No, this is not about the 949 I just got running as described in another thread. I did get that one running with a minimum of effort. Put the receiver in my main system for four speaker testing… long story short I’ve fallen in love with this quad receiver. So much so that I picked up a second one in non-working condition. It was described as lights up but no sound. Figured this was a protection/power amp board issue since there are several suspect transistors in the 4 channels of the power amp boards.
So, here’s what I’ve done. Measured output on power amp board of each channel (pins 5 and 23 on each board.) Pin 23 on the rear board (“B” board in the schematics) was reading -38VDC, and all the rest were reading in the mV range. I disconnected the wires on the outputs for that channel (pins 22B and 23B), and the unit came out of protection. So, this seemed to narrow down the issue to that channel. Replaced 2SC1451 with KSC3503 and the two 2SA726 transistors with KSA992FBU, but I was still getting high voltage (about -36VDC). I was also now getting high voltage on the other B board channel (pin 5). So, replaced 2SC1451 and 2SA726 on that channel on the board. Everything seemed ok, but I did not double check the transistors (and was not using a dim bulb tester!). Powered up and smelled burning, and next thing I know the 10µF/50V capacitor on the power supply board between pin 14 and ground blew.
So, I assessed the situation. Apparently I had put in KSC3505 switching C and B. KSC3505 was fried, also fried the 2SC869 on that channel. I also noticed that one fuse was blown, and one fuse was missing completely! I don’t know how I missed that before. The 3A fuse had oxidation inside it, so decided to get all new fuses that connect to pins 1, 2, 3, 4, and 5 on the power supply board. Replaced the blown cap on the power supply board between pins 14 and 15 (ground). On the power amp board replaced the 3503 making triple sure it was in right and replaced 2SC869 with KSC2383YTA on both channels of the B board. Finally made a dim bulb tester!
Plugged the receiver into the DBT, all seemed ok, bulb went dim. However, still had -36VDC on pin 23B. Had the parts, so I recapped both channels of the B power amp board. Still no change.
Measured voltages on all pins of the power supply board:
Pin: voltage
1: 46.4VAC
2: 46.4VAC
3: 7.4VAC
4: 37.9VAC
5: 38.0VAC
6: 13.3VDC
7: 0
8: 31.1
9: 0
10: 0
11: 8.8
12: 8.8
13: 0
14: -56.9 <--
15: 0
16: 0
17: -29.3 <--
Pins 14 and 17 are the two out of spec. 14 should be -42VDC and constitutes –B3 on the schematic. 17 should be -13VDC and constitutes –B4 on the schematic. Also found two problems on the power supply board: C19 should be 330µF/35V and C16 should be 220µF/50V. However, what I found was 100µF/75V in both positions. I had the parts, so I decided to recap the board and replace Q6 (2SA720) with KSA1013YTA as recommended in the bad transistor lists. No change in voltages on pins 14 and 17. These are both on the same circuit, along with the Q6. The other transistor on this circuit is 2SB507P. Measured voltages:
Leg: measured (expected)
E: -56.6V (-44.8)
C: -61.0 (-62.5)
B: -56.9 (-45.3)
So, I’m thinking this transistor has gone bad and should be replaced. Replacement is KSA940TU from what I can tell. Clearly, the power supply is giving too high a voltage (from pin 14) to the power amp boards (pins 18, 19 and 8, 9). However, it is unclear to me why this should affect only the front left channel (pin 23B output) on the power amp board. Shouldn’t all four channels be affected? I will need to place an order for some KSA940TU transistors, but before I do, I’m wondering if there is something else I should be checking? Is it likely other transistors on the power amp board in the FL channel are damaged and should be replaced? If so, what should I be looking at? If the 2SC1541 caused damage, the C of that transistor connects to Q6 (2SC869, which did go bad and I replaced), but also to C of Q10 (2SD358). What is the replacement for that?
Fortunately, I do have a working QX-949 to compare to and measure “good voltages”, but I’m a little stumped at the moment of what these measurements are telling me.
So, here’s what I’ve done. Measured output on power amp board of each channel (pins 5 and 23 on each board.) Pin 23 on the rear board (“B” board in the schematics) was reading -38VDC, and all the rest were reading in the mV range. I disconnected the wires on the outputs for that channel (pins 22B and 23B), and the unit came out of protection. So, this seemed to narrow down the issue to that channel. Replaced 2SC1451 with KSC3503 and the two 2SA726 transistors with KSA992FBU, but I was still getting high voltage (about -36VDC). I was also now getting high voltage on the other B board channel (pin 5). So, replaced 2SC1451 and 2SA726 on that channel on the board. Everything seemed ok, but I did not double check the transistors (and was not using a dim bulb tester!). Powered up and smelled burning, and next thing I know the 10µF/50V capacitor on the power supply board between pin 14 and ground blew.
So, I assessed the situation. Apparently I had put in KSC3505 switching C and B. KSC3505 was fried, also fried the 2SC869 on that channel. I also noticed that one fuse was blown, and one fuse was missing completely! I don’t know how I missed that before. The 3A fuse had oxidation inside it, so decided to get all new fuses that connect to pins 1, 2, 3, 4, and 5 on the power supply board. Replaced the blown cap on the power supply board between pins 14 and 15 (ground). On the power amp board replaced the 3503 making triple sure it was in right and replaced 2SC869 with KSC2383YTA on both channels of the B board. Finally made a dim bulb tester!
Plugged the receiver into the DBT, all seemed ok, bulb went dim. However, still had -36VDC on pin 23B. Had the parts, so I recapped both channels of the B power amp board. Still no change.
Measured voltages on all pins of the power supply board:
Pin: voltage
1: 46.4VAC
2: 46.4VAC
3: 7.4VAC
4: 37.9VAC
5: 38.0VAC
6: 13.3VDC
7: 0
8: 31.1
9: 0
10: 0
11: 8.8
12: 8.8
13: 0
14: -56.9 <--
15: 0
16: 0
17: -29.3 <--
Pins 14 and 17 are the two out of spec. 14 should be -42VDC and constitutes –B3 on the schematic. 17 should be -13VDC and constitutes –B4 on the schematic. Also found two problems on the power supply board: C19 should be 330µF/35V and C16 should be 220µF/50V. However, what I found was 100µF/75V in both positions. I had the parts, so I decided to recap the board and replace Q6 (2SA720) with KSA1013YTA as recommended in the bad transistor lists. No change in voltages on pins 14 and 17. These are both on the same circuit, along with the Q6. The other transistor on this circuit is 2SB507P. Measured voltages:
Leg: measured (expected)
E: -56.6V (-44.8)
C: -61.0 (-62.5)
B: -56.9 (-45.3)
So, I’m thinking this transistor has gone bad and should be replaced. Replacement is KSA940TU from what I can tell. Clearly, the power supply is giving too high a voltage (from pin 14) to the power amp boards (pins 18, 19 and 8, 9). However, it is unclear to me why this should affect only the front left channel (pin 23B output) on the power amp board. Shouldn’t all four channels be affected? I will need to place an order for some KSA940TU transistors, but before I do, I’m wondering if there is something else I should be checking? Is it likely other transistors on the power amp board in the FL channel are damaged and should be replaced? If so, what should I be looking at? If the 2SC1541 caused damage, the C of that transistor connects to Q6 (2SC869, which did go bad and I replaced), but also to C of Q10 (2SD358). What is the replacement for that?
Fortunately, I do have a working QX-949 to compare to and measure “good voltages”, but I’m a little stumped at the moment of what these measurements are telling me.
