Hi Gary , the new side trim pots arrived today, I’ve fitted just the one on each board ( P1 ) as I think it’ll be easier to access P2 with the regular trim pot .
OK, see how you go, adjust the new P1 DC offset trimpot to mid travel as you had done previously. P2 at max resistance to start with.
The amp is now finished and ready for powering up .
I’ve just got to gain the courage to actually go through all of the procedures, it’s quite daunting tbh .
I’ve just got to gain the courage to actually go through all of the procedures, it’s quite daunting tbh .
Take your time, and don't rush, follow the steps and stop if a problem arises.
Perhaps insert some power resistors, 10 ohm or so, in series with psu supply, makes it easy to observe current draw also
I’m going for it gents ! ( one channel only of course)
All is well so far
Voltages are all equal on the caps as I increase the voltage.
I’m now at the final 240v on the variac and into the amp ….
All is well so far
Voltages are all equal on the caps as I increase the voltage.
I’m now at the final 240v on the variac and into the amp ….
I’ve noticed some heat coming from the 2 transistors on the board ( attached to their heatsinks) ! Should I stop ?
Had you inserted preliminary resistors, could you check current draw on each rail..
I wouldn’t have a clue how to do that , I’ve followed instructions given and all seems well but I don’t know if those transistors should give off heat at this point
Do you have a meter to put in your various? You know the kind you use to check how much your electrical appliances use. This could also tell you a bit about current draw
First start up on every amp , bias pot needs to be turned all the way down, and gradually raised while observing voltage over emitters
No I don’t have one of those mate .
I set both pots at their required level before switching on , I was about to to adjust the P2 pot which is currently at maximum resistance
Q6 and Q7 on your new front end boards? Dissipation looks to be ~1.5W on each of those (.020 amps X 73 volts). Will get warm but shouldn't be unreasonable.
I’m basically following this ….
Thoughts on biasing:
1) I would bring things up on a variac, confirming that the bias is set to the minimum. Do say, 20% of line voltage and look over things. Then try 40%, 70% looking for smoke or the transformer to be buzzing like crazy... Use a multimeter to check the AC voltage as you bring it up and another multimeter to check across a emitter resistor to confirm the bias is set to 0
Andy, you probably don’t have a variac, so we will need to just apply normal power and see if all is stable, check DC rails for +/- voltage first up, 4 caps to the star point ground – you will get 2 pos and 2 neg voltage readings. All should be very similar.
Once the amp doesn't release any smoke and is at line voltage, turn the bias up enough to turn the transistors on, maybe 20mv across the resistor. Then you can check the DC offset to confirm that it is not above say 200mv.
After that check that each output transistor is pulling the same current within say 10%. I suspect the matching on these old amps are not as precise. You can do this by checking the emitter resistor on each mosfet one at a time. It is not a bad idea to write your results down.
Once that is done, move the bias to a higher level. Keep the multimeter hooked up to a emitter resistor, set the top on and let the amp warm up while watching how the bias tracks with temperature. I tend to keep another multimeter checking DC offset while I am doing this to confirm that things are behaving.
Keep raising the bias according to temp while checking the DC offset from time to time to confirm that everything is behaving.
Adjust the the bias up or down by maybe in 10mv intervals every 15 minutes to start while the temp is far from the goal. Then adjust the intervals to 5mv every 20 minutes once the amp gets within 5C of your 50C goal.
At this point, you can turn the amp off and let it get to ambient to see what the starting bias is when cold.
Biasing info would help a lot. what quiescent current (mV across the 1R emitter resistors did you settle on). NP says to adjust till the heatsinks settle at 50 deg C (but at what ambient temp is this for)?
I have a S/500 so the biasing should be different. Stock, the amp had around 70mv across the 1R resistors.
Keep a close eye on the bias as it warms up. Yes, adjusting until the heatsinks are at 50C is the way to go. The mV of bias will mostly depend on the amount of heatsinking and rail voltage the Stasis 2 has.
I think starting at say 35mv and letting it warm up to see where the temperature ends up is a good starting point. Then increase it by 5 or so mV at a time until you get to 50C. If you are further from your goal, it probably only needs like 15 minutes between adjustments. Once you get within say 5C of your goal, let it rest for a good 30 minutes with the cover on between adjustments.
Thoughts on biasing:
1) I would bring things up on a variac, confirming that the bias is set to the minimum. Do say, 20% of line voltage and look over things. Then try 40%, 70% looking for smoke or the transformer to be buzzing like crazy... Use a multimeter to check the AC voltage as you bring it up and another multimeter to check across a emitter resistor to confirm the bias is set to 0
Andy, you probably don’t have a variac, so we will need to just apply normal power and see if all is stable, check DC rails for +/- voltage first up, 4 caps to the star point ground – you will get 2 pos and 2 neg voltage readings. All should be very similar.
Once the amp doesn't release any smoke and is at line voltage, turn the bias up enough to turn the transistors on, maybe 20mv across the resistor. Then you can check the DC offset to confirm that it is not above say 200mv.
After that check that each output transistor is pulling the same current within say 10%. I suspect the matching on these old amps are not as precise. You can do this by checking the emitter resistor on each mosfet one at a time. It is not a bad idea to write your results down.
Once that is done, move the bias to a higher level. Keep the multimeter hooked up to a emitter resistor, set the top on and let the amp warm up while watching how the bias tracks with temperature. I tend to keep another multimeter checking DC offset while I am doing this to confirm that things are behaving.
Keep raising the bias according to temp while checking the DC offset from time to time to confirm that everything is behaving.
Adjust the the bias up or down by maybe in 10mv intervals every 15 minutes to start while the temp is far from the goal. Then adjust the intervals to 5mv every 20 minutes once the amp gets within 5C of your 50C goal.
At this point, you can turn the amp off and let it get to ambient to see what the starting bias is when cold.
Biasing info would help a lot. what quiescent current (mV across the 1R emitter resistors did you settle on). NP says to adjust till the heatsinks settle at 50 deg C (but at what ambient temp is this for)?
I have a S/500 so the biasing should be different. Stock, the amp had around 70mv across the 1R resistors.
Keep a close eye on the bias as it warms up. Yes, adjusting until the heatsinks are at 50C is the way to go. The mV of bias will mostly depend on the amount of heatsinking and rail voltage the Stasis 2 has.
I think starting at say 35mv and letting it warm up to see where the temperature ends up is a good starting point. Then increase it by 5 or so mV at a time until you get to 50C. If you are further from your goal, it probably only needs like 15 minutes between adjustments. Once you get within say 5C of your goal, let it rest for a good 30 minutes with the cover on between adjustments.
I think they have settled now . I’m now supposed to reduce the resistance from the P2 trim pot but when I turn the pot nothing happens 🤷♂️
I earned the name 20 years ago when working in construction.
Everything was going so well 😩