WEM Copicat Custom

[Techman]

So what is the result of C9 disconnected?

Edit - post crossed with Alistair D.

[Alistair D]

Forgot to add; please double check what I have said as it is difficult to be sure from the pictures.

Al

[Station X]

I too thought that the wiring on the smoothing caps was wrong, but having checked it from the pictures I came to the conclusion it was correct. What confuses matters is that a red sleeved wire goes to the common negative tag. You can see a green/yellow wire going from there to the chassis.

The other three tags are the three sections of the cap with a resistor between each pair of tags. It's easier to see in the first picture with the shattered resistor.

If the resistor was wired directly across the HT it would get hot regardless of whether the wires to the rest of the circuit were connected.

[Techman]

Good advice from Al.

I've actually got one of these early 60s all valve echo boxes stashed away somewhere. I seem to remember that it had audio through, but no echo - this thread is making me think that I ought to dig it out sometime soon and have a go at it.

[evilmoisture]

Ok so I reconnected both wires to R27 and (with C9 left unconnected) powered it up and R28 doesn't get hot anymore. So I guess the next step is replace C9 and see what happens?

[Techman]

What resistance reading do you get in ohms across C9 with it disconnected and mains OFF.

[evilmoisture]

zero, short circuit

[Station X]

Eureka! Replace it with a new component and see what fault shows up next.

[evilmoisture]

@station X it was me who added the green earth wire to chassis. Before that it was only a 2 pin plug. Very unsafe according to this ebay warning I read. I hope I did it correctly, pretty sure I did because audio ground also goes to the chassis via the pot cans. Here's what ebay say about it: http://www.ebay.com/gds/Watkins-WEM-...7449661/g.html

[Techman]

They were maybe thinking about hum caused via an earth loop with the original un-earthed design, but the unit should really have an earth of its own for safety.

Then there's all the legal issues of an un-qualified person altering an original maufacturers design should alteration fail for any reason - perhaps best not go there!

[evilmoisture]

It's a little hard to make out on the schematic, is that 200 pf? Pretty sure I don't have this exact type though, would another type like ceramic be ok if the voltage rating is high enough? I'll see what I can salvage...
Thanks so much for all this!

Edit all I have are 180pf or 220pf ceramics, quite small ones...maybe better to order aluminium foil ones?

[Station X]

The eBay article advocates the fitting of a three core mains lead. Nothing wrong with that from a safety point of view.

The green/yellow wire of the mains lead should be connected to a substantial point on the chassis such as the mounting screw for the toroidal transformer.

If you fitted the short length of green/yellow wire from the cap negative to the sub chassis I'd be inclined to remove it. If the sub chassis is not electrically connected to the main chassis then run a wire from the sub chassis to the mains lead earth connection point.

[Station X]

Originally Posted by evilmoisture

It's a little hard to make out on the schematic, is that 200 pf?

Any markings on the cap?

[Station X]

Looking at another diagram it is 200pF.

This cap has the full HT voltage across it, so I'd recommend a 400V, preferably 500V replacement or higher.

[evilmoisture]

Oh ok, I'll have another search but if not will have to order one, so will let you know how it goes in a couple of days! Do you have a link to that other diagram, if it's online?

Incidentally no it wasn't me who fitted the green wire from the sub chassis to the negative terminal, and I thought that was a little odd too, will remove it and do as you said. The sub chassis does connect to the main chassis but only by a bolt which touches it when the pots are tightened up.

Originally Posted by Station X

The green/yellow wire of the mains lead should be connected to a substantial point on the chassis such as the mounting screw for the toroidal transformer.

I think the rules for a safety earth require that it is not a fixing used for anything else, i.e. a dedicated earth fixing.

[Station X]

Fair enough. I'm not well up on the rules, but connecting the earth wire to its own connection point will mitigate against it being left disconnected if the transformer is removed then replaced for some reason.

but connecting the earth wire to its own connection point will mitigate against it being left disconnected

Indeed, at least this rule has some sense behind it.

As someone once said, "If you want to cheat, know the rules first"

[evilmoisture]

Hi again.
I replaced the cap C9 with a 600v 200pf silver mica, but there is still no echo. Nothing blew or got hot though, which is progress I guess? Also I tested the new cap after to see if it had gone short circuit but it seems OK.
I'm not sure if it was doing this before, but there is a slight rhythmic clicking sound coming through at the output, which I at first assumed was from the motor but actually it does it when the motor switch is off too (and it's the same with or without the tapeloop). It's possible that something else in my place is making it and it's coming through on the earth I guess, not sure. Also when I touch the wires on the back of the record head (first head from the left) I hear a quiet buzz, but nothing from the other 3 heads. I have 2 spare valves so I swapped them but it seems to make no difference.
What's next, do you think?

[Techman]

Good progress.

Unfortunately, now you'll have to do some serious fault finding - which is what I'll have to be doing when I dig mine out!