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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 14th Dec 2014, 10:09 am   #1
evilmoisture
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Default WEM Copicat Custom

Hi there.

I am attempting to repair a Copicat Custom for a friend. The first thing I did was to change the old unearthed two wire mains plug to an earthed wired one, with the earth wire going to the chassis (which I verified also goes to audio ground via the pot cans and other metal parts), after reading some safety warnings.

Upon plugging it in, all three tubes light up and sound passes through the unit (and both input volumes work) but there is no echo, no sound at all from the heads.

I then took a careful look at all the components, and discovered that one resistor had become unsoldered, so fixed that, and then noticed that one of the large resistors on the big 3-cap can (the 2.2k one) was blackened and had actually cracked in half. I replaced this with a 2.2k wirewound 5 watt resistor which I had in my spares, and powered it up again, but there was still no echo and after a minute the new resistor became RED hot, so I quickly unplugged it.

I'm thinking this *might* mean that the big cap (actually 3 caps) is dead and shorting, but, as I said to my friend, there's no guarantee that other caps haven't died too, not to mention the selenium rectifier, which I hade read elsewhere is a weird component which is better to replace with a 4007 diode (and resistor?).

I suggested to my friend that the only sure way (and also good for the future anyway) is to replace ALL the caps, the recifier, and maybe the resistors too (why not?), but I decided it might be worth asking your advice on here before proceeding, just to see if I am missing anything.

Another point worth mentioning is that the power transformer has been replaced several years ago with a more modern looking device, but that it was working fine with this until a while ago. I can post photos if it will be useful.

Thanks in advance for any tips!

-Andy
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Old 14th Dec 2014, 1:37 pm   #2
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Default Re: WEM Copicat Custom

Photos would help as don't think many readers will have seen one of these. What test gear do you have?
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Old 14th Dec 2014, 1:37 pm   #3
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Default Re: WEM Copicat Custom

Hello Andy and welcome,

Usual practice with valve gear that's possibly faulty is to try it initially with a 100W lamp in series with the mains feed, so that if something goes wrong, the bulb just lights up more brightly, as opposed to something in the set glowing or going bang. This is known as a "lamp limiter"

To help those of us not au fait with this particular machine, could you post a photo of the chassis? This would help us tell you which caps are suspect and which might well be OK.

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Old 14th Dec 2014, 2:49 pm   #4
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Default Re: WEM Copicat Custom

As it is a valved Custom copicat it will most likely be a MK 3. The schematic can be found here.

http://www.vintagehofner.co.uk/brita...s/wemwire.html

Al
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Old 14th Dec 2014, 4:41 pm   #5
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Default Re: WEM Copicat Custom

As it is passing through input signals, there is at least some HT reaching V1, so it is unlikely that any of the caps in the reservoir/smoother can is actually shorted. That is not to say they might not benefit from replacement, but the cause of the 2.2k overheating and the lack of signal off tape is more likely to be around the bias oscillator V2b. Do you know offhand if it is the play or record side that is dead?

My advice would be to try and trace the fault, rather than a shotgun repair. It is not a complex unit and was working recently. If you then take the working machine and replace caps for future reliability, when something goes wrong during your overhaul (as it sometimes does, with the best will in the world) you can stop, locate the problem and work back to a functional condition again.

For the very best in audio fidelity (within the inherent limitations!) you might want to change some resistors but I doubt any are stopping it working, except the one burnt out by the fault elsewhere. The same is true of some caps, and this is where pics would help.
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Old 14th Dec 2014, 5:29 pm   #6
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Default Re: WEM Copicat Custom

Apologies if I'm stating the obvious, but do ensure the magnetic side of the tape loop
(usually matt finish) is in contact with the actual front of the heads.
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Old 14th Dec 2014, 9:36 pm   #7
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Default Re: WEM Copicat Custom

Hi there and thanks so much for the replies. I also forgot to say hi to the group, so, err, hi! Here is a photo with arrows showing the resistor which blew, and a second with the ones I added (I didn't have 2,2k so I put 2k and 200ohm in series).
In terms of test gear I'm very limited, just a small ohm meter/ voltage tester. My knowledge of tube circuits is also very limited, which is why I thought about just replacing everything.
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Old 14th Dec 2014, 9:38 pm   #8
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Default Re: WEM Copicat Custom

Actually, Restoration73 took the words out of my mouth! Well partly anyway. Last year I got a melos echo chamber, which has sort-of a mini 8-track cartridge for the tape loop, I think also used on revox echo machines? When I first plugged in the machine, the audio was very quiet, and there was no echo at all! My first thought was that there was something quite wrong with the machine. However, I thought that the best thing to do first was to check that the tape cartridge was in good order. I found that the pressure pads had broken down, meaning that the tape wasn't coming in to contact with the heads. I found on ebay a service where they re-ferbed these cartridges for I think around £18. So I sent off the cartridge, and when I got it back, I cleaned the heads and tried it again. Amazing, the echo chamber now works perfectly! I hadn't realised that having bad pressure pads could result in no audio from the tape at all, I had thought that it would be muffled or low output!
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Old 14th Dec 2014, 10:02 pm   #9
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Default Re: WEM Copicat Custom

I have the tape the right way round for sure; if only it was just something like that! @Timvintage I also own one of these Melos echos and a common problem is as you described, due to the tape not touching the heads because the foam pads inside the cart have disintegrated.
Thanks for the schematic link Alistair D, yes I'm pretty sure it's this model, with one transistor (is that a FET?)...curious about this transistor, and I'm also going to double-check that it's not just a problem with the footswitch...
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Old 14th Dec 2014, 10:27 pm   #10
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Default Re: WEM Copicat Custom

For the 2.2K to overheat it'll have to be dissipating in excess of 5W which equates to a current of about 47mA. With that current flowing the voltage across the 2.2K will be about 103V, so a sizeable amount of the HT has been eaten up.

Try disconnecting the wires on either side of R27 (4.7K). If the resistor still overheats the problem is with the smoothing capacitors. If the resistor is now cool then reconnect the wires one at a time to try and localise the fault.
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Old 14th Dec 2014, 10:30 pm   #11
evilmoisture
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Default Re: WEM Copicat Custom

A few more photos. My friend also mentioned that it had sounded "weird" since the transformer was replaced with the "donut" as he called it.
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Old 15th Dec 2014, 11:39 am   #12
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Default Re: WEM Copicat Custom

Hi again,
following your advice, Graham, I disconnected both wires from R27 (4,7k) and upon powering up R28 (2,2k) didn't get hot. As I assume is a wise thing to do, after unplugging it I discharged the smoothing caps with a large screwdriver, and a spark was produced. This probably also is a sign that the caps are working if they are storing a charge, I presume?
So next I reconnected the wire which connects one end of R27 to the small transformer T1, and on plugging it in R28 got hot. When I did my screwdriver trick again there was NO spark, which might be useful info.
Finally I removed the wire to T1 and connected the other wire back to R27, and R28 didn't get hot when I plugged it in. This time there WAS a spark when I discharged the caps.
So I'm thinking the problem lies somewhere with T1, the 2200pf cap C8, or after...what do you think I should try next?

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Old 15th Dec 2014, 11:39 am   #13
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Default Re: WEM Copicat Custom

The obvious question has to be asked - why did the transformer need to be replaced and was the fault that knocked out the original transformer sorted at the time - and, is the replacement transformer the correct one for the job?

You need to do the tests as said first before you do anything else.
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Old 15th Dec 2014, 11:54 am   #14
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Default Re: WEM Copicat Custom

I've messaged my friend asking the reason for the new transformer. Looking at it it does seem to be the right value though: The input is 220v (that's the voltage here in France), Winding 1 is 250v (the red wires) and winding 2 is 6.3v (green wires). That's the same as on the schematic in any case...it also says "P=50VA".
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Old 15th Dec 2014, 12:06 pm   #15
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Default Re: WEM Copicat Custom

The fact that, with both wires disconnected, the resistor doesn't get hot and you can draw a spark from the smoothers, shows that the smoothers are OK.

The fault will be somewhere in the circuitry fed by T1 primary. Try disconnecting the wire from R27 to T1 and measuring its resistance to chassis. It should read open circuit. This may show up the fault, unless the full working voltage is required to break down a leaking cap.

I'll hazard a guess at C9 being the problem. Try disconnecting it.
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Old 15th Dec 2014, 12:43 pm   #16
evilmoisture
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Default Re: WEM Copicat Custom

Hi Graham, measured that and with the mains plugged in it's 500 ohms, with the mains unplugged it goes down to about 440-445. With C9 removed it's open circuit.
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Old 15th Dec 2014, 12:43 pm   #17
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Default Re: WEM Copicat Custom

Sorry, I seemed to have somehow missed your bit about disconnecting the wires and getting the spark.

I would go with what Graham says above.

You shouldn't be measuring ohms with the mains connected.

Edit - do you mean you just disconnected the transformer - I'm getting confused (again).
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Old 15th Dec 2014, 12:50 pm   #18
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Default Re: WEM Copicat Custom

Yeah I wondered about that, ooops. Anyway no harm done. And sorry my posts seem to be slow to appear, possibly because I'm a new user so the moderator is being careful...

Edit no I did just plug and unplug the mains. I should've remembered. Very cheap multimeter anyway so no worries
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Old 15th Dec 2014, 12:57 pm   #19
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Default Re: WEM Copicat Custom

Ah, ok, looks like you got away with it this time.

Yes, I did notice the posts seemed to be getting out of order.
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Old 15th Dec 2014, 1:01 pm   #20
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Default Re: WEM Copicat Custom

Quote:
Originally Posted by evilmoisture View Post
Hi there and thanks so much for the replies. I also forgot to say hi to the group, so, err, hi! Here is a photo with arrows showing the resistor which blew, and a second with the ones I added (I didn't have 2,2k so I put 2k and 200ohm in series).
In terms of test gear I'm very limited, just a small ohm meter/ voltage tester. My knowledge of tube circuits is also very limited, which is why I thought about just replacing everything.
The wiring to the electrolytic can looks wrong.

The bottom of your 2k2 resistor is connected to the same tag as the red wire from the transformer. That would put it directly across the HT supply. I am not sure whether it is the red wire or the resistor that is in the wrong place.

The red wire must go to the common negative of the capacitor. Hopefully the terminal details are marked on the side of the can. If the red wire is correctly connected then the bottom of the 2k2 resistor should be connected to the tag to the left of the negative. This tag should also have the bottom of the 4k7 resistor connected to it.

The blue electrolytic beside the rectifier also looks like it may not be an original component.

Here are some pictures that may help.

http://www.amp-fix.com/copicat1%5B1%5D.htm

Al
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