Repair ZP120

Lyricist III

Wait... I think your PWM is fine and I’m thinking that the secondary winding voltages just may not be measured properly. Looking at your secondary voltages downstream, everything looks nearly perfect. You shouldn’t be seeing the voltages if that secondary wasn’t working... so my guess is that your power supply is working fine. Getting back to the location where you had water damage... did you test the components near all that damage? Check diodes near that location as they are usually the first to go...

I’m just trying to think this through to help narrow the location...

Lyricist III

@gruv2ths:

RE: I can only think of one thing left to do, and that is measure the output of the buck converter on the bottom of the logic card. There is a inductor with a (normally) yellow electrolytic cap. Solder a wire to the striped side of the cap and put the card back in and plug in. You can put the red lead on the wire and the black one to chassis. Should have 1.5V output.

I was able to get a wire on that cap (a bit of a test of my soldering skills) 😳 Anyway, I do have 1.5 VDC on that buck converter.

Also RE: if you are getting the 3.3,8ish,15 on the secondary then the bias converter PWM is working correctly.

I am actually NOT getting those voltages on the secondary. I measured for AC and DC and both are 0V. Also I keep wondering about that P-FET voltage you said you have 15.4 volts on. I do NOT have any voltage o at that test point. Could it be that the PWM is not getting a signal to turn on the switching transformer voltage? Or is the fact that I have voltage on the buck converter and the other caps I mentioned on the logic board an indication that the PWM and the switching power supply are functioning normally?

Lyricist III

Yeah... then something isn't right with that PWM. So for gruv2ths and me, we see 13.03V on pins 1 and 3 on the PWM. You are showing 15v. That's an interesting delta, but certainly not a deal breaker outside the spec. Its possible that voltage is higher because the switching isn't working quite right (when I had no switching at all, I read 44v). Just for giggles, is the PWM hot?

Did you test the P-FET? (see https://www.youtube.com/watch?v=gloikp9t2dA)

Lyricist III

I am actually NOT getting those voltages on the secondary. I measured for AC and DC and both are 0V. Also I keep wondering about that P-FET voltage you said you have 15.4 volts on. I do NOT have any voltage o at that test point. Could it be that the PWM is not getting a signal to turn on the switching transformer voltage? Or is the fact that I have voltage on the buck converter and the other caps I mentioned on the logic board an indication that the PWM and the switching power supply are functioning normally?

When you are checking the secondary, are you setting your ground on the RTN and the positive lead on the others? (I'm just dotting I's and crossing t's here)

Lyricist III

@m0untainman -

RE: Wait... I think your PWM is fine and I’m thinking that the secondary winding voltages just may not be measured properly. Looking at your secondary voltages downstream, everything looks nearly perfect. You shouldn’t be seeing the voltages if that secondary wasn’t working... so my guess is that your power supply is working fine. Getting back to the location where you had water damage... did you test the components near all that damage? Check diodes near that location as they are usually the first to go...

I am getting all of the proper voltages on the secondary side EXCEPT the pins on the transformer and the P-FET voltage. See the pic below for explanation.

Also RE: When you are checking the secondary, are you setting your ground on the RTN and the positive lead on the others? (I'm just dotting I's and crossing t's here)

Yes I am using the RTN points. I have also tried putting the black lead on the copper colored test point labeled "CPU_GND" as they are the same electrically as the RTN points (Verified by checking continuity)

Like you, I keep going back and forth between a power supply problem or something on the logic board. As far as checking the components I replaced on the logic boar, yes I have checked them and other components around them. All seem to be fine. This one is a real head scratcher. I am hoping either you or gruv2ths can provide some more insight.

Thanks so much for all the help you have provided so far!

Lyricist III

Ok. That is helpful. My comment at this stage is carefully check your solder connections and traces around the PWM. You may want to put in another PWM if you have one if the traces are fine. That secondary really needs to have voltages if the PWM is working. That PWM is sensitive and rather easy to blow. Be thankful that you have more 🙂

Lyricist III

In order to do some more troubleshooting I wanted to check some of the voltage regulators that are mounted on the frame of teh amp, so I took the main board completely out and started looking around. All seemed normal until I got to 2 of the Schottky Barrier rectifiers near the 2 other transformers on the board (see pic for reference). I was taking some continuity readings and found that all 3 pins on both rectifiers seemed shorted. The traces on the board are hard to read but it looks like they are supposed to be that way? Anyway, to be thorough, I removed one of the rectifiers to test it and it tests good according to the data sheet. Positive lead on anode and negative lead on cathode gives beep on diode test. The other way around is open. I am puzzled by what this accomplishes

since it seems both rectifiers are shorted to each other and one of the 2 wires coming from the bigger transformer. M0untainman or gruv2ths: Any ideas what this circuit does?

G

gruv2ths

Trending Lyricist I

Yes, that is the 36V supply that comes on when you push play. It is kind of in parallels with the 15V supply.
Good catch. Definitely remove them. Once out see if the short is still there or was it the part. Lots of stuff is in parallel with that part of the circuit (caps and the output amp ICs).

Lyricist III

Yes... and you certainly don't want a short on a rectifier. That's one component that you should theoretically be able to test on board. After you remove it and test them (jut to be sure), look around for what else can be causing the short. Do you have an LCR meter to test the caps too?