BJT output device substitution

[ben12345]

I picked up two new identical 'Professional' stereo power amplifiers for cheap. They are items which have shipped to customers, found to be DOA, and returned for replacement.
I figured that if I can't fix them, they'll be good for their trafo's (1200va!) and cases.

They are supposedly rated at 385Wrms into 4ohms per channel, and the trafo is 2 x 67vac.

There are several blown output devices, which is where my concerns are, they are both the same make and model amplifiers, yet one uses 2 pairs of Sanken 2SA1695/2SC4468 per channel and the other amplifier uses PMC-Sierra made 2SA1294 and ISC made 2SC3263.

I've tried to contact the manufacturer with regards to the schematic, to see what it specifies on there, but they never replied.

Going by the Sanken datasheets, the 1294/3263's are the 'better' devices, and I would guess the more suitable.

Now, I have some 'free' onsemi NJW0281/302, so before I go ordering 8 pairs of 1294/3263's, would the onsemi's be a suitable replacement?

[megajocke]

A transformer voltage of 2x 67V AC sounds quite high for a 385W / 4 ohms amplifier. Are you sure it's correct?

[Aldotin]

Originally Posted by megajocke

A transformer voltage of 2x 67V AC sounds quite high for a 385W / 4 ohms amplifier. Are you sure it's correct?

Maybe they are rather rated 385W/8r and 600W/4r!

Anyway 2 pairs of output transistor sounds not much...

[megajocke]

No, trying to do 600W or even 400W in class A/B with just two pairs of 100W Pd (2SC44689) devices or fakes isn't going to work very reliably or even at all.

[nigelwright7557]

Originally Posted by ben12345

There are several blown output devices, which is where my concerns are, they are both the same make and model amplifiers, yet one uses 2 pairs of Sanken 2SA1695/2SC4468 per channel and the other amplifier uses PMC-Sierra made 2SA1294 and ISC made 2SC3263.

You need to know why the output transistors blew.

It could be fitting new output transistors fixes the problem or it could be the circuit has faults futher back which might just blow up the new transistors on power up.

Bias problems will kill multiple output transistors.

Personally I have found most often the amp has blown due to a short and replacing the output transistors worked fine.

On other occasions the fault has been in the ltp, vas or bias circuits.

[ben12345]

Originally Posted by megajocke

A transformer voltage of 2x 67V AC sounds quite high for a 385W / 4 ohms amplifier. Are you sure it's correct?

Sorry, I was a little out, it's 350w into 4 ohm, 270w into 8 ohm and i'm quite sure about the transformer voltage - it's printed on the label stuck to it.

Originally Posted by Aldotin

Maybe they are rather rated 385W/8r and 600W/4r!

Anyway 2 pairs of output transistor sounds not much...

I know, I think that's why they blew!

Originally Posted by megajocke

No, trying to do 600W or even 400W in class A/B with just two pairs of 100W Pd (2SC44689) devices or fakes isn't going to work very reliably or even at all.

Again, I think that's why they blew, the design doesn't appear to be the best thought out.

Originally Posted by nigelwright7557

You need to know why the output transistors blew.

It could be fitting new output transistors fixes the problem or it could be the circuit has faults futher back which might just blow up the new transistors on power up.

Bias problems will kill multiple output transistors.

Personally I have found most often the amp has blown due to a short and replacing the output transistors worked fine.

On other occasions the fault has been in the ltp, vas or bias circuits.

Don't worry - i'll be replacing lots of components on these PCB's. They are mostly constructed with poor quality components, which could have attributed to the early death of them. The drivers for one are blown beyond all recognition, as well as several resistors and diodes have just vaporized.

Another reason I'd like to try to use the onsemi 281/302 is that they are rated at 150w power disssapation, instead of the original devices 100 or so watts.

This is a link to the amplifier we are talking about here:
https://dreammachine-radio.nl/imgsta...ml?language=UK

[Aldotin]

I had a Monacor ST-700 amplifier given to add a soft-start (400VA mains trafo inside)
Chinese crap!

(Sorry for the low quality of schematic.)

[event horizon]

The only way you'll make the amplifier safe is to give the poor thing a decent amount of output transistors so it can dissipate power & still not cook the components. Amps can get away with enough transistors on an output stage that can dissipate double the output power, in this case you are looking at 700W+ of transistors per output stage (350W RMS x 2).

However they'll last a lot longer by doubling this to 4 times the output power, or 1400W of transistors per channel. Take it from me that with an output power of 350W RMS that isn't a great deal

As yours is a PA amp (& i'm glad i bought a Mackie M3000 & not a stage line amp ) then look to fit 4 x 100W NPN & 4 x 100W PNP transistors as a minimum.

It sounds like your amp has a cheap power supply, IE low storage capacitance & the PSU sags badly under even a moderate load.

[nigelwright7557]

Originally Posted by event horizon

The only way you'll make the amplifier safe is to give the poor thing a decent amount of output transistors so it can dissipate power & still not cook the components

Its quite a juggling act to get this right without spending a fortune.

There is the basic transistor power rating to consider.
Then there is derating depending the tempertaure they run at.
Heatsink size comes into the matter here.
Then you have to consider the transistor is only conducting for half a cycle.
Then are you going to use a fan(s).

I find I can get away with twice the transistor power rating per output pair with a good heatsink and a couple of fans. I run a mobile disco and this has worked reliably for three years having done this.

[Xoc1]

Hi Ben I don,t know if your amp internals look like the photo posted by Aldotin.
But I bought a cheap chinese amp (Warrior DA800) on ebay recently that looks remarkably similar internally.
After fixing the broken rail fuse, I found that it was not reliable in PA use, as it quickly overheated. I ended up fixing 2 extra fans in the front panel (existing fan was 24V)
I also added some extra capacitors I had in my junk box.
It still sounds fairly poor, but has so far been usable as a vocal monitor amplifier without failure.