Allison Research Gain Brain LED problem

[hereforever]

I have 4 of these, 3 are work great but 1 has issues.

On the broken module, with no input signal & the IN/OUT switch in either position the -12 LED remains lit

I've replaced all the electrolytic & tantalum caps.

The sidechain voltage on the broken unit seems to be in a similar range to one of the working units (~0.9v).

The schematic (especially the metering circuit) is really confusing me and I just seem to be working in circles.  It doesn't help that there seem to be a couple of versions of these.  The most legible schematic I found seems to be for an earlier version than what I have.  I redrew the portion of schematic to include the revisions (attached).  On the next post I'll attach a scan with some of my voltage readings marked.

I'd really appreciate any leads in the right direction...thanks!

 

[hereforever]

Schematic with voltage readings added...

The voltages in parentheses are on the broken module.  All voltages taken with no input signal

 

[Bo Deadly]

It's probably a bad solder joint. If you press on the PCB with something plastic or tap on it around the 3 transistors for that LED, does it flicker off and on?

With the unit off and unplugged and with supply caps drained, measure resistance between the LED cathode (the minus end) and ground. If it's low, the transistor is dead short and needs to be replaced (which would be a bummer because it looks like a weird MPSA13 darlington transistor). If it's not, sanity check resistance from the base of the transistor the LED is connected to and the far end of the parts connected to it. If it's too high, one or both of the solder joints is bad. If it's too low, it's a dead short transistor or resistor.

To fix a bad solder joint, retouch it with the iron and a dash of new solder to get some fresh flux into it.

Or just try retouching all of the solder joints of the 3 transistors for that LED. If that doesn't work retouch the the solder joints of all the resistors connected to those, etc.

 

[JohnRoberts]

I have 4 of these, 3 are work great but 1 has issues.

On the broken module, with no input signal & the IN/OUT switch in either position the -12 LED remains lit

I've replaced all the electrolytic & tantalum caps.

The sidechain voltage on the broken unit seems to be in a similar range to one of the working units (~0.9v).

The schematic (especially the metering circuit) is really confusing me and I just seem to be working in circles.  It doesn't help that there seem to be a couple of versions of these.  The most legible schematic I found seems to be for an earlier version than what I have.  I redrew the portion of schematic to include the revisions (attached).  On the next post I'll attach a scan with some of my voltage readings marked.

I'd really appreciate any leads in the right direction...thanks!

is just the -12dB LED lit, or -12dB and all lower level LEDs?

JR

 

[hereforever]

Just the -12 when there is no signal

When I apply enough signal to the broken module to get heavy compression I can get the -18 LED to light, but none of the LED's before -12 light up

 

[JohnRoberts]

Just the -12 when there is no signal

When I apply enough signal to the broken module to get heavy compression I can get the -18 LED to light, but none of the LED's before -12 light up

Check solder connections first for components attached to that one LED... If solder connections look good, check diode drops of associated transistors.

JR

 

[hereforever]

Turns out to be Q29 (2N5486) is out of spec and was throwing off the metering.  I swapped Q29 on the working module into the broken module and the problem goes away.

Now I have to find a suitable 2N5486 (mosfet) - schematic shows Vgs(off) 2.3-2.9v

I tried swapping in one from a Kepex module I've been using for parts but then the meter has the reverse problem (shows less than the actual amount of compression).

Is there a simple way to test Vgs(off) if I buy a batch of 2N5486's?

 

[Khron]

Something like a J112 should still be readily available, and the Vgs(off) and Idss look to be in the same ballpark (considering there's a huge spread to begin with).

https://stompville.co.uk/wp-content/uploads/2012/03/107SVrev1.png
https://stompville.co.uk/wp-content/uploads/2012/03/108SV.png

 

[hereforever]

Update-
I bought a small batch of J112's and tested using the battery trick.  Unfortunately they all tested too high
I'll have to keep searching for a suitable replacement

 

[Bo Deadly]

What does R73 do? If it's a trimmer it looks like it would control the bias of that transistor.

 

[hereforever]

Yes R73 is meter adjust trim. It doesn't have enough range to get the meter working quite right

 

[rackmonkey]

I don’t know where you’re located, but Jameco has 2N5486 in stock:

https://www.jameco.com/z/2N5486-Fairchild-Semiconductor-Transistor-2N5486-N-Channel-RF-Amplifier-25-Volt-TO-92_253577.html

 

[MidnightArrakis]

GREETINGS!!! Fans of "The Allison Research Gain-Brain"!!! --- I have a question that I was wondering if each one of you could answer a question for me? I think you will like the question!!!

"Many, many, many, many moons ago", I met up with Paul Buff (the designer of the "Gain-Brain") at an AES convention in L.A. Times were way different way back then and Mr. Buff agreed to sell me a pair of - bare, unpopulated - "Gain-Brain" circuit boards because, at the time, ALLISON Research was introducing the "Gain-Brain II". So, since the new "Gain-Brain" was to replace the original "Gain-Brain", Mr. Buff was happy to get rid of a couple of extra circuit boards!!!

Since I was a beginning "Equipment Designer" at that time, I was ecstatic in being able to receive the actual bare PCB's of the "Gain-Brain"!!! When they arrived at my house, I immediately set about to - replicate - the bare "Gain-Brain" circuit boards, so I could have my own PCB artwork of the "Gain-Brain". At this point in time, I was also teaching myself how to - manually, hand-tape - Printed Circuit Boards. I was able to successfully and identically replicate the original "Gain-Brain" PCB and I dropped-off my artwork at a local PCB fabrication shop. I say "dropped-off" because.....my replicated PCB artwork was done on a piece of - glass - with the crepe-tape (that was used at the time) placed onto the glass in order to duplicate the actual PCB. A "Labor of Love" you might say.

Since I already had a schematic, I made up a Parts List and purchased all of the parts for 2 circuit boards. An electronics friend of mine came up with the design of a 24VDC power-supply. After I had assembled the two circuit boards and wired together the power-supply, I then packaged the "Stereo Gain-Brain" into a 1U rack-enclosure. It worked perfectly and I even took the unit on-the-road with me when I was a "sound guy" with a local band on tour. Sounded great!!! In fact, here it is 45-years later and I still have those "Gain-Brain" circuit cards sitting just behind my left-shoulder here right now as I write this!!!

What all of this is leading up to is this.....How many of you out there in "GroupDIY-Land" would be interested in an "8-Channel 'Gain-Brain'" in a 1U rack-chassis? Go ahead and ask around, ask your girlfriends and wives, your next-door neighbors and your 2nd cousins that you've never met!!!

In order to meet today's usual studio wiring, I was also thinking of adding in a set of "Balanced" input and output chips to the original circuitry. What do you guys think of that? Any other ideas?

Since the time that I had met with Mr. Paul Buff, I have gone on to become an "Electronics Mechanical Packaging Designer" and I have designed all manner of electronic equipment for aerospace/avionics companies, defense contractors, medical electronics firms, R&D laboratories and others. I perform all of my own - combined - mechanical designs - and - PCB layouts and put all of the electronics together as a product. I find it to be a lot of fun!!!

By using "Surface Mount Devices" and other miniature components that didn't exist back during the 1970's when the "Gain-Brain" was originally made, I can imagine that I could easily fit 8-channels of circuitry, LED's and potentiometers in a 1U rack-chassis. Then again, I was thinking that if I stuck with the standard "Thru-Hole" components, then maybe I could offer an "8-Channel 'Gain-Brain'" as a 1U rack-chassis "kit" that you assemble yourself!!! How's that?

Anyway.....this is all just an idea of mine. I know that there are quite a few "Gain-Brain" fans out there in the recording world, but you cannot buy any new units anymore. Plus, the original chassis was a 4U rack-chassis with 16-channels, so I could provide 16-channels in only 2U of rack-space.

Some Of My Rack-Mount Chassis Designs
Click above to view some of my equipment and large-scale systems designs

And, finally.....the "Gain-Brain" schematic that I have also included a text section that details how to calibrate and setup the "Gain-Brain" for proper use. Does anyone have this on their schematics?

Looking forward to your feedback and responses!!!

JBW

/

 

[Jason Baldock]

GREETINGS!!! Fans of "The Allison Research Gain-Brain"!!! --- I have a question that I was wondering if each one of you could answer a question for me? I think you will like the question!!!

"Many, many, many, many moons ago", I met up with Paul Buff (the designer of the "Gain-Brain") at an AES convention in L.A. Times were way different way back then and Mr. Buff agreed to sell me a pair of - bare, unpopulated - "Gain-Brain" circuit boards because, at the time, ALLISON Research was introducing the "Gain-Brain II". So, since the new "Gain-Brain" was to replace the original "Gain-Brain", Mr. Buff was happy to get rid of a couple of extra circuit boards!!!

Since I was a beginning "Equipment Designer" at that time, I was ecstatic in being able to receive the actual bare PCB's of the "Gain-Brain"!!! When they arrived at my house, I immediately set about to - replicate - the bare "Gain-Brain" circuit boards, so I could have my own PCB artwork of the "Gain-Brain". At this point in time, I was also teaching myself how to - manually, hand-tape - Printed Circuit Boards. I was able to successfully and identically replicate the original "Gain-Brain" PCB and I dropped-off my artwork at a local PCB fabrication shop. I say "dropped-off" because.....my replicated PCB artwork was done on a piece of - glass - with the crepe-tape (that was used at the time) placed onto the glass in order to duplicate the actual PCB. A "Labor of Love" you might say.

Since I already had a schematic, I made up a Parts List and purchased all of the parts for 2 circuit boards. An electronics friend of mine came up with the design of a 24VDC power-supply. After I had assembled the two circuit boards and wired together the power-supply, I then packaged the "Stereo Gain-Brain" into a 1U rack-enclosure. It worked perfectly and I even took the unit on-the-road with me when I was a "sound guy" with a local band on tour. Sounded great!!! In fact, here it is 45-years later and I still have those "Gain-Brain" circuit cards sitting just behind my left-shoulder here right now as I write this!!!

What all of this is leading up to is this.....How many of you out there in "GroupDIY-Land" would be interested in an "8-Channel 'Gain-Brain'" in a 1U rack-chassis? Go ahead and ask around, ask your girlfriends and wives, your next-door neighbors and your 2nd cousins that you've never met!!!

In order to meet today's usual studio wiring, I was also thinking of adding in a set of "Balanced" input and output chips to the original circuitry. What do you guys think of that? Any other ideas?

Since the time that I had met with Mr. Paul Buff, I have gone on to become an "Electronics Mechanical Packaging Designer" and I have designed all manner of electronic equipment for aerospace/avionics companies, defense contractors, medical electronics firms, R&D laboratories and others. I perform all of my own - combined - mechanical designs - and - PCB layouts and put all of the electronics together as a product. I find it to be a lot of fun!!!

By using "Surface Mount Devices" and other miniature components that didn't exist back during the 1970's when the "Gain-Brain" was originally made, I can imagine that I could easily fit 8-channels of circuitry, LED's and potentiometers in a 1U rack-chassis. Then again, I was thinking that if I stuck with the standard "Thru-Hole" components, then maybe I could offer an "8-Channel 'Gain-Brain'" as a 1U rack-chassis "kit" that you assemble yourself!!! How's that?

Anyway.....this is all just an idea of mine. I know that there are quite a few "Gain-Brain" fans out there in the recording world, but you cannot buy any new units anymore. Plus, the original chassis was a 4U rack-chassis with 16-channels, so I could provide 16-channels in only 2U of rack-space.

Some Of My Rack-Mount Chassis Designs
Click above to view some of my equipment and large-scale systems designs

And, finally.....the "Gain-Brain" schematic that I have also included a text section that details how to calibrate and setup the "Gain-Brain" for proper use. Does anyone have this on their schematics?

Looking forward to your feedback and responses!!!

JBW

/

This is something that I'd be very interested in. I do already own a handful of Gain Brains, but they're in various states of repair. They're my favourite limiter/compressor, so I'd love to have 8 of them, fully working.

 

[MidnightArrakis]

This is something that I'd be very interested in. I do already own a handful of Gain Brains, but they're in various states of repair. They're my favourite limiter/compressor, so I'd love to have 8 of them, fully working.

It has been nearly a year since my comment posting and -- YOU -- are the first person to respond to my comment!!! So.....I can chalk up a "|" on the list of people who would be interested in an "8-Channel Gain-Brain" chassis. Any others out there???

/

 

[Matt Syson]

In around 1975 a review of the Gain Brain (I forget if it was original or version 2) found poor distortion performance when lowish frequency signal was causing one of the LEDS on the bargraph chain to fluctuate, which I would expect to be one stage having a quick burst of oscillation which is then coupling back into the audio path. Whether this translated into a real use problem I have no idea but sometimes 'heavy' switching currents for LEDs can intrude onto adjacent audio paths (or poor ground trace routing). Sometimes accurate copies of some old gear can be a mistake.

 

[MidnightArrakis]

In around 1975 a review of the Gain Brain (I forget if it was original or version 2) found poor distortion performance when lowish frequency signal was causing one of the LEDS on the bargraph chain to fluctuate, which I would expect to be one stage having a quick burst of oscillation which is then coupling back into the audio path. Whether this translated into a real use problem I have no idea but sometimes 'heavy' switching currents for LEDs can intrude onto adjacent audio paths (or poor ground trace routing). Sometimes accurate copies of some old gear can be a mistake.

This problem you describe -- may -- have been with the "Gain-Brain II" unit. I seem to vaguely remember reading about some LED problems with that version. The "original" Gain-Brain had already been on the market and had sold like crazy for at least 10-years before the "Version II" came out, so I would think that if the problem like you described had existed in the "original" unit, it would have been eventually "discovered" by someone during a recording/tracking/overdubbing session.....ya think???

/