Slightly Odd behavior with frequency sweep in Studer 169 type EQ

[Potato Cakes]

Hello, everyone,

A while back I built some of the Studer 169 recording channel 500 series units with the various mods to the circuit. All of them seem to work great except all but one have this curious thing that happens when I sweep the mid band frequency. At the top and bottom of the range seem to be giving me the appropriate amount of gain for those particular frequencies but when I sweep towards the middle of the range (800-3k in particular) there is about a 6dB drop in signal and the bandwidth widens as you would see in a proportional-Q equalizer. One of the units does not do this, and once again I am a bit baffled. I've changed the pots involved with this part of the circuit, I've compared components with the working unit, swapped op amps (using DOAs) and all of the other usual things one might do. To me it almost feels like there is something phase cancelling at the DOA in the feedback path, but I don't know how that could happen looking at the schematic. Plus according to what I am seeing, the mid band section is an island all it's own before it heads to the output section. There isn't much to it, so I don't know what I could be missing.

I've attached the full schematic. I would appreciate some new ideas where to look or even understand more how what I am experiencing could be possible.

Thanks!

Paul

 

[pucho812]

When I was working as head bench tech for pro audio company”leaving name out@ we did a production run of say 300 of a Particular unit during a quarter. In testing 5 units out the lot tested funny in the eq section.  Everything looked as it should, but clearly we had an issue. So I had myself and my underling test every component and compare between a known good unit. All it takes is a decimal spot in the wrong spot and your 1k resistors are a different value causing havoc in your eq. While I can’t say that is the case here, if you have a working unit and a unit that misbehaves, test and compare between units and the schematic to see where the fault lies.

 

[Potato Cakes]

I definitely have been comparing the values to the known working unit as stated above, not to say that I'm still not missing anything. I thought for a minute I had found the problem, but realized I was misreading the board layout. I'll poke back at it sometime next week so during the interim I'm trying to gather ideas on where to look as I'm currently just going in circles in my troubleshooting.

Thanks!

Paul

 

[PRR]

> towards the middle of the range

Crappy tracking in P4.

 

[Potato Cakes]

> towards the middle of the range

Crappy tracking in P4.

I thought something similar, but I swapped that out with new pot and the same thing happened. Looking at the schematic again I see that the feedback path is indeed fixed so it was doing something to cancel the signal it would be for everything, so P4 is the only thing that makes sense. I'll have to swap the pots with the working one to see if that confirms this.

Thanks!

Paul

 

[gyraf]

I've had something similar with a batch of pots where some of them had an almost-short from track to metal housing (and thus to chassis>ground).

Quite hard to find, as error disappeared every time it was disassembled for testing

/Jakob E.

 

[Potato Cakes]

I've had something similar with a batch of pots where some of them had an almost-short from track to metal housing (and thus to chassis>ground).

Quite hard to find, as error disappeared every time it was disassembled for testing

/Jakob E.

That may be very well what's going on here. I'll have to order some more pots and see what happens.

Thanks!

Paul

 

[PRR]

> swapped that out with new pot and

Two bad-track pots from the same batch would not shock me.

There's no value or taper on that drawing(?) so I will guess. Grab 5% match pairs of about 3k, 10k, 30k resistors and tack them in place of the pots. If that gives expected response, your pots don't track. If it still sucks, something else is wrong.

I don't like the apparently-paralleled '5534 chips. It should not be needed. But I don't think that is your problem. Only that they will tend to run hot, for no good improvement.

 

[Fuzz Face]

I don't like the apparently-paralleled '5534 chips. It should not be needed. But I don't think that is your problem. Only that they will tend to run hot, for no good improvement.

The schematic looks that way because the PCB allows the builder to opt to use ICs or DOAs, but in practice they are not paralleled.

I like your idea to use resistors in place of the pot for testing

 

[Potato Cakes]

I've had something similar with a batch of pots where some of them had an almost-short from track to metal housing (and thus to chassis>ground).

Quite hard to find, as error disappeared every time it was disassembled for testing

/Jakob E.

Just wanted to let you know that I got back to working on this as I had a new batch of 100k pots and it was bad tracking of the old pots. Put in new pots for two of the 5 five that are problematic and both of the ones with new pots have consistent mid gain across the frequency range. Thanks for pointing to this. I was definitely scratching my head trying sort out this mystery.

Thanks!

Paul