Potato Cakes
Well-known member
Hello,
On the eBay, there is a guy out of Ireland selling 1073 filter and preamp boards, who is a nice fellow and with whom I've had quite a bit of email dialog. I've been helping him debug and figure out wiring for several of his PCB and I wanted to share what I've found with other people here who are holding on to some of these boards but not quite sure how to proceed or are stuck like I was. There probably won't be an revised boards anytime soon, but there have been many wonderful projects done here that required alterations to the PCBs but no one really minded as it yielded a great sounding piece of gear. It's just another reminder that this is DIY. I've spend countless hours going over the original schematic, the one provided with the boards, and just following and metering traces to figure all of this out, but now I and others here can build these PCBs with confidence and get a working unit the first time. Or close to it.
I've attached a photo of the filter board in question for reference. It's pretty banged up from testing and trouble shooting. Also I'm using MPSA18's instead of BC550's. It's the 1073 EQ filter section with the two make up gain amps from the BA284 as shown on the original schematic, so it can work as a standalone circuit. With the fixes the insertion loss is about -3dBu, which I believe can be adjusted by changing what is R32 (10k) on the EH10023 portion of the full 1073 Amplifier schematic. This also gives you headroom for boosting frequencies, which is why you would have a 1073 EQ in the first place. Also, you should keep in mind that there are discrepancies between the BOM and schematic, so I used the values on the latter. I know there are a number of changes that can be made to this circuit to suit one's taste and to perform "better" with modern audio equipment, but those changes are discussed elsewhere on this forum.
First correction to make is the connections for G and H pins on the 9050 inductor. These are backwards as they relate to the MF selector switch. All traces touching both pins have to be cut. Then a jumper needs to be installed from C1E to Pin H. Pin G is then tied to ground. Also, the ground connection from R51X has to be reconnected to ground as it's original connection was through Pin H, which is now cut to make this correction.
The next thing relates to the MF 10k potentiometer. This board and the provided schematic are missing a 6k8 resistor from CCW to the CW connection as shown in the original schematic on the B211 board. The quickest way that I could come up with (at 4am) was to lift the side of R1E closest to the 9050 inductor and the one leg of R2E towards the inside of the board. An additional 6k8 resistor is added from the lifted R1E pad and those two resistors are tied together above the board. A jumper wire is then used to connect the lifted sides of R2E and R1E. Probably a cleaner way to do this but I don't think I'm going to spend anymore time trying to do so. I know this way works.
Next, the SW1 (1a) on the LF switch. It is supposed to tie to the LF wiper with SW7 (1b) to pass audio in the OFF position. Cut the trace at switch pin 1 and then run a jumper to the LF wiper connection.
The last thing to be aware of is that the connection points that are labeled "SHIELD" only connect to other points with the same label, but do not go anywhere else. To use these points as intended you will have to tied one of them to the EQ-GND connection.
On the eBay, there is a guy out of Ireland selling 1073 filter and preamp boards, who is a nice fellow and with whom I've had quite a bit of email dialog. I've been helping him debug and figure out wiring for several of his PCB and I wanted to share what I've found with other people here who are holding on to some of these boards but not quite sure how to proceed or are stuck like I was. There probably won't be an revised boards anytime soon, but there have been many wonderful projects done here that required alterations to the PCBs but no one really minded as it yielded a great sounding piece of gear. It's just another reminder that this is DIY. I've spend countless hours going over the original schematic, the one provided with the boards, and just following and metering traces to figure all of this out, but now I and others here can build these PCBs with confidence and get a working unit the first time. Or close to it.
I've attached a photo of the filter board in question for reference. It's pretty banged up from testing and trouble shooting. Also I'm using MPSA18's instead of BC550's. It's the 1073 EQ filter section with the two make up gain amps from the BA284 as shown on the original schematic, so it can work as a standalone circuit. With the fixes the insertion loss is about -3dBu, which I believe can be adjusted by changing what is R32 (10k) on the EH10023 portion of the full 1073 Amplifier schematic. This also gives you headroom for boosting frequencies, which is why you would have a 1073 EQ in the first place. Also, you should keep in mind that there are discrepancies between the BOM and schematic, so I used the values on the latter. I know there are a number of changes that can be made to this circuit to suit one's taste and to perform "better" with modern audio equipment, but those changes are discussed elsewhere on this forum.
First correction to make is the connections for G and H pins on the 9050 inductor. These are backwards as they relate to the MF selector switch. All traces touching both pins have to be cut. Then a jumper needs to be installed from C1E to Pin H. Pin G is then tied to ground. Also, the ground connection from R51X has to be reconnected to ground as it's original connection was through Pin H, which is now cut to make this correction.
The next thing relates to the MF 10k potentiometer. This board and the provided schematic are missing a 6k8 resistor from CCW to the CW connection as shown in the original schematic on the B211 board. The quickest way that I could come up with (at 4am) was to lift the side of R1E closest to the 9050 inductor and the one leg of R2E towards the inside of the board. An additional 6k8 resistor is added from the lifted R1E pad and those two resistors are tied together above the board. A jumper wire is then used to connect the lifted sides of R2E and R1E. Probably a cleaner way to do this but I don't think I'm going to spend anymore time trying to do so. I know this way works.
Next, the SW1 (1a) on the LF switch. It is supposed to tie to the LF wiper with SW7 (1b) to pass audio in the OFF position. Cut the trace at switch pin 1 and then run a jumper to the LF wiper connection.
The last thing to be aware of is that the connection points that are labeled "SHIELD" only connect to other points with the same label, but do not go anywhere else. To use these points as intended you will have to tied one of them to the EQ-GND connection.
