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

[eriksunding]

SOLVED! Faulty cable going from main pcb to ratio pcb. Two of the small wires had changed places with each other. Factory fault I assume.

 

[hrasmus]

Hi, i need some help fixing my Rev. A build, it works correctly most of the time, but some times it starts to act up. 
The GR- meter slams to the left and it compresses heavly, then it slowly recovers and all of a sudden compresses again, totally unrelated to the input signal.
When ever i try to investigate what might be causing this behavior it works fine. My guess is it is some how related to the ratio switches, because i was able to opperate with out problems for a long time as long as i kept it on 4:1. The GR meter slams to the left when i switch between ratios and then recovers after a few seconds, i don't know if it's supposed to be that way?
Thanks for your help

 

[Echo North]

Hi, i need some help fixing my Rev. A build, it works correctly most of the time, but some times it starts to act up. 
The GR- meter slams to the left and it compresses heavly, then it slowly recovers and all of a sudden compresses again, totally unrelated to the input signal.
When ever i try to investigate what might be causing this behavior it works fine. My guess is it is some how related to the ratio switches, because i was able to opperate with out problems for a long time as long as i kept it on 4:1. The GR meter slams to the left when i switch between ratios and then recovers after a few seconds, i don't know if it's supposed to be that way?
Thanks for your help

Could be a bad switch.  You could try replacing that.

Intermittent issues are always the trickiest.

Mike

 

[Spencerleehorton]

Hi All,

I'm having problems with the calibration of the rev A, setting the Q-bias seems fine, but onto the next step when adjusting R75 and putting MM on R74 i cant get the voltage any lower than 0.488v?
Have checked all resistors in the meter driver circuit and all come out correct.
The Vu meter wont adjust to 0db either whatever i do?
Could really do with some new ideas on how to fix this and get it all working please!!!
I might try and change Q12 and Q13?

regards

Spence.

 

[Spencerleehorton]

I changed Q12/13 and still the same, there seems to be a lot of distortion present in the sound, but not when i look with a scope?

Voltages around the meter driver circuit are off, at TP10 i'm getting 3v rather than 9.5v at the emitter of q13 i am getting the -1.4v, but im having to adjust the q-bias to get the meter anywhere near where it needs to be, which puts the threshold out!!?

I am using the OEP 2:1 on the input txf and the ED5002 as the output txf.

Could it be a dud meter or maybe the meter is the wrong type?

 

[Spencerleehorton]

right, ive put a 0db (0.774v) sinewave through diode bridge and put a series trim pot, ive adjusted trim pot so Vu meter reads 0db, i've then measured the trim pot and i have 120ohms.

Im now at a loss, do i have to change R67 to 120ohms?

 

[Spencerleehorton]

Have played around with putting trim pot on R77, R18, shorted out R67 and put 120ohm resistor in series.

nothing makes any difference, so i have taken pcb out and im checking all resistors and connections!!!

just found a 2n3705 in Q10 rather than 2n3707, dont know what difference that makes?

this is the Vu meter i have at the moment.

http://www.ebay.co.uk/itm/Set-of-Clear-Plastic-Audio-Amp-Panel-VU-Volume-Unit-Level-Meter-Indicator-/182339155354?hash=item2a7442b59a:g:JHsAAOSw5cNYGIcK

 

[Spencerleehorton]

well all i can think is that i am getting the distortion from having a wrong VU meter?

no way of knowing as, no one has seen any of my posts yet.

I have completely rebuilt the meter circuit and ive set it up as per the voltages rather than by using the vu meter, it sounds awful!!
loads of distortion, the signal is really hot?

when i probe the input with a 0.775v AC 1k signal going in i see a nice waveform, i follow the waveform through to Q3 and it looks good, probe on the output pot and signal is massive!!
Then probe to Q5 and signal looks good, and similar still at q5 to input, then after Q6 the waveform is huge!!
I can barely turn up my output past 7 o'clock, i have my input half way (12 O'clock)

Ive done a spreadsheet of all the transistor voltages and the only voltages that look off are on Q11, ive changed q11 and the voltages are still off as the meter circuit wont setup right.

its definitely moving towards the bin now!!!

 

[Spencerleehorton]

hanged the VU meter this morning and adjusted a few things, now i have a pretty good sounding rev A, nice and smooth if attack and release up to about half way, distortion monster if i drive the input and slam with the attack release right up.
will be ordering me some sifam Vu meters!!!!
teach me to be a cheap skate!!!

 

[Phrazemaster]

ER. I always had problems calibrating the meter in step 3.  I have the version 1.0 pcb with the suggested mods.  I ended up getting frustrated and just calibrating the meter to about 7 or 8 db reduction.  My unit compresses like it should, but something is funky with the meter that I never figured out

Hey all I'm just in the midst of calibrating a blue face - quick shout out to Mike/Hairball for STELLAR SERVICE!!! I'm using an old toroid at the moment but he is speed shipping me a replacement. Many thanks!

Anyway, I get past the first 2 steps flawlessly, and on step 3 I have the same issue as Phatmateo quoted above. I tried both Hairball and Mnats methods multiple times, unit warmed up over an hour each time. The reduction only goes to about 7-8 db shown on the meter. I can actually calibrate it both internally and externally to the same reading, but my concern is something is wrong. I run out of "pot" room to reduce it further.

How would I go about discovering what the issue might be? I haven't tried passing audio through it yet.

Thanks for any thoughts. I have of course searched this and read about 1/3 of this voluminous thread (~50 pages), but so far Phatmateo is the only reference to the issue that I have found.

I used 2N5088's with almost exact same Hfe readings on an Atlas DCA Pro, so the BJT's should not be an issue, and the FETs are Hairball matched.

This doesn't rule out something still wrong with the transistors, however. I used Carbon Comp on all signal path and metal film in Meter Tracking section.

All resistors and parts were not only double checked, but triple checked, sometimes multiple times with Fluke meter. I have OCD - not a bad thing for this hobby!

Other notes. I'm using a real O-12 at input. Followed the original schematic for wiring it up, using pins 1 & 5 and 6 & 8.

I have the 1.2.5 rev PCB.

I followed the hairball/mnats instruction but never found any instruction for wiring up pad 22, GRN, BLK found on the meter pcb. Subsequent reading seems to indicate this is not used when you have the attack pot with switch - is that right? Or have I gaffed and missed a major set of connections?

Any ideas as to what to look at here?

Thanks Kindly,

Mike

 

[scott2000]

, and the FETs are Hairball matched.

This doesn't rule out something still wrong with the transistors, however.

I've read there can be issues with these fets not being matched .....

There's this thread that touched on putting in a different pot value?????
https://groupdiy.com/index.php?topic=50547.0


And this guy

http://trentonblizzard.blogspot.com/2013/06/1176-gain-reduction-stage.html

Seems to be pretty knowledgeable about these.....Maybe you could reach out to him if you don't make any progress???

Good luck!

 

[Phrazemaster]

Scott that is really helpful thank-you. I verified all resistor values again but two - I couldn’t read the codes on a couple. Couldn’t measure them in circuit to verify so I removed one to check. It was right but I lifted a trace darn it.  Actually ruined two pads and traces! Continuity tests show the traces are still OK but they look like hell. Will need to route a couple wires for that one and lesson learned - for me - don’t remove components to test them, wow. I even used a Hakko desoldering station! Should have sacrificed the resistor to remove it!

Did attempt passing audio and the meter shows good movement on all GR settings except for “4” - the needle stayed at 0 and doesn’t move.

Cannot yet evaluate sound or if it’s even compressing. But audio does pass. High distortion when output is cranked but maybe this is normal?

Will repair traces and reflow some solder joints and report back. I may also just buy a pile of FETs, match them myself, and replace both the J309s and the 2N5457s (if memory serves those are correct names). I made a FET matching Jig (PRRs design) so that should be easy.

Thanks for support - great community!! This is helping me get over a horrific major unexpected girlfriend breakup. DIY therapy.

Mike

 

[scott2000]

I'd definitely take troubleshooting any day over heartache. Worst case I ever had was about two years. Time does heal . In fact, I can't think of anything good that came out of the certain times when I was being sad during the heart break.  Except maybe weight loss....lol


Don't ever break up with us!

We need you...

This is an interesting video about matching...

https://www.youtube.com/watch?v=hSFqkJn67eI

Good Luck!

 

[Phrazemaster]

I'd definitely take troubleshooting any day over heartache. Worst case I ever had was about two years. Time does heal . In fact, I can't think of anything good that came out of the certain times when I was being sad during the heart break.  Except maybe weight loss....lol


Don't ever break up with us!

We need you...

This is an interesting video about matching...

https://www.youtube.com/watch?v=hSFqkJn67eI

Good Luck!

You're the best Scott. All you guys. Thx for the community. I will check out the video. I'll be going to my studio tonight to do some more hand-wringing and attempts to repair and troubleshoot my Rev A beast!

I'll post back any results. Thx!

Mike

 

[Phrazemaster]

Hi Scott et al,

Just a status report. I was able to fix the damaged trace/pads, and also reflowed solder joints in ratio and meter boards.

Same presenting issue - cannot get a drop more than about 8db when doing the final calibration.

Scott I read that thread you copied - thank-you. They speak of changing the "bias pot" for 5k - do they mean the Q-bias pot? I have the 2k trimmer in there; it works fine for the 1st calibration. I'm not sure how that would help this step? I'm loathe to replace that pot for nothing.

I'll do it if it might help. Again to repeat, all FETs and BJTs were  matched. I matched the BJTs (2N5088's) using an Atlas Pro DCA 75; Hairball matched the FETS.

Would replacing the FETS with another matched set help perhaps? All my 5088's had HFEs of 620 - 625.

Thanks for thoughts in advance. I know I'm close on this and would love to get this handled!

 

[scott2000]

this was mentioned....???

Q bias adjustment. The pot was not centered before the adjustment and it was causing too much gain reduction - after correcting the Q bias, I now have full GR functionality.....


post #5293

https://groupdiy.com/index.php?topic=20058.5280


and brought up at #5313 later...

 

[Phrazemaster]

Thank-you Scott.

If that is the case then why do all the step 3 calibration instructions start with telling you to turn the Q-bias adjustment fully CCW? In other words don’t center that pot?

From the theory we want the FET just barely going into reduction.

I’m not saying you’re wrong I’m just saying I’m confused.

 

[scott2000]

I hear you....I'm just posting what I'm reading..... I've been following these threads a bit and don't mind looking around when I can....


Maybe back up a little on the posts I mentioned to get a few more bits......

I found this in the faq

http://groupdiy.com/index.php?topic=20058.msg461269#msg461269

 

[Phrazemaster]

Thanks Scott.

I can't get past the first part of the step 3 calibration, getting the -10 drop with front panel controls only. I can only get to -8db drop using the front panel controls. I run out of room on my input (t-pad) control; it's maxed out. I've tried this so many times now. R44 is a 5K trimmer on my build.

I found a great youtube video with yet another method for step 3 calibration, by (I'm assuming) a member here: https://www.youtube.com/watch?v=pgSWfsEzU40

This was the easiest method yet.

However again I can't get the -10db drop. Could it be I need to replace the FETs? They were hairball matched.

Unit passes audio and sounds good, but now ratios 4 and 8 don't work for GR. 12 and 20 work fine. It's like the unit isn't hot enough. I have to turn up input pretty high to even start getting compression. That's a Q-bias setting issue isn't it? Or...? Corn-fused!

I reflowed solder on ratio and switch boards, and really it's a clean build and done very carefully. Never had so much trouble with a calibration on a build before.

Could it be the Q-bias needs to be "hotter" for this to work? I read a post where someone had done this with 1.5db drop on initial Q-bias setting to get the needed room for calibration, but I'm a little worried to just change the tried-and-true procedure in case it means something else is wrong.

I'm also realizing there are literally thousands of posts on all this stuff...the one you linked to Scott was over 5000 in number. It would be so great if there was a better organizational system to all this than having to read, and poorly search, literally thousands of posts across multiple threads.

Thanks for any thoughts in advance.

Mike

 

[scott2000]

making sure you saw this mention in the video......(attached screenshot)




Great video! The comments section has some more bits of info too looks like....

I hear so many issues with the fets so, who knows...... maybe start back from the qbias and see if you missed something?????

You'll get it..... No problem


Hopefully it's just that part right between your monitor and chair.......lol...at least that's been my issue more than I care to count....

Man these things are riddled with calibration fiascos..... intense!