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I am repairing a Crown Macro-Tech 2402 that has channel one not working. Good thing there are two channels to compare readings with!
I've attached a schematic for the 2400/2401, but it seems they renumbered components when they went from 2400/2401 to the 2402.
First problem I found is that one of the 0.2 ohm 5W resistors (R18 on the board, one of R306-308 on the schematic) reads 80k in the circuit. I pulled and replaced that.
Then I measured across all of the PNP transistors and read 22 ohm (same readings as channel 2 equivalents). All looked good on this half so far. All of the NPN transistors matched channel 2 readings. While still not powered, I continued to test components in the circuit and compare them to channel 2 same points. It appeared that D06? (same diode type as D09 on the board right above it) was shorted. After lifting a leg, the diode tested good. I pulled the transistor (Q10 on board marked C8188-2, one of C306-308 on the schematic) associated with the failed resistor and tested it. The two pins measured as expected, but now the short that I was chasing on the board was no longer there. Then I tested continuity from each pin of Q10 to the case and found a short between one of the pins and the case.
For test purposes, I reinstalled the transistor with a mica pad and used insulated washers to secure it to the heatsink and remeasured resistances and diode checks across the components that previously were reading bad. I powered up and have solid signal passing all the way thru each channel, though I did not push the signal to full power.
So now I got to thinking more on this. Even though it's back to "working", Q10 transistor is just not producing power since I have isolated the transistor case, thus it's not providing any operation of that transistor. What would a suitable transistor replacement be? Can I get by with only replacing this one? If not, which pairs or sets should I be replacing? All of the high side C8188-2 PNP transistors? Both high and low PNP transistors? High side PNP and NPN? Other? Thanks!
I've attached a schematic for the 2400/2401, but it seems they renumbered components when they went from 2400/2401 to the 2402.
First problem I found is that one of the 0.2 ohm 5W resistors (R18 on the board, one of R306-308 on the schematic) reads 80k in the circuit. I pulled and replaced that.
Then I measured across all of the PNP transistors and read 22 ohm (same readings as channel 2 equivalents). All looked good on this half so far. All of the NPN transistors matched channel 2 readings. While still not powered, I continued to test components in the circuit and compare them to channel 2 same points. It appeared that D06? (same diode type as D09 on the board right above it) was shorted. After lifting a leg, the diode tested good. I pulled the transistor (Q10 on board marked C8188-2, one of C306-308 on the schematic) associated with the failed resistor and tested it. The two pins measured as expected, but now the short that I was chasing on the board was no longer there. Then I tested continuity from each pin of Q10 to the case and found a short between one of the pins and the case.
For test purposes, I reinstalled the transistor with a mica pad and used insulated washers to secure it to the heatsink and remeasured resistances and diode checks across the components that previously were reading bad. I powered up and have solid signal passing all the way thru each channel, though I did not push the signal to full power.
So now I got to thinking more on this. Even though it's back to "working", Q10 transistor is just not producing power since I have isolated the transistor case, thus it's not providing any operation of that transistor. What would a suitable transistor replacement be? Can I get by with only replacing this one? If not, which pairs or sets should I be replacing? All of the high side C8188-2 PNP transistors? Both high and low PNP transistors? High side PNP and NPN? Other? Thanks!
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