Watkins Copicat Tape echo servicing &mods

[snk]

Hello,

In a couple days, I should get a Watkins Copicat tape echo Super IC. It is supposed to work fine, but it's a rather primitive design with its own quirks. While I enjoy tape imperfections, I'm considering some mods and improvements (other the basic servicing like cleaning the heads and demagnetizing it).



I have found the schematic : https://el34world.com/Forum/index.php?action=dlattach;topic=28596.0;attach=96773;image
As well as a video with the guts exposed : https://www.youtube.com/watch?v=6JThrswo5W0
And a very nice website with several explanations : https://amp-fix.com/Copicat%20Super%20ic.htm

I have a couple questions regarding the modifications I am planning to do (of course, I still haven't got the effect yet, and I am waiting to try it before blindly doing anything).


1_ Recapping
I think that recapping the electrolytics would be a good idea.
I know that it might be wise to increase the values of the caps from the power supply filtering, but for the others (I see a 6,8µF, a 15µF, etc), should I stick with the original values, or increase them as well (I guess I should stick with the values used in my machine, but i'm asking for confirmation) ?


2_ Impedance
I have read several things about the input and output impedance of the Copicat, but sometimes the comments were made about the earlier (tube) models.
The input impedance is 50k (i see the 47K resistors on the schematic), but i am unsure about the output impedance.
My questions are the following :
- What is the output impedance ?
- Is the unit featuring input and output buffers ?
- I am not a guitarist. My plan is to use it mostly with the AUX of a mixing desk, and I fear to have impedance issues (I have read several comments about people trying to use a Copicat direct from or to a soundcard without much luck). The unit features 4 inputs, so it shouldn't be an issue to change a couple resistors if it can help the Copicat to play well with a mixer and make it more versatile. Are the stock values good enough, or should I mod one or two inputs to make it more compatible with the mixer aux send and receive ?


3- basscut switch
How easy would it be to add a basscut switch on one input, so i could remove the bass frequencies coming into the unit (for instance, cutting below 80-120Hz) ? Would it be as easy as on a stompbox, where you add or change the input capacitor value ?


5_ Varispeed.
Now, the biggest part : the Copicat is a fixed delay time unit, and I wish I could change the delay time. I have seen this video, where a light dimmer is used to slow down the motor. It seems fine and an easy mod, but some comments point out that without either a PWM dimmer or a light bulb in parallel, it's rather risky for the motor... and I wouldn't want to harm a venerable motor from the 70s.
https://www.youtube.com/watch?v=38DBLGdnlJE

So, what are your opinion and advices about this mod ? How to know if a light dimmer would meet the requirements (I am not talking a bout the voltage it can handle, but if it is PWM or not) ? Or should I just use a lightbulb in parallel ?

Thank you in advance !

[anotherjim]

For the electro caps, I would use the next highest available. 15uF becomes 22uF etc.

Output impedance will be pretty much the 1k feeding the output cable.
Normal input is 47k + the input control pot which depends on the wiper setting but the Echo In-Out TRS jack input has 47k added in parallel if nothing is plugged into the input 4 jack. Either way, input is over 10k which any line level mixer or audio interface should be good with.
It looks like the Echo jack is intended to be used with a TRS send/return insert cable from a mixer channel. Input to tape on tip (send from mixer) and return from tape to mixer on the ring.

A bass cut cap could be added in series with an input, but the way the volume pots are wired means the frequency of the filter depends on the pot setting. I don't know why they didn't design a standard virtual ground mixer there.

The motor & transformer is all in one (not seen that before). Any attempt to use a dimmer/speed controller will also affect the circuit supply voltage and possibly inject some nasty switching noises. I doubt it would damage anything but we know that light dimmers can interfere with audio gear even at some distance.

[snk]

Thank you very much, Jim!

For the electro caps, I would use the next highest available. 15uF becomes 22uF etc.

Yes, this was my intention.
For the power filter caps, I know that size was an important factor back in the day, but now I can double the values easily.
For the other capacitors in the circuit (15µF and 6.8µF), I will follow your advice.

Output impedance will be pretty much the 1k feeding the output cable.

This is what I suspected but I wanted confirmation : thank you
In theory, no need for a buffer or DI, then?

Normal input is 47k + the input control pot which depends on the wiper setting[...].

The 250k pot! I had forgotten about it  So yes, input impedance may roughly vary from 47k to 300k.

Either way, input is over 10k which any line level mixer or audio interface should be good with.

Thank you

the Echo In-Out TRS jack input has 47k added in parallel if nothing is plugged into the input 4 jack.

Yes, it is designed to be used as a FX insert, I like that!

A bass cut cap could be added in series with an input, but the way the volume pots are wired means the frequency of the filter depends on the pot setting. I don't know why they didn't design a standard virtual ground mixer there.

Putting a capacitor to ground after the 47k resistor and before the 250k pot would do the trick ? Or a capacitor in serie between 47k resistor and the 250k pot?

Any attempt to use a dimmer/speed controller will also affect the circuit supply voltage and possibly inject some nasty switching noises. I doubt it would damage anything but we know that light dimmers can interfere with audio gear even at some distance.

Thank you for your insight. I suspected the video to be a quick and dirty hack (despite being very cool), and some comments under the video (from people who tried the mod) said the same than your warning : adding a light dimmer produces noise.
Would a regulated solid state speed controller be a safer bet?

[anotherjim]

For bass cut, the capacitor goes in series. In the wire from the input jack tip to the PCB should be easiest. I think 47nF would be a good one to try first.

This is an AC motor and the AC supply frequency sets the motor speed. DC motor controllers won't work.
Lamp dimmers and AC motor controllers work by reducing the AC average voltage with SCR/Triac circuits but the side effect is a fast jump in voltage instead of the smooth transition of a sinewave and that's the cause of the noise. The motor will slow down when it doesn't get enough power to keep up with the supply frequency and it can't be made to go faster. I don't know if you can get separate access to the motor terminals. In tube days, it was common to use the motor winding to drop the voltage in series with the tube filaments, but this is another thing I haven't dealt with before. The motor and transformer windings could simply be sharing the iron laminations but I have a suspicion the motor winding is also the primary transformer winding in which case you can't control the motor independently of the transformer.

I think all you can reasonably do to play with the speed is a friction brake on the drive hub, which was also the only way to wobble the pitch of a Mellotron.

[snk]

Thank you, Jim.

For bass cut, the capacitor goes in series. In the wire from the input jack tip to the PCB should be easiest. I think 47nF would be a good one to try first.

I will try with 47nF, and see how it goes.

I think all you can reasonably do to play with the speed is a friction brake on the drive hub, which was also the only way to wobble the pitch of a Mellotron.

Ah ah! I hadn't thought about such a possibility! I haven't got any clue how I could manage to do it (in a rather elegant way, and safe for the machine), but it's well worth investigating.

[PRR]

> motor & transformer is all in one (not seen that before).

This became common on the cheapest solid-state record-players.

As you say, on this class of AC motor, speed=frequency, if not over-loaded to lugging. (And I have burnt-out a large AC motor by lugging it for hours-- stinks!) These phono motors are designed to run very UN-loaded (not doing any real work) so may not like being dragged.

In concept you could replace the amplifier supply with a little filament transformer, the power the motor with variable power oscillator. IME the voltage has to vary with the frequency. And 45 to 78 (104Hz) was a strain on a good turntable. VFD units are now common but tend to be industrial price, or junk; I have not played with one.

> The 250k pot! I had forgotten about it  So yes, input impedance may roughly vary from 47k to 300k.

At full-up it faces the near-zero of the summing node. So 47k to 109.5k to 47k.

I do think you could do a low-cut outside the box.

I do not like the 0.01u cap from opamp output to ground. That kinda load "can" make the opamp sing 1MHz. But it won't hurt the chip, and it has been that way after 40+ years and thousands in use, so I would not touch it.

In general: why do you think it needs improvement? Are our parts, or our understanding, much better than Watkins'? Yes, the power filter caps can go sour but they look to be about 10 times bigger than needed so that can rot a lot before hum/buzz gets obvious. And I'd be afraid that odd/old caps might be some of the "mojo tone".

[snk]

Thank you, PRR.

I don't think it "needs improvement" : I am just waiting to receive a tape echo unit, where some maintenance will be required (like cleaning heads, etc). I also know that the unit is not properly grounded, so i might add a 3 pins plugs...  While i'm waiting for the unit, I am gathering informations, and when I'll have the effect with its guts open, I am willing to add a couple mods if my first experience with the echo isn't satisfactory. But I am not thinking that anything is mandatory prior to receive it, and I will not do anything before playing with the echo for a while.

Well, to be perfectly honest, I'm a bit torn about two mods and two point of views : on one hand, I am eager to add a sound on sound feature (by adding a switch to the erase head) because I like to play with loops, and I feel that fixed delay times are a big limitation so the varispeed mod would be a real plus. But on the other hand, I often enjoy things the way they were designed, and most of the time limitations are what make the identity of something. So, if it gives me new musical ideas (or if i enjoy the "full of mojo sound", I'll stick with the original design), but i feel that something is slacking, I may try to mod it to suit my needs better.

[anotherjim]

I used to lust for a Copycat back in the day but they were too expensive for me although cheaper than any imported brands. My tape echo lust was satisfied by an Akai 7" RTR that had 3 heads and could do sound-on-sound dubbing (my first home studio!). The Akai cost about the same as a Copycat. Speed control was nonexistent apart from finger dragging a reel or changing the tape speed by fitting different diameter sleeves over the capstan spindle.

The speed thing brings up an Ampex 1455 7" RTR I have for repair. It's only a domestic model but it was brought over from the states by a friend who cherishes it as it belonged to her late father who used to work for Ampex. Her husband bought a 110v transformer to run it but I pointed out that the speed will be slow on our 50Hz AC power. The motor hub pully for the drive belt is a stepped affair with a clever mechanical finger thing that switches the belt to different diameter steps. I don't know what to do about this. There doesn't appear to have been a European export model so no alternative pully for 50Hz. There is a possibility we might get one machined.

[snk]

It was a bit of an impulse buy... I've been lusting over one for a while, and found this one just after seeing the varispeed mod video, so I bought it. The fixed delay time annoys me a bit, but maybe i can work with it (instead of against it) if the varispeed mod isn't wise to try.

Tape has its quirks, and its charms...
I don't know that Ampex, but I love my Revox

[pacealot]

which was also the only way to wobble the pitch of a Mellotron.

That's not actually true; even from the beginning with the Mk I/Mk II, the Mellotron had control over the motor speed in the form of a giant AC-generating pitch knob on the front (the lone exceptions were some — but not all — of the M300 models, which for some reason did not have player-accessible speed control). And thank goodness, because keeping that thing in tune with any other instrument requires feats of superhuman ability. Or as Robert Fripp said, "tuning a Mellotron doesn't." 

But to your point, "wobbling" is not something that rapid movement of the pitch knob of a Mellotron is likely to accomplish — the motor can't respond to the frequency changes quickly enough, so it will generally just sound seasick for a moment and then blow a fuse — ask me how I know...

[anotherjim]

^ Interesting, I had no idea the 'trons had fine-tuning capabilty. I have heard recordings that sounds like the pitch is deliberately being dropped and raised. Was it a 2 person task with one playing the keys and one twiddling the tune control while applying the handbrake? Or, had the motor somehow been switched on & off as can be done with a tonewheel organ? On one occasion, Fripp was in the studio but only credited for guitar.

[pacealot]

Mike Pinder from the Moody Blues probably did the most famous Tron pitch-bends in "Legend Of A Mind", and you can even see him do it here, around the ten-minute mark:

https://www.youtube.com/watch?v=C09d8skPowY

It sounds like it requires Bodie & Doyle Ford Capri skills, but it works out okay with just the knob. But only just.

And Fripp was probably the main Mellotronist (?) for Crimson between '70 and '74, so he ought to know!

Your Ampex story reminds me of the one about Trident Studios and their first Ampex 400 8-track. The Sheffield brothers (proprieters and later subjects of Queen's "Sheff—er, I mean—Death On Two Legs") didn't know to ask Ampex to modify the motor to factor in the difference between 60Hz US and 50Hz UK power, so when they first got it and started using it, they discovered that its 15IPS was more like 12.5IPS! They used it for several months that way before they could get it retrofitted. I believe "Hey Jude" was one of the recordings that was tracked with it running too slow! 

Closer to on-topic (sorry!), it'd be nice to be able to add such a knob to the Copicat (or a Binson for that matter) but I suspect those motors run on DC, and that complicates matters.

[snk]

Hello
I received the echo this weekend.
The circuit features some strange things, but it's a lovely machine



First, I think i'm going to change the power supply capacitors