Potato Cakes

eBay 1073 boards
« on: March 15, 2019, 02:37:00 PM »

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.
« Last Edit: March 15, 2019, 03:21:47 PM by Potato Cakes »

Potato Cakes

Re: eBay 1073 boards
« Reply #1 on: March 15, 2019, 03:21:55 PM »
I also have the 283NV and AV preamps boards from the same guy to make a full 1073 preamp unit. The NV connections correspond to the first amplifier stage from the BA284 board and the AV is pretty much verbatim to the BA283AV as on the original schematic. The components with the "X" designations refer to off board components shown in the EH10023 schematic. This applies to the first post. And again, I would start off by using components values shown in the schematic and not the BOM.

I built this using an EK20033 switch, which seems pretty intimidating but once I got to looking at it closely it didn't seem that bad. (Side note: I have been building and learning nonstop from projects and people on this site for the last 5 years with all of it's pain and agony, so I'm at a point now that able to follow schematics a bit easier, but just a bit). I've attached the switch schematic with some easy to see connection points. These labels however are for a different set of boards, but I've complied a list of how it will translate to the NV and AV boards:

Stage 1 Input: AV Preamp In
Stage 1 Boost: AV F Gain
Stage 2 In: NV In
Stage 2 Out: NV Out

The Mic, Line, and ground are as is.

On the AV board, there are some user preference gain adjustment resistors (R10X and R44X) to allow this board be implemented in a myriad of applications including adding passive filter section. For use with the EK20033 switch, these are to be left empty as well as R Gain. On the NV board, the Boost connection is also to be left empty. This will give you unity with the switch set to "0." Also if this is being used as just a preamp, the Preamp-Out on the AV board needs to be strapped to the Stage 3 Input.

Shielded cable is necessary when interconnecting the boards. Each board should have a separate ground connection from 0V to the start ground point. When jumping boards together, the shield needs to be connected to only one side of the cable (i.e. do not connect shield to shield between boards).

External PSU or switched internal are a must for this design. I have built other units that used a different Neve style preamp card with the toriodal transformer on the opposite end and experienced no issues, but this does not seem to be the case here, even with Mu-Metal shielding that I'm using for my build. JLM audio has some very clever SMPS units and PSU boards that address this issue. I have some on order that I will swap with the PSU I currently have installed when it arrives.

The bias procedure is set like any other single ended +24V preamp. That is covered elsewhere as well.

Right now I'm trying to put all my boards together in a case as I've finally have all of the individual ones fixed and working. I hope this information is of use to anyone who is holding on to these boards. They're a great way to build a 1073 style project with the EK20033 switch without the need for using the original cards and the intricate off board wiring network that is required.




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