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Old 19th Sep 2021, 1:18 pm   #1
The Philpott
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Default Farnell DMM131 PSU drop..

Probably shouldn't have left it on all night, but anyway... LED display was blank this morning, which isn't surprising since the PSU has fallen from it's 5.00v to 0.75v.

I have narrowed it down to 5 components, C1,C2,C3,Z1 and IC1. To get proper readings off the caps i will need to desolder the board, but C1 and C2 seem to be good insofar as a basic Uni-T DMM can tell. I am looking with suspicion at IC1-TDA1405.

Does anyone have a better inkling of which component has shut down?

Dave
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Old 19th Sep 2021, 1:50 pm   #2
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Default Re: Farnell DMM131 PSU drop..

Ah- there's certainly a corroded solder pad at one end of the zener.
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Old 19th Sep 2021, 2:15 pm   #3
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Default Re: Farnell DMM131 PSU drop..

What diagnostics have you done?

need a bit more information before narrowing it down from just about any component in circuit!!

Are you sure the load on the PSU is not pulling the volts down?
What voltage on input to IC1?
What volts on either side of resistor?
If you pull the fuse, what voltage on live side of fuse?


Output is floating, so make sure you are using correct point for 0V lead on your multimeter. (0V not Earth)

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Originally Posted by The Philpott View Post
Ah- there's certainly a corroded solder pad at one end of the zener.
Zener is over voltage protection, so corroded pad will not affect its influence on circuit. Only if zener is short circuit will it be a problem.
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Old 19th Sep 2021, 5:04 pm   #4
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Default Re: Farnell DMM131 PSU drop..

Thanks Peter. I will desolder the '5v' output at the next opportunity and see if there (was) an abnormal load on it.
Dave
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Old 20th Sep 2021, 2:17 pm   #5
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Default Re: Farnell DMM131 PSU drop..

With + output lead disconnected the PSU output does indeed rise to 5v. Might have known it wouldn't be simple.

Prior to this failure there were isolated occasions when the display failed to initially illuminate on moving the selector from 'off' to 'volts', and on one occasion mains voltage was erroneously shown as 10% of the genuine value. Resistance readings from, say, 10k upwards have always cycled up and down slightly without settling. I was advised in the past this was unlikely to have a cure- something to do with thick film component/s..?

Dave
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Old 20th Sep 2021, 3:14 pm   #6
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Default Re: Farnell DMM131 PSU drop..

Check C1 (by substitution or even paralleling) - if it`s gone high ESR or open the supply will have little output.
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Old 20th Sep 2021, 6:04 pm   #7
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Default Re: Farnell DMM131 PSU drop..

Solved- the assortment i bought a year ago had a 2200uF 50v 85C in it, which in parallel with the WIMA (16v) does the job. Although it's pretty much the same size unfortunately it's radial so i'll see if i can grab a 2200uF 25v 85C or 105C axial. Thanks for the guidance!
I am seeing 'forever' 'Nover' and 'Unicon' on ebay..
Dave

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Old 1st Oct 2021, 6:03 pm   #8
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Default Re: Farnell DMM131 PSU drop..

- A fiddly job, having extracted the PSU board for attention,whichever wire is soldered back to it's post first then obstructs iron access to the adjacent one- and so on..

Axial cap. of 25v fitted into the space vacated by the 16v. All good, now the interesting thing is that the fault on the resistance range mentioned in Post No.5 has improved massively- it can measure 1.4 meg without drifting or hunting- but it can't resolve 5.1 meg. Somewhere between those values must be the point at which it becomes unreliable.

The tracks had corroded on the PSU board, this could be down to fumes from the faulty cap but there is evidence of having been kept in a damp environment elsewhere in the meter, so this could have accelerated it..? One (quite wide) track had to be bridged having rotted all the way across.

Dave
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