[BUILD] 1176 Rev A - Back to the beginning...

[Echo North]

Yep - haven't deviated from 20:1 ratio in all of this testing, except to see if changing the ratio had a different effect, which has not been the case.

That's not what I'm seeing (20=3.8VAC, 12=1.8VAC, 8=1.2VAC 4=05.VAC) at the anode. 

What is your VAC @ pad 22?  Does it change with ratio?

0dBu input; 12o'clock; GR on.

 

[steve_h]

Are you getting those readings at CR2 or CR3?

Readings at pad 22:

20:1: 0.262 VAC
12:1: 0.154 VAC
8:1: 0.1 VAC
4:1: 0.053 VAC

 

[Echo North]

Are you getting those readings at CR2 or CR3?

Readings at pad 22:

20:1: 0.262 VAC
12:1: 0.154 VAC
8:1: 0.1 VAC
4:1: 0.053 VAC

They should both be the same.  Are they not?

I think there is something happening after the GR control amp and before the FET gate.

 

[steve_h]

Per my earlier post, they're pretty close:

CR2 anode - 1.363VAC
CR3 anode - 1.359VAC

Close enough to consider more or less the same, I take it? Where would you suggest I take measurements at this point? Any theories as to the culprit?

 

[Echo North]

Per my earlier post, they're pretty close:

CR2 anode - 1.363VAC
CR3 anode - 1.359VAC

Close enough to consider more or less the same, I take it? Where would you suggest I take measurements at this point? Any theories as to the culprit?

But they are not changing with ratio changes?

 

[steve_h]

Sorry, that's at 20:1. I tested pad 22 at each ratio, but not CR2/3. Here are those readings (tested at anode of CR3 specifically):

20:1: 1.335 VAC
12:1: 0.792 VAC
8:1: 0.52 VAC
4:1: 0.275 VAC

 

[Echo North]

Are your FETs in sockets?  Can you pull them out?

 

[steve_h]

Nope, as I noted earlier, they're not socketed, but I measured Q12 and Q13 in the past to make sure they were the same hfe and that 7-10 measured at 250 or above. I can carefully attempt to remove one or more if you think it will help troubleshoot the issue.

 

[Echo North]

Actually try this.

Usual test scenario.

does your gate DC on QI decrease when you increase the input?

 

[steve_h]

Q1? I get -0.9VDC at the gate, which doesn't seem to change with the usual test configuration when input is increased.

 

[Echo North]

Q1? I get -0.9VDC at the gate, which doesn't seem to change with the usual test configuration when input is increased.

Ya.  The -0.9VDC is probably your QBias, which is reasonable.

The GR control amp outputs a DC voltage (cathode of CR2 and CR3) and feeds it back to the Q1 gate moving more positive which in turn opens FET channel wider shunting more signal to ground (reduction).  Think of the drain/source as a trap door for the signal controlled by DC at the gate.

I think your problem is this control amp DC is not reaching the gates of Q1/Q11.

You control amp transistor voltages are fine.  There is a negative voltage trick happening at the output on the control amp. This is CR2/3 where your AC voltages seem a little low.

Check the value/connection of:
R58, R61, R62, R63 on the ratio board.  Check R52/R53 and C21 on the main board (values and C21orientation).  Check everything to do with your attack and release pots.

I'm off to bed.  Tomorrow night I'll look around and get some values to test for.

 

[steve_h]

I'll check on these and report back tomorrow.

Mike, thanks so much for your determination in helping me track down the source of this issue. You spent practically the whole day today following up with my posts and I'm majorly appreciative. Your work is a huge asset to the DIY community, which often relies on members to help one another further their projects and understanding. Thanks again. Catch up with you tomorrow!

 

[Echo North]

Also just double check that the FET/QBias is all working well.  You should be able to adjust your Qbias from no GR on the output (0dB drop), to a ton of GR on the output (like 10dB+).  If that is working, then set it back to the 1dB drop calibration.  You should see some meter movement if you do this in GR mode.

If all that checks out I'd say your FETs and Qbias are fine.

I'll do a little reading on the control amp, specifically the negative offset and post later tonight.

Mike

Edit: Ya I really want to test the part of the ratio board that controls the offset/threshold.  If that's off that could be the culprit.

 

[steve_h]

Ok - here's where I'm at. I pulled the ratio board and tested the resistors, as well as touched up all of those connections. All resistors measured fine, if not a little off from the expected reading, which I think would be chalked up to their being in circuit and standard tolerance level differences. the 10M resistor wasn't giving me a close reading, but as with all resistors, I checked them out prior to installing.

FET/QBias is all working well - I did this step a few times in the past to make sure this wasn't the culprit - this step has never been an issue and the changes to the Q Bias adjust trimmer are reflected on the meter in GR mode. FETs and Q Bias seem ok per earlier testing anyway, correct?

R52/53 measure 47k and I've got a 100uf 35v cap oriented correctly in C21. Regardless, I touched up these connections with a little bit of new solder and tried again to get a GR reading with usual test settings - no luck.

Here's a picture of my attack/release section, which I verified against Mako's guide and have tested all connections for continuity, as well as ensured that it all looks ok and is adequately connected. The other image shows the meter driver section:

http://imageshack.us/photo/my-images/807/56078075.jpg/
http://imageshack.us/photo/my-images/7/76952956.jpg/

Will keep poking around. Looking forward to your response and taking more readings to hopefully close in on the problem.

 

[Echo North]

When I get home I'll poke around some more and post.

So to summarize (in case someone wants to chime in):
- Preamp and Lineamp seem to be working fine.  Unit passes audio fine.
- FETs are getting the proper QBias voltage and appear in working order
- Control amp transistors have the proper DC voltages
- Ratio ladder B seems in working order and is supplying signal to the input of the control amp
- Signal is reaching the rectifiers CR2/3, but seems a little low.

Problem: Unit is not compressing.

Next step will be to check the ratio ladder A.  It supplies a negative offset voltage to the CR2/3 anode.  I'll get some proper voltages for you tonight. 

Mike

 

[Echo North]

Read and check this if you have time Steve:
http://www.groupdiy.com/index.php?topic=29981.msg524243#msg524243

Seems to have a very simular issue to yours.  Probably not it but something to triple check.

Thanks,

Mike

 

[steve_h]

Thanks Mike. Just checked and they look ok - R41 measured 265R and the release pot resistor measured 257.7k.

I've definitely searched through this thread and the Rev D thread for reports of issues similar to mine and in cases where they found a solution, checked against my unit to see if I could replicate their fix. No luck unfortunately.

 

[Echo North]

Cool.  I'll have something in like 3 hours.

 

[Echo North]

Try this real quick.

Measure the anode of CR2/3 for DC.  No signal and don't worry about the controls.  Measure the 4 ratio's.

You should see a negative DC for each one.  Are they the same for each diode?  What are they?

I think I might be geting this sorted (maybe).  I've become obsessed with this one...I think the issue maybe somewhere completely different.

 

[steve_h]

Great! Just took this measurement and got -9.84VDC. This was consistent with both anodes, but didn't change when ratio was changed.