Behringer T1952 Tube Compressor

[Tejaus]

I recently picked one of these up for cheap. Unfortunately Channel One is acting up and triggers the limiter and pegs the output meter without any signal applied. It will pass audio if the In/Out is switched Out. The tubes are OK. I believe they come in after the compression part of the signal anyway. I re-seated the ribbon cables at the Input Board on the back, no help. Tried switching the two cables to the opposite inputs, nothing worked doing that. It seems that something is shorted and is creating a signal after the input.

Channel Two works fine however. Anyone dealt with this before of have any ideas?

thanks

[Enzo]

Look for an op amp with DC on an output pin.

[Tejaus]

yep, found DC on pin 7 of IC7b and one underspec 47uf cap so far. Thanks Enzo.

http://music-electronics-forum.com/a...schematics.pdf

I'm trying to figure out why one NJM4580 is more than the other.

Electronic Components & Supplies

[Enzo]

One costs more than the other so I know which one to buy. I buy the less expensive one. I also am careful to note what form the part is, if I need a DIP8, an SOIC won;t fit.

[Tejaus]

I found out in the spec sheets that the "D" ranked ones have been binned for Maximum Input Noise for RIAA use. http://semicon.njr.co.jp/njr/hp/file...?_mediaId=9530

The ones I need are the DMP-8. Not quite the smallest one but still gonna' be tough soldering those tiny pins.

[Tejaus]

Well I replaced the opamp and the DC disappeared on Pin 7 but it still wasn't working properly. So, I finally broke down and bought an 100Mhz Oscope and a Leader LAG-125 off Ebay to chase the signal from the input. Too hard to do with just a meter.

Now after tracing the circuit it looks like this http://www.weclonline.com/downloads/pdf/07-04-0024.pdf Bestar BT-24S bypass relay is not passing the signal. It clicks but there is no signal on the output. I also discovered that the IC numbers on the schematic do not match the silked screened numbers on the PCB as well.

Anyone know where to source that part or an equivalent?

thanks

[Tejaus]

Finally traced it down to the THAT 2159 VCA. After tracing out the circuit using a magnifying glass turns out is was the culprit. A little tricky trying to diagnose since the PCB silk screen doesn't match the schematic. THAT Corp sent me two 2159 VCAs and after replacing the suspect one it's now functioning. Is the a procedure for setting the voltages with the Unity Gain and THD trims?

[Tejaus]

After reading the .pdf for the THAT 2150 Series it shows the THD 50K pot to be for adjusting the Symmetry between the bases of two internal transistors to reduce even order harmonics. It looks like I would need a THD Analyzer to really dial it in.

http://www.thatcorp.com/datashts/THA..._Datasheet.pdf

Page one of the schematic shows the pot and IC.
http://music-electronics-forum.com/a...schematics.pdf

[Jazz P Bass]

Then again you could use a scope.
Any harmonic distortion will show up as a deformation of the sine wave.
Check out this excellent link: Harmonic Distortion Pictures

[Tejaus]

Thanks, I'll try that out.

Thinking further I could probably use both channels of the oscope, send the a signal to both the compressor and the oscope and overlay the two wave forms to compare. Then adjust the trimmer to so the output signal matches the signal on channel two.

[Enzo]

Or invert one channel on the scope then display the two channels together with the ADD function. That way the one cancels the other - at least after levels are matched. And only the differences between the two will move the trace. You would then be seeing the distortion signal on the screen.

[Jazz P Bass]

Cool!
Thanks.
Another 'scopes are useful' tip.

[Tejaus]

Thanks again Enzo! That's a great idea.

I'm still have a long way to go learning all this stuff but it's fun.