Wem Custom Copicat (tube version)

[thomasdf]

Hi all

I recently got myself a Wem custom Copicat (tube version) very cheap because it was broken... or supposed to be broken.
I ended up putting a missing tube (6BR8) in it, and fixing a few soldering points that looked tired.

It is now working fine except that I have a whole lot more wet signal than dry signal so it's pretty hard to use it with a guitar and an amp for instance.

Do you guys know what could be the problem here?

My final goal is to turn the Swell pot into a dry/wet pot, do you know if it is possible? Being a sound engineer as most of you I would like to have this feature so I can use it as a parallel FX (100% wet) while mixing in studio.

Thanks a lot in advance

Thomas

 

[thomasdf]

Problem solved! For those who are interested, the schematics is way simplier than it looks. For some reason part of the internal wiring was done in a wrong way and I was missing a resistor...
I fixed everything and added a on/off switch to cut the direct signal if I need to use it with a mixing desk.

My only concern now is that it is pretty noisy (sounds like DC hum) when the motor is off, and it's even worse when the motor is running! I added a 3 lead power cable and linked the earth and the chassis' ground but the problem is still the same...

I'll need to take a deeper look into that, but any advices are welcome!

Cheers

 

[L´Andratté]

Did you change the electrolytics?

 

[thomasdf]

Haha, no... Everything is STOCK (except the tape haha)... Which I like... But I know I'll have to change them at some point.
Maybe I should start with those related to the PSU

Here is the schematics of my unit (Custom Copicat, the last tube model) :

 

[thomasdf]

I changed the electrolytics and it's better.
I have a bias problem I think, the echo is very saturated. I changed the bias caps and it's better, but still not very clean.
I am having impedance troubles, the echo sounds good with a guitar but very weak and no bass at all when inserted on my soundcard... I was told the output transistor was here to help fixe impedance problem...
Any clue on how to use it properly or tweak it? I haven't tried with a reamp box + DI but that's a lot of gear, I'd prefer something like a resistor swap or small buffer add, etc!

 

[Tubetec]

One of the issues that causes alot of 50Hz hum in the copicat MKII and custom valve units , is the fact that it has a half wave rectifier in the power supply ,what I did with my MKII was take out the old metal oxide rectifier,and replace it with a modern high voltage bridge rectifier.Peak voltage at switch on was the same ,however actual HT voltage was up by about 35 volts during normal opperation ,in the MKII this isnt a problem ,although I did add an extra filter RC filter in the ht line to the early preamp stages .The result is much less circuit noise ,as the ht capacitors are more effective at filtering out 100hz as opposed to 50 hz.So now tape noise is  predominant over mains hum ,and the unit is much more useable.I also removed the bias trap from between the first and second stages of the echo recovery side ,more like the copicat custom .I decided to add one further embelishment ,I replaced the push button switches with three 25k linear pots and a tie down resistor of 22k across the input.Works a treat this allowing independant variation of echo  from all three playback heads.I can post the circuit if anyone is interested ......

Just one more point ,the copicat MkII has an unbuffered valve output stage ,so it needs to see a IM ohm minumum input resistance when you hook it up to something,a soundcard or line input on a mixer will be way to low,and it will not sound right, a high impedence DI input will fix things ,or you could feed it through a spare guitar amp and mic that up ,for a bit of extra grit .

 

[abbey road d enfer]

My only concern now is that it is pretty noisy (sounds like DC hum) when the motor is off, and it's even worse when the motor is running!

that would suggest the motor's magnetic field is interfering with the PB heads. Try adding complementary magnetic shielding. Often a simple steel plate will deflect the flux lines enough to reduce hum.

 

[abbey road d enfer]

I am having impedance troubles, the echo sounds good with a guitar but very weak and no bass at all when inserted on my soundcard... I was told the output transistor was here to help fixe impedance problem...

Transistor Tr1 operates as a voltage follower so has rather low output impedance, but the capacitor (C14?) is notably low in value (10nF). You may want to try replacing it with a much higher value like 10uF. It will be an electrolytic so you'll have to orient it correctly, i.e. with the negative connected to the transistor's emitter. You may also want to add a high-value resistor (22-100k) across the output in order to allow capacitor discharge.

 

[Tubetec]

One issue I had that increased hum level induced in the heads was someone ,a previous owner, had soldered one of the heads in reverse polarity with the rest ,once I reversed the wireing , hum was reduced further by cancellation.A small pink painted dot on the back of the heads denotes where the red wire should be connected.Using a screened cable from the output of the pots as in my mod above ,or the three way switch  ,helped a bit also .Lifting the yellow wire going from the board to the high end of the swell pot ,up and away from the other wires and chassis ,improved matters further .

 

[abbey road d enfer]

I have a bias problem I think, the echo is very saturated.

In order to properly check that, you must first run a test tape, that will tell you what the nominal PB level is. You don't need a certified test tape, just something that you record with 1k at nominal level. If you record it on a 2-track machine, the level will be marginally lower than nominal.
If you record the test tape on a 1/4 track machine, the level read will be about 6dB lower than nominal.
That is supposing the heads on the Copycat are full-track.
Indeed you need to turn off the erase and record electronics. this can be done by lifting V2.
Then when recording, you must adjust the record (input) level in order to match the nominal operating level. Then you must check distortion.  Typically on a tape echo unit, bias is set for max output at 500Hz. There is no apparent control for bias; you may proceed to substitute a fresh tube for V2, then maybe the core of transformer T1 can be adjusted.

 

[Tubetec]

Before I modded my cat, there was so much mains hum  getting onto tape,from the input electronics and bias stage, that when I increased the reverb control,the unit had a tendency to feedback at 50hz and the sound became very muddy ,interesting noise ,but not very musical. Once I had dealt with ht supply issue,which basically masked all other sources of noise ,then it was possible to start working on reducing the magnetically induced hum  ,from the playback electronics.

The heads on my cat only record onto less than half the track width,the input gain is the record level control ,no provision is made to adjust the bias to tape either,and the bias oscillator transformer is air cored .

The full wave bridge mod only required removal of the old oxide rectifier,and installation of the bridge rectifier ,which I took from a computer switch mode ,its fits neatly into the mounting bracket of the old rectifier too ,and only required changeing a few of solder joints. See photo....

 

[FriquitaFrancis]

Hey guys, I am not sure if this is the right place to post but I got a watkins copicat black cat.. its the one they made for the U.S. it runs fine and seems to be working but I can't get any effects out of it through my amp.. I was wondering if anyone could help me troubleshoot? thanks, Friquita

 

[CJ]

working on one too.

missing the tape.

just splice some 1/4 inch?

this is a transistor version, has Joe Meek written on the chassis,








jus kidin, 

 

[FriquitaFrancis]

Ha ha. Mine is running fine with tape and all but I can't seem to get any effect out of it through my amp, I need someone to troubleshoot.

 

[abbey road d enfer]

Ha ha. Mine is running fine with tape and all but I can't seem to get any effect out of it through my amp, I need someone to troubleshoot.

Do you have the schemo? Is it solid-state or valves?
Troubleshooting is relatively easy once you have the schemo; the traditional method of following the signal flow apply.
It seems the direct sound works, so that part works. You need to find if it's the record section or the playback that is duff.

 

[thomasdf]

(Refering to my (OP) Unit : Wem Custom Copicat, which is likely a Copicat MK3, tube unit with a single transistor on the output)

Long time, I thought I'd give everyone here an update... The unit has been fully recaped in 2015 but the echo remained very distorted and almost unusable ... and yesterday I decided to get back to it.

I recorded a 500hz sine wave tone tape on my mono full width Nagra 3, and played it back on the Copicat with V2 6BR8 out and the magnet out of the way, it seemed hard to tell on a sine thru tubes and guitar amp but it didn't sound that much distorted. I'll retry with more complex tones and guitar signals. Since the playback path is far from hi-fi I'm pretty sure that if it going to be hard to check on a scope, with the playback distortion, wow of the tape, etc. So my hears will be the judge I guess.

In the meantime, I've found this great page How it works and read that the bias signal is 44Khz.

Before hooking up my scope at 6BR8 pin 2, I measured DC and AC. DC is around 375VDC at startup and comes down to around 280VDC in operation ... but the AC... I am scared, according to my multimeter it is ABOVE 600VAC and it made my multimeter whistle !
There's definitely something wrong around here ? It should read a few volts tops, right ?
I didn't hook it up to my scope (max 400V old Hameg) so I don't know what the signal looks like ...

Maybe I should call an Exorcist at this point ?

 

[abbey road d enfer]

Multimeters can act crazy when submitted to HF signals.
I suggest you really do use the scope.
Actually, the AC voltage on the plate of the oscillator has a swing of 2 times B+, which is probably about 600V.

 

[thomasdf]

That's 200V above my max oscilloscope input voltage... I don't want to risk frying it ! Maybe with attenuator probe, I'd have to venture into that as I've never used it and not sure how to...

 

[abbey road d enfer]

Most attenuator probe withstand 600V. Just make sure it's in the X10 position if it is a switchable type.

 

[Tubetec]

The mixer/feedback network is prone to picking up lots of hum due to high z unscreened wiring ,
I was able to reduce noise a lot by using screened cables on the feedback and dry signal wires .
They run the full lenght of the chassis in close proimity to other wires , try lifting them up and away from the other wires to see how noise is effected .

Did you make sure to demag the heads and tape path ?

Check any of the high value series resistors in the direct signal path ,470k,1M at the inputs and near the echo control.

The 6BR8 oscillator is run at full HT volts/current at or above its maximum , the tube wears out in a few years ,
Check the caps around the osscilator 200pf, 2200pf and 0.02uf , replace with modern high voltage parts
if theres any doubt,

I converted mine to full wave bridge rectifier , I padded down the HT line with extra series resistance so the start up voltage surge isnt so abrupt
It also means the PSU hum freq shifts to 100hz from 50hz in the case of half wave .
the psu filters work better and less hum ends up in the chassis, heads and on tape going round and round in an ever building wall of noise .