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STK0040 module in JVC JA-S22 amplifier

Diver300

Well-Known Member
Most Helpful Member
I'm trying to repair a JVC JA-S22 amplifier. It's a class AB amplifier with Darlington output transistors. The power transistors are blown in one channel.

The transistors are contained in a module called an STK-0040, which has an internal circuit of this:-

1726262938501.png


It's TR4 and TR5 that have blown and are shorted collector - emitter.

There are lots of STK-0040 module on ebay, but I am fairly sure that they are all copies. The original STK-0040 has semiconductor dies bonded onto a circuit board, so it would be quite easy to make an equivalent with a very different technique, but it's likely that the copies don't perform properly.

This video (
which is in Spanish) show someone repairing the same amplifier and uprating the transistors on a copy STK-0040.

I have opened up the blown STK-0040 and think everything except the big transistors is working. I think that I could cut the base and emitter connections on the big transistors, and mount equivalent NPN and PNP transistors in T0220 packages somewhere else on the heatsink, and run wires to the module or the circuit board.

So I've got a choice of:-
1) use existing module with the big transistors fitted remotely.
2) use a module from ebay that is probably a copy and hope for the best.
3) get a module from ebay and change the output transistors, as in the video.

Has anyone got experience of these or any suggestions?
 
These sorts of modules are a nightmare, as they were rarely available for long - but at least this is a fairly 'minimal' module, so less to replace. In the past amplifier manufacturers often produced 'repair kits' to replace a duff full module, with an alternative type - these were often fairly complicated, consisting of a new (different) module, a small PCB, and a good number of components. They usually came with a very poorly photo-copied instruction leaflet.

As the module is so simple, I'd suggest making your own - use darlington transistors to replace the drivers and outputs (four for the price of two :D ), and a small NPN for the Vbe multiplier. Add a pot in the Vbe multiplier, so you can adjust the quiescent current.

Even if you replace just the output transistors (either for the existing module, or a copy) I'd still add a pot, as it's unlikely the quiescent current would be correct with different transistors.
 
Well I tried assembling a circuit, and it hasn't worked. I used a pair of TO220 Darlingtons, BDX33C and BDX34C and a TO126 NPN transistor bolted to the same heatsink as TR1.

The problem with that idea is that there is no way of adding an resistor in the R5 position, and the thermal coupling between the two transistors in a darlington pair is far, far better than the coupling to TR1. The quiescent current wasn't stable and varies very quickly when the temperatures changed. There was a thermal run-away when I tried to put some audio through it.

While adjusting the quiescent current, I had a thermal runaway, but I was monitoring the current and I had my hand on the power switch so I manage to catch it that time.

I also couldn't find very generously rated Darlingtons. If I use discrete transistors I have a much wider choice as well as being able to have a resistor in the R7 position. I'll probably use the power transistors in TO247.
 
Well I tried assembling a circuit, and it hasn't worked. I used a pair of TO220 Darlingtons, BDX33C and BDX34C and a TO126 NPN transistor bolted to the same heatsink as TR1.

The problem with that idea is that there is no way of adding an resistor in the R5 position, and the thermal coupling between the two transistors in a darlington pair is far, far better than the coupling to TR1. The quiescent current wasn't stable and varies very quickly when the temperatures changed. There was a thermal run-away when I tried to put some audio through it.

While adjusting the quiescent current, I had a thermal runaway, but I was monitoring the current and I had my hand on the power switch so I manage to catch it that time.

I also couldn't find very generously rated Darlingtons. If I use discrete transistors I have a much wider choice as well as being able to have a resistor in the R7 position. I'll probably use the power transistors in TO247.

R5 isn't needed, as for darlingtons, TIP142/147 would be fine, and plenty more power than required for a 40W amplifier. The Vbe multiplier mounted on the same heatsink as the output transistors should be fine.
 
I don't know if the emitter resistors should be larger to use with Darlingtons. The resistors are 0.22 Ohms each.

I had a transistor Vbe multiplier on the same heatsink, but the quiescent current was certainly not stable. There is quite a time lag between the Darlingtons heating and the Vbe multiplier transistor heating.

Are there any particular changes you would suggest to make the circuit stable? I don't want to make what would essentially be the same circuit again if it's just going to run away again.
 
The original transistors were in a darlington configuration anyway, but you could certainly try increasing the emitter resistors - however, they will be thermal lag between the transistors heating up and the Vbe multiplier receiving the heat.

The Velleman 100W amplifier uses 0.47 ohms emitter resistors:


Schematic at the end of the User Manual.
 

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