I diverted an electric piano from the skip the other day. The blown transistor is one of balanced pair in the pre-amp, it is split in two. Does a balanced pair just mean that the gain of both the BC238 transistors needs to be the same or is there more to it?
Thanks
misterroy explained on 14/10/2012 :
It usually means that all of the parameters of two transistors are checked and matched as closely as possible, one to another.
two. Does a balanced pair just mean that the gain of both the BC238 transistors needs to be the same or is there more to it?
Odd to have a balanced pair in the pre-amp.
Or to shove so much juice down it to split it in half. Your best bet is to buy two (or more) new ones from the same batch and select the two that seem to be closes on any way you have of measuring them.
Trouble is you want similar DC AND AC gain. Having said that its a purist thing. You probably won't notice more than a shade of extra hum buried in the noise.
What is far more worrying is how on earth a preamp transistor drew enough current to split...
Are u sure? its a EU designated device..
I had a thought: does this piano have an integral POWER amp? because that can indeed draw enough current if much of the output stage goes bang to kill the drivers. And it is conventional to have a balanced pair of PNPS as the first stage to drive the phase splitter.
If that is the case, you are advised to remove ALL the transistors one by one and check that they still have diode junctions and not dead shorts.
Replace any that have popped and any obviously burnt resistor and if the whole thing has a DC fuse remove it and put a meter in there. Should draw no more than 20-40mA
in two. Does a balanced pair just mean that the gain of both the BC238 tran= sistors needs to be the same or is there more to it?
Balanced pair doesn't mean the same gain. Tranny gain varies widely, and ev= ery circuit has to deal with that, normally by using nfb and bias stabilisa= tion.
If a preamp tr has split in 2, the fault isn't due to tr failure, its due t= o something else allowing that much current to flow. If its a power amp tr,= as sounds much more likely, its not as simple as assuming the tr is what c= aused the fault. But if you can't trouble shoot, its always worth putting a= similarish tr in there to see if it works, it might.
BC238 is a general purpose tranny, lots of gen purp trs would substitute fo= r it ok. Data:
Balanced pair doesn't mean the same gain. Tranny gain varies widely, and every circuit has to deal with that, normally by using nfb and bias stabilisation.
If a preamp tr has split in 2, the fault isn't due to tr failure, its due to something else allowing that much current to flow. If its a power amp tr, as sounds much more likely, its not as simple as assuming the tr is what caused the fault. But if you can't trouble shoot, its always worth putting a similarish tr in there to see if it works, it might.
BC238 is a general purpose tranny, lots of gen purp trs would substitute for it ok. Data:
in two. Does a balanced pair just mean that the gain of both the BC238 tran= sistors needs to be the same or is there more to it?
Just replace both transistors with two more from the same modern manufactur= er. The old obsession with matched pairs made sense when (especially for ge= rmanium) transistors were made by a "craft" process of diffusion and where = their parameters varied according to their times in the oven. With modern = structures and manufacturing controls, there's just not that much differenc= e to begin with.
If you still really need a matched pair these days, then you'll also want t= o have them fabricated as part of an array, so that they're thermally coupl= ed too.
Except that he was after BC 238 not BC 328
For an initial try-out, buy transistors with the same suffix. BC 238 a, b & c each have reasonably close gain balance. They shouldn't be more than a pound or so each, even from your local friendly retail supplier. If the unit doesn't work, you've not wasted much, and if it does, then you can buy matched pairs or just buy a batch and match your own.
The worry is about why the original transistor split, and whether the same thing will happen to a replacement. All the other components in the circuit need to be checked thoroughly.
If its a power amp phase splitter better matching will give you two things: less DC offset on the loudpseaker if its DC coupled and better common mode rejection of power supply ripple.
Damn that ythrew me
two. Does a balanced pair just mean that the gain of both the BC238 transistors needs to be the same or is there more to it?
What a goof I have been. I only noticed that the blown bc238 is not part of a matched pair when I looked for the schematic on-line. (There was a paper copy stapled inside the keyboard) The image is here
The supply is supposed to be 9v, probably 12 volts supplied. I don't have the power supply that was used. Thank you all for your help. I might still need a matched pair of BC550C
in two. Does a balanced pair just mean that the gain of both the BC238 tran= sistors needs to be the same or is there more to it?
Having looked at the circuit board I dont feel quite as stupid as I did. Be= side the battery cables there is a BC108, then beside the on/off switch the= re is the split BC238, and close to the split one there is a BC550c. Thhttp=://
two. Does a balanced pair just mean that the gain of both the BC238 transistors needs to be the same or is there more to it?
matched pair when I looked for the schematic on-line. (There was a paper copy stapled inside the keyboard)
power supply that was used.
The BC 238 has blown because something is drawing excessive current, or the input voltage has risen too high. Probably, the 100 microfarad smoothing capacitor has failed short circuit, as all other current paths are limited to a total of less than 2 milliamps by resistors. There may be a short on the circuit board, which you could spot by using a magnifying glass. Also, check the zener and the 1.8 K resistor in the circuitry round the BC 238.
The BC 550 transistors do not need to be matched, as their gain is set by the feedback loops round them.
More to the point one really needs to know a bit more about this amplification stage. Are you saying there are two amps feeding different paths, or what. What purpose does it serve exactly? are you absolutely sure that you do not have two different transistors, one pnp and one npn used for some odd purpose like phase splitting or some such?
Brian
I'd be tempted to stick a couple of equivelents in and see what happens. Its not going to blow it up after all.
Brian
In the circuit that the OP has now posted a link to, the BC 238 is part of an emitter follower type voltage regulator, and the BC 550s form a pre amp and tone control circuit. Standard, very basic open collector type circuit, typical of the period. Matched transistors are not required, as the gains are set by feedback to be much lower than the hfe of the transistors and the BC 238 could be replaced by any small signal NPN transistor with reasonable gain. The only critical parts in the circuit are the feedback capacitors, resistors and inductors.
Seems to be the only possibility unless a short has occurred on the copper side of the board. STray wire come loose or washer rolling around
- that kind of thing.
Replacing the 100uF capacitor and BC238 with almost any NPN transistor ought to fix it. PSU noise might be improved slightly by putting a modest capacitor in parallel across the zener diode too.
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