Picture 1
Picture 2
Here's the pics.
However i;ve been doing my own tests n tweaks.
I soldered the setup resistors back on, and did another reading of the bias/quiescent current.
On both sides it was now 40V!.
Im not sure whether this is because (i suspect) that q8-q13 have died. I get weird continuity on them.
Picture 2
Here's the pics.
However i;ve been doing my own tests n tweaks.
I soldered the setup resistors back on, and did another reading of the bias/quiescent current.
On both sides it was now 40V!.
Im not sure whether this is because (i suspect) that q8-q13 have died. I get weird continuity on them.
bah nvm the 40v comment. It's doing that because somethings dead, VR1 doesnt change the output at all.
mmmm.
also im getting something similar to this i think.
http://www.diyaudio.com/forums/show...68986&perpage=10&highlight=sc480&pagenumber=1
though mine didnt short onto the heatsink, and are attached nicely ...
mmmm.
also im getting something similar to this i think.
http://www.diyaudio.com/forums/show...68986&perpage=10&highlight=sc480&pagenumber=1
though mine didnt short onto the heatsink, and are attached nicely ...
I've got to say what an utterly rubbish and stupid way of getting you to set the bias current, a recipe for disaster if you are a novice not using their exact power supply recommendation.
Set the bias back to minimum by turning the bias pot VR1 so that the resistance measured across its outer pins is as low as possible.
Then find the fault causing excess output stage current to be drawn. Once all good, set the bias current by putting a volt meter across any of the 0.22 ohm resistors and turning the bias pot slowly until you read about 11 mV. This equates to 50 mA flowing through that output transistor.
Did you use the PTC in the output line? That is also a stupid rubbish idea. It will do nothing to protect the amp and make output linearity with temperature quite horrible. And your damping factor variable and poor.
Set the bias back to minimum by turning the bias pot VR1 so that the resistance measured across its outer pins is as low as possible.
Then find the fault causing excess output stage current to be drawn. Once all good, set the bias current by putting a volt meter across any of the 0.22 ohm resistors and turning the bias pot slowly until you read about 11 mV. This equates to 50 mA flowing through that output transistor.
Did you use the PTC in the output line? That is also a stupid rubbish idea. It will do nothing to protect the amp and make output linearity with temperature quite horrible. And your damping factor variable and poor.
okay should i do that with or without the resistors attached to the fuses?.
Im im not sure there was any fault, or at least none i can diagnose without replacing the output transistors( i will replace the bd139 mje340 and mje350 for good measure).
was one of the transistors a thermal vbe or something like that?, and needs to be attached to the heatsink?. Maybe that was the problem, that transistor not mounted correctly?.
Im im not sure there was any fault, or at least none i can diagnose without replacing the output transistors( i will replace the bd139 mje340 and mje350 for good measure).
was one of the transistors a thermal vbe or something like that?, and needs to be attached to the heatsink?. Maybe that was the problem, that transistor not mounted correctly?.
Q7 should be mounted to the heatsink to track the temperature of the output transistors. However, that is unlikely to be the cause of your problem.
My advice on setting the bias was with the fuses in place as a finished amp would be. As long as you set the bias pot to minimum as I said, before applying full power with the fuses, you will be fine.
My advice on setting the bias was with the fuses in place as a finished amp would be. As long as you set the bias pot to minimum as I said, before applying full power with the fuses, you will be fine.
okay just thought id make sure we are on the exact same wavelength.
ive been replacing the blown transistors q10-q13.
on removing them i've tested them with a jaycar transistor tester kit i bought today.
showed me that q11 and q10 were fine, but q13 and q12 were short circuits (confirmed with multimeter on continuity test).
I bought enough to replace them all for good measure (there may be other problems when in use that the kit wont detect). but if i blow up the amp again, ill have spares handy
.
now just to desolder and test q7-q9. (btw all these transistors passed in circuit on the original reverse polarity board, which leads me to believe maybe the electrolytics or some other cap karked it).
hopefully when i replace these transistors and reset the bias. (ill first boot up with the resistors, and tune it to minimum. Then power down, desolder, place fuses, power up, and do what richie says.
Im not as sad as i could be, those transistors cost about 15 aud, which is cheap if it fixes it in my eyes, i've grown to love this lil amp, and amp building.
Thanks for all your help, ill post with cheers and joy, or grumble and gloom in a few hours if it works or not XD
ive been replacing the blown transistors q10-q13.
on removing them i've tested them with a jaycar transistor tester kit i bought today.
showed me that q11 and q10 were fine, but q13 and q12 were short circuits (confirmed with multimeter on continuity test).
I bought enough to replace them all for good measure (there may be other problems when in use that the kit wont detect). but if i blow up the amp again, ill have spares handy
.now just to desolder and test q7-q9. (btw all these transistors passed in circuit on the original reverse polarity board, which leads me to believe maybe the electrolytics or some other cap karked it).
hopefully when i replace these transistors and reset the bias. (ill first boot up with the resistors, and tune it to minimum. Then power down, desolder, place fuses, power up, and do what richie says
.Im not as sad as i could be, those transistors cost about 15 aud, which is cheap if it fixes it in my eyes, i've grown to love this lil amp, and amp building
.Thanks for all your help, ill post with cheers and joy, or grumble and gloom in a few hours if it works or not XD
hmm ill do it in the morning. its a bit late.
Id just thought id ask, wheres the best place to apply thermal compound?
i bought a small 15g tube of silicon thermal compound and i thought id dab a little onto the transistors.
I was thinking the metal collector that gets pressed against the insulating pads?
Id just thought id ask, wheres the best place to apply thermal compound?
i bought a small 15g tube of silicon thermal compound and i thought id dab a little onto the transistors.
I was thinking the metal collector that gets pressed against the insulating pads?
okay ive got it running through a full range speaker. Louder than i would normally listen to.
Just kinda burning in/ testing now.
I set the bias richies way, ranges from 9.6-10.7 mv across all four resistors.
When its playing full bore at in this speaker, i get 30-40mv across each resistor?. (once saw about 55mv too)
Too much? just observing stuff as i go along, in case it runsaway again.
Currently the heatsink is slighty warmer than the transformer, but its only been playing for abuot 15 minutes.
Just kinda burning in/ testing now.
I set the bias richies way, ranges from 9.6-10.7 mv across all four resistors.
When its playing full bore at in this speaker, i get 30-40mv across each resistor?. (once saw about 55mv too)
Too much? just observing stuff as i go along, in case it runsaway again.
Currently the heatsink is slighty warmer than the transformer, but its only been playing for abuot 15 minutes.
SC480 woes
Bojo2
You mentioned previously that you hadn't mounted Q7 (VBE multiplier) on the heatsink. Have you NOW correctly mounted Q7,Q8 and Q9 to the heatsink using the insulators and mounting hardware supplied with the kit?
I partly disagree with Richie about the PTC in the output. It just may help save your speakers from burning out in the event of a fault.
However, after a suitable running in period, including careful checking of heatsink and O/P device temperatures after a period of continuous use, you are satisfied that the amplifier is working correctly, slightly improved performance will be obtained by bridging out the PTC, or removing, and replacing with a wire link.
SandyK
Bojo2
You mentioned previously that you hadn't mounted Q7 (VBE multiplier) on the heatsink. Have you NOW correctly mounted Q7,Q8 and Q9 to the heatsink using the insulators and mounting hardware supplied with the kit?
I partly disagree with Richie about the PTC in the output. It just may help save your speakers from burning out in the event of a fault.
However, after a suitable running in period, including careful checking of heatsink and O/P device temperatures after a period of continuous use, you are satisfied that the amplifier is working correctly, slightly improved performance will be obtained by bridging out the PTC, or removing, and replacing with a wire link.
SandyK
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