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Old 1st April 2014, 11:10 AM   #11
Mooly is offline Mooly  United Kingdom
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Originally Posted by jomor View Post
yes I did measure the voltages:

I dont have my notes with me (I am at work now) but if I am not mistaken Q14 has 3.8V (Q14 source to ground) while Q114 (the other channel) has 4 volts.

Can't remember Q12, I will let you know as soon as I return home. Thanks again for being helpful
No problem but remember its the voltage between those points thats important, not the voltage from each to ground.
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Old 1st April 2014, 11:12 AM   #12
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Originally Posted by jomor View Post
pressing the cap with the probe pushes the inflated skin back to flat so it could also be just a failed protective thin film cover
Hmm... OK, that does sound a bit strange but I wouldn't jump to conclusions just yet. I'll look at what it does later.
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Old 1st April 2014, 11:21 AM   #13
jomor is offline jomor  Greece
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Originally Posted by Mooly View Post
Vgs is a measure of how the D-S current varies with a change in G-S voltage. And I think that is where your problem lies, the circuit has a very limited adjustment range and its outside of the Vgs of the replacement devices.

I'll have another look at the circuit later today (its on a different PC) and see if anything can altered.
this enhances my intention to replace all mosfets with STP40NF10L, because now I have questionable origin and specs mosfets on the left channel and factory mosfets on the right channel.
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Old 1st April 2014, 01:30 PM   #14
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Originally Posted by jomor View Post
this enhances my intention to replace all mosfets with STP40NF10L, because now I have questionable origin and specs mosfets on the left channel and factory mosfets on the right channel.
I can understand your thinking on that. I would still measure and confirm the voltage I mentioned though.

Remember there can be a wide tolerance on Vgs. An IRF240 (really common power device) has a min of 2 volts and a max of 4 volts quoted as thresholds in the data sheet. Maybe Creek used selected devices of a certain low Vgs.
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Old 1st April 2014, 02:43 PM   #15
jomor is offline jomor  Greece
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Vd_q14 - Vd_q12 = 52.8 volts

Vd_q12 = -49V (black probe to ground)
Vd_q14 = 3.8V

Vd_q114 - Vd_q112 = 54 volts (on the good channel)

if this makes any difference, Q12 & Q12a are zvp2120 on the destiny instead of zvp3310 mentioned on the 5350's schematic.

Last edited by jomor; 1st April 2014 at 02:56 PM.
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Old 1st April 2014, 05:58 PM   #16
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The only close equivalents I could find are STP40NF10L and I don't see another option but to replace them with these.

edit: According to this article, the circuit demands "logic level" mosfets which means low Vgs-th (correct me if I am wrong, I am just finding out what these mean) and STP40NF10L must be ok on this (min 1V max 2.5V while the 76639p has 1-3V ), are they?
You are right, output mosfets have to be logic level...one of the good candidate is Siemens/Infineon SPI47N10.
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Old 1st April 2014, 06:37 PM   #17
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Quote:
Originally Posted by jomor View Post
Vd_q14 - Vd_q12 = 52.8 volts

Vd_q12 = -49V (black probe to ground)
Vd_q14 = 3.8V

Vd_q114 - Vd_q112 = 54 volts (on the good channel)

if this makes any difference, Q12 & Q12a are zvp2120 on the destiny instead of zvp3310 mentioned on the 5350's schematic.
Easy one first. C20 is just a rail bypass. If the 15 volts is present and correct across it then its going to be OK.

Your voltage readings. It was this voltage I was after. If getting FET's with the correct Vgs range is an issue then it still might be possible to alter the circuitry to suit but it would have to be done carefully and with a bulb tester in place to limit current. Adding another diode in series with D8 (circled) would add another 0.65 or so to the Vgs range available.
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Old 1st April 2014, 06:57 PM   #18
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yes the voltage in your drawing is 52.8 volts

and I do have 15volts across C20 so I guess its ok.

I have ordered the STP40NF10L and let you know about the result

Last edited by jomor; 1st April 2014 at 07:04 PM.
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Old 1st April 2014, 07:11 PM   #19
Mooly is offline Mooly  United Kingdom
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Originally Posted by jomor View Post
yes the voltage in your drawing is 52.8 volts
OK... sorry that's my fault, its been one of those days lol I'm losing the plot a little bit with this one, forgetting its a quasi complementary stage... its such a strange circuit to visualise.

I still think the problem is down to the different Vgs characteristic of the replacements. Do you think you can get better devices than you are using now ? If not then it still might be possible to juggle this around a little to achieve the desired result with what you have but its all an unknown and would require care and a lot of thought.
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Old 1st April 2014, 07:41 PM   #20
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I have ordered STP40NF10L which are the closest equivalents to the factory mosfets (HUFA76639P3) from Mouser.com and I hope I will have them in my hands in a few days.

I would prefer to try a solution like this first, because i will have both channels identical, if this doesn't work as expected then I will have to try your modification advice

Another option would be to risk again and buy HUFA76639P3 from ebay but I doubt if they are originals.
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