![]() ![]() If your DMM does not have hfe (beta) measurement mode also do check for shorts between collector and emitter in the Ohm mode in the transistors, leaving the base open. If the final transistors are not mixed, the idle current would be close in the other pair of transistors. The idle current test only tests the current in one of the pairs in the final stage. 14W load in series with a 40W bulb would not give you a fully lighting bulb, it would be dimmed!Ī fully illuminated light would indicate a short, most probably in the final transistors. The current (and power) draw of the preamp stages can be neglected compared to this. This suggests that you need to adjust the idle current as described in the service manual.Īs for the dim bulb test, you have 4 times the 50mA idle current (200mA in total), loading the 70V supply in your amp. That 25mV is across the 0.47 Ohm emitter resistor(s), that means roughly 50mA idle current in one pair of transistors. The service manual for the plain (not D version) mentions 25mV on the idle current test points. ![]() Still, it suggests that the amp is basically fine, you just need to adjust the idle current. When checking if the PA is fine without input you do not need a speaker or output load, that would just muddy the results.ģ5mV and 25mV DC sounds OK on the outputs. I take you took the above measurements without a speaker. This will protect newly replaced final transistors from blowing up again.Īlso, do not forget to set the final idle current when the amp is working. Having a dim bulb tester (limiting the mains input current to the amp) is a prudent way to get to the point where the output DC voltage is close to 0V. When the PA is working fine the output should have no more than 20mV to 40mV DC without audio input.Ī way to debug such an amp is to check every and each of the transistors, not just the final and driver transistors, and then the components. If the amplifier fails, manifested by not having close to 0V at the output with no input and/or high current draw, it could come from any stage within the circuit. TIP41-42 are OK, but to be really on the safe side triple current rating for the final transistors are preferred.įor this safety reason I would prefer to use the MJE15032-33 transistors as replacements for the finals. TIP31-32 are OK for drivers, but not adequate for final transistors. (This is not the first Sony amp where components were undersized!) With this in mind, the original transistors are still a bit undersized, no wonder they failed. The final transistors are two pairs in parallel, so the required current rating for a single pair is half that required. I could simply bypass the full-bridge rectifier on these amp modules and supply from the positive and negative rail to the 2200uf filter caps of the tda7293. The sony amp has a positive and a negative 35V DC rail.ģ5vdc/1.414= 24.75Vac, well within tolerance. Max supply voltage on tda7293 amplifier boards is 32V AC. If those output transistors don't cut it, I'll replace the power amp stage with 2 mono tda7293 amplifier boards I have in my drawer, and install this 12v 2 relay speaker pop circuit. It's not an exact replacement however it has better max characteristics than the original transistors and gain bandwidth of 15mhz compared to the original 8mhz. I'll try replacing all output transistors with D613/B633. When raising the DC Bias to 25mv (idle current value from the service manual), Q409 blew up, and Q410 smoked. The amp when testing worked with the DC bias at 10mv. Q409(H1061) and Q410(2SA671) blew up (VT409 & 410 in this schematic from MBuras build thread), those output stage transistors were not complementary as I thought.ĢSA671 is a good replacement for the original 2sa670 but h1061 is not a good replacement for 2sc1060. Yeah something is wrong in the Power amp. ![]()
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