Mnats 1176 Rev D No gain reduction

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JCN1218

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Hi everybody,
I've been looking around the forum, especially the official build thread for this unit, and so far I haven't found a solution, though I am a little farther along than I was.

So right now, on channel 2 of my stereo build with rotary switches, I'm able to complete calibration steps 1&2, but I can't complete the final step because my unit seemingly has no gain reduction, and I can't figure out why. I've re-flowed solder into all of my solder joints, and the voltages on all of my transistors check out.

I feel like it's either an issue on the ratio board, or an issue with setting the qbias. My qbias, set according to mmats directions and also double checked using Hairball's directions, puts about -1.5 VDC at pads 7&18 with my black probe on CT/ground lug. My trim pot maxes out at about -3 VDC, so I usually go through half it's range before I see any volume drop.

It's hard to tell if my ratio switch is working since I'm getting no GR. But when I switch between the normal ratios I see the vu meter needle twitch as I'm used to seeing on my Hairball Rev A. However, when switching to 'all' mode I don't see any needle movement. It stays over zero instead of slamming hard right which makes me think an issue might be there, or is perhaps indicative of a problem elsewhere.

I'm probably going to check out the ratio board a bit closer when I get  home to see if there are any major issues.

Any help would be greatly appreciated. The unit passes audio just fine, and both input and output controls with as I would expect them to. I feel like I'm so close to finishing this unit and getting into the rack
 

scott2000

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I don't know anything about this unit but am interested in it for down the road......

I looked and think I saw some talk of similar issues starting around page 260 or 261 of the build thread ??? From hfe matching transistors.... etc...
You'll get it sorted out.....

Good luck!!

 

JCN1218

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So I finally managed to get channel 2 to 100% functionality. As it turns out, I was simply missing a connection between the ratio board and the output pot. It had happened because I had initially had the stereo link boards installed and was following a wiring diagram based around them, but then I decided to remove those boards to make troubleshooting easier and I never reconnected the one pad.

So it ended up being a stupid mistake on my part, but now I'm really scratching my head over channel 1.

So far, to both boards, I've rewired my output  to meter board connections, reconnected the one pad I had forgotten about, and resoldered most of the meter and ratio board connections. Unfortunately, none of these things have fixed channel 1's problem. When I send a 1kHz test tone through that channel, I'll have about a quarter turn on the input before I start to hear a high pitched whining sound overshadow the signal. This tone is also audible coming from the unit when my interface is muted. The meter on 'GR off' (pad 22 to ground) mode only ever shows a maximum of around -10 dB, which is usually around the first quarter turn and then doesn't move throughout the rest of the turn.

Channel 2 used to behave this way also, but I found the issue to be a transistor placed on the board backwards. Once I fixed that, it passed a clean signal. I've been over the channel 1 board many times looking for something stupid like that, but have yet to find anything. To make matters even more difficult, all transistor voltages on Channel 1 match up to the documents provided by mnats as well as the voltages on the working channel. I also checked the input of Channel 1 into the Channel 2 board and got normal results, so that rules out any problems with the input transformer/ t-pad attenuator.

scott2000 said:
I don't know anything about this unit but am interested in it for down the road......

I looked and think I saw some talk of similar issues starting around page 260 or 261 of the build thread ??? From hfe matching transistors.... etc...
You'll get it sorted out.....

Good luck!!

Thanks for the reply! I had matched the hfe of my transistors before installing, but a few pages after that post I found a few things that ended up leading me to double check my ratio board where I discovered the missing connection as well as some less than stellar solder joints. So far though, looking through that thread I can't seem to find anything that matches my current problem.

At this point, I'm totally stumped as to what could be wrong with Channel 1. I at least have one working channel now, but the whole idea of the stereo unit was to have two. Any ideas as to what could be causing this issue would be greatly appreciated. Thanks in advance!
 

scott2000

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I guess you are not alone in trying to get to the finish line. Seems a lot of people get hung up around this stage from what I've read. Have you tried posting in the build thread?

It still seems pretty active with the various builds. Even the older ones that aren't "supported"...

I know with 2 channel devices I've worked with in the past, I could sometimes compare readings to narrow down an area that looks hokey but, I'm not familiar enough with these. Even some respected builders I've seen on the thread have been having issues but thankfully they get them figured out. A large part is just missing some direction although there have been some things guys have had to do to make things work the way they need.

I'm still looking around. Will message you if I come across anything... I did search mnats 1176 high pitched whine and it kinda brought up stuff on page 84 of the thread......

Good luck!!!
 

JCN1218

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Thanks so much for the support Scott2000, I really appreciate your help so far. I might try posting on the official build thread if this post goes on for too long without a solution. I've been looking through the official thread for the past week or so and have seen some other posts that described similar issues but it seems like they either never got resolved, or they never posted their solutions to that specific problem. My next thought is that maybe I mixed up a transistor in the wrong spot or something. I'll check on that tomorrow.
 

Spencerleehorton

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I have also just put together a 1176 rotary switch version and i'm stuck!!!
at point 28 im supposed to get 10v but only getting 1v? strange thing is when i turn on the unit i can see 9.5v there for a millisecond then it goes down to 1v?
also when i turn the unit off, for a split second after i can see signal gets bigger and Meter comes to life?

any ideas?
 

scott2000

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If the build thread isn't helpful, I'd suggest posting schematics or whatever relevant diagrams  with the areas that are giving you trouble so someone not familiar with these units may be able to help out more .  May be a shot in the dark.....


Good Luck!

 

Spencerleehorton

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I’ve done just that, printed off a schematic and noted the areas where I need to know the correct voltages.

I’ve emailed hairball but no reply yet, my problem doesn’t seem to be documented.

With strange voltages appearing on one end of the same lead? One side it’s 0v and the othe end it’s -0.245v?

Very weird?
 

scott2000

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Spencerleehorton said:
I’ve done just that, printed off a schematic and noted the areas where I need to know the correct voltages.

I’ve emailed hairball but no reply yet, my problem doesn’t seem to be documented.

With strange voltages appearing on one end of the same lead? One side it’s 0v and the othe end it’s -0.245v?

Very weird?

some type of Phantom voltage? IDK what we're talking about tbh.....
 

scott2000

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Sorry. It's all foreign to me right now. Hopefully someone with experience can chime in with some help and numbers.

I did come across this somewhat unrelated topic regarding linking which was interesting .......

https://www.gearslutz.com/board/so-much-gear-so-little-time/650485-strange-attack-release-behaviour-two-linked-1176-a.html

Good luck!
 

JCN1218

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Spencerleehorton said:
This is what I’d like to find out.

All the marks in red is what I need.

I can check those voltages for you tomorrow since I have at least one working board. Sorry to hear that you're also having issues. Hopefully we can both get our problems straightened out soon. I've actually been taking a break from this particular project for the last few days to clear my head.

By the way, are you building a stereo unit? If so, are you having this issue across both channels?
 

Spencerleehorton

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Hi ya, no just building mono unit.

If I take off pad 19 lead the unit nearly functions correctly?
No GR though  :(

Get good level through with no pad 19 and voltages look fairly similar to schematic.
As soon as I reconnect 19 the unit cuts signal down to very little?
 

scott2000

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Spencerleehorton said:
Hi ya, no just building mono unit.

If I take off pad 19 lead the unit nearly functions correctly?
No GR though  :(

Get good level through with no pad 19 and voltages look fairly similar to schematic.
As soon as I reconnect 19 the unit cuts signal down to very little?

Taking off 19??? So that disables the dc from  getting to the fets that control the gain reduction and gain metering

When connected it feeds through your attack and release pots???
 

Spencerleehorton

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Progress finally!!
Changed the 6.8uf tants from 25v to 35v type, had 1uf tants in there which I’ve changed for electrolytics, now getting a little further.

Can setup q-bias fine, setting 0.00v on R74 fine but can’t get -10db drop yet? And my lorlin has just fallen apart on the ratio!! Tried to push on the knob a little too hard!
Earlier in the build the lorlin was setup wrong!! So I had to take apart and put it back together correctly!
Manufacturing problem!
Gonna use hot glue this time as once those tabs are messed around with the case to the lorlin is a bit loose.
 

scott2000

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Spencerleehorton said:
Is it possible that I am not meant to have DC after  C19?
Is this cap used to block the Dc going into the arrack pot?
And C20 doing the same?

If yes I shall replace these.

It's supposed to be rectified DC after the diodes (cr2 and cr3)going to the pots if I'm understanding it right....
and those 6.8caps should be low leakage types.....

You should contact this guy. I think he may have some info you might find helpful in this.... He was supposed to have uploaded some voltages on his blog a few years back but I don't think he ever got around to it..... He seems very helpful...

https://trentonblizzard.com/
 

Spencerleehorton

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At least I can connect pad 19 now!
Still not out of the woods yet as I have tested bass guitar through it and sounds distorted.
Think I’ve got input txf wired wrong, will change tomorrow.
Then do another voltage sweep to see what’s left to do.
 

Spencerleehorton

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have swapped the input txf round and unit is sounding good now, nice and clean with attack and release at half way, distorted and mean when turned up, havn't checked voltages out again yet but im gagging to record some stuff so i'm just gonna roll with it!!!
how you guys getting on?
 

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