Advice on AC-AC Stepdown transformer substitute: 120 VAC - 24v 70VA - Using Teensy for control

T

Treebeard

Member
Hi all,

I am currently repairing some Linak Controllers for some hi-low massage tables, and I inserted a teensy into the control board so that it can be controlled remotely.

The entire set up is a unit consisting of two PCBS: PCB1, power input and stepdown. PCB2, control board that sends DC out to linear actuators of massage table.

The control board PCB2, have been repaired with equivalent parts. They are from 2009 or so, so some of the capacitors were dried up. Further testing shows the teensy appears to be working correctly when inserted into the circuit, but I'm now running into a problem that the air-core stepdown transformers will work for a bit and read ~28VAC (prior to the full-bridge rectifier circuitry) and then plummets to 0.25VAC after one or two uses. I took the teensy out of the equation to confirm that the intial repairs were correct, and everything on board 2 seems fine: it's just not getting enough juice. If i unplug the unit for a few seconds and plug it back in, I'll get ~24AC before it gets used a few times and the voltage drops. Did readings with PCB2 attached and disconnected from PCB1.

The transformers in question are air-cored and appear to be custom ones that came with the unit. No descriptors or anything on them.

Can I exchange these with a more easily accessible metal core transformers as long as the specs match and the output VA equals or exceeds the maximum of the current unit?

Here are the specs for the unit as a whole:
  • Type: CB09L0-2T-24
  • Item: CB9L00020T-1145
  • V in: 120V AC, +/- 5%, 60Hz.
  • I In: Max 1,6 A (Assuming this is 1.6A since LINAK is from Denmark).
  • U OUT: 24V DC, max. 70 VA
I'm used to dealing with lower voltage stuff, or off the shelf PSU's, so I'm not sure if I am over thinking this or not.

Photo 1: PCB1 Image 1 Top: Cable connects to transformer via pins 1, 2: brown, blue respectively.
PCB1-Image 1.jpg



Photo 2: PCB1 Bottom. Nothing going on here aside from a little break off line for an LED.
PCB1-Image 2.jpeg


Photo 3: PCB1, enclosure, and transformer. Red and Black cables connect to Full Bridge Rectifier on PCB2. Yellow cable is unused.

PCB1-Image 3-PCB and Transformer.jpeg
 
That's not an air-cored transformer, its a toroidal transformer(*). Could it be a thermal-cutout switch operating? Or a damaged winding internally responding to thermal changes?

(*) the core is ring shaped within the windings.
 
Hi Mark,

You are correct. It is a toroidal transformer!

I was informed of this today when I was on the phone with a few transformer manufacturers. This is what I get for staying away from this side of tinkering due to fear of getting zapped. (This has inspired me to do some more learning about this part of electronics to rectify my own ignorance and not do something stupid—hence the forum post and phone calls.)

Their thought was that there is internal winding damage to thermal damage. Whether it is something with the insulation or something else, no real conclusion without tearing it apart or sending it in. We did some quick-ish tests over the phone, but ultimately, they recommended either doing a rewinding or replacing the unit. They said it would cost more than getting a replacement, so that's where I'm leaning.

From what we can tell, they are extremely old, even relative to when the units were purchased on the massage tables back in 2009. They got a lot of use in 17 years. Additionally, the control unit as a whole is not grounded at all. It is just a two-pronged IAC connector. Both units took some power spikes over the years, and got used a lot. Given the enclosure as a whole, they probably got toasty.

So at this point, my main dilemma is to either replace it with another toroidal or a different type of transformer all together. Then the main concern is whether the replacement should be grounded even with the original chassis not being grounded nor metal.
 
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