Electronics (not rocket related) help

[tsmmiller]

I have a BOSCH (corrected from Samsung that I originally placed here! Thanks kuririn!) Convection/Microwave combination oven. Installed new when I built my home 6 years ago. About 2 to 2 1/2 years in the LED display began to get dim. I found a replacement board installed it and all was well.
Now here it is about 3 years later and once again the display is dimming, though now it is so dim to be non existent.
Looking for boards again, there are none to be found. I did however find a page that describes how to repair said board.

https://www.diychatroom.com/threads/bosch-hmc80251uc-microwave-oven-has-dim-display.645121/
Out to the garage, looked around and found the original board. (Something to say about hoarding?)

Looking at the photos on post 6 this is my board, but not quite as charred.
To me it looks like the heat is coming from the center post of the *rectifier*(that black part next to the capacitor).
I figured to replace the diode, the cap and the rectifier. I have sourced the cap and have found a 3000 hour part, where most seem to be 2000 hour. Is there a difference?
I am not sure of how to size the diode, are there sizing differences in the 1n4148. Mine without having removed for clearer view looks to be printed with 58, but that could just be board positioning.
Post 7 deals with the same issue but replaces the 16v cap marked by he circle. He also mentions the 25v, is he replacing the 16v 220uf with a 25 volt? OK to do this. I can get either size of that cap with no issues.
Also Looking at post 7 photos you can see discoloration on the board common to that 470uf cap.
Would that charring And heat be normal, or signs of a different issue, related to replacing the 16 v cap with a 25?
Any help or ideas appreciated.

 

[Brian H.]

Just about any 1N4148 with leads should be fine. It does not have to be pushed clear down to the PCB and the leads can be bent to go into the holes. Some have a slightly higher continuous amps/power rating than others. I would choose a higher rated one if available, but it might not make a difference. Quick search at mouser.com shows anywhere from 100mA to 300mA continuous current ratings available.

The 16V rating of the original cap may have been pushing the limits causing it to fail and taking out the rectifier with it. Going with a higher voltage rated cap is no issue. Charring and heating of the PCB around the cap is typical signs of an internal short that would overload and damage the rectifier. Higher hour ratings indicate better quality components.

 

[OverTheTop]

Electrolytic caps have very short lifetimes, as per what you have stated. If they are kept cool (there's not much heat near an oven, is there?) and the voltage on them well below rated voltage it can really help extend their life. Going for a 105degC rated cap instead of an 85degC cap is also worthwhile IMHO. Cap with a higher voltage is fine. Larger capacitance would usually work as long as you don't go stupid bigger.

Just looked at the earlier link. If the cap has gone short/leaky it might have loaded the voltage regulator thus causing some extra heat and browning on the pcb. Given there is a transistor, diode and resistor there i would be checking if this circuit is a discrete voltage regulator. If that is the case then the diode is likely a zener and not a 1N4149/1N914 signal diode. I can't trace the circuit from here so I am just pointing you in the right direction.

If you replace the cap and it works just check the joints around the diode and see if it keeps working.

 

[tsmmiller]

Thanks guy, pointing me to what I was leaning on.
I believe it may be a zener, just looking at the board (from memory now, since I am at work) the silk screened area that I can see next to it is z*** the rest is buried under the neighboring parts. The photo in the link is silk screened Z103.
Looking in Mouser I find the Zener, voltages 9.1v to 200v. How do I determine the voltage of the one I have. The capacitor it is next to is 25v, so...?
This is what I plan to replace, 4 parts, the 220µf 16 v, up sizing to 25v 3000hour part, the 470µf 25v cap (bulging) the D367a rectifier (if I can find one, not sure I am looking at the right part number yet) and the z diode once I figure out the correct voltage.

 

[tsmmiller]

Looking at the silk screened z103, searching for z103 I found on flea-bay FMB z103,
https://www.ebay.com/itm/263591592260Not trusting a $40 purchase for "reclaimed" items from there!
I found what I hope to be the same item at Mouser
https://www.mouser.com/ProductDetail/Vishay-Semiconductors/TZX9V1B-TR?qs=8Acm1RElxHIfWwHJEteAjA==I'll still take any advice to my errors on thinking.
Now the last item for my Mouser shopping cart is the rectifier, having issues find that item.

 

[jderimig]

The z103 could be the schematic symbol. Usually at 2 factor of safety for a cap is good. So if it's a 25v cap it may be a 12-15v zener

 

[tsmmiller]

The z103 is not likely the part number but instead is the schematic symbol .

Yep, I am aware of that. I may end up pulling it to test, just to figure it out. Since I am plannig on replacing it anyhow, no harm in doing that.

 

[OverTheTop]

Z103 is likely to be the designator on the schematic, and not a component manufacturer part number. Don't purchase the FBM-Z103 as it is a different style of diode and not likely the correct part.

If you are lucky the zener will still be working anyway, as a shorted supply rail usually just shunts current away from a zener (assuming it is in a shunt regulator configuration) and if it is a transistor boosted regulator the additional current draw will again have no effect on the zener.

If you are really keen there is likely to be a number on the diode which you might be able to read. Typically 1Nxxxx. Then you could get a new part.

I would just replace the cap first in your situation. Once it is up and running you could then measure the voltage across the zener and purchase a new part if you were really keen.

 

[tsmmiller]

I got home from work today, did my chores then sat down with circuit board and looked everything over.
I removed the two capacitors, the 16v to upgrade to the 25v and the 470 due to the bulging condition. I found the rectifier par number was much easier to read without the swollen cap in the way.
Having all the part numbers I located everything the two caps at Mouser, and the rectifier from eBay. I placed an order for qty of 5 of each part.
Looking at the zener under a 60x lens I couldn’t see any damage so taking Over the Top’s advice and leaving it alone.
All I can do now is wait for the two weeks for everything to arrive.
Thanks all for the advice, much appreciated!

 

[kuririn]

Looking for boards again, there are none to be found.

Is this the board you're looking for?
Part # is Bosch 00771183
Bosch 00771183 Microwave Electronic Control Board (partsimple.com)

EDIT: oops, my mistake. This is for "Repair Service". You send in a defective board, they repair it and send it back to you for installation.

 

[kuririn]

PS Is it common for a Samsung Microwave to have a Bosch control board?
Non electrical savvy guy here.

 

[tsmmiller]

OMG!!! BOSCH!!!! How in the h e double hockey sticks did I type SAMSUNG where I should have put BOSCH.
So my BOSCH micro/convection combo!
Yes, this is the correct board, and obvious the board is a repaired unit, (repair service) it has the charred area just as mine and the others seem to have!

 

[tsmmiller]

Ordered three parts and spares, so I can repair the board that is installed now, and put it out in the garage for next four years until the next burn out. (remove, repair, replace!)
Replacing the 2k hour cap with a 5k hour cap should give me at least another year of use before the fade out.
Interesting note, this am while it was dark I attempted to take a photo of how dim the display is. Using my phone camera, the display was bright and visible, to the naked eye, couldn't see it at all!

 

[tsmmiller]

Just did a quick search again and found two places with that board is stock. $170 and $175, one with the LCD screen one without.
I think I will still attempt my less than $10 repair first and see how it goes. Should be just fine.

 

[tsmmiller]

My Caps came in today from Mouser.
Since I had my original board I de-soldered the two caps thought to be the issue.
I replaced the 16 v with a 25 volt part and the 470uf 2k hour cap with a 470uf 10k hour part. Both were twice the size of the OEM parts but fit the board like they were made for it.
Replaced all the panels and placed the microwave back into the wall where it took me longer to plug it in again than it did to remove and replace the board!
Powered back on, display is vivid and bright and I received a hot slice of pizza in 45 seconds.
Thank you all who helped point the way for me!

 

[OverTheTop]

Good outcome. Why somebody would put a 2khour lifetime component in an oven that is expected to be on 24/7 is beyond me. Not a good engineering decision to save a few cents. The harm done to a brand name by customers having unreliable appliances is not worth the cost saving.

 

[Grog6]

Those hours are the rating for certain conditions, usually 105degrees C. lifetime increases exponentially for lower temps. Power surges tend to fail them pretty routinely.

 

[OverTheTop]

Those hours are the rating for certain conditions, usually 105degrees C. lifetime increases exponentially for lower temps. Power surges tend to fail them pretty routinely.

Yep. Typically for each 5degC lower than the operating temperature the lifetime doubles. A truly exponential improvement. The problem is that the ovens get hot during use, and the PCBs are usually above the hot bits so they see significant temperatures, not just ambient.