Redesign a daughterboard that already exist but my don't work - Page 1
Forum section: electronics / projects

author: Vicus
author_rank: Regular Contributor
timestamp: March 31, 2021, 12:40:06 pm
content: Hi, I'm in trouble with a repair. The patient is a powered speaker model Wharfedale titan 12A. The schematic is attached.wharfedale_titan_15_active (1).pdfThe part I have to redesign is a small daughterboard that produce the pwm signal and drive the output stage. The schematic is in page 8. I have to replace it beacuse heat deteriorate the fiberglass substrate and tracks almost lift. The board work but I want to replace to be safe (LOL).So I redesign the board using kicad, my first time with kicad, and of course I did some error, but I don't think thats the problem. One of the problem was how kicad manage the sot23 pinout and the other the footprint of the logic IC. I modwire the board and resolve the first. The speaker works great for about 2 or 3 hours. Then stop to work. I found a short cap (C39 if I correctly remember) on the 5V rail and one logic IC, also one mosfet fail. So I decide to replace U7 U8 U9 U10 to stay on the safe side and the mosfet that fail. After turn it on again it wors for minute than U7 and U8 overheat and fail again. In the next try the mosfet failed again. Now I waiting for new mosfet that is quite difficult to find. So the question is:Due to the wrong footprint that I did use I choose different logic IC, insted of TC7WH04https://www.mouser.it/datasheet/2/408/TC7WH04FU_datasheet_en_20200205-845893.pdfI use 74LVC3GU04https://www.mouser.it/datasheet/2/916/74LVC3GU04-1319931.pdfand insted of TC7WH08https://www.mouser.it/datasheet/2/408/TC7WH08FU_datasheet_en_20200205-845691.pdfI use 74LVC2G08https://www.mouser.it/datasheet/2/916/74LVC2G08-1319483.pdfthat to my eyes they look similar or almost identical. There is something that I did miss, and the two parts are not interchageble? If yes what could be? I check every single component value (from the original board) and also copy R45 R46 that to me is useless.Thans.

author: sahko123
author_rank: Frequent Contributor
timestamp: March 31, 2021, 01:30:30 pm
content: Why would you re-design a completely new board? why not reverse engineer and make an identical layout? In terms of pinout make sure your using identical parts

author: SilverSolder
author_rank: Super Contributor
timestamp: March 31, 2021, 01:38:49 pm
content: That looks like an ambitious repair, I like the spirit!It is not clear to me which circuit diagram is for the daughterboard. Can you screenshot just the relevant circuit?I would try measuring some DC operating points around the circuit on the original board. Then, do the same with your board. This should at least reveal if there is a difference between the two.If the circuit processes audio signals, it is possible it may be oscillating at an inaudible frequency due to the new layout, which might cause it to overheat. Have you tried probing with a scope?

author: Vicus
author_rank: Regular Contributor
timestamp: March 31, 2021, 01:52:36 pm
content: Quote from: sahko123 on March 31, 2021, 01:30:30 pmThe schematic of the board is the same, the board is quite simple and I think is faster to redraw the pcb then try to generate gerber file from an existing pcb.Quote from: SilverSolder on March 31, 2021, 01:38:49 pmPage 8 of the schematic, attached. DC voltage it ok, + and - 15 and -70 to GND is ok. Also +15V to -70 it's ok. Also +5V to -70 it's ok, it's the output of U10.Autooscillation or a drift in the oscillating frequency (it's a self oscillating systems with oscillationg frequency set by the instrinsic delay of the system and the value of the feedback network of U6). I did not measure it, I don't know how to measure if there is oscillation. Also my problem is that I have only scopes that is not isolated from ground so not really shure how to measure for example the output of U8 that it's reference is -70V.

author: 2N3055
author_rank: Super Contributor
timestamp: March 31, 2021, 02:15:36 pm
content: You need to clone circuit EXACTLY.TC7WH08FU is nowhere close to being equivalent to 74LVC2G08 for instance. Circuit seems to rely on timings related to gate propagations delays and such.Stop wasting time redesigning same timings with different components.Unless this is not about most efficient repair but a learning experience..

author: Vicus
author_rank: Regular Contributor
timestamp: March 31, 2021, 02:31:26 pm
content: Thanks, what's the parameters that differ and metter between the two components? Yes propagation delay is a ey factor in setting the working frequency. Thanks.

author: 2N3055
author_rank: Super Contributor
timestamp: March 31, 2021, 02:47:57 pm
content: Quote from: Vicus on March 31, 2021, 02:31:26 pmWell just look at datasheets.It has different propagation delays and different rise and fall times. Also TC7WH08FU has a GUARANTEED symmetrical propagation times for rise /fall. That is needed to guarantee proper timings between output mosfets and symmetric drive.... Also dead band (to prevent cross conduction of mosfets) is defined by propagation delay of two TC7WH04 gates. If dead band is too short (it is because you used pretty much fastest 74 family you could get) and you have both mosfets conducting at the same time. Pretty much quick short circuit across power busses. It will stand it for some time and then it will die..This type of circuit relies on intrinsic properties of particular chip for proper function. You need original chips. Or something really equivalent, but you will need to characterize it yourself. Across temperatures and full working envelope..And suddenly original chips look as simplest, nicest and cheapest solution....

author: Dave8266
author_rank: Contributor
timestamp: March 31, 2021, 07:06:26 pm
content: Quote from: Vicus on March 31, 2021, 01:52:36 pmFor something like this, where a schematic is provided, it's good to also double-check it against the board in hand to verify that there were not any revisions or changes made in production, which didn't get updated in the service manual.Cloning the board layout exactly would have been a great way to do this - because if a trace didn't or couldn't be routed via the same path, it would have been an immediate clue that something was "off", and that the schematic had changed.