2sc5200 400 watt mono amp not working !!

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Hi everyone how you all fine !!
please i need help about my diy 2sc500 amplifier
few days ago i found a site where some stuff was available about 400watt mono amp, i downloaded that material nd start making it, i made pcb nd buy all the components required for this amp nd finally completed whole circuit
Tomorrow when i connect the transformer to the circuit (55-0-55)ac 12amp and +/-76 volt dc , after few second rectifyng diode became hot nd the wire connect to the transformer to rectifier became red nd broken .
i review all the copper tracks for miscontact but didnot found such an error. i desolder all the transistor including a1015,c2229,2sc5200,tip42 nd check with dmm the result is positive
i solder all transistor except power transistor i.e 2sc5200
and started testing the circuit with bulb testing method(connecting 220v bulb in series with primary of transformer)
when i power up the transformer the bulb did not glow (REMEMBER THERE IS NO POWER TRANSISTOR IN CIRCUIT)
now i solder all the negative cycle power transistor i.e power transistor driven by b688 , the bulb glow slighty that is the good sign
now after soldering all negitive pwr transistor i solder only one positive cycle pwr transistor i.e driven by d718 and the bulb glow extreme brightly
i desolder that transistor nd connect another transistor in place of it the result remain same
iam unable to daignose the problem in positive cycle section of circuit , the i tested the pwr transistor which i connect to the positive cycle section by dmm , the result which came is positive
(sorry for my bad english)


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I know this amp, but I didn't build it. You say the wire became red...Are you sure there are no shorts on the PCB and you are connecting the transformer wires correct ? The center wire on the PCB (right connector) is not GND (transformer 0V) GND is on the bottom of PCB (tap) Is the rectifier correctly placed ? + left top - right bottom

Check rectifier Transformer and caps connections first. Measure caps voltage without power transistors. The caps connect on the middle holes.

Don't rely on the red drawing, check correct polarization of components by the schematic on the left.
Sorry to say it but that schematic is full of 'wishful thinking' circuitry, simply don't bother and avoid it. Yes i know it's Soooo tempting becasue 'it's simple and cheap' - which is also why it's not very good even when it works (and it will not because that TIP42 can't stand the voltage). Desolder your parts, salvage what still works and build a proper design.
Also, you really need to work on your method of testing. 'The bulb glows slightly' is NOT a good sign for this amplifier, it has no bias current in the output stage and should not have any difference in current consumption with or without the output stage. Except that without the output stage the circuit does not operate as an amplifier so that way of testing says nothing really useful.
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That kinda differential VAS with Q3/Q4 is interesting, how exactly would it work?

Underbiased quasicomp output stage (since there is no thermal compensation)... I bet distortion figures will be pretty abysmal. This one is strictly PA-only.

2SA1015 is only rated at 50 V breakdown, there is no way it can safely be used in an amp with +/- 75 V rails. You need half supply plus a bit at the very least, so 80 V would be the very minimum here (e.g. BC556 - careful, EBC pinout!). And that's not even taking fault conditions into account. One might use 2SA970 or 2SA992 here instead, or alternatively employ a cascoded input stage like many high-power amps tend to do.

And yes, that TIP42 would have to be a TIP42C at the very least (rated at 140 V vs. 80 V), and even then its voltage rating would remain pretty tight.
TIP42C is rated 100V CE, 140V CB (the latter is not a relevant spec for this design). It looks cobbled up from parts someone had in the bin, without looking or understanding the specs, and 'declaring' it works because it did not immediately explode. There is a TON of these 'projects' on line and you cannot kill them no matter what because they appear simple and invariably attract ambitious but ignorant beginners like flies to... well, sh*t.
The VAS works just fine, it uses a current mirror on the op side converting one side of the differetial to a topside VAS half. However, this means that side of the differential must withstand the full 150V, even the most stingy manufacturers that used this knew that side must be cascoded at ground, to distribute heat evenly across all the transistors.
And fercryingoutloud, you do not test an unknoen project by soldering it together and then just plugging in, not even with a dim bulb tester which was only used as an afterthough. First you solder the power supply and test it, then you solder the power amp up to the VAS and test it by connecting the VAS to the output - no load, only to establish proper NFB for operation, then the driver stage (same thibng with NFB) and ONLY then, having verified the bias voltage for the output stage (E to E of drivers) do you solder in the output stage. A scope would help. Or at least a DVM plus a set of 'sacrificial' headphones connected over a large resistor, to test if music is coming through without loading the amp - for starters.
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