Novice attempting Allison Research rack

[BrianJSiegel]

I have an Allison Research Rack with 6 Gain Brains and 10 Kepex and am wanting to service them myself although I don’t know much about it. I understand signal flow, a little about components, can use a mumtimeter and know how to solder but I just want to know if theres a “standard” step one to take when testing a circuit board, like the ones on the Allison Research stuff. Thanks for any input you may have

 

[Rob Flinn]

Be careful there are high voltages in there for the Kepex gates.

 

[ruairioflaherty]

I just want to know if theres a “standard” step one to take when testing a circuit board, like the ones on the Allison Research stuff.

Standard step one is to test them in the studio.  Are they working?  If not make detailed notes of what the fault is and the conditions under which it occurs.

 

[BrianJSiegel]

Be careful there are high voltages in there for the Kepex gates.

Thank you. Good lookin out! Excuse my ignorance here. What would be being ‘not careful’? Other than the obvious rattling around with metal in there or getting it wet or something. What am I looking for? Thanks again btw

 

[JohnRoberts]

I wouldn't expect unusually high voltages but the old circuits boards are probably fragile, so be confident in your soldering skills before undertaking significant rework.

JR

 

[Brian Roth]

The original Kepex gates used neon bulbs for the metering, and thus had something like a 100 VDC supply rail fed to each slot in the frame.  Later ones used LED's.

Bri

 

[iampoor1]

Whats wrong with them? Thats always the first question. If they are just old, a standard step is to recap them. Just do one at a time in case you make a mistake, then its easier to isolate.

 

[Rob Flinn]

Yes 100v d.c.  Enough to wake you up a bit if you touch it !!

 

[pvision]

Best way to start is to check supply voltages. If the cards are in a rack voltages should be the same on each board but the edge connectors will be old and may cause problems. If you don't have a card extender to bring the board out of the rack then you'll need to make up a couple of edge connectors on flying leads & connect then to the power supply

Then feed a test tone to two units, one working and one not, and trace the signal through the board comparing the two units. As soon as you find a difference between the two you're on the way to finding the problem

From my research the Gain Brain is a relatively simple transistor-based compressor using an FET for gain reduction. The schematic is confusing but most of it is metering. The actual active audio part is pretty small

Checking DC voltages across transistors may be instructive as you compare boards but be careful not to let your probe slip & short things

If you print out a schematic and mark measured DC voltages on it from a good board you may be able to jump to finding faults quite quickly

And, of course, replace all old electrolytics and tantalum caps...

Nick Froome

 

[BrianJSiegel]

Hey guys, thanks for the help btw. Much appreciated. I’ll let you know my findings