

[Joe]

Just a final word, for now, on this. R42 is original and actually measures 1.07MΩ and has 53V across it. R32 is a replacement which measures 1.012MΩ and has 182V across it. All these readings were taken with a digital meter.

I've now put the set back together and will investigate further when I've got a new EM80. It certainly sounds very good indeed.

I'm still a bit puzzled about how the wave-switch went wrong so suddenly but can't get it out to fully investigate. By-passing the HT part certainly isn't a problem as it'll be used on VHF most of the time anyway. I am curious as to why it was connected like that in the first place. It isn't an arrangement which is unique to this model or even a peculiarity of Ekco. If as Steve says it's better for the ECC85 to have HT on it all the time why make the switchery needlessly complex?

- Joe

By the way, if you right click on the thumbnail and then click on "Open Link in New Tab" the picture can be seen at full size.

[pwdrive]

Thanks Joe, i can see the diagram better now, I learn something everyday. Just as a note I have a Y63 magic eye in a R1155, it's about as green as the grass in the Gobi Desert, the valve is definately shot but as a last ditch attempt to get some kind of display I boosted the heater volts across it, it came up reasonable for about half an hour and that was about it. A NOS or a known good S/H one is in order at some point.

Lawrence.

[Steve_P]

Of course another old trick was to make the HT a bit higher. Go on the other side of the resistor across the smoothing block. A few more volts can bring the Green up, but please remember to put it back when you get a new valve.

Don't up the heater volts. Not a good idea.

Cheers,

Steve P.

[pwdrive]

No worries Steve the Y63 in my case was well shot to start with, uping the target anode voltage did not do anything with mine, it being well shot, I still have the afformentioned Y63 magic eye in situ to save staring at an empty hole in the receiver front panel which I would find even more depressing.
By the way has anyone used the Russian made octal base magic eye that is supposed to do for a 6U5G, they seem reasonably priced and wondered what they wouid be like.

Lawrence.

[Skywave]

[In association with something that should be green] . . . . it's about as green as the grass on the Gobi Desert . . .
Lawrence.

Now that is a metaphor I've never met before ~ but I like it! And the alliteration gives it a distinct bite, too. Definitely one to add to my collection.

Al.