Curious MF behavior on Audiox 169 Recording channel

[Potato Cakes]

Hello, everyone,

I'm going back through and debugging some 169 recording channel 500 series units that Audiox put out a while back. The one thing that is making me scratch my head is on some of them, when I boost the mid gain and sweep the frequency to test for functionality, the gain of the boosted frequency decreases in the middle of the range by about 10dB compared to top and bottom of the frequency sweep. Checking the usual errors (soldering, components, etc) has produced no answers as of yet. I even swapped DOAs thinking there was something wrong with one of those and it was being loaded improperly as the frequency boosted as in the vicinity of 1kHz, but that also doesn't seem to be the case. I do have some units that give the same mid boost level through the entire frequency range, so I at least know it's not in the design. As I continue to look for errors on my part, I'm wanting to know if there is something else going on in the physics department that I am not thinking of which could lead me to my answer to the problem?

Thanks!

Paul

 

[Potato Cakes]

I've been doing more poking around and still haven't found the culprit. I've been comparing voltages between a fully functional and one with the MF problem. With both units' MF set to 1k and maximum MF band gain, signal being traced is the same up to where C21 and R26 connect. The one other side of R26 is where I seem to have discrepancies. Here's what I've done so far:

Replaced gain pot
replaced frequency pot
new opamps
swapped opamps from working unit
reflowed solder joints
component check

I've built a total of 6 of these units and 4 of them exhibit this strange behavior. I keep being drawn to area around the frequency selection as adjusting that it causing the issue, but I've pulled that potentiometer and measured it as well as swapped it out for a new one. I know the circuit works as I do have two of them that work, so I've messed something up four times on the others but managed to do a very good job hiding it from me on a very simple board. Something about when P4 is moved to the middle of its range the gain boost decreases to about 6dB less than CW/CCW. Either the feedback loop is being affected or some how an inverted polarity signal is being bled into the non inverted input of DOA2, but I'm not sure how the latter would be possible unless I purposely changed the circuit to do that. I'd like to avoid pulling apart a fully working unit to do more in depth comparisons, but unless I find something that seems to be the next step for me.

Thanks!

Paul

 

[Potato Cakes]

One of the problematic units I was working on did indeed have a faulty pot for the frequency select. When I would wiggle the shaft it would create very loud pops at different frequencies. Once I let it settle I saw that I was getting the proper amount of gain in the 1k-3k range. However when I would sweep the frequencies it would start to freak out so the pot had to be replaced. I had one more 2x100k log left and when I swapped it out it then was back to have the issue as before where the gain of the frequency would decrease as it approached the middle of the 2x100k pot's range (actually more like 1 o'clock - 2.5kHz) and then back up when turned fully CW/CCW. I'm going to order a new set of pots and see if that does it, but I am skeptical since when I pull the pots out they seem to measure fine. I'm not sure what else I could be missing that would cause 4 of these units to have the same incorrect behavior and the potentiometers are the only variable that remotely makes sense as I am using the same components on the fully functional and the ones that have the issue at hand.

I'm definitely open to suggestions as I think my trouble shooting process is starting to get circular so might be missing something obvious.

Thanks!

Paul

 

[Gustav]

One of the problematic units I was working on did indeed have a faulty pot for the frequency select. When I would wiggle the shaft it would create very loud pops at different frequencies. Once I let it settle I saw that I was getting the proper amount of gain in the 1k-3k range. However when I would sweep the frequencies it would start to freak out so the pot had to be replaced. I had one more 2x100k log left and when I swapped it out it then was back to have the issue as before where the gain of the frequency would decrease as it approached the middle of the 2x100k pot's range (actually more like 1 o'clock - 2.5kHz) and then back up when turned fully CW/CCW. I'm going to order a new set of pots and see if that does it, but I am skeptical since when I pull the pots out they seem to measure fine. I'm not sure what else I could be missing that would cause 4 of these units to have the same incorrect behavior and the potentiometers are the only variable that remotely makes sense as I am using the same components on the fully functional and the ones that have the issue at hand.

I'm definitely open to suggestions as I think my trouble shooting process is starting to get circular so might be missing something obvious.

Thanks!

Paul

Just offering a shot in the dark here, but I had a run of boards at one point (more than 2 year ago, but could be an old board), where the gold finger edge connector was not milled properly, leaving shorts in between the connectors at the very edge. Hairlines you wouldn't notice, but you'll see them clearly, if you look for them.

Edit:Never mind, recording channel - no boards ever made by me, I think.

Gustav