Studer 169 EQ in API 500 format

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OneRoomStudios

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Joined
Jul 8, 2004
Messages
287
Location
Minneapolis, MN, USA
Yeah, to echo Ricardus, your tech is wrong. R1 isn't anywhere near either of the DOA's.

Ref this schematic: https://drive.google.com/file/d/0B_...M0Yi00M2IzLTkwYTYtOWE4Zjg2YTk0MDE3/view?hl=en

R1 is on the inverting input of U1. When using a transformer, that should be disconnected (open), and the path to the non-inverting input to U1 should be shorted (usually R2, but a wire-short with a transformer). In any case, R1 has nothing to do with either DOA (which replace U2 and U3), and yes, it really should be disconnected (not shorted).

Based on your earlier comments, I'm assuming you're only using a DOA in the DOA1/U2 position. If the DOA really was fried, the only real possibilities are a bad DOA to begin with or a short or wrong resistor somewhere around DOA1/U2.
 

Ricardus

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Dec 2, 2018
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982
Location
Upstate NY
Weird, so tech says I blew the transistors on the DOA because R1 wasn't in and it overheated. But on the transformer pdf it says no R1should be in, plus the other unit works perfectly, or so it seemed? Confusing. Think I'll revert to 100% 5534's on both for now. Does it need R1?
Have you uploaded any hi-res photos of both sides of the board yet?
 

karloff70

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Joined
Aug 2, 2007
Messages
254
Location
London
I've built 6 of them according to the "With Transformer" PDF, and they all work fine without R1. I've tried them all with DOAs and 5534s, so the tech is wrong. There is something else going on.

No R1 means nothing though. No jumper, no nothing. Is that what you did?
Yup, had nothing. And yes, tech is wrong, as it didn't come back working either........lap 2 next......
 

karloff70

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Joined
Aug 2, 2007
Messages
254
Location
London
Lap 2 complete. Turns out I had managed to burn loose a track when soldering one of the caps around the output 5534 in without noticing. Sorted. What lovely EQ's!! Going to put some sort of DOA in for the mids as well, and add shelf freq choice switches, but have to move house first.
 
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