Hi
This amp had no sound at all. Upon checking, I realised that both channels have blown transistor, The first channel had boths its power and driver transistors blown whereas the other channel had its power transistors OK but the driver transistors blown. The power transistors are batwing transistors(2SB616 & 2SD586). The drivers are 2SB548 & 2SD414. I replaced the driver transistors with 2SA1220A and BD237 on both sides and the power transistors were replaced by BD243C and BD244C on one side. The other side had its original power transistors. The problem is the BD244C and BD243C keep blowing on one side, whereas the driver transistors on both sides are OK. Can someone suggest what proper power transistors should I use to replace the batwings.
Thanks
Carlos
This amp had no sound at all. Upon checking, I realised that both channels have blown transistor, The first channel had boths its power and driver transistors blown whereas the other channel had its power transistors OK but the driver transistors blown. The power transistors are batwing transistors(2SB616 & 2SD586). The drivers are 2SB548 & 2SD414. I replaced the driver transistors with 2SA1220A and BD237 on both sides and the power transistors were replaced by BD243C and BD244C on one side. The other side had its original power transistors. The problem is the BD244C and BD243C keep blowing on one side, whereas the driver transistors on both sides are OK. Can someone suggest what proper power transistors should I use to replace the batwings.
Thanks
Carlos
Looking at the service manual, this amp is quite basic. I'd suggest MJE243/253 for drivers and TIP2955/3055 as the outputs.
Make sure you adjust RT206/RT256 to their minimum resistance first, which sets minimum bias current. Check the trimpots arent damaged. Then if everything checks out, adjust for 3.5mV voltage drop over one emitter resistor (for 10mA of bias). Id probably adjust for 9mV (nearer 30mA) if it doesnt get too hot from doing this.
Make sure you adjust RT206/RT256 to their minimum resistance first, which sets minimum bias current. Check the trimpots arent damaged. Then if everything checks out, adjust for 3.5mV voltage drop over one emitter resistor (for 10mA of bias). Id probably adjust for 9mV (nearer 30mA) if it doesnt get too hot from doing this.
Carlos you are never going to make it like this ...This will never work
Bias scheme of this amplifier is way too generic and it will not allow properly biased transistors .... either bias will be to little or too much
You need to start it up with a variac and observe what is going on plugin it in str8 and will blow
Choice of parts beyond characteristics is to be done in the same or close capacitance cause otherwise stress will appear to the previous stages
Obviously drivers are to be from the same family to operate properly
You are in the wrong path even if it works it will not last in time
Bias scheme of this amplifier is way too generic and it will not allow properly biased transistors .... either bias will be to little or too much
You need to start it up with a variac and observe what is going on plugin it in str8 and will blow
Choice of parts beyond characteristics is to be done in the same or close capacitance cause otherwise stress will appear to the previous stages
Obviously drivers are to be from the same family to operate properly
You are in the wrong path even if it works it will not last in time
D548 B669 and in general this style of transistors are a good choice of drivers for this amp easy to get also
BD 243 though will never make it there as an output
Wise think to do is to mod it for 4467/1694 outputs B669 D667 drivers Question is that a pair of 667-669 will drive a 4467-1694 the problem is if the previous stages will handle the 667-669
Done it a million times before as a combo for many other and similar amps but not for this one ...
Kind regards
Sakis
BD 243 though will never make it there as an output
Wise think to do is to mod it for 4467/1694 outputs B669 D667 drivers Question is that a pair of 667-669 will drive a 4467-1694 the problem is if the previous stages will handle the 667-669
Done it a million times before as a combo for many other and similar amps but not for this one ...
Kind regards
Sakis
Sakis, MJE243 and MJE253 are good driver transistors and easy to get here. The 2SD548 and 2SB669 are not easy to source, and quite frankly given the design of this amp, not worth the hassle.
I agree on the bias circuit being poor in this amp... I suspect whoever has just fitted the BD243/244 has not adjusted the bias at all, and it is far too high. If I was doing the repair I'd probably add a Vbe multiplier arrangement in place of D201/D251, and change the existing bias pot for a fixed resistor eg 220 ohms to make an EF type 2 output stage.
I agree on the bias circuit being poor in this amp... I suspect whoever has just fitted the BD243/244 has not adjusted the bias at all, and it is far too high. If I was doing the repair I'd probably add a Vbe multiplier arrangement in place of D201/D251, and change the existing bias pot for a fixed resistor eg 220 ohms to make an EF type 2 output stage.
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