1073 style filter board puzzlement

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Potato Cakes

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Hello, friends,

I have been working on developing a product for live use and one of the features is a 1073 style EQ. To show the concept to potential customers and to give guidance to the designer, I've cobbled together some boards that would perform the same function  so I and others can see it all work as one unit. However, the boards I'm using are causing me a bit of trouble at the moment as I have been finding and correcting discrepancies between the BOM and schematic provided, which also corresponds closely with the original 1073 circuit. Right now I'm focused on the filter board. This particular PCB has the make up gain circuits from the BA284 card included, so you can use it as a standalone EQ if desired. Where I seem to be stranded is getting signal to the output from the second amplifier stage. The culprit(s) seem to be related to C25B which has a large red arrow pointing to it in the attached PDF. With the standard issue 1kHz tone at 0dBu connected to the EQ input and C25B installed, signal dies where R2E and R3E join and feed the input of the amplifier in question at C18B. When C25B is removed, I get 5.2V on it's postive solder pad and 0.26V on the negative pad, which is what I get at the EQ output. I verified I was not getting a false reading (which I had been) by noting that the aforementioned voltages changed proportionally with the adjustment of the output level of the oscillator. Output is increased further with R32X (10k) removed. I also tried removing the resistor that feeds the HPF section to see if something downstream was causing an issue, but I still had the same results. So good news I have at least isolated the location of the problem, but I don't know how or why. The 0.620V that are present at the CCW and wiper (depending on wiper position) of the MF-Level are not affected by this process. With the exception of C2X and C3X, the amplifiers are identical circuits according to the schematic. I even checked continuity on all the connections in the second amplifier section in hopes I would find an issue with the board but I did not. I am getting the same VDC on the transistors respective of the first amplifier.

I have built several other versions of 1073s and I have yet to experience anything like this.

I usually come here asking for this kind of help as a last resort. I've honed my troubleshooting skills to the point of being able to solve most of my problems, even sometimes thinking of the solution as I'm about to post a question here. But I believe my limited electrical knowledge has been pushed a little further beyond it's limits with this one. If the attached portion of the schematic seems in order, then I'll have to go back and stare at the traces some more. If anyone does see something wrong please let me know. Again, this is pretty much straight from the filter and ba284 makeup gain section of the 1073, so I'm a bit lost as to what could be killing the signal.

Thanks!

Paul

 

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  • 1073 Filter Trouble.pdf
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