Magnavox DRV power supply SMPS

[momaka]

Re: Magnavox DRV power supply SMPS

There are tiny smd tranistors on the secondary side, i think it might be one of those.

I see only two transistors on the schematic that could possibly be SMD: Q2209 and Q2221. I doubt Q2203 is SMD, but correct me if I'm wrong.
In any case, none of these should cause the PSU to malfunction even if they are bad.

Can you be specific on the rectifiers to check?

D2201 through D2209. Remove from circuit, if possible. Each diode/rectifier should show conductivity in one direction (250-800 mV) and no conductivity at all in the other - that is, out of circuit. In circuit, this may not apply.

Early on, i lifted the transformer, plugged it in and there were zero shorts, primary looks good.

I don't know why you keep wasting your time with this when we've mentioned multiple times in this thread before that this is a pointless test.

Furthermore, you never mentioned that you replaced those Zener diodes on the primary side and with what. So I am much more inclined to think the problem is still on the primary side.

Better yet, again, have a read at the PSU thread I mentioned in my last two previous posts above and seriously consider doing a PWM-chip mod to this PSU. It only requires a few components and may be more simple and more stable in the end than troubleshooting this POS design.

[johnfin]

Re: Magnavox DRV power supply SMPS

Early on, i lifted the transformer, plugged it in and there were zero shorts, primary looks good. There are tiny smd tranistors on the secondary side, i think it might be one of those.
Can you be specific on the rectifiers to check?

[momaka]

Re: Magnavox DRV power supply SMPS

Better? I don't know . It just means at least the 431 feedback shunt IC is working. So if the optocoupler is okay, technically that is all you need on the secondary side for the PSU to regulate properly. Just double-check there aren't any shorted rectifiers on any of the other rails. If not, then that means the problem is probably on the primary side.

[johnfin]

Re: Magnavox DRV power supply SMPS

Ok, I may have messed up because I was working upside down. On the 431, pin1(ref) = 2.5, pin 3(cathode)=1.8. , resistor 2234 removed. Is that better results?

[momaka]

Re: Magnavox DRV power supply SMPS

Problem with home brew power supply it that it will always be running with fans on. Original one was in sleep mode until timer turned it on.

But you won't be building a home-brew power supply. I merely pointed to the thread above to show you the type of chip circuit you could make to replace the 2-transistor circuit in your power supply. This is probably the best bet to keep your original power supply and fix its problems for good. So you might want to give that thread above a more in-depth read. The circuit is not too involved.

[johnfin]

Re: Magnavox DRV power supply SMPS

431 regulator is a to-92 case.
2234 and 2226 are SMD's and are close to the 431. I have a feeling its one of those micro components that is the problem. There are smd transistors on the secondary side.
I will runs the tests again.
Problem with home brew power supply it that it will always be running with fans on. Original one was in sleep mode until timer turned it on.

[momaka]

Re: Magnavox DRV power supply SMPS

Volt reg with 12V & 5v on board. Pin1 ref=1.8v ,Pin2=0 Pin3 cathode 1.8 (10.6 with just 12 volts connected to board)

817b optocoupler Pin1=3v , Pin2 1.8V , Pin3=0, Pin4=0
817b with just 12v Pin1=11.6v , Pin2 10.8V , Pin3=0, Pin4=0

Hmmm, that reference pin on the 431 regulator (IC2201) just doesn't look right at 1.8V.

Is yours and SMD device? If so, what is the marking code written on it? According to the service manual again, this IC should be a KIA-431AF (i.e. a SOT-89 SMD chip with a marking of "3B" on top). If that's not the case, let me know. I'm curious if your power supply's 431 shunt is a version with a lower reference voltage (hence the 1.8V reading you got on the reference pin).

As for the 817 optocoupler readings, they do look normal more or less, with approximately 1V drop across pins 1 and 2 due to the LED diode inside it.

If it's not a big hassle, maybe try removing either resistor R2234 or capacitor C2226 and repeat the above tests. I Just want to make sure that RC network is not causing the low reference reading on the reference pin. Just double-check that R2234 / C2226 are near the 431 shunt regulator to make sure these are the same components in your power supply (due to me using a simila model's service manual).

Other than that, I'm starting to run out of ideas. It's possible that building a PWM mod board like this...
https://www.badcaps.net/forum/showthread.php?t=28271
... might be easier than doing a "proper" repair. Give that thread a read and see if you can make something like this. I personally think it might be a simpler solution now.

[johnfin]

Re: Magnavox DRV power supply SMPS

Waiting for my next set of tests

[johnfin]

Re: Magnavox DRV power supply SMPS

Man these voltages are messed up. Hope you can make heads/tails over this.

5V is connected to 2205 cap

Volt reg with 12V & 5v on board. Pin1 ref=1.8v ,Pin2=0 Pin3 cathode 1.8 (10.6 with just 12 volts connected to board)

817b optocoupler Pin1=3v , Pin2 1.8V , Pin3=0, Pin4=0
817b with just 12v Pin1=11.6v , Pin2 10.8V , Pin3=0, Pin4=0

[momaka]

Re: Magnavox DRV power supply SMPS

You need to have 5V connected to the board as well while the 12V supply is connected. That opto needs to show around 2.5V on the reference pin with a steady 5V supply in the EV+5V pin.

[johnfin]

Re: Magnavox DRV power supply SMPS

I put 12vdc at cap at 2204 and have 12vdc at pin 1 of the opto-coupler.

IC2201 (kai431 regulator) pins =1ref=0 , 2=gnd annode, 3=10.8 cathode to opto coup.

I did not have 5vdc connected to the board.

whats next

[momaka]

Re: Magnavox DRV power supply SMPS

oh and these often wont run right on a dim bulb tester.

I've seen a PC PSUs do the same as well. But I've had success running PC PSUs like that with two to three 60 Watt bulbs in parallel rather than just one (or as I often do when I am too hasty to put 3 bulbs in parallel: a 450W heating element).

Ok, I looked at the board closer, it did not have bad pad on C2204. C2204 goes to pin 27 only. Similar to pin 30 on schematic, also goes to center pin of transistor Q2203.

That should be connected to the EV+12V rail then.

In any case, what I really want to see is 12V from your external PSU eventually making its way onto the top part of the opto-coupler and 5V into the resistive divider network formed by resistors R2229, R2230, R2231, R2232, and R2235. This should allow to verify if the opto is receiving the correct feedback.

Though going by kc's advice, looks like you still have something amok on the primary side. But do the test, and we will see.

[johnfin]

Re: Magnavox DRV power supply SMPS

Ok, I looked at the board closer, it did not have bad pad on C2204. C2204 goes to pin 27 only. Similar to pin 30 on schematic, also goes to center pin of transistor Q2203.

[johnfin]

Re: Magnavox DRV power supply SMPS

You are 100% correct. Took a dump when i was running the house on a generator. I could not find a match for some of the chinese rectifiers but they tested ok on various testers. Of all the parts in the seconday side, where would you focus your attention?

[kc8adu]

Re: Magnavox DRV power supply SMPS

just a warning about these funai power supplies.
they are "conditionally stable"
any line disturbance can blow them up.
the paralleled diodes on the secondary can break down even if they test good.
this is a common fail point right behind a simple power blink.
any anomaly = BANG! and loss of everything on the primary side at a bare minimum.
you have to carefully check everything and get it right 1st time.
esp anything that leads in any way to mosfet gate.this includes the opto.
i tend to shotgun these as its faster and the parts are cheap.
oh and these often wont run right on a dim bulb tester.

[johnfin]

Re: Magnavox DRV power supply SMPS

C2204 is connected to 12vdc and schematic is similar. Regardless the cap is not connected to the trace. Once i connect it, whats the next test?

[momaka]

Re: Magnavox DRV power supply SMPS

Seems like 2204 + solder pad is not connected to the trace.

Okay, STOP here and please verify if capacitor C2204 is actually physically supposed to be connected to the EV+12V line or not (follow the board traces and jumpers of your PSU board). Remember that I am using the schematic uploaded by budm in post #50, so there may be differences in it that I don't know about. Thus, you have to verify that all steps I ask you to do actually apply to your power supply. Again, this is the reason we always ask to upload good quality pictures of your actual power supply rather than someone else's pictures or schematic - that way, we can directly see any differences (if there are any). Otherwise, it's almost as if I am troubleshooting this PSU blind-folded.

[johnfin]

Re: Magnavox DRV power supply SMPS

Seems like 2204 + solder pad is not connected to the trace. Of course when i put voltage right at the cap it measured 12vdc. So i guess we will go one step at a time, 1. Connect cap to board and repeat first set instructions with atx supply.

[momaka]

Re: Magnavox DRV power supply SMPS

Okay, repeat this experiment, but insert the 12V supply directly on the positive lead of cap C2204 and report the results here. If you still don't get anything close to 12V at all on C2204, there may be a problem with something being shorted on the EV+12V line. But before jumping to conclusions, also double-check your power supply / adapter connections - that is, make sure that you are also connecting a ground wire from your PC PSU / power adapters, in addition to connecting the 5V and 12V wires to EV+5V and EV+12V.

The reason I am having you repeat this experiment is because the voltage on cap C2204 is almost non-existent, and the voltage across cap C2205 seems a bit low. Also, the voltages on pins 1 and 3 of IC2201 are low. Thus, we need to find if this is due to a circuit fault or a wiring mistake. If IC2201 doesn't have the correct voltages we are looking for, that's one possible reason why your power supply won't work.

[johnfin]

Re: Magnavox DRV power supply SMPS

RESULTS of test:
2204 cap = .764 volts - checked it multiple times
2205 cap - 4.5 v
ic2201 pin1= 1.9v pin3=1.25v