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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 2nd Jan 2018, 8:36 pm   #1
MerlinMags
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Default Repair of 1959 WEM Copicat guitar echo unit

I'm attempting to repair a tape loop unit with valve amplifiers, made by Watkins Electronic Music. I think we have the 1959 model, as seen here - http://www.amp-fix.com/image%20file%202/Copicat.jpg - with blue colour, and three buttons for tape head selection.

The circuit seems to match this diagram - http://www.amp-fix.com/image%20file%202/Copicat.gif - but the wiring is often point-to-point so hard to decipher. We have followed about half of it through, and not found any discrepancies, but there may be a lump of extra solder making a short somewhere that we cannot see.

We have desoldered and checked every capacitor, and they seem to be functioning correctly when tested in isolation.

One new valve was purchased (a 6BR8) and the other two valves (both ECC83) seem to be functional but have not been scientifically tested. All show the expected glow from heater and the glass tips are silvery.

The positive DC rail shows approx 160V.

Plugging it in, all you hear is mains hum.

I have not bothered with checking the four tape heads, or fitting a tape, as we cannot even get the input signal to make its way to the output socket yet.

If anyone has advice I would be most grateful. Hopefully we have done all the obvious things, and are not wailing for assistance too early!
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Old 2nd Jan 2018, 9:00 pm   #2
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Default Re: Repair of 1959 WEM Copicat guitar echo unit

Hi,
It is possible that the main smoothing cap may need changing, but first you might need to find why you are not getting any direct signal through.
If you were able to check the dc voltage at the anodes of the first valve then this would help indicate where the fault might be. I wonder if you have checked the jack sockets as I seem to remember that corrosion on them can cause problems with intermittant signals. if you are not used to high voltages, do please be careful when taking measurements.
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Old 2nd Jan 2018, 9:16 pm   #3
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Default Re: Repair of 1959 WEM Copicat guitar echo unit

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Originally Posted by MerlinMags View Post
We have desoldered and checked every capacitor, and they seem to be functioning correctly when tested in isolation.
Did you test the capacitors at the full working voltage, which would have shown up any electrical leakage, or just use a capacitance tester operating at low voltage?
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Old 3rd Jan 2018, 7:40 am   #4
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Default Re: Repair of 1959 WEM Copicat guitar echo unit

These are pretty simple devices. It could be the rectifier has failed or the HT smoothing capacitors.

In view of the present financial value of the early Copicats, it might be worth getting professional help.
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Old 3rd Jan 2018, 11:02 am   #5
MerlinMags
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Default Re: Repair of 1959 WEM Copicat guitar echo unit

Thank you for these suggestions. I will look into all of them at the weekend.

I assumed the components in the AC-to-DC converter were functional as I could measure 160V DC in the right place, but perhaps the AVO (multimeter) was not truly releaving how stable/erratic the 'DC' was?!

We did not test the capacitors at high voltage; we just stuck the meter across them to check for the obvious signs of a breakdown.

Yes, I am sensibly terrified of these voltages, and aware that the Copicats have no earthing.

Will report back in a few days...
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Old 3rd Jan 2018, 11:25 am   #6
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Default Re: Repair of 1959 WEM Copicat guitar echo unit

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Originally Posted by MerlinMags View Post
We did not test the capacitors at high voltage; we just stuck the meter across them to check for the obvious signs of a breakdown.
That's a waste of time. The voltage from a normal meter won't break down anything. Only a very, very leaky capacitor will show up as faulty on such a test.

Leaky capacitors will show up as incorrect measured voltages, Positive voltage on a control grid or low voltage on a screen grid for example.
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Old 8th Jan 2018, 12:17 am   #7
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Default Re: Repair of 1959 WEM Copicat guitar echo unit

I would start by checking that you have an anode voltage on pins1 & 6 of the first ECC83 in the schematic. If not check that you have h.t. voltage on the two 100k resistors, these often go open circuit. You don't say whether you have access to a scope to check for a signal on the grids of the valve. You only need this stage to work to get an output from the unit. Other things to check are the resistance of the gain pots, the 1.5k resistor and contacts on the jack sockets. Is the output a length of screened cable with a jack plug, if so buzz it and check it is not shorted?
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Old 8th Jan 2018, 6:28 pm   #8
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Default Re: Repair of 1959 WEM Copicat guitar echo unit

I've got one of these which from memory looks exactly the same as yours. Mine is totally original inside and has the usual types of lower value paper capacitors whose age related leakage will by now cause problems in critical parts of the circuit. This thread is giving me yet another nudge to dig it out and get on with fixing it.
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Old 12th Jan 2018, 6:47 pm   #9
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Default Re: Repair of 1959 WEM Copicat guitar echo unit

The circuit diagram you give looks right, but my eyes see the gain controls as being 500k and 300k - they are both supposed to be 500k.
If you aren't getting any of the direct signal to the output, that is going to be because the two halves of the pre-amplifier ECC83 are dead or aren't passing any current, as sortedradio says. I would check the power-supply first; the voltages on the positive ends of the 20uF capacitors ought to be present and get lower as you go from the capacitor closest to the HT rectifier to the one feeding the 100k resistors on the ECC83 anodes - take care as the voltages ought to be around a hundred or so volts and can hurt! If there is HT present on the anodes of the ECC83 (pins 1 and 6), then the cathode circuitry, which is common to both triodes (a cheapskate technique) is likely suspect. You say that the heaters of the valves are all glowing - make sure that you can see two glowing heaters in the ECC83. I wouldn't mind betting that once you solve problem of not getting signal to the output, you will probably have a working Copicat.
One problem that can arise with these valve Copicats, while a highly desirable bit of kit, is that they can run hot and wax will melt out of the mains transformer when that happens. They also, like most tape-based echo units, suffer from noise coming from the tape-loop splice, which can be disconcerting at times.
While the unit, as you say, doesn't have a connection to mains earth, this is because the output cable will connect to the earthed chassis of the amplifier that you are running it into, thereby effectively providing an earth. This wasn't particularly good practice, either. It would have been better to properly earth the Copicat to mains and to break the ground connection of the output cable. This would have provided a proper earth and eliminated any earth-loop between the Copicat and the connected amplifier. Oh well, you could make this modification for better safety, but it would detract from the re-sale value.....
Colin.
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Old 16th Jan 2018, 7:57 pm   #10
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Default Re: Repair of 1959 WEM Copicat guitar echo unit

Replace all of the wax Hunts capacitors as a matter of course (that's what I always do & have resurrected scores of Mk 2 Copicats) assuming that you have tested the valves have HT & heater present. They are fairly simple to fix but if the Marriott heads are knackered you will have a problem of a different nature. Why not send it to me to establish what is wrong?
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