WEM Copicat Custom

[Station X]

It's not unusual for valves to glow very brightly for a second or so on switch on, looking almost like a flash bulb. If you're concerned about it check the voltage on the heater winding of the mains transformer.

I think I'm right in saying that the base bias voltage of the transistor is derived from a potential divider which the volume controls form part of. Does the base voltage change as you move the controls?

[evilmoisture]

Ok then. the green wires from the transformer (heaters) measure 6.3v AC so that's as it should be.
With my meter on 20v DC and the wires clipped to chassis and the base of the transistor there is -2.7v but it doesn't change at all when I move any of the controls.

[Station X]

Back to the experts then.

[evilmoisture]

Clutching at straws here but how abut this? When I first got the copicat from my friend I noticed that the 1k resistor was sticking up and it looked like it had become unsoldered from the wire next to it (sticking up also) -see pic. So I soldered them back, and had forgotten that this looks rather odd anyway. There are some unused holes on the board as you can see. Also looking at the other side of the board -see the other pic- someone has added a bridging wire, but isn't it going to the wrong point on the board? I'm struggling to see where this is on the schematic, would the 1K resistor be R15?

[evilmoisture]

Looking again possibly 2 of the unused holes are for an option of a cap from pin 3 of V3 which is present on some schematics

[Station X]

The only way to determine where the 1k resistor is connected in the circuit to visually trace the PCB tracks or wires it's connected to, to some identifiable point such as a valve pin. You can then compare this with the circuit diagram to see whether it's in the right place.

[PJL]

As Graham recommends, if its been messed with you have no option but to trace out the circuit before you go any further.

My suggestion was to remove the direct feed so that you would only hear the echo but I believe you must have disconnected all of the input to the transistor. If you can now hear an echo you have something to work with. I am sure there will be alignment screws for the heads and these will need to be carefully adjusted for maximum output but take note of the adjustments you make so you can put it back if you get into trouble.

[vidjoman]

May I suggest that, at this point, don't add possible further complications to the problem by twiddling the head alignment.
You have to get the electronics sorted first. If there is no sound from the electronics there still won't be even if you play with the heads.
As already mentioned - make a drawing of the layout, or take clear photos of both sides of the board, and trace against the original circuit. You seem to be going round in circles because of the probably user applied faults as well as an original problem.
When the amplifiers are working - you might need to tweek the heads to improve the sound quality, but only if they have been already got at.

[Station X]

One thing strikes me a odd here,

When you removed the direct feed you couldn't read any voltages at all on the transistor, yet there is a potential divider on the transistor base formed of a 470K resistor from the base to the -25V supply and a 1M?? resistor going to the wiper of the swell control, The track of the swell control then goes down to the chassis or positive rail as far as the transistor is concerned.

In parallel with the swell control is a foot switch presumably containing another pot?

This gets a mention in posts 125, 127 and 145.

[evilmoisture]

Thanks for the suggestions, yes it seems like today's task is to draw both sides of the entire board, number the components and see how they correspond to the schematics. The closest one to my version is http://www.vintagehofner.co.uk/brita...opicatmk3.html - i also attached a better quality pdf which I found elsewhere- .The problem is that the schematics differ (Omegaman's and 2 versions of the Watkins schematic), especially in the way the switch section around V3 is wired, and some resistors are of different values, or are omitted. On one diagram the switch resistors R19, 20, 21 go to the wiper lug of the reverb (or"sustain") pot, on the other they go to ground.
@station x no the footswitch is exactly that, a switch. However my friend said it was broken, and he cut the cable, so there are just 2 wires which can be twisted together to switch it "on". I'll do another test on the transistor moving the pots just to double check.

[Omegaman]

Hi Guys,
Just to clarify...
The heads on the Copicats are fixed and not adjustable in any way. There is no adjustment for height or azimuth etc. Bargain basement stuff!!

Do you get a 'straight through' signal that is loud and clear and adjustable in volume with the volume pot/s?
If yes, then V1a/b stage/s and the transistor are working.
Do you get a buzz (on the output) if you touch the connections on the back of each head with your finger (with the push switch pressed in to select each particular head)? The swell control needs to be turned right up for this test.
If yes V3a/b are most likely ok which leaves the bias oscillator and record head amp (V2a) suspect.
The footswitch is not required and the connections should be left open. This merely grounds the playback from the tape to switch off the echo.

An oscilloscope would be most useful. Can you get hold of one?

Regards all

[evilmoisture]

Hi Omegaman, yes I do get a straight through signal which is adjustable with the gain pots. No buz on the pegs on the back of the heads, or at least nothing I can distinguish from the way the background hum changes when I touch the chassis etc.

I *may* have figured out the problem(s) after spending all today drawing it. I decided to go for overkill and just draw everything and relate each component as closely to the numbering in the schematic as possible. I attach a .jpg here but it may be too small to read clearly, so please see the .png at this url http://evilmoisture.free.fr/copicat/copicat_layout.png
PLEASE NOTE this is a schematic of the way MY machine is wired up, not necessarily the RIGHT way, so beware if you use it for repairing another one.
I left out a few things such as the pots (and their wires to the board), also didn't go into depth with power or switch mechanism, and I think I forgot to include the 470k resistor which connects to the centre lug of the "reverb" pot, ground, and the wires for the footswitch.

In mapping all this out this I noticed the following:
1) On my machine the connections to V2B are different to those on the schematic where C9, C10, and T1 connect; on my machine everything connected to pin 1 on the schematc is connected to pin 2. I did wonder if the valve can be connected either way round or not. Anyway you can see the difference in the connections by comparing my drawing with the schematic
2) The strange sticking up 1k resistor I mentioned a few posts ago seems to be some kind of "bodge" replacing C5, R11, and R12, which (and I've wracked my brains but correct me if I'm wrong) are missing on my machine . I've added blue lines and vertical text on the drawing to point out the missing components.
3) On my machine there is a small electrolytic between the transistor and R31 which is not on the schematic as far as I can tell. Maybe just to reduce hum?
4) R17 is 4,7k on my machine as opposed to 47k on the schematic.
5) It's a mess!
Let me know your thoughts, it took me ages . I couldn't find a pic of an empty board online so I ended up taking a photo and then drawing the lines / erasing the components etc. I guess these might be useful for someone in the future so here are links to them: http://evilmoisture.free.fr/copicat/...onent_side.jpg
http://evilmoisture.free.fr/copicat/board_A.jpg

[evilmoisture]

Hard to get at but here's a photo for reference which may help when looking at my drawing. You can see the 1k resistor "bodge" and the spaces where missing components should go, unless this was some revised money-saving version. Looks more like 'circuit-bending' to me though.

[PJL]

C12 should go to the 100K (drawing error?).
C5/R12 are not needed as this is for the 6BR8.
There should be a link between V3 pin 7 and the junction of R8/R9 (drawing error?).
The fault is probably the missing R11. Is the 1K shown on your drawing the one you soldered back? If so then put it in the correct hole for R11 and I suspect the fault will be fixed.

[evilmoisture]

THANKS!!! You were correct in all three, first two were drawing errors which I've corrected. I moved the 1K so that it connects to the trace which leads to pin 7 on V2 and now I hear the sound of the tape passing! However there still seems to be a problem around V2/record, because the input signal does not pass to the record head, or only *very* faintly. When I touch the pegs on the back of the record head briefly the "pop" gets echoed, so I know all three play heads are working too. I also hear a hum now when I touch the pegs on the back of the play heads.
I've checked 3 times very carefully over the board to ensure there are no solder bridges, and reflowed the solder over the caps I replaced before. Also tried swapping the valves, I have 5 in total, and I think I've tried every combination and they all sound the same i.e. they all work. At one point while testing with a jack cable plugged in, touching the end to make a buzz, I had very loud crackly and distorted sounds trying to break through as if there was some bad contact or half dead component.
Almost there I think, any advice?
Thanks again for all the amazing help, I didn't realize this would be such a tricky one!

P.s. in answer to your earlier question, don't know anyone who could led me a scope, but I will ask around...

[evilmoisture]

Actually just realized it should be pin 8 not pin 7. AND, drumroll please...
IT WORKS!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!
Super happy, seriously thanks for all your help everyone, it dragged on a bit but it was worth it for how much I learnt. CHEERS!!!!!!!

[Omegaman]

Super result!!

PJL beat me to it... I noticed the same.

It looks to me if it had been previously 'modified' to use as a valve overdrive unit. I have seen one before like this. Why go to all the bother with altering the circuit? Just remove the tape or switch the heads off!!

Glad you fixed it.

Regards

[Station X]

Well done. It's always difficult repairing stuff which has been "got at" and very difficult indeed to diagnose such faults remotely. So well done Omegaman and PJL.

[Timvintage]

Wow!! I've been keeping up-to-date with this thread since the start, with growing interest. I think all who contributed should give themselves a pat on the back! Well done all! Evilmoisture I bet you're pleased with yourself. It's always so satisfying when you get something working isn't it?

[ColinTheAmpMan1]

Originally Posted by evilmoisture

@station x no the footswitch is exactly that, a switch. However my friend said it was broken, and he cut the cable, so there are just 2 wires which can be twisted together to switch it "on".

IF you take a close look at the schematic that you posted, you'll see that when the foot-switch is in the closed position, the echo signal is shorted to ground. I believe that this is true of most (if not all) Copicats. So, twisting the wires together will kill the echo, not turn it on. Could that be your problem?
Regards, Colin.
Well, that serves me right. I should read all the posts before having a go myself. Omegaman beat me to it!