Tab U73 Threshold Pot/Trimmer Modification (Solved and done!)

[inputhone]

I tried to put out the pot for Makeup Gain and Threshold on my TAB U73 (Without B) and something does not work. As i understand from the Manual and Schematics the trimmer (pos. 62) does set the makeupgain in compressor mode and the trimmer (pos. 64) levels the threshold to keep the signal within a maximum level during regulation, as taken from the calibration process descriebd in the manual.

http://www.irt.de/IRT/publikationen/braunbuch/U73_U83.PDF

The 100k makup potentiometer i put on the front works fine, but the threshold seems not to work. It just starts to produce clicking noises when turned halfway full. So i assume it is not just the trimmer for the threshold (pos. 64)? What does it exactly do? 

Any opinions on that?

 

[Michael Tibes]

I I get things right, then you are on the correct path. Unfortunately this doesn't resolve your problem. Did you replace the 'lytics and adjust the balance trim? Try new tubes?

Michael

 

[inputhone]

The lytics are replaced, and it is working fine. I did not adjust the balance trim. I don't know how to produce an impulse with 1-2sec pauses as ask for in the manual. I only have a funtiongenerator to produce a steady signal.

But i assume it is good, because clicking noises normally are very low. But if you think i am on the right track, maybe i have done something wrong, as there is really no threshold thing happening when turning the knob - which is weird.

I have two new tubes laying around but not all 4. Might be stupid to replace just two of them..

 

[inputhone]

i have to add something: i just checked it all again and it seems like in "begrenzer" mode the knobs seem to work
i can adjust the threshold through the applied knob and also the make up.

when it is in compressor mode - i just can adjust the makeup and it is much louder at the lowest setting and the clicking occurs when i turn the threshold in.

strange cause: i thought for threshold in begrenzer mode you need to adjust the  pos. 79 for threshold?????

any ideas michael?

edit: i also tried replacing the e80cf and e88cc tubes with new ones and it did not solve the issue. weird, what could be causing this? 

 

[Michael Tibes]

... I don't know how to produce an impulse with 1-2sec pauses as ask for in the manual. I only have a funtiongenerator to produce a steady signal.

I have assembled this in wavelab. The absolute values are not too critical, you should be able to build this test signal in any daw. Just make shure not to have clicks in the burst.

Michael

 

[Michael Tibes]

i have to add something: i just checked it all again and it seems like in "begrenzer" mode the knobs seem to work
i can adjust the threshold through the applied knob and also the make up.

when it is in compressor mode - i just can adjust the makeup and it is much louder at the lowest setting and the clicking occurs when i turn the threshold in.

strange cause: i thought for threshold in begrenzer mode you need to adjust the  pos. 79 for threshold?????

any ideas michael?

edit: i also tried replacing the e80cf and e88cc tubes with new ones and it did not solve the issue. weird, what could be causing this? 

Pos 79 is only for 'begrenzer', 64 sets the general input level of the rectifier and should always work. Pos 62 is also always on.

I'd maybe start with a general setup and make sure the unit works as it should before you try to mod it? The clicking might be caused by a misbalance, if I remember right the clicking turns more into a plop when you adjust the balance.

Michael

 

[inputhone]

Well it was working good as i remember before is started to mod it. i now did the balance calibration on pos. 61 and in begrenzer mode it is possible to set the plop to nearly not existing, while in compressor mode you can always hear it more. i wonder what can be causing this. as it works well in begrenzer mode (also my knobs i put to the front panel) there has to be some issue wit the compressor section.

if you set the threshold to fully off than you dont get the clicking in compressor mode. maybe it was set to something like that before, because the limiter always grabs in even in compressor mode, so maybe i did not realize something was not working.

is there something in the circuit you can think of that might be working wrong affecting only the compressor mode? if a tube was broken it would effect both right?

i had all electrolytics recapped so there is nearly no chance it can be that.. 

edit: here you can listen to what it sounds like:

this is begrenzer mode makeupgain-poti set 75% clockwise turning slowly the threshold in until fully clockwise: http://www.funkstudio-berlin.de/begrenzer_gain75.mp3


this is compressor mode makeupgain-poti set 0% clockwise turning slowly the threshold in until fully clockwise: http://www.funkstudio-berlin.de/kompressor_gain0.mp3

 

[Michael Tibes]

Doesn't the compressor mode have 10 db or so more gain at the input? This might lead to more gain reduction at the same input level, therefore louder pops?

Michael

 

[inputhone]

yeah, that's right but it seems like not only be a normal pop issue.

i just did the calibration from the manual and it is not possible to set pos.64 for the maximum threshold level as it starts to click frequently very fast when turning it in in "kompressor mode". even just with the 1khz signal set to the makeup-level as said in the manual. that means something is damaged (but what?). it sounds quite the same as the sound samples i just posted (look above). there is no such thing when i switch to "begrenzer mode" and the knobs seem to grab!

any ideas where to look?

 

[inputhone]

i put the original trimmer back in place and suddenly the issue is gone. it seems like something was not working with the new poti! very strange..

i measured that the originial trimmer actually says 130kohm not 100kohm as in the manual but i guess that's just due to tolerance. i used a linear alpha 100kohm poti and wired it to the parts exactly as it is now. why could that have happened? is it possible that the long twisted wires i applied did cause that problem? i have no idea..

anyhow the real trimmer works fine and i am able to adjust everything. so no broken u73, but now i just have to find out why i cant wire it to the front!  :-\

 

[inputhone]

For everyone who is interested: I did solve the issue today. it turned out that the weird clicking sound only started to appear when the wires that led to the makeup-gain-poti and the wires from the threshold-poti came close together. separating the wires solved the issue and i now have access to the threshold and makeup on the front! yeah!

ps. i also calibrated the unit to the manual and marked the settings for the knobs where it is set up like in the manual, so i can choose how far to go depending on the input level. mhhh, sounds amazing on vocals and percusive bass and guitare!

 

[opacheco]

For everyone who is interested: I did solve the issue today. it turned out that the weird clicking sound only started to appear when the wires that led to the makeup-gain-poti and the wires from the threshold-poti came close together. separating the wires solved the issue and i now have access to the threshold and makeup on the front! yeah!

ps. i also calibrated the unit to the manual and marked the settings for the knobs where it is set up like in the manual, so i can choose how far to go depending on the input level. mhhh, sounds amazing on vocals and percusive bass and guitare!

Dear inputhone,
Could you do a description about the differents pots that you add to your U73 in order to get more control please(Threshold control, Make up Gain, Ratio Control, Knee control...etc)??

Thanks
opacheco.