Zapco SP 2000.1

Hello, today I bring the fateful DWM1216X_V20 driver back to the surface. I have a Zapco SP 2000.1 in my hands with a blown channel.
After ordering the IRF640 (non-N) and IRF9640 mosfets I would like to review the drivers as well.
I would like to replace the following:
  • MMBTA42 (1D)
  • MMBTA92 (2D)
  • 2SC4672 (DK QU)
  • 2SA1797 (AG QS)
I can recover the two MMBTA42 and MMBTAA92 transistors without problems, instead, the 2SC4672 and 2SA1797 cannot. Can they be replaced with 2SA1213 and 2SC2873?
Has anyone ever had a similar problem?

PS: Wandering around the Internet I found on a Russian forum a schematic of the aforementioned card that mounts on Hifonics.
On that scheme instead, the BCX56 and BCX53 are used.

Some idea?
Hello, using your advice I ordered everything on digikey, through octopart. After replacing the 2SC4672 and 2SA1797 I also replaced the MMBTA42 and MMBTA92 with the following:

The amplifier sounds but, at minimum with all mosfets connected, the idle current exceeds 5A and without mosfets it exceeds 1.3A. So much so that without a heatsink the transistors burn out in less than 3 minutes.

PS: I redid all the soldering on the driver circuit, checked the capacitors and resistors (especially the 470 ones). I also checked all the diodes and replaced the two integrated LM211 and TL072CD.
Questa è la scheda:


  • 20230902_202055.jpg
    587.8 KB · Views: 61
Last edited:
The image isn't available. Maybe the file is too large. If it's good quality, it's doesn't need to be larger than what will fit on an average computer monitor (1920x1080).

It's odd that only one transistor is heating up. Did you look at the gate drive signal for ALL of the output transistors to confirm that they have a clean signal?
It is actually strange. Power amps don't heat up evenly, each time you reboot the board they heat up different FETs. The welds of both boards have been completely redone, vacuuming the old tin.

I tried to mount all the old driver circuit 1D and 2D transistors except the two blown ones, and the minimum current drops significantly. It would appear that they are the problem, but the only difference is the hFE.

The two new ones in place of the burnt ones are the Q8 and Q4.
Hello, following in-depth tests, the problem was precisely the too high HFE of the two new transistors replaced previously. Only by changing two transistors (Q8 and Q4) the idle current decrease by ± 400mA. I have to replace four more, but I don't expect the idle current to change much because the originals on the circuit have a gain value very similar to these new ones.

For those who will read the discussion in the future, the new transistors that I went to assemble and solved the problem are the following:


Through the thermal camera, I noticed that the Q11, which previously heated up to 70°, now barely reaches 45°.

I'll keep you updated as soon as I replace the others and mount the mosfets.
Good evening, after having mounted the new mosfets and the amplifier on its heatsink with new pads and new thermal paste, I started the tests again to understand the excessive absorption at idle. In reality, it is the driver transistors (Q12, Q9, Q6, Q5 and Q20, Q21, Q16, Q23) that heat up really a lot (+80°±). The mosfets now heat uniformly, but do not become very hot (35°±). Having mounted the heatsink, I started to increase the current through the potentiometer of my power supply, until I brought it to a maximum of 10A. The amplifier consumes 6A at 13.6V at minimum WITHOUT ANY LOAD. The hottest part of the circuit are the two large output inductors, which after just 5 minutes at idle, exceed 110°C. I tried to move them but they are well glued and the idle current does not change...