Yes. I think, you have checked them earlier - no shortages were observed.
Then, set the pots to the position, giving minimum voltage q3c-q5c (2.18V).
Connect the V-meter between pins 17 and 18.
Connect the big transistors' plugs - one channel for the beginning.
Switch it on, looking at the V-meter. If the voltage you see is higher than 200mV - better switch it off.
You need to set the voltage to, 70-80mV.
Be careful, as big transistors can produce high currents if something goes wrong - when adjusting the bias, turn the pot slowly and carefully.
Wait 5-10 minutes - the voltage will go down a little. Correct it carefully to see 70-80mV again.
If the first channel is set successfully - do the same with the second channel.
Don't hurry - setting the bias is a "sensitive" procedure 😉
Then, set the pots to the position, giving minimum voltage q3c-q5c (2.18V).
Connect the V-meter between pins 17 and 18.
Connect the big transistors' plugs - one channel for the beginning.
Switch it on, looking at the V-meter. If the voltage you see is higher than 200mV - better switch it off.
You need to set the voltage to, 70-80mV.
Be careful, as big transistors can produce high currents if something goes wrong - when adjusting the bias, turn the pot slowly and carefully.
Wait 5-10 minutes - the voltage will go down a little. Correct it carefully to see 70-80mV again.
If the first channel is set successfully - do the same with the second channel.
Don't hurry - setting the bias is a "sensitive" procedure 😉
Correct
OK, thanks for the heads up regarding high currents.
Pins 17,18 will be blocked by the conncetor, but I can get the same reading at R25,R26, if I read the schematic correctly...
I will be a little while, until I have mounted the heatsink/output transistors.
Yes, or - probably even better - you can measure right on the pins of the big cement resistors (R16-R17 or R18-R19) - just select the right pins (not the ones connected to each other, but the other sides)... you can use either pair of those two pairs.
Ahhh, left ch. has the old original q6 & q7 mounted, I just want to replace them with new ones.
After this I will do the tests again to control I'm OK to continue...
After this I will do the tests again to control I'm OK to continue...
Right, you can measure either of them - R16, R17 (pins 17 and 18) or R18 and R19 (pins 16 and 19). Those are just two pairs of the same kind of transistors, working in parallel, so the measurements are going to be roughly the same for either pair of them.
OK, slightly lower range. I hope it will be still enough for setting the bias we need...
OK, a good start 🙂.
First right channel - VR1 at max resistance => 1,4mV between 17 & 18.
Adjusting carefully to about 75mV, which is is approx. middle position on VR1.
Voltage is slowly increasing - now about 85 mV.
Observing with my hand on the power switch.
edit:
Feeling carefully on PSU q1, q4 - getting a little worm, like can't keep fingers on them more than 1-2 secs.
First right channel - VR1 at max resistance => 1,4mV between 17 & 18.
Adjusting carefully to about 75mV, which is is approx. middle position on VR1.
Voltage is slowly increasing - now about 85 mV.
Observing with my hand on the power switch.
edit:
Feeling carefully on PSU q1, q4 - getting a little worm, like can't keep fingers on them more than 1-2 secs.
Last edited:
Not bad!
It will grow a bit, and then goes down a bit as soon as the power transistors warm up.
It will grow a bit, and then goes down a bit as soon as the power transistors warm up.
The diodes (D5 - STV4H) - I expect them to be mounted on the same heatsink where the big transistors are. Am I right?
This is a temperature sensor, providing a thermal feedback and stabilizing the bias this way. Without the D5 on a heatsink your voltage (and though the bias) would grow to infinity (thermal runaway)...
Yes! D5 are mounted on both channels, but only right have output trans connected.
88mV again.
Yes, if you mean big heat sink. It's slightly luke at the active side.
89mV
A lot (or some) discussion about the D5 (STV4H) can be found on Audiokarma as to how to get by if the are broken. This can easily happen as when the wire snaps at the component end. Happened to me, but I managed to do a good repair.
89mV
A lot (or some) discussion about the D5 (STV4H) can be found on Audiokarma as to how to get by if the are broken. This can easily happen as when the wire snaps at the component end. Happened to me, but I managed to do a good repair.
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