Waiting to get hot for readjustment!
"In spite of the thermal compensation in the circuit, you should assume that there will be drift as the heat sink temperature
rises, and you will need to readjust the values over the course of an hour or two. Usually it is best to start out bias
adjustment low, at maybe 0.4 mV across R11 and R12 until the amp is warmed up a bit." f5 manual
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This is the next thing to try ! Now its work time!
But first thing is to make the second channel work, I have to make alot of tests starting from the pots!!
The problem:
There is no way to make stable the bias between the two resistors! When the pots are to zero position the bias to the one resistor is 0.25v and zero to the other!
In addition the feedback resistors get really hot in a second if I leave the channel on more time then I am going to lose more parts, maybe I should try some measures with the bulb tester!!!!!!
But first thing is to make the second channel work, I have to make alot of tests starting from the pots!!
The problem:
There is no way to make stable the bias between the two resistors! When the pots are to zero position the bias to the one resistor is 0.25v and zero to the other!
In addition the feedback resistors get really hot in a second if I leave the channel on more time then I am going to lose more parts, maybe I should try some measures with the bulb tester!!!!!!
Prooptiki, it sounds like one of the out put fets or its current limit transistor has been damaged possibly. do you have spares you cn drop in for the side of the amp which measures 0.25V at 0 Ohm bias setting.
my opinion is this amp is simple enough to measure every part / connection to find the faulty one/ mistake.
- you have one working channel to compare it too...
-Dan
my opinion is this amp is simple enough to measure every part / connection to find the faulty one/ mistake.
- you have one working channel to compare it too...
-Dan
The PCB layout looks like P2 is not configured like the F5 circuit so its minimum setting is not the same as P1 at max clockwise.
Thank you all for that information on adjustment during warm up.
I was referring to;
"I operate mildly higher - around 0.55V but 0.5 is also fine, I would think".
It is also suggested (in Papa's PDF) "After the amplifier has been operated for a few weeks, it is a good idea to check and adjust the offset again after the parts have been burned in"
I'm still figuring this amp out.....
Ron
I was referring to;
"I operate mildly higher - around 0.55V but 0.5 is also fine, I would think".
It is also suggested (in Papa's PDF) "After the amplifier has been operated for a few weeks, it is a good idea to check and adjust the offset again after the parts have been burned in"
I'm still figuring this amp out.....
Ron
Prooptiki
As I see it you should measure on the legs that connect to the paralel resistor
That would be R3/R4
But Im not sure it works properly when not mounted, where it would have two legs shorted
Its usually recommended to measure directly on R3/R4, to lowest value fore powerup
It might be a good idea to note/write down which way to turn the pot, or maybe even make an arrow on print board
I would let the arrow point in the direction of increased bias
This way you will never turn it the wrong way
If you are close to optimal, its very annoying to accidentally turn the wrong way
The pot is ok, the only difference on my pcb is the R16 opposite to TH2 (thermistor) is this a problem?
The second channel now is working!!!!
The problem was on Q4! Now everything measures ok and stable!
Some extra questions:
When I fire up the amplifier dc offset reach for a second 0.27v and then starts falling to 0.008v and after half a minute goes to zero Is this ok?
When I turn of the amplifier I get a dc offset near to 0.37v for about 3-4 seconds Is this also a problem?
Tommorow the listening test! Here is 4.00am
Thanks for your help
I got rid of this problem by removing the thermistors.
I had the same issue of a wildly drifting offset for the first few minutes till the amp warmed up and it was only solved by eliminating the thermistors from the circuit. The tradeoff is that I have to readjust the pots every few weeks as the weather changes.
I had the same issue of a wildly drifting offset for the first few minutes till the amp warmed up and it was only solved by eliminating the thermistors from the circuit. The tradeoff is that I have to readjust the pots every few weeks as the weather changes.
I don't think that's a problem.
The difference of the PCB and the F5 schematic is that on the PCB, P2 pads 3 & 2 should have been connected with the copper trace and not pads 2 & 1. Just remember on this board design(post #7209) that adjusting P2 is the opposite of P1. So if 0 ohm is read on P1 by going full clockwise, P2 will be 0 ohm fully anticlockwise.
Pay attention to the small circles (like bolts) inside the Rectangle
In addition I measured the resistance between the legs (the two shorted and the other) I believe this is the better way!
Yes, but this can be easily overlooked by a diyer as the trimpot has all three legs lined up and can be mounted either way.
Yes of course that's the best way. Measure twice and smoke once (with a cigarette away from your circuit board
