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Hello all. I'm in the middle of repairing a Kalamazoo Bass 30, and there are a couple of things I found inside that I don't fully understand. Can someone more experienced provide illumination? I've uploaded a picture of the schematic and marked a number on it corresponding to each of the things I describe below.
(1) On the schematic there is an RC filter coming off the grid of "unit 1" of V1. (The data sheet for the 6eu7 calls that section, the one that's to the right on the schematic, "unit 1", so I'm rolling with it.) There's a 0.0047uF cap, followed by a 22k resistor and a 500pF cap to ground. Inside the actual amp, though, the 22k resistor comes after the 500pF cap. Has someone changed this from a low-pass to a high-pass filter, or is the order irrelevant here?
(2) This filter cap has been changed from a 20uF/25V cap to a 50uF/150V cap. How much does this difference matter? Should I take it as an indication that the stock value proved inadequate somehow, or is it likely just an over-spec replacement? I'm considering restoring it to the stock value, but I don't want to do that if there is an underlying problem that necessitated the change, especially if it remains unfixed.
(3) Off the plate of "unit 2" of V1 (to the left on schematic), there is a 0.022uF coupling cap. It's a ceramic cap, and the top is clearly blackened. What could have caused this cap to experience excessive current and/or voltage? I haven't taken it out of circuit to test it yet. I plan to replace it with a beefier part, but again, I don't want to ignore potential problems with the amp.
When I purchased the amp, it was working, although to my ears not sounding particularly healthy. Other than the 0.022uF cap mentioned above, the only part in the amp that I've found (so far) that looked dodgy was the first power dissipating / filtering resistor, marked 1k/1W on the schematic. It seemed to have some damage, and tested at 1.4k, so I replaced it. I also replaced the part marked "SA 1045", the dual diode rectifying the PT, as the leads on it seemed very corroded, but I haven't managed to coerce my cheap multimeter into reading a measurement from it. Replacement filter caps are on order; except for the one mentioned in (2) above, they all appear to be stock.
Since the amp WAS working, I'm tempted to simply restore the filter cap in (2) to stock, replace the 0.022uF coupling cap in (3), and call it a day. But I'm making a learning experience of this repair, and I want to understand what's going on as well as I can. And I don't want to leave any potential problem without investigation.
(1) On the schematic there is an RC filter coming off the grid of "unit 1" of V1. (The data sheet for the 6eu7 calls that section, the one that's to the right on the schematic, "unit 1", so I'm rolling with it.) There's a 0.0047uF cap, followed by a 22k resistor and a 500pF cap to ground. Inside the actual amp, though, the 22k resistor comes after the 500pF cap. Has someone changed this from a low-pass to a high-pass filter, or is the order irrelevant here?
(2) This filter cap has been changed from a 20uF/25V cap to a 50uF/150V cap. How much does this difference matter? Should I take it as an indication that the stock value proved inadequate somehow, or is it likely just an over-spec replacement? I'm considering restoring it to the stock value, but I don't want to do that if there is an underlying problem that necessitated the change, especially if it remains unfixed.
(3) Off the plate of "unit 2" of V1 (to the left on schematic), there is a 0.022uF coupling cap. It's a ceramic cap, and the top is clearly blackened. What could have caused this cap to experience excessive current and/or voltage? I haven't taken it out of circuit to test it yet. I plan to replace it with a beefier part, but again, I don't want to ignore potential problems with the amp.
When I purchased the amp, it was working, although to my ears not sounding particularly healthy. Other than the 0.022uF cap mentioned above, the only part in the amp that I've found (so far) that looked dodgy was the first power dissipating / filtering resistor, marked 1k/1W on the schematic. It seemed to have some damage, and tested at 1.4k, so I replaced it. I also replaced the part marked "SA 1045", the dual diode rectifying the PT, as the leads on it seemed very corroded, but I haven't managed to coerce my cheap multimeter into reading a measurement from it. Replacement filter caps are on order; except for the one mentioned in (2) above, they all appear to be stock.
Since the amp WAS working, I'm tempted to simply restore the filter cap in (2) to stock, replace the 0.022uF coupling cap in (3), and call it a day. But I'm making a learning experience of this repair, and I want to understand what's going on as well as I can. And I don't want to leave any potential problem without investigation.
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