Tape heads on a Dynacord Super62

[Noth]

Hi,

This is a folluw up of Capacitors on a Dynacord Echocord Super62
And Dynacord Echocord Super 61 tube tape echo replacement playback head

So, the preamp section of my Dynacord Super 62 works fine now.

Now, it appears that all 2 play heads and 3 record heads are cut (unsoldered and tested with an ohm-meter).
Worse, I did actually repair one of the record head that was also cut and.. it's dead again.

Play heads
As far as I can remember, at least one of the two was partially unsoldered. Thus on wire was having fun very next to one of the tube pin.
I supposed that, at some point it went in contact with high voltage and died.

Record head
I suspect that the circuit is sending too much voltage/tension which ends up burning the wire.



I'm lacking a bit of methodology here, what should I do ?

I'm thinking replacing the heads with a fixed 440 Ohm resistor and measure the tension/voltage across it.
Does that makes sense?
What are regular values that I'm supposed to expect (I'm totally unfamiliar with tape circuits)?

Once everything looks fine, i'll have to fixe all those heads.
Since it's quite hard to find replacement ones, I'm thinking about two options:

1. dismount the tapes heads, unwire, resolder and rewire
2. dismount the tapes heads, unwire, rewire with a brand new wire

Does it make sense? Is that a "regular" practice?
I'm not looking for a super crazy audio quality (if I need a hi-res delay, I have other options)

Thanks for any advice!

 

[cyrano]

I test tape heads by attaching them to a hi-gain mic preamp and dragging a piece of recorded tape over them.

Alternatively, I've used a known-good tape head to inject signal, just holding these together with a rubber band. Any signal generator or PC headphones output has enough signal to drive this.

I tried doing frequency plots with two heads, but that's not reproducible. The slightest shift in position wrecks the results. I don't do it often enough to build a jig...

 

[Noth]

So, here's some updates:

I probably found why all my play/record heads are dead (even the ones that were originally working fine).
I replaced the selenium diode bridge with a modern one. I read somewhere that the selenium bridge were delivering a lower output voltage than new ones.
I should have checked this before connecting everything back.

The output voltage of the power supply part was around 225V instead of the expected 185V. This probably killed the remaining tape heads that were still working.

So my plan is:

- Replace R38 with a higher value 5W resistor. It seems that 5K is fine, but I need to order a 5W version (my 2W test setup reached 100°C after a few minutes )
- Since the original tape heads cannot be found anymore, I plan to dismount and rewire mines

I will post some updates for the rewiring setup, this sould be interesting. My setup is:

- 3D printed coil frames
- An arduino with a motor shield
- A stepper motor for winding the coil
- A servo motor to properly dispatch the wire on the coil
- A loop counter (used for winding and unwinding)

The 3D printed coils are ready and properly fir the tape head frames.
The wiring machine components have been ordered and should arrive soon.

 

[Noth]

Here's some progress!

My machine is almost ready:

- An arduino Uno for the CPU
- A motor shield
- A stepper motor (behind the big black wheel)
- A servo motor to properly dispatch the wire all along the coil frame
- A rotation counter (a bit useless since I can know the number of rotations using the stepper)
- An OLED screen to display the progress
- A push button to stop/start

Following steps:

- Update the big wheel, It's currently a bit too loose
- Calibrate the servor motor
- Run a first test and check the wire tension. I'm thinking about passing the wire through two holes on the servo motor instead of just one to get the proper tension