EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: PiotrMisiuna on December 02, 2024, 10:57:09 pm
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Could not locate any photos of the PCB, attaching best I could make, maybe will help someone else.
There is several versions of the box but I don't believe they will differ much in layout, especially on the primary side of the psu.
Killed by a drop of water, I did some reverse engineering, but with my low experience I don't believe I can make it without help.
Components on the schematic marked in red are the damaged ones.
Seams obvious that all the carnage was caused by arc under the C37.
I hope that blown components broke the circuit before melting the transformer, it does not appear to me melted.
Not sure how to determine the value of the Z7 zener diode...
list of components I wish to find markings for:
R114
Q32
Z7
R85
U10
(http://attachimg=4])
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UC3842?
https://www.ti.com/lit/ds/symlink/uc3842.pdf (https://www.ti.com/lit/ds/symlink/uc3842.pdf)
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Well, finally! One person who got together and redrew the circuit from the PCB.
It will be a pleasure to help you!
I'll take a look a little later.
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Hi!
Could you carefully clean R114 and try to read the color code.
Also make a good photo of Q32, U10 area.
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R114 has the bands visible on the other side but I think the paint pealed off due to temperature and what's left is just a discolored substrate.
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Check Z13, Z14 for short and open.
I don't like that the base Q32 is connected only Z14.
Please measure C56.
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Q32 doesn't make any sense to me, nor does the feedback circuit involving the optocoupler.
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Z13 and Z14 open circuit, tested out of circuit.
R112 - 10K,
C56 - 2,4nF
did some edits following your order
third attachment is just a back lit photo of the pcb, maybe useful.
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Did some corrections to the schematic.
As to the optocouppler, I believe it is just a voltage sense for a controller. Tracks disappear under the row of sockets and run deep into secondary side.
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The Vcc pin needs to charge up to 16V before the IC starts pulsing the MOSFET. I don't see any path for this charging current.
Z7 must be a Schottky diode.
Why is the base of BC807 grounded?
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Weird circuitry around Q32.
Normally Vcc is directy feed from HV (Yeah, 300V++) through a 200K-1M resistor.
This greatly limits the current, so the Vcc capacitor charges slowly.
When reachingthe threshold voltage the PWM IC will turn on, pulsing the driving transistor, and start feeding itself though the auxiliary winding.
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Did U10 pop part of its top off?
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U10 did popped a piece of i
t.
More corrections. it gets better once the shorts get removed. BC807 was also shorted
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Hello PiotrMisiuna,
More corrections. it gets better once the shorts get removed. BC807 was also shorted
Nice holes in U10 & Q32 : 'magic smoke' generators :P
Your latest schematic starts to match with the UC3842 reference design (figure 8-2 (https://www.ti.com/lit/ds/symlink/uc3842.pdf#page=22))
Opto on the right of your schematic is for the SMPS feedback : you should also find a TL431 (https://www.ti.com/lit/ds/symlink/tl431.pdf) in that area.
Opto on the left of your schematic is probably for checking the mains presence or grabbing 50/60 Hz synchronization ticks : is it connected to the Fujitsu µC located on the far right side of the PCB ?
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Yes, the opto on the left goes to the large Fujitsu. Above it there is a motorola Eprom. There is also a MAX 232 next to the DE9 port. There is really not that much stuff on this board, and the service guys ask long waiting time and a small fortune for the replacement.
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The oscillator frequency according to ST datasheet is around 90kHz.
Two chips are currently under consideration:
UC3842B max 100% Duty Cycle/ 90kHz drive operation frequency
UC3844B max 50% Duty Cycle/ 45kHz drive operation frequency
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Z13 and Z14 open circuit, tested out of circuit.
Might be high voltage zeners (tens of volts), even if one junction drop should be found in one way : any marking on them ?
Package is known as 'melf' https://services.taiwansemi.com/storage/resources/datasheet/ZM4728A%20SERIES_E2301.pdf (https://services.taiwansemi.com/storage/resources/datasheet/ZM4728A%20SERIES_E2301.pdf)
and wattage seems to be 1W https://www.mouser.fr/c/semiconductors/discrete-semiconductors/diodes-rectifiers/zener-diodes/?mounting%20style=SMD%2FSMT&package%20%2F%20case=MELF (https://www.mouser.fr/c/semiconductors/discrete-semiconductors/diodes-rectifiers/zener-diodes/?mounting%20style=SMD%2FSMT&package%20%2F%20case=MELF)
Check if your parts size correspond to a 1 W zener in melf package.
The circuit around Q32, Z13 and Z14 needs more attention/refinement, because it goes beyond the application note.
On this page (https://www.ti.com/lit/an/slua143/slua143.pdf?ts=1733295288730#page=5) :
- Figure 5 gives interesting information about the required voltage to power the UC3842, but this will be useful later.
- Figure 6 explains how most SMPS controllers like the UC3842 are powered : startup power comes from the mains DC rail (about 324 VDC in Europe) via 'Rin'. Once running, an auxiliary winding 'Waux' provides power through D1.
Let's go back to your latest schematic.
The 'aux winding' is present and the main power for U10 comes from D12, and this seems OK.
R114 is probably 'Rin' and even if the paint is peeled off, the value you measured (100 k) is probably correct, because the UC3842 asks for at least 1 mA to wake up.
Additionnal information : at 100k, the current is around 3.1 to 3.2 mA, which is more than enough to wake up the PWM controller.
R114 seems to be a 2W resistor. However, it is wise to replace the part with a new one.
Two chips are currently under consideration :
UC3842B max 100% Duty Cycle/ 90kHz drive operation frequency
UC3844B max 50% Duty Cycle/ 45kHz drive operation frequency
You are right, and to solve the riddle, the transformer ratio and primary inductance is probably required :palm:
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Please check the both zener connection.
PS
Seems they are the same 18-20V to get 17.3-19.3 start at Vcc.
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:palm: yes, you are correct. I need to stop making this edits at 1:00am.
as to the transformer, it has a sticker, no meaningful results in the search engines thou.
markings at the top just refer to the type of the core.
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as to the transformer, it has a sticker, no meaningful results in the search engines thou.
Yes it is for sure a custom made part and we won't find any datasheet.
If we really need to know the transformers' ratio and primary inductance to validate the PWM controller reference, those intrinsic parameters have to be measured... :scared:
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Some more thoughts.
There is no point in installing Q32 just to start the controller.
Most likely something else is being powered there, take a look.
If transistor Q32 is shorted on all pins then it is most likely a MOSFET.
Resistor R114 is marked with the first brown stripe, please check it. I set it to 100k but when applying high tension it will not burn like that. Looks like he's about 10 kilos.
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I corrected the circuit digram.
Due wake up resistor is replaced here by stabilized power supply on Q32 the controller should be with lower start threshold i.e. UC3843 or UC3845.
Seems all the zener diodes Z1,Z4, Z7, Z13 and Z14 are the same.
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I'm ordering the parts. it will take a while, Cyprus shipping is like 3 times the usual. I'll share results as soon as possible.
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Cyprus shipping is like 3 times the usual.
Yes, life is't easy for you :)
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Don't forget to replace the blue bulged electrolytic cap.
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Zeners can be 13 volts. This is enough to start the controller (Vcc=9V max).
The main thing is that after start-up Vcc voltage rises slightly (up to 10-11 volts) and turns off Q32.
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Buy also a replacement for R114 (100 k).
10 k seems pretty low (lead to a start current of I = 31 mA)
If the controller does not start with 100 k, you can still try 10 k in a second attempt.
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100k resistor will not burn out as much as 10k in case of Q32 short.
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100k resistor will not burn out as much as 10k in case of Q32 short.
I already noticed your explanation above.
Let me just allow to have different opinion than yours.
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G'day All,
I am not sure if the attached is a help or a hinderance but it contains similar components to the damaged PSU.
I have seen similar damaged components in these as well.
I do not have a schematic.
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G'day All,
I am not sure if the attached is a help or a hinderance but it contains similar components to the damaged PSU.
I have seen similar damaged components in these as well.
I do not have a schematic.
Hi Poroit!
Yes, yours PSU is mostly similar.
PFC provides its own Vcc supply. That is why low power start-up UCx845 is used there.
Also 50% duty cycle is usually used with the high power transformer.
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Hello Poroit,
G'day All,
I am not sure if the attached is a help or a hinderance but it contains similar components to the damaged PSU.
I have seen similar damaged components in these as well.
I do not have a schematic.
Can you confirm that the document you provided shows an elevator door controller from Prisma ?
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About Z14 (Vz = 11 V in the latest schematic)
@PiotrMisiuna Any marking on the zeners ?
Minimum PWM controller startup voltage depends on IC reference :
UC3842 & UC3844 : 16 VDC : Z14 value is underrated, PWM controller will not start
UC3843 & UC3845 : 8,4 VDC : Z14 value is OK
Interesting sentence in chapter '7.3.5 Undervoltage Lockout (https://www.ti.com/lit/ds/symlink/uc3842.pdf#page=15)' of the datasheet :
During normal circuit operation, VCC is developed from auxiliary winding NA with DBIAS
and CVCC. At start-up, however, CVCC must be charged to 16 V (*) through RSTART. With a start-up current of 1 mA,
RSTART can be as large as 100 kΩ and still charge CVCC when VAC = 90 V RMS (low line). Power dissipation in
RSTART is then be less than 350 mW even under high line (VAC= 130 V RMS) conditions.
(*) Actual minimal startup voltage depends on IC model, see above.
Reminder : as PiotrMisiuna is located in Europe, VAC = 240 V RMS @ 50Hz (hence DC rail voltage = 339 VDC)
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G'day timeandfrequency,
Unfortunately not.
It is an SMPS from a 25 year old Air Data Test Set.
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Unfortunately not.
It is an SMPS from a 25 year old Air Data Test Set.
Thanks Poroit for this information.
These UCx84x switchers (https://www.ti.com/lit/ds/symlink/uc3842.pdf) have numerous applications and were used in many different designs.
This thread is about the repair of a 'Prisma elevator door controller'.
Unfortunately, the board you showed can not really be used as reference to identify the fried parts we have on that elevator controller.