Wharfedale SPC 12 SMPS Repair - Help with Identifying Replacement Components - Page 1
Forum section: electronics / beginners

author: rstor22
author_rank: Regular Contributor
timestamp: June 21, 2022, 07:14:05 pm
content: I have a fault with what appears to be the SMPS on a Wharfedale SPC 12 subwoofer. The unit does not power on. The IR2153 has blown, both mosfets (IRFP460A) Q1 and Q2 test bad. Also C14, C18, and C10 electrolytics have high ESR/open and/or capacitance out of spec (located around the IR2153). I have already ordered replacement parts for the mosfets and capacitors. I decided to test the diodes and found that diode D3 (appears to be connected to gate of mosfet Q1) and zener ZD1 (near the IR2153 that had blown) are shorted (confirmed out of circuit). I also confirmed out of circuit that NPN Q3 (also near the IR2153) has its base-emitter shorted (65R).I don't have experience with SMD rework. From what I found online, the diodes appear to be SOD-80 packages (see attached pictures). NPN Q3 has the markings as 2F or ZF (not exactly sure). According tohttps://alltransistors.com/adv/pdfdatasheet_kec/2n5551s.pdfit makes reference to the 2N5551S having the marking of ZF. I have attached images of the three components. One image I have included a ruler to provide a scale (in that one ZD1 was already removed).Someone earlier had made a schematic of the circuit where ZD1 and Q3 are located. They labeled ZD1 as a 17V zener (with a question mark):https://www.diyaudio.com/community/attachments/schematic_wharfedale-spc12-power-supply_circuit-diagram_20200117172512-png.809501/I am hoping someone could help me identify suitable replacement SMD parts for Q3, D3, and ZD1?I found the following on digikey / mouser however I would appreciate feedback if these would be suitable replacements:Q3:https://www.mouser.ca/ProductDetail/onsemi/MMBT5551LT1G?qs=HVbQlW5zcXXlUyy8htIqAQ%3D%3DD3:https://www.digikey.ca/en/products/detail/central-semiconductor-corp/CLL2003-TR-PBFREE/5325131ZD1:https://www.digikey.ca/en/products/detail/vishay-general-semiconductor-diodes-division/TZM5247B-GS08/4874715I have also posted this question on diyAudio as some of the members had experience with this unit:https://www.diyaudio.com/community/threads/tricky-smps-fault-in-wharfedale-spc-12.348101/page-2

author: rstor22
author_rank: Regular Contributor
timestamp: June 22, 2022, 04:52:45 pm
content: I went over my suggested replacements for Q3, D3, and ZD1 with someone locally and it looks like the parts I have chosen for D3 and Q3 would be overkill. ZD1 that was selected should be ok. I will continue with these selected parts.I will update with the results of the attempted fix.

author: eblc1388
author_rank: Frequent Contributor
timestamp: June 23, 2022, 05:24:05 pm
content: D3 and D4 at the gate of the MOSFET are usually just plain 1N4148.

author: rstor22
author_rank: Regular Contributor
timestamp: June 28, 2022, 07:49:50 pm
content: After replacing the parts I mentioned earlier I measured the output voltage (expecting to see +15/-15V) however there was no voltage present. The voltage on pin 1 of the IR2153 is measuring 11.08V.On the schematic that was posted (on diy audio), VDC for the VCC Supply was stated at 340V. For me, I am measuring 320.8V.With the IR2153 removed voltage where pin 1 would be was measured at about 16V which appears correct.Tried lifting ZD4, Q5, and R21 (measured 362.1K out of circuit). Also lifted pins 5 through 8 on the IR2153. Pin 1 on the IR2153 still low at around 11.5V. Connected the IR2153 such that only pins 1 and 4 were connected. Voltage at pin 1 still measuring low at 13.5V.At this point I have given up on the repair.

author: eblc1388
author_rank: Frequent Contributor
timestamp: June 29, 2022, 11:31:10 am
content: Quote from: rstor22 on June 28, 2022, 07:49:50 pmThat's a bit unfortunate, but there might be things you can try, hopefully.I did a LTspice simulation on part of the 320V to 16V circuit and noticed that it can't provide sufficient current, which your actual measurement proved to be the case. The voltage drops as the load current increases.I have made some changes(remove 6K8 and change both 180K into 100K) and now the circuit can provide 5mA before the voltage starts to decrease. Hope this will give you some ideas. The simulation file is also attached.

author: rstor22
author_rank: Regular Contributor
timestamp: July 03, 2022, 02:24:20 pm
content: Quote from: eblc1388 on June 29, 2022, 11:31:10 amoh wow! thank you for taking the time to put together a simulation for this. I just saw your reply last evening (forgot to turn notifications on). I was thinking about adding a resistor in place of the IR2153 to test the circuit for a 5mA load to see if there was an issue with the replacement chip. I had simulated using falstad.com earlier to see the voltage at where pin 1 would be but never applied a load to the circuit in the simulation. I never thought about simulating the circuit to see how it behaves under variable load.I will give it a try with resistor modifications (though the end user wouldn't be using it anymore) it will be nice to see if I can get the smps / subwoofer running and narrow down the issue to this. I wonder what changed in the original component values that has resulted in the voltage drop under load. I tried setting the simulation to 340V (as that was the voltage that someone else had measured when repairing their SPC12) however it looks like this doesn't have much of an effect at 5mA load.I installed LTSpice and tried the simulation you created. There were some component issues with the 100uF capacitor, Zener and transistor. I used the polcap capacitor model, MMBZ20VALY for the 17V zener, and 2N5550 for the transistor. For some reason the graph of V(out2) with respect to the load current starts off at around 19V as opposed to your graph that starts at about 16V (see attached picture). I am unsure why this is.

author: eblc1388
author_rank: Frequent Contributor
timestamp: July 03, 2022, 05:11:59 pm
content: Quote from: rstor22 on July 03, 2022, 02:24:20 pmIt starts at 19V because the wrong zener model is being used in your simulation. MMBZ20VALY is not a zener diode but a Transient Voltage Suppressor diode, which often specify with the normal working voltage(at which it should not conduct) so the actual breakdown voltage will be a little higher, as you have found out.I'm sorry I forgot to include the zener diode model in the .asc file. Please copy the following zener diode model and pasted into your .asc file. The simulation will then run.Code:.MODEL 1N5247 D(Is=5.398f Rs=4.945 Ikf=0 N=1 Xti=3+ Eg=1.11 Cjo=63p M=.4188 Vj=.75 Fc=.5 Isr=1.453n+ Nr=2 Bv=17 Ibv=25.923m Nbv=1.1189 Ibvl=324.66u+ Nbvl=.86905 Tbv1=823.53u type=zener 17v)

author: rstor22
author_rank: Regular Contributor
timestamp: July 03, 2022, 06:16:18 pm
content: Quote from: eblc1388 on July 03, 2022, 05:11:59 pmThanks again. I've added the zener diode model to the standard.dio file and set the simulation to use this diode. The graphs are now similar.

author: rstor22
author_rank: Regular Contributor
timestamp: July 15, 2022, 03:32:23 pm
content: Changing both 180k resistors to 100k 3W and leaving the 6.8k resistor in place resulted in pin1 of the IR2153 to be 15.73V. I recall also measuring pin 3 (CT) of the IR2153 at about 7 to 8v.Unfortunately there is still no voltage when measuring the output of the smps supply..