WEM Copicat Custom

[Station X]

Looking back again at the pictures in post #7, there's no obvious ORIGINAL connection from the common negative of the smoothers, other than that to the transformer, only the green/yellow wire which you added.

Without a connection there's no way the negative side of the supply can reach the rest of the circuit.

If the connection is missing you'll measure HT between the common negative of the smoothers and the valve anodes, but no current can flow through the valve without a negative connection.

Measuring between the negative rail and the anodes would show no voltage.

[evilmoisture]

Added the wire back (and did just see if it was working, but still no echo) and tested the valves again, here are the radically different voltages:

V1
1: 180v
2: 0v
3: 1.2v
4/5 x
6: 172v
7: 0v
8: 1.2v

V2
1: 249v
2: 0v
3: 2.2v
4/5 x
6: 221v
7: 0v
8: 2.1v

V3
1: 320v
2: -24v
3: 0v
4/5 x
6: 198v
7: 0v
8: 2.0v

[Station X]

Looks better. I'll have a closer look tomorrow.

[evilmoisture]

Ok cheers

[Station X]

If you've designated the valves as in post #112, the voltage on pin 2 (the control grid) of V3 looks all wrong. It should read 0V. Lack of cathode voltage indicates no current is flowing through the valve which is hardly surprising if the grid is at -24V.

Check the resistors on pins 2 and 3.

Make sure the cap on pin 3 isn't short-circuit.

Make sure the valve is making good contact with the holder.

As all three valves are the same type you could try swopping them.

[evilmoisture]

Sorry, my bad, I labelled them from left to right looking at the copper side of the board, so going by the schematics my list should be V1, V3, V2. V1 being the inputs, V3 being the record head, and V2 is for the playback heads. Last 2 swapped round in other words.

My measurements according to the valve numbers on the schematic should be:

V1
1: 180v
2: 0v
3: 1.2v
4/5 x
6: 172v
7: 0v
8: 1.2v

V2
1: 320v
2: -24v
3: 0v
4/5 x
6: 198v
7: 0v
8: 2.0v

V3
1: 249v
2: 0v
3: 2.2v
4/5 x
6: 221v
7: 0v
8: 2.1v

[PJL]

The HT is still too high but otherwise the DC conditions look about right. The oscillator is running although we can't tell if the frequency is correct.

I suggest disconnecting the direct connection from the input (R6,R7) to the base of the transistor so that you only get the delayed signal. It could just be the heads are out of alignment.

[Station X]

evilmoisture.

Having sorted out the valve designations, as PJL suggests, your voltage readings look reasonable.

I was going to suggest moving a piece of pre-recorded tape across the P/B heads, which is kinder than tapping them, to see if you get any output.

My knowledge is of valve circuits in general and not specific to Copicats. I think I have brought you as far as I can and will leave you in the hands of forum members with knowledge more specific to Copicats.

[evilmoisture]

I can try, but I have the loop set up and there is no sound at all. In my experience of tape heads there will be some sound even if they are misaligned. I'll try as PJL says and report back. Good news if you think the oscillator is running as this coil is a rather strange component...

[evilmoisture]

No difference with that wire disconnected PJL.
I can push the tape up and down when its moving so that it covers different parts of the head surface and there's definitely no sound at all. A small hum when I touch the wires connected to the record head, but nothing from the play heads. Instinctively it feels like it's recording but there's no playback. I have a reel to reel, so I could try a long loop which passes over the copicat heads and the reel to reel heads (already have this set up for another project), just to see if the copicat is recording onto the tape, which I would be able to hear via the reel to reel .

[vidjoman]

If you are getting a hum when you touch the record head but no sound from the playback heads it might be that the heads are wrongly connected. Have you checked the wiring from the heads to the switches?

[evilmoisture]

Sorry, correction, no hum from any of the heads, I think I was just hearing a change in the hum from touching the chassis. I replaced C9 (again) with a 1000v cap just in case because it looked slightly black, but still no luck.

[evilmoisture]

Checked over the switch wiring and wires to and from the heads and it all looks fine. It's very unlikely that they are wired wrong because it was working before and my friend woudn't have changed things like that.

[Station X]

Back to generic stuff. Do you get any output if you touch the tags on the P/B heads?

What voltages are present on the transistor?

[evilmoisture]

No output at all when I touch the P/B head tags, or the record head.
Just measured the transistor pins to chassis, meter on DC on various settings, no voltage at all on all 3 pins.

[Station X]

OK.

-24V from V2 pin 2 is fed to the transistor's collector by what looks like an 82K resistor.

What voltage do you see from chassis to each side of this resistor?

What is the voltage across the resistor?

[evilmoisture]

-26v (pin 2 side)
-6.7v (other side going to transistor)
voltage across resistor = 19.4v

[Station X]

The -6.7V side should go to the transistor via a wire or PCB track, so I can't see why you're not reading -6.7V on the transistor.

[evilmoisture]

I read it when the wire was removed, as suggested above, sorry. Transistor readings to chassis now show:
Collector: -6.7v
Emitter and Base -2.5v
(the voltages fluctuate a bit)

I notice that when I power it on V1 glows very brightly for a second. Will go back to using the lamp limiter perhaps

[evilmoisture]

Huh, I don't know what happened but I do now hear a feint echo, very quiet but it's there buried in the hum (I have the volume up quite hugh on my mixer/amp to hear it.