Ekco A239/1: A Sorry Tale.

[ppppenguin]

In that position the cap is exposed to full HT plus maximum audio swing. It can also get odd transients if the output valve is driven into clipping and helps to absorb transients if the speaker is disconnected. If it fails short circuit you probably say goodbye to the output transformer so best to be conservative with the rating. Modern caps are 400V, 630V or 1000V, all DC. A 400V will work but not recommended. 630V should be fine, 1000V best if you have one to hand. If you have a class Y cap that will be excellent. Class X is probably OK but unlikely to be found in that value.

These days most caps don't show an AC rating. The exceptions are class X and class Y which are specifically safety rated for use at mains voltage. Class X for use across the mains, class Y for use where failure would give a shock hazard.

[Joe]

Thanks for that - it explains why tracking AC ones down was so darned difficult.

Cheers
- Joe

[Joe]

Cheers, Trevor, that does make more sense.

[This was advise to move it to across the OPT]

I won't be doing anything to it yet a while though as today is a dog-sitting and furniture restoring day.

- Joe

[Joe]

My capacitors from Mark arrived this morning and turned out to be the posh yellow jobs so THANKS VERY MUCH to him; I hope I can reciprocate his kindness in some way in the future.

I fitted one of these easily in the configuration suggested by Trevor and when the old waxie was out of the way I could get at and change a Hunt's thing that was cracked.

I also changed the capacitor suggested by Rob but VHF still doesn't work. I'll leave off that until I can positively rule out the wave-change switch.

The other evening I found that the EM80 IS working but it's very dim. This however is a root vegetable of no consequence.

The other interesting thing I discovered while fiddling was about 555 timer ICs: "They don't like it up 'em." I discovered this by connecting my 'frequency generator' cross the HT by mistake. I've got another ten on order.

As ever, thanks for all the help and advise.
- Joe

[Joe]

I've done it again.

I'd had it playing all morning so after dinner thought I was safe to put the chassis back in the case. I took the chance to have a look at the wave-change switch but couldn't see any broken contacts or any that were obviously failing to make.

When I switched it back on I heard a nasty cracking noise while it was warming up so switched it off quick thinking I must have disturbed something of pinched a wire.

After getting the chassis out again I couldn't see anything so powered it up again via a 100W lamp. This only glowed very faintly but I found that for all I could hear the sort of noise you normally get between stations I couldn't tune into anything.

I then decided to see if the PU input was still working. Luckily while putting the plug into my DAB radio I touched it and got a belt off it. I then measured two-hundred-and-some volts on this.

Fiddling about with something that doesn't work is one thing but messing with something that's plain dangerous is a different matter so I'm leaving it well alone for a while. I can't see anything obvious wrong; there's no wires off or anything that looks like it's over-heated and haven't a clue what to check first.

- Joe

[Mark R]

something is shorting to the chassis, make sure any caps that you have put in are not touching anything else and are sleeved.

cheers Mark

[Joe]

Thanks, Mark. I'll have another look tomorrow.

- Joe

[Steve_P]

You'll put capacitors in test leads in future. One of the two sets could have died as a result. Oh, and are you using an isolating transformer? Using one wrongly is more dangerous than not having one.

Cheers,

Steve P.

[Paula A]

Actually.. a dim tuning indicator can be diagnostic in some circumstances.. check the target volts and have a look at the circuit and see where it is getting it's drive from. A duff cap here can cause all kinds of demonic head scratching because it can muck around with the agc line and/or any auto lock tuning arrangements. Same goes for resistors which tend for some reason to go very high.

[Joe]

The anode voltage on the tuning indicator is, or was until things went wrong, 26V between stations and 96V on a strong one.

You're obviously right about the capacitors in test leads, Steve; I'd mistakenly thought that the 100µF one already fitted to the output would do the same job.

I don't have an isolating transformer but this wouldn't have protected me in this case anyway as I got the shock by touching both the tip and ring of the 3.5mm jack plug - the shape of the scar on my finger confirms this. It was also handy that it's got a pair of 10 kΩ stereo to mono mixing resistors in. I'm just pleased I didn't manage to connect it.

I've just been having another look but can't see anything shorting. The test lamp lights briefly, then goes out but as the set finishes warming-up it comes back on dimly as the voltage appears on the PU input. I'm loathe to risk leaving it on for long as I don't now what harm it's doing.

I'll keep looking as it's too easy to not end up being able to see the wood for the trees. It's so annoying as it was working fine, apart from VHF, until I put it back in the cabinet. At least if I knew what I'd done I'd have an idea of what to do about it and even if it was irreparable would have learnt a lesson for the future.

Joe