dmp

Re: 525 build thread
« Reply #620 on: December 03, 2019, 12:49:10 PM »
Can you  measure for the control voltage?
If you feed a audio wave to the unit and measure the DC voltage at the attack pot, you should see the control voltage go up as you increase the signal level feeding the unit.


duantro

Re: 525 build thread
« Reply #621 on: December 05, 2019, 12:37:16 PM »
I swapped out a fresh vcr2n, and no difference. DMP are you saying measure the dc voltage at the attack pot, while increasing signal level?

dmp

Re: 525 build thread
« Reply #622 on: December 05, 2019, 01:40:35 PM »
Yes - I would start out measuring on the attack pot (the side that faces the VCR2N) for the control voltage.  You probably will not have any voltage there. If you don't, then work your way back through the sidechain to see where things are getting messed up.
The basic operation is an audio signal goes to the ceiling control, which sends a signal to the sidechain (the ceiling changes a dual voltage divider to adjust the threshold and output together) . The signal to the sidechain  is amplified and rectified to a DC control voltage. That voltage changes the resistance of the FET (VCR2N) in the first 2520 amp circuit, to modify the gain of the amp.

duantro

Re: 525 build thread
« Reply #623 on: December 16, 2019, 06:53:28 PM »
Thanks so much DMP, for the detail explanation. I was sick over the weekend and now am backed up with work, but will check this asap!

duantro

Re: 525 build thread
« Reply #624 on: January 07, 2020, 06:00:18 PM »
I finally got around to looking at this. Spent about 5 hours searching for components and retracing wiring, etc. Still the same results. I'm not getting any dc voltage at the att connection on the meter board. I am getting between -2vdc to -12vdc on the top lead of r50 depending on how hard I send input audio. I double checked the wiring and solder joints going to the release time board. Input pot changes the meter reduction, but does nothing to the sound. Output pot is working, but when it is ccw there is still like 30 % audio coming through. Sw5 just makes out put gain hotter as turned cw. So frustrating...

dmp

Re: 525 build thread
« Reply #625 on: January 08, 2020, 11:09:19 AM »
You have a negative control voltage on R50 but not on the attack pot???
Which side of R50: CR5 or CR12/CR11?

duantro

Re: 525 build thread
« Reply #626 on: January 08, 2020, 12:42:26 PM »
You have a negative control voltage on R50 but not on the attack pot???
Which side of R50: CR5 or CR12/CR11?
On the CR11/CR12 side. I'm wondering if I should just jumper a wire from R50 to the inside att/c3 junction on the meter board, as it seems to die en route somewhere. The previous owner attempted a few repairs, perhaps a solder trace got too hot.

dmp

Re: 525 build thread
« Reply #627 on: January 08, 2020, 12:48:12 PM »

From the schematic the R50/CR11/CR12 node should connect to the Attack pot / C3  - so yeah something seems funny
I don't remember what the routing looks like and I don't have one of mine with me to look at now, so can't be much help.

duantro

Re: 525 build thread
« Reply #628 on: January 08, 2020, 12:59:31 PM »
That fixed the side chain! Compression at last! ....However, I'm still getting around 20-30% signal passing through, with the output pot turned off. Time to search for that and calibrate.

dmp

Re: 525 build thread
« Reply #629 on: January 08, 2020, 01:08:23 PM »
Nice!

The ceiling also attenuates the signal


duantro

Re: 525 build thread
« Reply #630 on: January 08, 2020, 01:18:04 PM »
Nice!

The ceiling also attenuates the signal
Right. I'm talking about the output pot itself. Regardless of the ceiling control position, I still get signal with output off. I think it might be time to double check the calibration now. Thinking of making sure R7 isn't way off.
Also, i need to do the mounting screw mod to disconnect gold finger 5 and 13 from each other. Does this image look correct to cut the red line?
Edit: i cut that and the same section from pin 13.
« Last Edit: January 09, 2020, 01:50:04 PM by duantro »

duantro

Re: 525 build thread
« Reply #631 on: February 11, 2020, 04:44:32 PM »
I finally got some free time to check these out, and I'm now getting low output. Signal drops down after R1. Q5 checks out E-B @.6VDC. 2520 voltages are 15.6VDC. Did I cut the wrong traces at gold finger 5 &13? DMP did you cut the trace on the top or bottom? I cut the bottom. I'm still getting continuity to chassis from pin 5 and 13, so thinking this is the issue.
EDIT: this cut on pin 5 is incorrect!
« Last Edit: February 12, 2020, 01:24:48 PM by duantro »

dmp

Re: 525 build thread
« Reply #632 on: February 11, 2020, 05:20:53 PM »
I went back to try to figure out what you were asking and thinking it is this mod, 10 years ago so I don't remember details. But the 5&13 had to do with the ground plane - should be pretty easy to see what is connecting to the ground plane.

From the 500 series specs, pin 5 is audio ground and pin 13 is power ground.


I think I found the proverbial smoking gun with these 525s.
The ones I built both had hum and a non-musical distortion (same as discussed on  pg 14 by Mich and Peter). The distortion was not caused by the FET - it was caused by the meter circuit. After turning R55 all the way off (so the meter would not light at all), the distortion disappears. There's a ground problem when the meter circuit shares the same ground as the in/out pot grounds. I think the current from the meter circuit was causing a ground loop between in/out pots and A3. Easy fix - I cut the trace leading from the neg side of CM1 to the GND on the meter board, and ran a wire from Neg CM1 to audio ground at the hole by gold finger 5. With this change, both of my 525s are quiet & distortion free. Even at heavy compression they sound smooth - a huge improvement. And the hum is gone
A second mod I'd recommend - after tightening down the mounting nuts a few times (or a zillion in my case), the silkscreen over the ground plane developed shorts, which connects audio ground to the chassis, not desirable. Easy mod was to cut the traces by gold fingers 5 & 13 to disconnect the pcb ground plane from audio ground. Then the short to the chassis through the mounting bolts makes the pcb ground plane a chassis ground, which is better for noise rejection.
With these mods the 525s are fantastic. I just played through them with guitar and singing and they are smooth and clean.

duantro

Re: 525 build thread
« Reply #633 on: February 11, 2020, 09:34:37 PM »
I went back to try to figure out what you were asking and thinking it is this mod, 10 years ago so I don't remember details. But the 5&13 had to do with the ground plane - should be pretty easy to see what is connecting to the ground plane.

From the 500 series specs, pin 5 is audio ground and pin 13 is power ground.
Crazy it’s been that long ago!
Yeah I understand everything about the mod, except deciphering where the trace cut goes. Do you remember if the ground plane that pin 5 &13 need to be  disconnected from are on the top, bottom, or both side?

dmp

Re: 525 build thread
« Reply #634 on: February 12, 2020, 01:21:13 AM »
do the nuts on the top side sit on a ground plane?  should be able to trace to the connections with a multimeter

duantro

Re: 525 build thread
« Reply #635 on: February 12, 2020, 01:22:16 PM »
Yes they do. I've deducted that the trace I cut on pin 5 underneath wasn't correct. I've soldered it back and cut the top trace to the left of pin 5. Audio and power ground now are unconnected from the chassis ground plane. I'm still not getting any gain reduction now  and output is low and intermittent. If I push on the output pot it comes back in, however it is still quiet. I'm thinking I may have heated up the solder pins so many times that the pots might be dead. I'll order some new pots and see if that will save this thing from the trash can!

dmp

Re: 525 build thread
« Reply #636 on: February 13, 2020, 12:08:41 PM »

The calibration of trimmer R7 adjusts the amp gain. If you are getting low signal with the output all the way up, you need to adjust R7

Quote
Regardless of the ceiling control position, I still get signal with output off.

Either your output pot was dying, or it was not connected to ground.  Make sure once you get the output pot replaced, that it is seeing audio ground (pin 5)


 

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