Hi to all, I have a Bush 2025 amplifier, it's OK through headphones but very low and distorted on the left channel through speakers, seems like the power transistors have failed and a check with the meter shows the resistance readings on the 2 transistors on the left side are different to the right channel ones. The transistors are Sankey 2SB1685 & 2SD2641, a quick look through the excellent all transistors website shows equivalents but I'm struggling a little as to which might be the nearest, I was advised to check the emitter resistors first, but a check on these shows them to be fine,the original ones are darlington transistors, and reading around has suggested that they will have to be replaced with darlington transistors, can I replace with higher value rated ones, i.e 80 watt, 150 volt, 8 amp, instead of the 60 watt 110 volt? I would assume that would be OK so long as I don't go below 'spec'. Understand I wouldn't get the higher wattage output as the transistor voltage supply would still be the same.
I'm hoping someone on here with a better under standing might be able to give some advice.
Regards Alan.
I'm hoping someone on here with a better under standing might be able to give some advice.
Regards Alan.
I wouldn't just condemn the output transistors without further tests. 99% of power transistor failures involve them failing short circuit and that doesn't seem to be the case from what you are saying.
I think we need to see a circuit to offer detailed advice... without and you need to check the output DC offset (or if AC coupled design, the mid point voltage of the output pair) and also the quiescent current of the output devices.
I think we need to see a circuit to offer detailed advice... without and you need to check the output DC offset (or if AC coupled design, the mid point voltage of the output pair) and also the quiescent current of the output devices.
As far as replacements go, I would suggest you look into Sanken's 2SB1559 and 2SD2389, which are 150 V, 8 A parts (they don't make any 6 A ones any more).
Sanken catalog
You could actually still buy the 2SD2641 fairly easily, but the 2SB1685 seems to be long gone.
Sanken catalog
You could actually still buy the 2SD2641 fairly easily, but the 2SB1685 seems to be long gone.
Hi Guys thanks for all the advice so far, Ive checked with the mulitmeter on the 20k setting, amp not switched on, The readings from my multi meter are as follows:-
Right channel (working OK), B1685, B-E= 14.99....B2641, B-E= 13.6
Left Channel (distorted), B1685, B-E= 1.7.......B2641, B-E= 000.Obvoiusly something amiss there, hopefully then this is repairable, it wasn't an expensive amp when new, but I will change all 4 transistors as well to 'balance' the left and right sides, I have checked the emitters too and they seem to be fine so it's pointing to the transistors.The person I got the amp from used it as well to do a little bedroom DJ' ing, so I suspect it may have been wound up a bit, a visual look at it though looks looked after though.
Regards, Alan.
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Right channel (working OK), B1685, B-E= 14.99....B2641, B-E= 13.6
Left Channel (distorted), B1685, B-E= 1.7.......B2641, B-E= 000.Obvoiusly something amiss there, hopefully then this is repairable, it wasn't an expensive amp when new, but I will change all 4 transistors as well to 'balance' the left and right sides, I have checked the emitters too and they seem to be fine so it's pointing to the transistors.The person I got the amp from used it as well to do a little bedroom DJ' ing, so I suspect it may have been wound up a bit, a visual look at it though looks looked after though.
Regards, Alan.
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Don't simply replace the outputs then power it up.
Not nessesarily every single time, but generally when outputs fail there will
be other components in circuit which will be damaged as well. It could be other
transistors, resistors, diodes ect..
That's the fun with complex DC-coupled amplifiers. Overlook one problem, and poof!, those were your shiny new output devices. A bulb tester is highly recommended.
BTW, many bias setting solutions are not inherently safe and will have bias current going up when the trim pot develops contact issues. Secondary breakdown eventually does the rest. So that is something I would always recommend checking.
BTW, many bias setting solutions are not inherently safe and will have bias current going up when the trim pot develops contact issues. Secondary breakdown eventually does the rest. So that is something I would always recommend checking.
Well I chnaged the output transistors today, to the ones recomended by 'sg', and they seemed to have done the trick, further testing shows that the 2SD2641 was the one that had failed, but I changed all 4 anyway, thanks to all on here with the help & advice, much appriecated.
Regards, Alan.
Regards, Alan.
Hi JB, I picked this amp up second hand with the matching CD player for £10, ($15), for both, reason was, I already had the matching tuner, so it was a bargain,. it looked in good condition too, and is, to be fair, but the lad I got it from did mention he had used it with a couple of turntables as a 'DJ', so I suspect it had been slightly over driven, or wrong type of speakers connected, too small or too large, it's now driving a pair of hi fi speakers, (Mission 760i), which again were picked up for £10. Main thing is it's now operational, and was a nice little project.
Always a good feeling bringing something back to life, and a great way of finding bargains, as many on here will testify, thanks for all the info, a great site.
Regards, Alan.
Always a good feeling bringing something back to life, and a great way of finding bargains, as many on here will testify, thanks for all the info, a great site.
Regards, Alan.
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