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An illustrated guide to building an F5
An illustrated guide to building an F5
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Old 25th September 2018, 08:40 PM   #1011
Corpius is offline Corpius  Netherlands
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Default Short circuit to ground

Hi,

I haven't been on this forum for some years now, but a recent problem with my F5 brought me back here. I've been happily using the amplifier 5 years until it recently stopped working directly after I turned it on. I checked the amp for burn marks or other visible damage to the components, but I couldn't find any. It turned out that the fuse was blown, so I swapped it for a new one and powered it back up while still having the lid off.

This time I heard a hiss and saw smoke coming from the thermistor TH1 between the PSU and ground. So there must be a short circuit to gnd, but I can't find it... or I don't know how to find it.

So I'm hoping that somebody could guide me in the right direction to find the error. Any ideas on to what could cause this sudden short circuit?

Thanks!
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Old 25th September 2018, 09:25 PM   #1012
Corpius is offline Corpius  Netherlands
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I did some more measurements with my DDM and I might be on to something.
One of the rectifier board short circuits the pin 1 and 3 of all rectifiers on the board to the chassis. Meaning that it short circuits one of the outputs of the transformer to the chassis. The other board with rectifiers seems still fine. I wonder how this could happen?

The chassis acts as a heatsink for the rectifiers, so perhaps it insulation got damaged somehow???
Tomorrow I will remove the rectifier board from the chassis and measure it again, but first I need some sleep.
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Old 25th September 2018, 09:29 PM   #1013
Zen Mod is offline Zen Mod  Serbia
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An illustrated guide to building an F5
care to post some pictures?
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Old 26th September 2018, 12:49 PM   #1014
Corpius is offline Corpius  Netherlands
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Sure. I'm at work right now, so it's an old picture, but nothing changed.

The red arrows point to the soldering pads that connect directly to the chassis.
Attached Images
File Type: jpg IMG_9514.jpg (905.8 KB, 202 views)
File Type: jpg IMG_9514_close_up.jpg (120.4 KB, 200 views)

Last edited by Corpius; 26th September 2018 at 12:52 PM.
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Old 26th September 2018, 02:42 PM   #1015
Zen Mod is offline Zen Mod  Serbia
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An illustrated guide to building an F5
desolder bloody thingie (meaning - rectifier board(s) from rest of circuit and check them solo

then you'll know for sure where shortie to case is

if diode is dodo , just replace it , don't fret with cause

I see they're big , but indulge me - write exact type here , so we can think about possible current (doubt that, due to size) inability or - more likely - sissy voltage

if both are 'enough' ...... blame Gremlins

or me - Papa seems finding that lately as most convenient explanation , for 42 and more

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Old 26th September 2018, 04:50 PM   #1016
Dennis Hui is offline Dennis Hui  Canada
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An illustrated guide to building an F5
I recall a member having a smoking thermistor caused by a short in a power
supply cap. If no other issues present themselves perhaps this can be investigated?
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Old 27th September 2018, 06:44 PM   #1017
Corpius is offline Corpius  Netherlands
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Quote:
Originally Posted by Zen Mod View Post
desolder bloody thingie (meaning - rectifier board(s) from rest of circuit and check them solo

then you'll know for sure where shortie to case is

if diode is dodo , just replace it , don't fret with cause

I see they're big , but indulge me - write exact type here , so we can think about possible current (doubt that, due to size) inability or - more likely - sissy voltage

if both are 'enough' ...... blame Gremlins

or me - Papa seems finding that lately as most convenient explanation , for 42 and more

I will desolder the rectifier board this weekend and check it.
The diodes are FEP30DP-E3/45

Quote:
Originally Posted by Dennis Hui View Post
I recall a member having a smoking thermistor caused by a short in a power
supply cap. If no other issues present themselves perhaps this can be investigated?
Thanks for the tip! I'll look into that if needed. What steps do I need to take to check for a faulty pwr supply cap?
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Old 27th September 2018, 06:50 PM   #1018
Rewind is offline Rewind  Sweden
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I remember the components costing like €150. You will be fine.

Btw, how do you test a diode?
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Old 27th September 2018, 07:04 PM   #1019
mrdave45 is offline mrdave45  United Kingdom
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I think that if your diodes have failed that may well be the short to ground.
If your psu caps have shorted, a continuity test with a multimeter may reveal that. Although, if its failed, with the amount of current going through it I would expect something visable on one of the capacitors.
Youre safety earth wiring looks suspect.
Are you sure that the earth tag on the iec inlet is directly connected to where you attached the safety earth in the back chassis plate? As far as I'm aware the safety earth should go from the tag directly to the chassis. This point on the chassis would also form your star ground point. The ground going from psu board through the thermistor should then be fastened to that star ground bolt.
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Old 27th September 2018, 07:12 PM   #1020
Corpius is offline Corpius  Netherlands
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Quote:
Originally Posted by mrdave45 View Post
I think that if your diodes have failed that may well be the short to ground.
If your psu caps have shorted, a continuity test with a multimeter may reveal that. Although, if its failed, with the amount of current going through it I would expect something visable on one of the capacitors.
Youre safety earth wiring looks suspect.
Are you sure that the earth tag on the iec inlet is directly connected to where you attached the safety earth in the back chassis plate? As far as I'm aware the safety earth should go from the tag directly to the chassis. This point on the chassis would also form your star ground point. The ground going from psu board through the thermistor should then be fastened to that star ground bolt.
Thanks.
The earth tag from the iec inlet is directly connected to the perforated bottom plate, but you can't see it in the picture. It's just behind one of the psu caps. The back plate is connected to the bottom plate with a wire, because it's coated and it would otherwise not connect to the rest of the chassis
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