Transistor replacements needed

[gekkehenk]

Hi!

I have an old (1976) Japanese made amplifier from Eagle International. One of the power stages and driver stages are blown.

The following transisitors are broke.

2x C789.
1x C853. (driver)
1x A545. (driver)

The C853 and A545 are complementary!

If i'm correct, the most common replacement fot the C789 is the BD243C

I don't know the replacement's for the C853 and A545. I do have some specs:

2SC853. SI-N 70volts 0.2A 0.4W NPN
2SA545. SI-N 70volts 0.2A 0.4W PNP

Who has the replacments for me?

Besides this problem I have another question.

Why isn't there a NPN and PNP transistor used in the power stage? I found it strange beqause the drivers were NPN and PNP. Is this common for these amplifiers?

Thanx!

[Bone]

When this was designed there were no PNP power silicon transistors available for sensible money. So they used what is called a quasi complementary output, where the drivers are complementary and the powers were both NPN. I would probably use BC139 (PNP) and BC140 (NPN) to replace the drivers. Watch the pinouts as they are not the same.

Tony

[jaycee]

In those days, power PNP transistors were rare, expensive, and not very good... so they used an output stage type called quasi-complimentary.

2SC789 seems to be a TO-220 package. The recommended BD243C would work well here.

As for the drivers, BD139/140 should work well. They are not as fast as the originals, but they should work well.

[gekkehenk]

Originally Posted by Bone

When this was designed there were no PNP power silicon transistors available for sensible money. So they used what is called a quasi complementary output, where the drivers are complementary and the powers were both NPN. I would probably use BC139 (PNP) and BC140 (NPN) to replace the drivers. Watch the pinouts as they are not the same.

Tony

Hi Tony,

The Collector voltage is higher in this amplifier, 56 volts on the 2SC853. The ones you named only go to 40 volts. Also is the BC119 complementary to the BC139 and not the BC140.

[gekkehenk]

Originally Posted by jaycee

In those days, power PNP transistors were rare, expensive, and not very good... so they used an output stage type called quasi-complimentary.

2SC789 seems to be a TO-220 package. The recommended BD243C would work well here.

As for the drivers, BD139/140 should work well. They are not as fast as the originals, but they should work well.

As I was saying to Tony, the Collector voltage is not high enough. Following the sheets it say's that they are not complementary to each other, is that not a problem then?

I really don't know if this is possible.....Is it possible to change the output driver to an NPN and a PNP driverpair?

EDIT: Sorry, you were saying BD and Tony BC types. So with the BD139/140 types it will work right?

[mjf]

hello.
my quick reference list shows this:
bd 243c is the substitute for 2sc789
and bc639/bc640 are for 2sc853/2sa545
greets

[Bone]

Sorry I should have said BD139/140 (typo!!)

[jaycee]

BD243C has VCEO of 100V versus the 2SC789's 70V and should be adequate.

The BD139/140 have VCEO of 80V which is adequate also. Also the BD139/140 package should fit well in the original pinout. BC639/640 would work for drivers too but you might need to fix some heatsinks to these small TO-92 packages.

Another thought - you might want to replace both channels even if the other is working. It would be better if they matched.

[gekkehenk]

Ok! Thanx for all the help guys. I will buy the parts this afternoon.

[Mooly]

And remember to power up with a light bulb in series with the mains (60 to 100 watt) first, to avoid any problems if there should be any other fault on the amp. It can save blowing the outputs again.

When it's all fixed, check and reset the output bias.