What does a wire in parallel to a trace cause? - Page 1
Forum section: electronics / beginners

author: Ober EE
author_rank: Newbie
timestamp: October 20, 2021, 06:37:09 am
content: So I am learning repair work and I was working on an Fisher MC-4040, not an easy one for a beginner specially with the lack of service manual (had to buy it) and information of others working on it. The previous person who was in this device had done a repair where first I found they replaced a cap but instead of putting it into the normal spot they put it on the back of the board and then shrank wrapped the legs so they did not short anything else(), but the more odd thing was the wire I found going from the inside leg of a one component over the leg of the collector of the main mosfet on the power board to the inside leg of the next component - so they just jumped over the collector with a wire in parallel to the trace and nothing else. When I tried to ask people what this might do, a couple suggested it wouldn't do anything, I do not have any sources I can confirm with but I am thinking maybe it splits the current and maybe they were trying to keep the mosfet from overheating, they had drilled holes in the board to add a heatsink that just crumbled away when touched so I replaced with a fan and its staying cool without the "jumper wire" and now everything is up and working great!Trying to understand what the wire in parallel was doing exactly....

author: drdm
author_rank: Contributor
timestamp: October 20, 2021, 07:29:00 am
content: The track is probably to thin and too long to carry all the current trough. This will cause not only heating, but also excessive voltage drop and the pin it is supposed to power may not get enough voltage.There is also a possibility that the track was damaged.

author: Ober EE
author_rank: Newbie
timestamp: October 20, 2021, 11:18:32 am
content: So am I understanding correctly that it basically does something similar to a voltage divider except only using the resistance of the trace and wire? This also would answer why I was getting only 12v where I was wanting 25v at one junction later on in the amp too. I appreciate the information!

author: drdm
author_rank: Contributor
timestamp: October 20, 2021, 11:43:21 am
content: No, it's not a divider. It is a bypass of the track.If you can share some pictures, maybe I can give you a more detailed answer.

author: Ober EE
author_rank: Newbie
timestamp: October 20, 2021, 12:54:13 pm
content: Unfortunately it looks like I did not take any pictures of this device, I will make sure to do so next time I am curious. Now that its repaired its back in the owners hands.But if it was a bypass, would the mosfet be putting out anything? The mosfet in that path was also giving 12v when it should have been giving 25v. I did have to change the mosfet and some other parts down the path in the mean time to get full functionality back to a chip in the phono preamp section that was neededing 20v+ to run and was only getting that 12v until we fixed that and then the phono roared back to life...while cassette died for a bit and that turned out to most likely be related to dirty record switch. My goodness I wish I had a better source to ask in person. Glad to have started up on this board, hopefully I can get some things figured out here!Thanks for your help!

author: Capernicus
author_rank: Frequent Contributor
timestamp: October 20, 2021, 01:06:11 pm
content: It could be a divider, but that would cause a bypass as well, could be doing both. the load that the short is bypassing is the "R2" and the load before the short, is the "R1" resistor. and the balance of the two causes the voltage reduction, 50/50 being exactly /2, what ur reading.