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Short in F5T
Short in F5T
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Old 31st March 2018, 01:18 PM   #1
dewdrop is offline dewdrop  Singapore
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Unhappy Short in F5T

Hi there,
I have built one channel of the F5T and tested with the main light bulb. Unfortunately, the bulb did not dim after start up and became brighter. I did test the PSU without load and there was no issue.

There does not seem to have any physical shorts on the boards. I will check again tomorrow.

I measured the resistance of the speaker output and it is 47.5K Ohms whereas my Arcam integrated amp was OL. Is this normal?

What else can I check?

TIA
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Old 31st March 2018, 01:25 PM   #2
6L6 is offline 6L6  United States
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Short in F5T
Make sure the bias pots are turned all down all the way. Measure ohms across R5 and turn P1 so those ohms read the minimum you can get it to

Do the same with R6/P2

Reconnect power and see if lightbulb does same thing.

That you tested PSU and fond no issue (an hopefully good voltage rails) is a good sign.
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Old 31st March 2018, 01:26 PM   #3
Zen Mod is online now Zen Mod  Serbia
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Short in F5T
for start - are trimpots in min position (shorted across pins ?)

if amp is biased even just 50% , bulb will not go dim m simply because amp is drawing current

so , condition for first start is , set trimpots to short , thus output mosfets not conducting

edit: toobhead was faster
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Old 31st March 2018, 01:46 PM   #4
dewdrop is offline dewdrop  Singapore
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Thanks guys. I did check P1 and P2 via TP1/TP2 and TP3/TP4 respectively. They are already at 1.2 ohms. Hmmm. This is frustrating.

What about the typical resistance at V+/V- relative to GND at the PSU? I assume it won't be at OL.
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Old 31st March 2018, 01:55 PM   #5
Zen Mod is online now Zen Mod  Serbia
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Short in F5T
dunno for that resistance ...... not overly informative , so I usually don't care for it , except in case of dead short

care to post some pictures ?

checked trice orientation of power rails , parts position , resistor values, mosfet/heatsink isolation ?
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Old 31st March 2018, 02:01 PM   #6
dewdrop is offline dewdrop  Singapore
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Quote:
Originally Posted by Zen Mod View Post
dunno for that resistance ...... not overly informative , so I usually don't care for it , except in case of dead short

care to post some pictures ?

checked trice orientation of power rails , parts position , resistor values, mosfet/heatsink isolation ?
Here you go. How do I know if the PSU has a dead short?20180331_215928.jpg20180331_215905.jpg20180331_215841.jpg20180331_215829.jpg20180331_215822.jpg
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Old 31st March 2018, 02:07 PM   #7
Zen Mod is online now Zen Mod  Serbia
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Short in F5T
sorry , didn't read properly your last post , thinking wrong that you asked ( again ) for impedance reading from output node to GND , as you already asked

anyway - no proper reading between rails and gnd ....... it goes from dead short , rising as your ohmmeter is filling PSU caps

not seeing much on your pictures ..... can you post proper ones , 1600xsomething pixels ?

checked what I wrote ?

thermistor legs not making shorts with washers ?
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Old 31st March 2018, 02:10 PM   #8
dewdrop is offline dewdrop  Singapore
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Originally Posted by Zen Mod View Post
sorry , didn't read properly your last post , thinking wrong that you asked ( again ) for impedance reading from output node to GND , as you already asked

anyway - no proper reading between rails and gnd ....... it goes from dead short , rising as your ohmmeter is filling PSU caps

not seeing much on your pictures ..... can you post proper ones , 1600xsomething pixels ?

checked what I wrote ?

thermistor legs not making shorts with washers ?
Sorry, I realized my photos did not come up nicely. Let me take again during the day.

Will check the thermistor legs too.
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Old 31st March 2018, 08:12 PM   #9
Gyuri is offline Gyuri  Hungary
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Are there enough space between PS wires connected to top of TB-218908 board and heatsink?
If not, then can be a short circuit to heatsink, as I can see:
Click the image to open in full size.
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Old 1st April 2018, 01:39 AM   #10
dewdrop is offline dewdrop  Singapore
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Originally Posted by Gyuri View Post
Are there enough space between PS wires connected to top of TB-218908 board and heatsink?
If not, then can be a short circuit to heatsink, as I can see:
Click the image to open in full size.
Hi Gyuri,
Thanks for the heads-up, I checked and the wires do not touch the heatsink.

Click the image to open in full size.
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Last edited by dewdrop; 1st April 2018 at 01:48 AM. Reason: fixed photo link
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