Disaster NEED HELP PLS!

[leander]

Disaster

Hi people, I am in urgent need of help before going mad!
This is the amp I’ve just made and it took me a whole week to do it and managed to make it working perfectly.
( http://www.ampslab.com/c300.htm )
I wasn’t so lucky the second time I hooked it up because as pumping up some music, the center tap wire (ground of amp) went loose and the thing just freaked out!
I switched of immediately and rehooked the ground properly, however only a loud buzzing sound was heard! A disaster!
Checked out the components, no components were carbonized.
Checked out two power transistors any hfe resulted to be fine (aprox150)
As a start I’m thinking of replacing all small power bjt transistors.
I’m not an expert in troubleshooting, PLS what is your opinion about the problem


Here below is a schematic of the amp with grounds illustrated with red boxes

[Jan Dupont]

Check the Vbe (Voltage between base and emitter) on all transistors like this:

Connect the input to gnd.
Do not connect any load.
Watch out for components that get hot.

Power up the amp, and meassure the Vbe's.
They should ALL meassure app. 0.6 Volt +/-50mV

Good luck

[dtrif]

may be damaged (it has happened to me). Your desicion to replace small transistors is quite reasonable, as they are more commonly damaged in such situations. In a situation like this, where I couldn't find a solution, I simulated the whole thing, found voltages in different places in the simulator, and compared them with real life. Wrong voltages existed around a small signal transistor (previously measured with a normal hfe). When I replaced it everything was ok.
I am not a repairer professionally, so this approach may be wrong. It's just what I did. Anyone else with more experience please contribute. It may be even a passive component (capacitor perhaps), which would be very unfortunate, because I thing it will be much more difficult to find it.

[jaycee]

Check Q5, Q6, Q8. Also check your filter capacitors arent damaged.

[janneman]

Did you check that your supply voltages are normal? Fuses?

Jan Didden

[Ultima Thule]

Hi leander,

just of curiosity to your amplifier, I may be a bit tired to think clearly as it's 2 a clock in the night here, but what is the idea behind the VAS formation around Q 9 and Q 10?

Cheers Michael

[Tim__x]

At first glance, Q9 appears to speed up turn-off of Q10. Though perhaps there is more to it than that.

[darkfenriz]

With mirrored LTP and EF buffer at clipping Q10 needs such limiting. Otherwise would saturate very deeply and potential on its base would be close to zener voltage of D1. I'm not sure if I'm right but it definitely looks to do nothing in 'linear' range.

[jaycee]

Q8 and Q10 form an EF VAS. Q9 is a safety cutoff, when the VI limiter activates it can result in Q10 being saturated. Q9 detects this and cuts the base current.

Leach explains this for his own amplifer here

[powerbecker]

Originally posted by jaycee
Q8 and Q10 form an EF VAS. Q9 is a safety cutoff, when the VI limiter activates it can result in Q10 being saturated. Q9 detects this and cuts the base current.

Leach explains this for his own amplifer here

Yes and no :collector of Q9 must go to base of Q8, R18 should be zero!