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repair Tribest Sedona SD-P9000 dehydrator -- low DC voltage from power board: maybe rectifiers or caps?

.brush

Oct 5, 2022
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hi @badger2 -- glad this thread was helpful, and i hope you were able to figure out your issue with the buttons.

i do have another update -- shortly after putting the dehydrator back in service (the reassembly was a pain), it stopped working again, same symptoms. i had had my son learn about soldering in replacing the D7 diode, and he'd cooked the PCB a bit too much, so i'd had to scrape some of the insulative sheath off the trace and solder to that, then glue it in place -- i assumed this had been a weak connection and had failed.

however, i pulled it all apart again, only to find that the replaced D7 diode and its connections tested fine. instead, the previous diode in the voltage-reduction chain was blown (ie. D6). so i replaced this one as well, this time using the long legs of the diode to jumper over the traces to add redundancy, and put it back together -- and it didn't work, no life in the control panel.

so i started testing voltages -- it was the full 15VDC after the FWB, but then dropped 10V over the next diode (D5), so that by the time it came out the end (D9) it was around 3V, which i'm assuming is just about too low to activate the control panel.

however, while i was poking around checking voltages, the board suddenly lit up. now it's working fully as expected. voltage drop across the series of diodes is about 3.5VDC. each is ~650mV.

so... any ideas what might be going on?

is this a sign that all the diodes in that chain are on their last legs and should be replaced? (but do diodes have a failure warning mode like that? i thought they didn't...)

or is it a sign that there's some other problem somewhere that's causing diodes to blow? (like maybe spikes of voltage from something?)

or could it be that my hackish soldering job is causing intermittent problems? (like maybe a poor connection that was adding to voltage loss?)

any thoughts very welcome!

i'm going to try running it for a while and seeing what happens...
 

kellys_eye

Jun 25, 2010
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Volt-drop implies resistance and diodes don't have a (pre) failure mode as such. We can only assume that the soldering wasn't up to scratch and that there was a dry joint somewhere. Physical movement seems to have corrected the issue but it will re-occur unless the soldered joints are properly made (re-wetted/reheated etc).
 

73's de Edd

Aug 21, 2015
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Your D5-6-7-8-9 string are ONLY working at ~1.5 % of their rated capabilities, so I also suspect an open / intermittent . . . SOMEWHERE ? . . . in that foil string diode/ chain.

THE NEXT ? PROBLEM AREA . . . .?

Also probably 100% ignored, was the heat sinked semiconductors at either end of the board . . .the center one is likely being O.K. . . .but check.
The viewed boards heat signature, tells you that they are handling some power . . . . with emphasis on the amperes aspect.
If those leads develop solder fractures, they progressively develop circular heat rings.
When so, there then is MORE current conduction, having to be made within an ever SMALLER effective peripheral /circular contact area then being available.
That quantitatively oxidizes contact surfactal area, with then even greater heating . . . until a floating solder ring fully develops.
Clean / suck/wick off all of the old solder, aided by liquid rosin flux's de-oxidixing action.
Apply new solder.
 

badger2

Oct 30, 2022
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Oct 30, 2022
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hi brush - so like yours i had 1 completely dead diode originally. it did fire into life when probing the diodes with power running. just as you describe. gave me a fright! ( of course i dont encourage anyone to play around inside it while the mains power is on )

i had read somewhere a theory ( i think on another forum about a different machine ) that it is reasonable to suspect that all the diodes are on the way out. in which case replace all is logical action.

or, it is equally reasonable to suspect that -i think this is what 73's de Edd is getting at .... that a fault nearby is contributing to the failure of the D7 diode and/or others, and indeed might be rapidly re-destroying the newly replaced diode. so by replacing diode only we are fixing a symptom and not the root cause ?
i am no artist when it comes to soldering and i did wreck a track removing the dead diode. i had to do a horrible bodge repair and ran a jumper wire back to next solder pad from leg of new diode.

so my machine works - partially. like it gets and warm and the fan runs. but timer doesnt function at all , and up-to-temperature doesnt ever kick in.
im not convinced that thermostatic control is working at all . who knows. is it because one or more diodes are still not right, so the voltage isnt right to allow the control system to run solidly and reliably ?

its not ideal but hey its good enough for me to dry a few herbs etc by eyeball/feel and looking at the clock. better than not working at all.

cheers
 

73's de Edd

Aug 21, 2015
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it did fire into life when probing the diodes with power running
Doesn't that just . . . SCREAM . . . at you . . . . . of microscopically finite . . . . poor/and /or intermittent connection(s) ?

Pull that units 5 diodes off the board ( NO makee BOO-BOO on pads ! ! ! ! ) and if you have 5 new diodes , laterally do a total lead peripheral scraping and use rosin flux and new fluxed solder to do a full tinning of all of those leads.
Then, after a full solvent cleaning of them and a linear run buffing with cotton Q tips, their tinning SHOULD then look as if being CHROME plated.
The same procedure relates to the used diodes, if you have to straighten them for rework and re bend to reuse then . . . . . AFTER an ohms and diode mode testing of their Vf's . . . even if using an old school analog meter, if that is all that you have.
Then clean up the related pads of old solder by sucking or wicking off and then re tin all of those pads using rosin flux solder and rosin flux, with a final molten solder / wooden toothpick tip punch through and twist, thereby, leaving your lead hole open in each pad.
Do the same lateral clean up procedure on all of those pads and the also identical " they SHOULD look as if being CHROME plated."
THEN you reinstall all of the diodes and re-flow solder each with fresh solder, aided by additional rosin flux.
Use the SAME solvent clean up and buffing procedure again to acquire "CHROME CITY"
You will THEN have made a noteworthy repair of the nature of a NASA or MIL SPEC rework . . . . . . where, any lesser repair, could result in a million dollar ++++++PLUS+PLUS+PLUS+++++ failure . ( F7-F117-F-15-F16-F22-F35 )

OLDWHATSHISNAME . . . . .

oh yes . . . NOW I remember . . . .

73's de Edd . . . . . . . . . .

This is my step ladder . . . . . . . . . . . (since I really never got to know my real ladder.)


.
 
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73's de Edd

Aug 21, 2015
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Brush . . . . when getting your unit operational again, could you also put a meter probe on ground of the - of the E can filter closest to the UL20000 series IC and then use + probe to read all of the voltages on the WHITE center connectors 10 pins.
That could then assist you if there are ever any future problems . . . but as for NOW . . . to assist the " Badgester" in his troubleshooting of his display board..
 

.brush

Oct 5, 2022
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Oct 5, 2022
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hi folks, sorry for the delay in response, busy times. the dehydrator worked for several hours at a stretch, control board with operable features. so looking good for now. i think that's a good idea to grab the voltages i'm getting as an aid to @badger2 ... though i'm not sure next time i'm going to get to it.

if i was seeing weird errors with the board, i guess i would tend to assume it was one of the chips that had failed, either that or some specific component on the control board itself. i guess having a "known good" list of voltages will help narrow it down.

as for my weird behavior -- certainly *could* be a bad joint that i bumped into working, but it doesn't seem like it. everything that measured continuity before still does, and vice versa. moreover, the weird reading was in a spot that was *before* any of the soldering jobs. so it seems like it's something else? though if it goes bad again, i'll definitely follow those instructions for a clean and shiny soldering job, to take it out of consideration!

but i did have one thought -- it was about 10-15 sec after turning power on that the control board kicked into life. maybe it was coincidence that i was probing with meter? is it possible that it took that long to charge up a capacitor, which was draining power in the initial period, then once full ceased drawing, reducing the voltage drop to spec?

esp. since @badger2 had the same experience, makes me wonder...
 

phillipsf

Jun 1, 2023
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Jun 1, 2023
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Hey guys, my sedona 9 tray dehydator just died after running for a few minutes. And i found this thread. Would love to do the same repair as brush and badger above, but can someone point me in the right direction of purchase the 5 diodes? So i can solder them in
Thanks
 

73's de Edd

Aug 21, 2015
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Sir phillipssf . . . . .

Initially why don't you refer to post # 15 for referencing and take a voltmeter, placed in its DC volts test position, and connect its neg lead to pictures extreme right corners Item 3 E-caps bottom right neg terminal . . .really stab it and firmly hold it into the solder blob.
Power up the unit and then measure with the meter positive probe to bottom center YELLOW A, take its voltage reading . . . . with others to follow . . . and then sequentially move to
B-C-D-E-F-G H and then up to J and finally to Yellow L at the E-cap + terminal.
If you loose voltage at any point, you may actually have a . . . . time related . . . . . oxidized diode lead that is floating within its solder blob . . . . . instead of an actual bad diode.

Thaaaaasssssit . . . . . . .

Post 15 Referencing . . . . repeated to this page . . . .

2-dehydrator-pcb-png.56521

73's de Edd

FACEBOOK just alerted me that someome is now following me . . . . . . . . so I'm going to start walking FASTER !

.
 

badger2

Oct 30, 2022
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Oct 30, 2022
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73's de Edd "oxidized diode lead that is floating within its solder blob . . . . . instead of an actual bad diode." aaaahhhh now i see what you were getting at ! too cryptic for me previously.... i hope phillipsf is able to get his machine running. thanks once again to all clever people on here!
 

2315Robert

Jun 28, 2023
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Jun 28, 2023
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Sir phillipssf . . . . .

Initially why don't you refer to post # 15 for referencing and take a voltmeter, placed in its DC volts test position, and connect its neg lead to pictures extreme right corners Item 3 E-caps bottom right neg terminal . . .really stab it and firmly hold it into the solder blob.
Power up the unit and then measure with the meter positive probe to bottom center YELLOW A, take its voltage reading . . . . with others to follow . . . and then sequentially move to
B-C-D-E-F-G H and then up to J and finally to Yellow L at the E-cap + terminal.
If you loose voltage at any point, you may actually have a . . . . time related . . . . . oxidized diode lead that is floating within its solder blob . . . . . instead of an actual bad diode.

Thaaaaasssssit . . . . . . .

Post 15 Referencing . . . . repeated to this page . . . .

2-dehydrator-pcb-png.56521

73's de Edd

FACEBOOK just alerted me that someome is now following me . . . . . . . . so I'm going to start walking FASTER !

.
My wife loves her Sedona 9000. Was shutting off after about 15 and control panel would go dark. All diodes checked good on both sides off the board. What did not check good was continuity on the traces. Desoldered and reflowed all the diodes working like a champ. Thank you.
 

garyjames916

Sep 22, 2023
1
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Sep 22, 2023
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Welcome to the forum! It seems like you're dealing with a complex issue involving an older dehydrator's control panel and power supply. The fluctuating 0.5VDC to the control panel does seem lower than expected, possibly indicating a problem in the bridge rectifier or capacitors. The multiple diodes and discoloration on the PCB are suspicious, suggesting a potential issue in that area. While schematics aren't available, exploring a workaround with analog controls might be an option if troubleshooting doesn't work. Hopefully, the community can offer some insights and suggestions. Good luck with your repair efforts!
 
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