Furman parametric eq distorted

[bigdrums925]

I have managed to get the second channel working more less properly, thank you all!
But I now need to make the adjustments to have the 2 channels work similarly.
With the EQ disengaged via front panel switches, the out puts are the same.
Engaged they are quite different in gain depending on the frequency.
There are a total of 6 trim pots on each board.
As labeled on the schematic, - (infinity) & zero adjust, 1 of each for lows, mids and highs, as labeled on the board not the schematic
The adjustment of these do not seem to have the effect one might expect, particularly regarding the lows, mids, highs.
I have been able to get the highs balanced pretty well, not so much on the lows and highs.
Anyone able to suggest a method of going about getting it adjusted?

[bigdrums925]

Still need help with this. The trimpots won't bring the two sides into similar response. I thought there must be some point where a resistor shunt to ground must be draining off more signal on one side than the other but found no such point. All the op amps are socketed so tried swapping one side to the other. None of these has worked. It's difficult to find the point where the signal attenuates with the schematic and the actual circuit being so different. There are similarities but its not all that helpful trying to chase the signal path.
BTW I can't seem to upload any pictures

[g1]

Are any of the pots dual (l/r) ?
If so, make sure the 2 halves are matched. I had a problem similar to post #16 once. I turned out to be a pot that was too far off spec from one half to the other.

[TimmyP1955]

Run the output of the bad channel into the input of the good channel with a polarity reversing cable. Monitor the output of the good channel. Set all external pots to "no effect" then adjust the bad channel's trimmers for maximum signal cancellation. (Use a high resolution spectrum analyzer if possible. Borrow a Behringer X series digital mixer - each channel has a 100 band analyzer.)

[J M Fahey]

Still need help with this. .......
BTW I can't seem to upload any pictures

Please describe how you do that.

Pictures might be way too large to be accepted here, both in pixel count (avoid any setting larger than 1600 pixels by 1200 something) or Megabytes or file size, check whatīs allowed.

Many Cellphones canīt be dialed down, then take picture at whatever size itīs set to but then downsize it to something Forum acceptable.

Or plain "attach" them as files, instead of images.
We open and check them in another page or downloaded to some image viewer.

[bigdrums925]

There are 3 stacked pots per channel for freq selection 1 pair per band rated at 10k measured between 98K to 138k end to end
They are mounted on the front panel and none of them are L/R

[bigdrums925]

[QUOTE=TimmyP1955;525427]Run the output of the bad channel into the input of the good channel with a polarity reversing cable. Monitor the output of the good channel. Set all external pots to "no effect" then adjust the bad channel's trimmers for maximum signal cancellation. (Use a high resolution spectrum analyzer if possible. Borrow a Behringer X series digital mixer


I really like this out of the box idea but there is too much of a gain difference between the channels I need to solve first.
Still I will use this once the two channels are similar

[bigdrums925]

Tried uploading a small 1.8K pic using the toolbar and still no good. How do I attach a file?

[g1]

Below the text box, there is a button that says 'go advanced', click it.
Then scroll down and click on 'manage attachments'.

[bigdrums925]

So what I can say for certain: Starting with a 100 Hz input signal (wanting to deal with the 1st lowest band first) split input to both channels, then starting with the High gain output working backwards to last op amp in chain, pin 1 on each channel is much different in gain. So check pin 3, same, pin 2 same. Check pin 7, both are strong and equal, pin 6 same.
I have swapped the good channel op amp with the one on the weak channel, no change. I don't get it!
Pin 1 in the picture is the one marked in green. Red is pin 2 circuit. The board in the distance is basically the same board turned. So the op amp in the lower center of the board is the last in the chain.
Green dots show where the pair of diodes end up, that feeds into the To92's center pins. Pin 2 (red marks) going up between the diodes feeds the small electrolytic. That feeds the white wire which is the high gain output jack.

[g1]

If there are series coupling caps check/replace them. Also any electrolytics in the op amp circuit.

[bigdrums925]

[ATTACH=CONFIG]53082[/ATTACH]
this is the best view of the problem board so far

[J M Fahey]

That view is very clear.
I guess in an idle weekend afternoon you will be able to draw the schematic.
I would also print the "official" schematic and keep it side by side as reference, drawing a small portion "fresh" and searching for similarities in the published one.
Different versions may very well have different part designators, as in, say: it has 60 resistors named "R1 - R2 - ..... R59 - R60", now they add a series resistor at the input and start counting again ... NO value will match because new R1 didnīt even exist before, old R1 is now R2 (different place and value than before) and so on ... but the basic circuit is still the same, what else? ... besides minimal mods here and there ... such as added R1.
If Furman designed an excellent and complex Parametric Equalizer, to boot using same controls, panel, chassis,etc. and same model designation, I canīt imagine they redesigned something *completely* different, it does not make sense.

Ok, hope you can draw at least one section, input, output, whichever you like, so we can compared it to "official" schematic

[bigdrums925]

I checked all 7 caps on the bad board by removing them and they were all within 10% of the printed values

[bigdrums925]

I found some variations in the readings I was getting related to the scope and probes themselves
After that was handled, and found much more similar output from the 2 channels except in the lowest range ~100 Hz
In the 400 & 2K ranges both sides were approx. equal.

I will take your advice and give some effort to drawing a schematic or at least parts of it.
So far trying to make sense of it makes my head swim, but does anyone know of a simple schematic
drawing program I can download to help in this early effort?