Repair ZP120

The transformer is just a bunch of coils of wire wrapped around a ferrite core, so yes with your DMM it will just look like it is all shorted together. That is normal.
As for the rectifier, i believe the outside pins are shorted together on the board and both outside pins should be high resistance to the center pin. Open the datasheet for that diode then google 'how to test a diode'. The package in question is just 2 diodes in one package. This is stuff that is good to know.
A short on the output of the 35V supply may explain why you have 0V on the Pfet. Maybe take your meter in ohms mode and measure (with the power off) the resistance from the pin of the Pfet that measures 0V to ground. I bet you will measure the same short you are measuring at the rectifiers.

I thought about that and is definitely a possibility, but I don’t want to think about it because then I fully unraveled an inductor when I didn’t need too! 

But, I did do a pretty good job of rewinding it .

It’s all setup and working!

I do have one heat sink clip I cant get back on, frustrating because I never needed to take it off, but there must be some special tool to spread this clip and get it over 2 transistor casings, I’ve read that it is a bear to get back on...and so far it is. 

Lots of great stuff here.
Looks like the shiort is not the rectifier.
Like I mentioned earlier, there could be a lot of thing to cause a short at that point. Anything on the 36V bus.

Start taking off the electrolytic caps by one. Start with the 2 Big brown radial ones right next to the rectifiers.
Hope it’s not the Amps.

Ok cool,
Which meter did you order?

I’m a bit stumped on a ZP120 and am looking for help.  The zp120 had a bad varistor, between VPRI and VMID), with lots of scorched bits when it caught fire.  It took out the PWM chip, the fuse and a board trace.  I’ve replaced those but still can’t get it working properly.

The attached image is from the underside of the zp120 board, below the large capacitors.

I have circled two small circuits in green.  The one on the left looks like a bleeder resistor that is intended to discharge the large 1000uf capacitor that sits between VPRI and VMID (about 150V).  The 68k and 240k resistors provide 308k of resistance across the capacitor.  I thought the small “p1.c” diode is some kind of snubber diode, but I’m not so sure any more, based on what’s going on in the other circuit...

The similar circuit one on the right seems to bleed the 150V between VMID and PGND.  That circuit also has the addition of D16035, which sits between the bottom legs of R16082 and D16041.  This additional chip is a 19V zener diode in a SOT-23 package.  (MMBZ5249B).  Pin 3 of this zener diode (which is presumably supposed to be at 19v when the circuit is working property) is the voltage that drives the PWM chip.

My board reads 0V at that junction.  I have replaced the zener with no luck.  When I provide 16V from an external source, the PWM operates as it’s supposed to.  The PWM chip needs 15V to get started, but operates ok with as little as 9V… if I remove my external 16V, the circuit continues to operate (and somehow the circuit now internally generates about 12.5V at the same junction of R16082 and D16041).

I can’t figure out how this circuit works.  Specifically, the junction between R16079 amd R16082 reads about 110V when powered up (this seems right given the resistor divider created by R16082 and R16079).  I understand that the zener is supposed to clamp that to 19V, but what is the P1C diode (D16041) doing?  If it is a regular diode then it wouldn’t pass any voltage to the zener.  If it isn’t a vanilla diode, then what is it?  I’m guessing D16041 isn’t working but I don’t know what to replace it with.  It does not read a voltage drop in either direction when tested (the other P1C reads 0.68V).  I can’t find a reference to that marking anywhere.

 

 

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.

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.

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.

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.

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.

I'm sorry, but I'm going to be stupid trying to find a suitable spare part.Can someone give me a link to a suitable spare part, e.g. at mouser or digikey?Many Thanks

No I haven’t plugged the board in since I have a bunch of components removed. I have been visually inspecting and checking with my meter however it is hard to see where the short is right now as there are a lot of components in that area. Also a lot of inductors.