Replace burnt resistance - Page 1
Forum section: electronics / repair

author: Ghoghnoos
author_rank: Newbie
timestamp: May 04, 2022, 09:54:27 am
content: Hello. What is the way to replace the resistor that is burned and its amount is not clear?Of course, in the case where the circuit map is not available

author: max-bit
author_rank: Frequent Contributor
timestamp: May 04, 2022, 10:41:56 am
content: Hmm good questionTake a picture, what's the circuit, etc ...Maybe something will work ...

author: alm
author_rank: Super Contributor
timestamp: May 04, 2022, 10:57:37 am
content: I would measure the resistance out of circuit. Just because it looks burnt doesn't necessarily mean the value is (far) off.

author: Ghoghnoos
author_rank: Newbie
timestamp: May 04, 2022, 12:52:28 pm
content: Resistance of the R3 that burned.When I check it out of the circuit It is 1 kilo ohm.But I do not know why it does not work properly when it is in the circuit?

author: Ghoghnoos
author_rank: Newbie
timestamp: May 04, 2022, 12:56:26 pm
content: This is the ST-303 TRA multimeter board

author: Spectrum
author_rank: Regular Contributor
timestamp: May 04, 2022, 01:11:50 pm
content: is it a carbon one ? In this case I scratch the paint of the resistance gently, when I arrive at the resistive material I measure from one end up to the scratched point, then on the other end up to the point. If the scratched point is in the middle, I expect the value divided by two. It won't be super reliable, but you can get an idea

author: Ian.M
author_rank: Super Contributor
timestamp: May 04, 2022, 01:46:24 pm
content: Once there's soot on the board and case, all bets are off, and measuring the remains of the resistor probably wont help as odds are, much of the resistive element has been destroyed and redeposited as fine particulates in its general neibourhood!Find a schematic, either of the exact same model or or a similar one that uses the same value resistors with the same designators (i.e. electrically identical) or a tear-down photo of the same meter at a high enough resolution to read the resistor color codes.Otherwise it should be possible to trace enough of the circuit to work out the resistor's function, and by circuit analysis, and knowing all the other resistor values, get some idea of its value. Then patch in a resistor substitution box or variable resistor set to the estimated value and check how far off the calibration of affected ranges is. Adjust till all affected ranges are within calibration, then replace with a fixed resistor of the required value. If you cant get all the affected ranges in spec. with a single value of substitute resistor, its likely that there is another fault present.

author: metrologist
author_rank: Super Contributor
timestamp: May 04, 2022, 04:21:24 pm
content: Your multimeter is probably based on some common chip with reference schematics out there.This may not be it but look for your main chip or model number and search for schematics.https://www.eleccircuit.com/digital-multimeter-circuit-using-icl7107/

author: Ian.M
author_rank: Super Contributor
timestamp: May 04, 2022, 04:38:27 pm
content: Looks like an analog multimeter to me! You don't see many DMMs with a thumbwheel ohms zero pot and a single sided SRBP PCB, even at the 'free gift' bottom end of the market.

author: max-bit
author_rank: Frequent Contributor
timestamp: May 05, 2022, 09:27:12 am
content: I assume that this resistor is some kind of shunt in the field of current measurement.We will not find a scheme for this "miracle".Enter the current range on which this resistor was. Looking at the multimeter, I assume that it is in the range of 250mA.On the basis of the given parameters, I assume that the measuring system itself has about 3kOm of internal resistance

author: max-bit
author_rank: Frequent Contributor
timestamp: May 05, 2022, 09:31:48 am
content: if I combine well, this resistor for the 250mA range will have a resistance of 6 Ohm.Of course I assume it is for the 250mA range!