Proceed HPA2 power transistor

[im1663]

I bought a Proceed HPA2 from internet. It use 6 pair of power transistor Motorola MJL3281A/ MJL1302A. However, I found some were replaced by 2 X Toshiba 2SA1302 and 4 x Toshiba 2SC3281 in one of the channel.

From datasheet, Pw of 2SC/A is only 150w while MJL is 200w. I measured the offset voltage, it is fine at below 10mw and the amp work properly.
Question
1) is it good to replace the 2SC/A to MJL.
2) I could only find ON MJL3281AG / MJL1302AG in the market, is it necessary to replace ALL or just the 2SC/A
3) Does anyone know where to buy Motorola MJL3281A/ MJL1302A

[satoru]

The difference of MJL3281A/MJL1302A and ~G version is the latter are Pb (lead) free, which is pretty much the requirement for new electronics devices these days (in Europe, USA, etc). Whether the difference makes any change in sonic characteristics, I don't know.

As the part number suggests, MJL3281 and MJL1302 are second source/improved(?) version of discontinued 2SA1302/2SC3281 comple pair transistors (made by Toshiba for audio amp output stage). It is a total mystery why the previous owner replaced with rather harder to find parts instead of readily available parts... May be the person had some of them in hand.

Motorola's discrete semiconductor sector was span off and became ON Semi. They have a reasonable sample policy so you may want to try their service if it is available in Hong Kong. There isn't any "Motorola" MJL3281/MJL1302 parts.

Good luck and have fun!

Satoru

[im1663]

I'm planning to replace the Toshiba by ON MJL3281AG/MJL1302AG.
Thanks

[tiefbassuebertr]

in this case this thread could be of interest:
bipolar (bjt) transistor families for audio power output stages

[im1663]

If after replacement, I found the output offset is hight, how can I adjust it.

[satoru]

Most of the offset should be coming from the input stage. There would be a trimmer somewhere near the input stage for offset adjustment.

I think you would worry more about the bias current at the output stage. There should be a transistor (or diode) mounted on/near the heat sink. Around there (or connected through wires) should be a trimmer. You may want to measure/experiment the bias current before changing the transistors.

Good luck!

Satoru

[im1663]

There are four trimmer, I think one for bias, offset...the rest two...potection cct.

I propose the following method..is it work..pls comment.

One ch is perfect, I measure the emitter voltage Ve first and take this as reference.

Use two muti-meter, one across the spk output to measure the offset Vo, the other across the emitter resistor to measure Ve.

Turn either one trimmer, notice the Ve & Vb, if no response, turn trimmer to original position.

Try other trimmer, if Ve response, adj the Ve to reference
Try other trimmer, if Vo response, adj to zero

[satoru]

Looks good. Good luck!

I'd recommend to wrap veneer tape around the DMM probe except the tip so you reduce the chance of "accidents"... Also, if the offset is not that big, you can leave it alone.

Satoru

[im1663]

Dear Satoru

Good Luck, I have another amp Krell KAV-250a, all R ch (5 pair) was replaced with MJL15024/25, the off set is 100mv which is acceptable so I leave it for long time and no bias was adjust, the amp work properly.

On krell, the circuit is much simple then Proceed.
I found there were two trimmer, one near the bias transistor 15031 mount on the heat sink, it is for bias. The other near the input stage, near MPQ6600A1 (quad complementary pair), this is for offset. This morning, the offset was adjusted to within 20mv. The L ch, also adjusted from 40mv to 20mv (the reading was readjust after one hour). I also check the emitter voltage, both around 10mv..resulting in 10/.33=30mA Iq and no near for adjustment.

Regards
Henry

[Telstar]

Originally Posted by satoru

There isn't any "Motorola" MJL3281/MJL1302 parts.

Are you sure? Because the datasheets from Motorola and On Semi are a little different. Unless it is all due to the no-lead construction.