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Old 23rd February 2010, 06:35 PM   #7161
alazira is offline alazira  United States
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Quote:
Originally Posted by acold7dusta View Post
Click the image to open in full size.

here is the affected area. ignore the red wire, as its the V- from V- confusing i know

Click the image to open in full size.

here is my setup if it helps any. Thanks
Hi,

In your photo the solder of R4 & R14 seem to be very close...

Oh, actually that won't make a difference if they were bridged

Last edited by alazira; 23rd February 2010 at 06:38 PM. Reason: jumping the gun
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Old 23rd February 2010, 06:42 PM   #7162
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by alazira View Post
Hi,

In your photo the solder of R4 & R14 seem to be very close...

Oh, actually that won't make a difference if they were bridged

Yeah, according to schematic they are supposed to be connected

R12 tho appears to have been overheated, and may be damaged
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Old 23rd February 2010, 08:13 PM   #7163
ichiban is offline ichiban  United States
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Default acold7dusta

Hi acold7dusta,
Just a suggestion but you may want to mount the 3 watt resistors about a quarter inch off the boards when you replace the burnt ones.
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Old 23rd February 2010, 08:53 PM   #7164
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Dunno, but 3 watts seems marginal for this part.
Double the value and parallel two.
Two 1 ohm are close enough. You can add a "trim" resistor if you need to pull the value down some... don't forget that unless you have 1% values, that the actual value of two 1 ohm in parallel might be the same as a 0.47 depending on tolerances...

Permitting air to circulate freely around the resistor(s) is required for them to meet their full power specs... you'll get a very hot spot between the board and the resistor (any resistor, any board) if they are mounted flush. I space them up using the end of a tweezer as the spacing guide. A little more is ok too.

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Old 23rd February 2010, 10:15 PM   #7165
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Quote:
Originally Posted by tinitus View Post
Yeah, according to schematic they are supposed to be connected

R12 tho appears to have been overheated, and may be damaged
I don't know if you saw my saga, but R12 is where smoke was pouring out of. I have since placed an order for extra 3W resistors, as well as a new 5k trimpot to replace the damaged one (mounted it backwards in the beginning and had to flip it and resolder it.)

Ichiban & bear, thanks for the advice, I wasn't aware of the hot spot issues. If I popped them up 1/16 would this be enough? I already clipped the leads and don't want to sacrifice the joint. I guess I was going by the philosophy that flushed components sound better than raised components (I've heard it multiple places, especially with caps (from vibration?) don't know if it's true though)
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Old 24th February 2010, 12:39 AM   #7166
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Quote:
Originally Posted by bear View Post
Dunno, but 3 watts seems marginal for this part.
It's adequate IF the amp isn't used in a laboratory, or some other industrial type environment, and it's the item specified in the AX article.
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Old 24th February 2010, 02:21 AM   #7167
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jackinnj, seems like a whole lot of folks have smoked these resistors due to operator error... so IF the mosfet survived, either they are good fuses or else they are maybe on the edge...

If I wasn't lazy and unable to calculate accurately in my head (what is in my head?) I'd figure the peak current through them and so the peak watts... if it was within 50% of the resistor's rating, I'd double it up myself... Ymmv.

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Old 24th February 2010, 11:37 AM   #7168
flg is offline flg  United States
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Many power resistors have a rating that is based on destruction over time, in addition to max voltage etc. Don't really know the details but, I don't like the idea of something inside my amp hot enough to start a fire or burn something else The resistor might survive 150C but that is to hot for me. On Source R type things this typically happens. I tend to use a derating more like 20%. That is, 5X the necessary power capability or more. Acttually I like the heat radiating area that this will provide. Parallel Rs ussually is better than 1 big one in this regard. Or, Rs designed with heat sinking properties like the Vishay Dale RH/NH aluminum cased, gold thingies with 2 mounting ears.
Also note, most film resistors go over the edge easily. If you heat them to much they burn the resistive film and change their properties forever.
just my 2 cents
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Old 24th February 2010, 12:26 PM   #7169
sekess is offline sekess  United States
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Default power supply question

Using the standard F5 power supply as an example --

1. Does anyone know how to calculate (or at least approximate) how long it will take for the filter caps reach full charge upon start-up?

2. Now, if you used a resistor w/ relay (instead of a thermistor) for soft start duties on the primary of the tramsformer (say a 20 ohm resistor for example) -- does anyone know how to approximate or calculate the time it would take for the filter caps to reach full charge after start-up?

Thanks,
Steve
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Old 24th February 2010, 12:55 PM   #7170
AndrewT is offline AndrewT  Scotland
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1F charges up 1V if we apply 1A for 1second
dv=di/dt / C
but dv and di change with time and state of charge.
If V is at the start (no charge), then after RC seconds the charge will have risen by ~70% of the starting V. R is what limits the starting current.

All done and charged in a few tenths of a second.

BTW,
the resistor in the soft start (primary) is there to limit the start up current while the transformer is pulling up it's socks. It is not a transformer until it is dressed.
This resistor only needs to be in line for a few cycles of mains frequency. It should not be used to slow charge the capacitors.

Use a current limit in the secondary circuit to slow charge the capacitors.
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Last edited by AndrewT; 24th February 2010 at 12:58 PM.
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