MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

[sententia]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

I received the UC3845B, and replaced it but no change! Resistance is still 1.9k from VCC to ground, although when the ic was removed i got a higher resistance!

I removed one pin of those zener diodes to check label and D30 is 150V and D84 is 200V so these seem to operate.

Bump again...

1. Should i try to reverse the diode i replaced that connects to the flyback transformer or that will result in great damage?

2. Is it possible it might need a battery to work even if manual states "it can be used as a power supply in float mode"?

I will be away for a few days so i won't be able to make more tests but any input of course is welcome!
Thanks

[budm]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

"2. Is it possible it might need a battery to work even if manual states "it can be used as a power supply in float mode"?"
I would try that to see what happen. I would not try reversing the diode unless we know what the function of that diode is.

[sententia]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

Unfortunately i don't have a good battery (or two for 2ch) to properly test if there is any difference. The one i tested was at 8.5V...

BUT i did have an interesting finding.

There is a PIC16f88 microcontroller near the output side where i measure an input voltage of 5V only when the battery is connected. If the battery is disconnected it goes to 0V.

I wonder if it could have problem to operate without any heatsink for a few seconds, as putting it back in the heatsink with thermal paste and clip is a pain!

[budm]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

I have a feeling that you hook up the battery that bad and has low Voltage or internal shorted cell, the unit will sense that low Voltage of the battery so it will not allow the charger to come on to protect itself. Most charger has the cut off around 10V for it not to try to charge if it is 10v or <.

[sententia]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

I finally managed to test the charger on site today with 2 good and charged batteries.

It does not work...

My hopes are getting down

I don't know what i might be missing or if i should try to reverse the diode...

[fzabkar]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

According to the datasheet, the UC3845 has "UVLO thresholds of 8.5 V (on) and 7.6 V (off)", not 32V or 15V. Therefore ISTM that the IC is trying to start, but is not getting a regenerated auxiliary supply from the transformer. This is evidenced by the 295V to 305V hiccuping. The failure to start could be due to a leaky capacitor on this auxiliary supply, or it could be due to an overload on one of the secondaries. Did you check for shorted DC outputs?

BTW, it is not necessary to repair the PFC circuit in order to repair the DC output section, provided that the bulk capacitor is getting 300VDC or thereabouts.

[sententia]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

I have tested all electrolytic capacitors and all test good except the main filter cap which measures 200uf instead of 220uf but with good esr. Also the X2 caps measure 1.25uf instead f 1.5uf. I doubt those caps are causing the problem especially without any load.
When you say "auxiliary supply" are you refering to the booster circuit on the primary side?

The 3 components on the output side 1x schottky 2x transistors test good (upper left corner of first pic).

[fzabkar]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

Examine the application circuits for the UC3845 IC. Look for the electrolytic capacitor(s) on pins 12 (Vcc) and 11 (Vc). This capacitor should charge up and take over the supply for the UC3845. If there is a short on any of the loads, then the capacitor doesn't receive a charging pulse from the transformer, and VCC then drops below the UVLO switch-off thresold.

[sententia]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

VCC and VC are connected. The supply capacitor for the UC3845 is the smd cap just above Vcc and GND which is 100uf...

[fzabkar]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

If you are measuring a resistance of 1.9K to ground at Vcc, even when the IC is removed, then ISTM that this 100uF capacitor or its associated diode must be leaky.

What is the resistance between the Vcc pin and the 306V supply? It should be of the order of 100K or so.

[fzabkar]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

This is how I envision the circuit:

      306V
      +
      |
      |
      .-.
      | |
      | |
      '-'
      |
  Diode  |      Vcc .---------.
  --->|----+------+--------| UC3845 |
      |   |   Vc '---------'
      |   |       |
   100uF ---  .-.      |
      ---  | | leak    |
      |   | | 1.9K    |
      |   '-'      |
      |   |       |
      +------+-------------+
      |
      ===
      GND

[sententia]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

I just removed the charger from its enclosure and i am trying to find where the VCC of power ic comes from. Unfortunately this is a 3 or 4 layer board and it makes it difficult to locate tracks visually... Tomorrow i will be able to remove the coil that budm mentioned and maybe try again to remove the main transformer. There are some parts under the coil ELY0704 i want to check and also if i manage to remove the main transformer maybe it would be easier to identify the correct direction of the smd diode i replace under the heatsink.

[fzabkar]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

I would think that Vcc comes from the positive terminal of the 400V bulk capacitor.

[budm]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

I would think that Vcc comes from the positive terminal of the 400V bulk capacitor.

+1, see application circuit to see how it gets the start up then it gets the running Voltage from the AUX winding of the power transformer to keep it running.

[sententia]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

I have some more photos for any input...

First of all i noticed that i get 1.9K resistance between VCC+GND only when probes are connected - to VCC and + to GND. If + to VCC and - to GND i get a rising resistance such as a capacitor is charging.

Only track i have found to be connected straight to VCC is the red line of the BAS20 anode side diode. The cathode goes to + of C80 just above it. The + pin of C80 connects with R80 (which looks like a G ohms resistor) to + of the bridge rectifier.

The two sot23 smds on the left of it are 2 voltage regulators.

The big filter cap + side connects to + of bridge rectifier through D5 and through the booster circuit with output sot-220 diode D7

[sententia]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

I am thinking of going down the way of reversing the diode i am not sure which side was correct.

I think the diode is there to protect from inductive spike from transformer???

Any ideas what kind of damage i should expect if it goes in the wrong way?

*EDIT* R80 is a 150K 1W? resistor (brown-green-yellow) but measures 163.7K. One end goes to + bridge rectifier and the onther end goes to the blue 100uf 63V capacitor next to UC3845. Is the resistance difference significant?

[fzabkar]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

The "leakage" appears to have been a red herring. You were probably measuring the forward resistance of the BAS20 diode. Sorry, I should have seen that.

IME with similar circuits, C80 often dries out, resulting in hiccuping faults. Also, startup resistors (R80) typically go open circuit, resulting in a no-start symptom. Yours looks like it may be on the way to failure.

[budm]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

"First of all i noticed that i get 1.9K resistance between VCC+GND only when probes are connected - to VCC and + to GND. If + to VCC and - to GND i get a rising resistance such as a capacitor is charging." that is a big missing info.
'
'blue 100uf 63V capacitor next to UC3845, by any chance the positive leg of that cap is connected to the VCC pin of the UC3845? I thought you have already changed ALL the caps except the main filter cap in the primary side.

[sententia]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

The VCC of uc3845 has a smd capacitor (in red rectangle) which is 100uf.

I have tested all caps but haven't replaced them except C80 "just in case" but didn't do any difference.

[sententia]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

This circuit is connected with VCC and GND! C82 has + straight to VCC.

Also there is a schottky diode there and a 10R resistor. Diode is MBRS1100T3.
Sorry for mixed info!

*EDIT*

I just found out that a 22R resistor (marked in pic) is open! Damn i was posistive i have checked all part 1 by 1! I think of replacing it and put everything back together to do a test again....