Repair ZP120

Lyricist III

It's kind of a cheap one but it got pretty good reviews: https://www.amazon.com/gp/product/B00EYYJRC0/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1

Also go this soldering iron as the $20 one I had stopped melting solder after using it 3 times: https://www.amazon.com/gp/product/B01DGZFSNE/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

Lyricist III

Hi gruv2ths - I still haven't received my meter and iron yet, but was looking around at the are where the caps are. I don't really see much that can be shorted and was wondering if you might have any theories. I am still going to remove the caps and test them, but wanted to think of the next steps if they check good.

Lyricist III

I took the caps out today and they checked out OK using a capacitance meter. With the caps out, I connected the 2 wires from the Xformer back to the board and all 3 pins of both rectifiers are shorted again. If I remove the wires, the pins measure what they are supposed to. Could it be that the 36V transformer is shorted out? There doesn't seem to be any damage to it.

Lyricist III

Good info ando1. The 2 wires will appear to short because its a winding. They should short as that's how a transformer works. See this to understand why:

https://en.wikipedia.org/wiki/Transformer#/media/File:Transformer3d_col3.svg

You should see a little bit of resistance on it (a few ohms?), but very small. If it has no continuity (i.e. full open), then it would be bad. Have a read on this to help on testing it:

http://www.ebay.com/gds/How-to-Test-a-24-Volt-Transformer-/10000000205661057/g.html

The rectifier will work by the switching (AC) back and forth of the polarities and current direction. Your rectifier numbers that you posted above look pretty reasonable. Check the data sheet to be sure the voltages that are going through as shown are within spec. But a cursory glance, it looks good to me.

What I would do is check the voltage on the primary winding when its powered up. Do you have any? This should be AC (or the transformer just wouldn't work). Again, if your secondary winding is showing 0v, then there is a switching problem somewhere as that transformer won't work without AC current of some form. You need to trace backwards until you start seeing voltage. That will likely indicate the general location of where the problem might be.

Lyricist III

This should also explain why you will see a short on your rectifiers while in circuit:

https://sub.allaboutcircuits.com/images/quiz/02311x01.png

So its normal.

Lyricist III

Thanks for the info m0untainman. I think I found a major issue as that 36V power supply circuit definitely has a short. I asked gruv2ths in a PM to measure the 2 sets of pins on that transformer in my pics above and he is showing that all 3 in each set are shorted together but the sets are NOT shorted to each other. This is where my issue is. My transformer readings show a short between the sets (0 Ohms). This means that something in that circuit is shorted which is I am assuming causing no output from the 36V supply.
I have visually inspected the entire board multiple times and cannot see any evidence that would indicate shorted components, so I think I now have to start removing components to check for shorts.

Lyricist III

Yep... the 3 in each set should be shorted since they are on the same winding (with a tap) and the 2 sets of 3 should certainly not be shorted as those are different windings. Sounds like you are getting close. 🙂. Yes... now is the time to start removing components to hunt down the short. You can do some of that on board like diodes and resistors... but not always. Look for shorts across components and if so, they are suspect for removal.

Lyricist III

OK so I made a little more progress today. I removed the transformer and checked it out of the circuit. It checks out OK. I them removed the inductor near where I removed the caps and found that the short disappeared from that part of the circuit. I then started to look at the diodes that are in the picture below. The first set checked out OK while in the circuit but the second set shows a short. I then removed the diodes and checked them out of the circuit and they check out OK. So something else in that part of the circuit is shorted. Also the 36V and GND 36 test points show shorted.

Lyricist III

Keep going 😉 You are getting there...

Have you felt any of the components getting warm? A short will typically be very warm somewhere and that will likely be the culprit.