F5 Power up bias problem - help please! - diyAudio
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Old 11th May 2013, 10:43 PM   #1
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Default F5 Power up bias problem - help please!

Helping my friend power up his F5 today, we ran into an immediate problem.

He has Peter Daniels tiny boards, that I meticulously populated myself, with the full circuit, and the original resistor values from the earliest schematic publication.


The power supply was tested prior to power up, and was reading 26.2v & -26.1v, but once connected to the amp board, for power up, was reading 19.2v & -19.2v.


The Problem:

On power up, across R11 & R12 values were reading 1.8v DC on both channels. Both P1s and P2s were set until clicking fully anti-clockwise. We did not test the speaker outputs yet, as we only have 2 multimeters, and we already had a power down problem. Searching the boards etc, yielded nothing but scratched heads. Can anybody help?

Many thanks
Lucas
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Old 12th May 2013, 04:06 AM   #2
Zen Mod is offline Zen Mod  Serbia
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depending of orientation of trimpots , clockwise or anti-clockwise means nothing

they need to be shorted ( minimal value ) as starting position for power up

turn them fully to other side , confirm with ohm-meter are they in proper position

there are several tutorials for F5 powering up and setting procedure
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Old 12th May 2013, 04:55 AM   #3
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Thanks ZM.

I have several tutorials. They all make the presumption that on first power up, ideal conditions of 0.0v are achievable with trimpots at starting position. I didn't get further than that before powering down, with a tiny whiff of smoke. With 1.8v on both channels, I should be at bias of about 3.6A which is too high, and we have probably blown something already. The question is, what has blown, and why can't I power up with the bias down to the normal level? Any ideas?
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Old 12th May 2013, 09:30 AM   #4
AndrewT is offline AndrewT  Scotland
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When the trim pots are set correctly for first start up the F5 does not draw significant current.
In this state the amplifier perfectly suits start up using a mains bulb tester.
If the bulb goes off, then you know the circuit is not drawing excessive current.
You can now measure voltages around thew circuit and PSU.
Once you KNOW that everything is OK, power down and restart direct from the mains.
NOW and only now are you ready to start biasing the output stage of the F5.

However, if in that "test" state the bulb lights up you know that something is not right.
You can measure the PSU voltages. They will be very low, maybe around +-2Vdc to +-8Vdc.
You can measure currents around the circuit to find what components are drawing current. From this you can ask questions or work out what is set/wired incorrectly.

How many times does this need to be repeated?
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Old 12th May 2013, 09:52 AM   #5
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Quote:
Originally Posted by LucasAdamson View Post
Thanks ZM.

I have several tutorials. They all make the presumption that on first power up, ideal conditions of 0.0v are achievable with trimpots at starting position. I didn't get further than that before powering down, with a tiny whiff of smoke. With 1.8v on both channels, I should be at bias of about 3.6A which is too high, and we have probably blown something already. The question is, what has blown, and why can't I power up with the bias down to the normal level? Any ideas?
easiest - check Jfets and mosfets out of circuit , with adequate test

from where you got input Jfets ?
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Old 12th May 2013, 03:13 PM   #6
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Nice info thankyou
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Old 12th May 2013, 07:13 PM   #7
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Well, it seems with the standard orientation on DIYAudio boards (CViller) the correct starting position is fully anti-clockwise, BUT on Peter Daniel's boards, with trimmers fitted as per the printing on the PCB, the correct starting position is FULLY CLOCKWISE. Thanks for the comments and guidance. Problem SOLVED.
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Old 12th May 2013, 08:44 PM   #8
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Thank you all for your advice, I am most grateful as it was my amp we were building. Fingers crossed that the faint whiff of smoke was a warning, rather than a signal from the morgue.
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Old 14th May 2013, 11:41 AM   #9
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Update - with pots turned in opposite direction we have ZERO Mv at start up I wonder if you have any thoughts regarding which component might have produced the smoke and would you recommend changing the mosfets? I confess I have a nagging doubt that I may have reduced the SQ by briefly frying something?

Thank you once more.
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Old 14th May 2013, 09:40 PM   #10
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Usually its the resistors that you are measuring bias from...are they a little off color? If not fully roasted they are liable to still work fine. BUT...listen to what others here are saying, I was lucky on my fireups, and fortunately have a variac. I assume you didnt use variac or lightbulb, so when the full on bias hit it could have damaged the Mosfets and other parts, hopefully you shut down quick enough.

Russellc
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Last edited by Russellc; 14th May 2013 at 09:45 PM.
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