Copicat MK2 - No Echo

[Michael Why]

Hi All

I have 2 what look like Mk2 valve copicats , both with the same fault - the circuits look slightly different so i can only assume there was some variation in the Mk2 run or they have been modified over the years, These were sold to me as use the parts from one to fix the other ( the older one is in pretty poor cosmetic shape) but as the hard to get parts seem to work on both i would like to repair them both. so for the purposes of this post ( cry for help) I will just deal with the older one ( and hopefully i will learn enough to fix the second one)

https://www.amp-fix.com/image%20file%202/Copicat.gif
https://www.amp-fix.com/image%20file%202/Copicat.jpg

Ok , so copicat 1 - it doesnt look to have been modified too much , the switch block is broken (doesnt latch) so i have temporarily jammed one switch closed - A few components look to have been replaced but these look old so guessing this was some decades ago

I am getting the clean guitar signal through and all the play heads seem to work ( i get a pop when i tap them ) , The record head is measuring about 470Ohm

The rectifier has been replaced and i am getting the following voltages across the smoothing caps 244v,216v,178v

Heater circuit looks fine and i have tested valves ( and have known working ones from another machine )

V1 ( left one on the drawing , but furthest away from the bias oscillator on the machine)

1 - 137v
2 - 0v
3 - 1.15v
6 - 113v
7 - 0v
8 - 1.15v
4,5,9 heater

V2 ( the 6BR8A)

1 -234v
2 - 0v
3 -235v
4,5 heater
6 -185v
7 - 0v
8 - 0v
9 - -0.5v

V3 ( the other ECC83)

1 -155v
2 -0v
3 - 1.5v
6 - 151v
7 - 0v
8 - 1.15v
4,5,9 heater

If the pass through is working and i am getting the pop from the playback heads i presume this would point to the record/bias circuit being the problem

Any suggestions on where to look next please ,

[TIMTAPE]

Not sure what you mean by a pop when you tap the play heads. A quick careful touching of the active head terminal with a finger should give a loud momentary hum at the output.

Are the tape head faces clean? Is the tape threaded with the oxide side against the heads?

Many later tapes had a black side and a brown side. The brown side should be against the heads, not the black.

If a brown tape, the dull side is against the heads.

Record erase and bias currents are ultrasonic and need to be measured with a suitable meter or CRO.

[llama]

Hi and welcome.
Looks to me like the pentode of the 6BR8 is drawing current (since its anode is lower voltage than the rail). Maybe its cathode capacitor is short-circuit.
Graham

[ronbryan]

With regard to TimTape's post #2, the erase function in that type of Copicat is achieved with a permanent magnet positioned in the tape tensioning arm.

The anode of the 6BR8A triode (pin 2) is shown as 0V and the triode grid (pin 1) is shown as 234V. If these are the true voltages measured, it looks as if the 0.002uF triode grid couplng capacitor has gone leaky, biasing the triode fully on. It is also posible that the oscillator transformer primary has gone open as well.

This fault would prevent the bias oscillator from running.

Ron

[Michael Why]

I cannot make sense of the schematic so either i am doing something wrong or its the wrong schematic

In the photos you can see the 6BR8 valve base ,left side with the yellow wire is pin 1 , following clockwise round to pin 9 which you can see one side of the resistor, is that correct ?

trying to locate the cathode capacitor, I think that would make the cathodes pins 3 and 8, but 8 goes to ground and 3 goes to the black 0.1uf hunts capacitor to the right of the erie red one. I cannot translate what i am seeing onto the circuit diagram as it looks like the cathode cap should be 25uf

Tape heads have been cleaned and demagnetized and tape is definitely the correct way round, what i meant by pop is the sound of touching a screwdriver tip on the face of the head

[ronbryan]

Are the valves in the right sockets? From the heater connections in the second photo, the 6BR8A (heater pins 4, 5) should be in the socket nearest the oscillator transformer, and the ECC83s (heater pins 4 and 5 joined, 9) in the other two.

Ron

[sortedradio]

Always worth cleaning the switches and pots on these.

[Michael Why]

Yup , 6BR8 nearest the oscillator other two are ECC83's

From the second photo , the 0.002uf you mention (triode grid coupling) is the little red one that is obscuring the yellow/purple one - That one was dead , no uf reading at all

I now get

1 254v
2 0v
3 256v
4 218v
7 0v
8 0v
9 -0.5v

I measured the coils on the oscillator , I am getting 40-60 ohms depending on which pins , no open circuits. The two caps on the oscillator , one from one side of the coil to the record head ( 200pf ) is measuring about 160pf , but the other that bridges the coil is supposed to be 2000pf , thats only measuring about 160pf as well

[ronbryan]

I think you are reading the 6BR8A pin numbers incorrectly. From the gap in the pin circle, looking at the soldering side of the valve socket, you should count round clockwise from pin 1. Doing this, as a check, the two orange wires should be on pins 4 and 5, and read 6.3V AC between them. Your reading in post #8 shows 218V on pin 4, which must be wrong. Can you re-measure the voltages on the 6BR8A.

Ron

[vidjoman]

There are some black Hunts capacitors in there that are almost certainly faulty and should be changed. They are possibly the problem.

[Welsh Anorak]

i second that. I had one in and eventually I found a small Hunts capacitor hiding away. However looking at the chassis, it wouldn't take too much effort or expense to change the lot. Then a quick check on the resitors is in order.
There are many schematics on the web, and after some detective work you'll find yours. As a rule of thumb, the hardest one to read will be the one you want!

[Michael Why]

Ron , you are correct ,error in my numbers

1 274v
2 0v
3 274v
4 htr
5 htr ( 6.3v) between
6 243v
7 0v
8 0v
9 -0.5
Its difficult to see on the photo's

1 goes to 200pf , yellow to bias coil , 002 to circuit board and record head
2 goes to first terminal on sustain pot
3 circuit board 0.1uf hunts
4,5, heaters
6 circuit board -100K res
7 circuit board - white hunts ( cant read value)
8 ground
9 200pf cap from bias coil , 47k res

I have ordered replacement caps and resistors, should be here tomorrow

[ronbryan]

It looks as if your Copicat has been wired, or re-wired to expect an ECF80 or ECF82 instead of the 6BR8A. Those valves have the following pin-out.

1 a (triode)
2 g1
3 g2
4 htr
5 htr
6 a
7 k,g3
8 k (triode)
9 g (triode)

Those pins seem to match the component placements you have listed, except that the white Hunts cap on pin 7 that you mention should have 680 ohms (or similar) across it to ground - could you have possibly overlooked this resistor?

Ron

[Michael Why]

Hmmm, if that’s the case I wonder why, availability of the 6br8’s at the time maybe. This one had no valves in it so I have no reference point.

Very possible I missed the 680k as it was tucked under the caps, I will have another look tomorrow, disconnect that cap and get its value and see what is hiding underneath

I will get an ECF 80/82 and see what happens.

Cheers

Mike

[TIMTAPE]

Originally Posted by ronbryan

With regard to TimTape's post #2, the erase function in that type of Copicat is achieved with a permanent magnet positioned in the tape tensioning arm...

Ron, thanks for the correction. I've not seen such an early (and valve) Copicat.

[Michael Why]

Ron ,correct again 680K coming off Pin 7

I have ordered an ECF80 and 82 so should have those in a couple of days

I have created a layout which hopefully will help show the changes that have been made to take the different valve. I am about 2/3 of the way through testing each component ( off the board, i started right side working left)

cheers

Mike

[ronbryan]

The resistor from pin 7 (pentode cathode) to ground, in parallel with the 'white Hunts' should be 680 ohms, not 680k. Was the 680k a typo?

Ron

[Michael Why]

Sory yes 680 ohms not k

Mike

[Craig Sawyers]

Vintage Charlie Watkins design. I spoke to him years ago regarding repair of a WEM Dominator Bass. A real gentleman, alas now gone to the great very loud audio in the sky.

He was making Copycat version a great number pretty much to his last gasp.

He designed sound systems for Pink Floyd, the Stones, the IOW festival, AC/DC and just about every rock band from the late 60's onward.

Other than that, I can't offer any pearls of wisdom to the OP

[Michael Why]

Hi Guys

Ok so tried both an ECF80 and 82 - Not that i was really expecting it too but still no echo - getting the same pass through and noises if i tap the play heads

I swapped the record head for one of the play heads so that should rule out a head issue

I have also swapped the majority of resistors and capacitors for new ones ( i missed a few where i didnt have the correct values ) a few things were way off

updated layout attached

voltages on the ECF80/2 - it was pretty much the same on each valve

1 235v
2 -.03v
3 71v
4
5
6 64v
7 1.25v
8 0v
9 -23v

Cheers

Mike