Thank you for sugestions, I'll follow this.
Now I remember that the first time I set to adjust the bias I had the lightbulb-mains cable which was lit once it started to draw current (biased up on the left channel first) and I noticed the + and - out of the PSU went down to as much as 10V when biasing the left and to 5V when biasing both! I thought the lightbulb-mains cable was not the case to connect the amplifier to mains when setting the bias. When I replaced it with "normal" mains cable everything set up nicely (+/- 23V constant at PSU outs and bias values as I mentioned before and no smoke whatsoever).
Was the light-bulb behaviour (i.e. lighting up) normal when biasing up due to current drawn in the circuit?
Just checked the drains (mid pin and front metal tab at lateral notches ) to case continuity and ... to my surprise left channel (-the faulty one) seems OK (i.e. infinite MΩ) while right channel is in contact (see photos).
As to the torque, I checked tightness - it seems tight enough and I don't want to overdo it. I loosened the torque and checked continuity again - nothing changed. I'll have to undo the mosfets and check mica and double layer of thermal coating underneath.
Any more suggestions?
Attachments
ZM - what do you advise as a sensible torque value?
[being heavy-handed I invested in a small torque lever many years ago! It save many a disaster with alloy engines!!]
Interesting question. I have a torque screwdriver but didn't actually use it. So just now I tested them starting at 5 in-lb and increasing 1 in-lb at a time. It seems like they were at about 10 in-lb. The guide that comes with the torque screwdriver says 12-14 in-lb for a 4-40 steal screw into aluminum. That doesn't actually answer the question of how much torque to put on the mosfet case but that's where mine are at.
Vishay Siliconix Torque Recommendations for TO-220 Devices
"Breaking Torque For The Assembly
This value was obtained last with a destructive test. The screw
was tightened with gradually increasing torque. The M3 screw
broke at around 26 in-lb to 27 in-lb.
CONCLUSION
Typically 15 in-lb torque is adequate for fastening a TO-220
device on the heat sink and obtains the best (lowest) possible
thermal resistance value. Use of a heat sink compound
improves the thermal resistance by almost 0.6 C/W, but
electrical isolation between part tab and heat sink increases
the thermal resistance of the interface by a factor of 3 to 6."
http://www.vishay.com/docs/72674/72674.pdf
"Breaking Torque For The Assembly
This value was obtained last with a destructive test. The screw
was tightened with gradually increasing torque. The M3 screw
broke at around 26 in-lb to 27 in-lb.
CONCLUSION
Typically 15 in-lb torque is adequate for fastening a TO-220
device on the heat sink and obtains the best (lowest) possible
thermal resistance value. Use of a heat sink compound
improves the thermal resistance by almost 0.6 C/W, but
electrical isolation between part tab and heat sink increases
the thermal resistance of the interface by a factor of 3 to 6."
http://www.vishay.com/docs/72674/72674.pdf
Any update on the last problem?
It's all gone quite, solution found or still struggling?
I am close to powering up a "vanilla build" and watching for guidance.
I am not happy with some resistance readings on Mosfets to ground last night but need to re-check this morning.
(9240 / 240 from Digikey)
It's all gone quite, solution found or still struggling?
I am close to powering up a "vanilla build" and watching for guidance.
I am not happy with some resistance readings on Mosfets to ground last night but need to re-check this morning.
(9240 / 240 from Digikey)
No, not yet. I tried doing a mosfet diode test I saw on the net but I think I have to desolder first the mosfets and then test. I saw that, apart from the R7 and R8, also the R13 (100Ω the gate resistance of the p-channel) also blew off. Any suggestions on this, I'm confused as what to check - should I go as back as the J-Fets?
Problem here.
First time power up with bulb protection, then without.
Powers supply all working well, approx 26v With German transformer from RS running ok everything cool.
Power applied to both amp boards (one at a time.) with bulb.
No magic smoke or pops and sparks. diodes light up.
The problem is that adjusting P1 P2 there is no change in voltage across R11 / 12.
Mosfets stay cold.
Mosfets are IRFP 9240 / 240 from Digi-key
ZTX 450 / 550
2sk170 / 2sj74 from the chap on here?
Polarities double checked. (I hope)
Any obvious places to start looking please?
First time power up with bulb protection, then without.
Powers supply all working well, approx 26v With German transformer from RS running ok everything cool.
Power applied to both amp boards (one at a time.) with bulb.
No magic smoke or pops and sparks. diodes light up.
The problem is that adjusting P1 P2 there is no change in voltage across R11 / 12.
Mosfets stay cold.
Mosfets are IRFP 9240 / 240 from Digi-key
ZTX 450 / 550
2sk170 / 2sj74 from the chap on here?
Polarities double checked. (I hope)
Any obvious places to start looking please?
