Trying to identify chip in ADR S24 ADT module limiter circuit

[musika]

I am back to trying to get this ADR S24 ADT module to work, I initially received no sound. When powered up, the red limiter led immediately lights up and stays on. After looking at the schematics and making sure the correct components are installed, I noticed that IC2 (which on the schematics reads MC144B, has a TL082CP installed. This is the limiter circuit. After researching for MC144B, I realized that this chip doesn't really exist (or maybe was a special IC for the module). Does anyone know what this IC is (MC144B)? Or what IC could possibly be substituted for IC2 in this module? If someone has one of these, could you possibly verify this chip for me?

Also I found a discussion on another forum about this IC in the S24 module:
Can someone educate me about the MC144B (or MC1448) dual op-amp? - Music Electronics Forum
Is there a possibility that LM101A could be used as someone has posted?

 

[JohnRoberts]

from looking at the schematic... the 144 looks like a dual op amp with same footprint as 1458, and many more modern duals.

JR

 

[musika]

Thanks… Can I assume then that an NE5532 would work there?

 

[musika]

The other difference I noticed is that where a 2N3820 is labeled there is a 2N5462 installed.

 

[JohnRoberts]

Thanks… Can I assume then that an NE5532 would work there?

not automatically, perhaps try one, also try bifet like TL072, they all have same footprint but different non-ideal behavior.

The other difference I noticed is that where a 2N3820 is labeled there is a 2N5462 installed.

without looking it up, likely another n ch JFET... with a bunch of them working from the same control voltage you may want to match them (for Vgs), not casually mix in different parts.

JR

 

[nashkato]

so the status quo is :
- no sound
- limiter always on ( or at least the led )
?
before anything i would go ahead and make the assumption that the module was working ( more or less ) with those components installed .
no use wasting time when you end up at the same point where you started .
look for more usual failures ( bad solder joints , electrolytics or maybe you even got tantalums in there , etc etc

 

[musika]

so the status quo is :
- no sound
- limiter always on ( or at least the led )

yes. both green(norm) and red(limiter) stay on. System led IN / Out pushbutton led indicated when pushed.

I have checked for solder... all looks good.

OK. Good advice. I've recapped except for 2 2.2uf 63V - need to order these. The tantalums - there are 11 or 12. I don't have a good way to test these. Should I just replace with same?

 

[JohnRoberts]

tantalums have a bad habit of failing short circuit.. may check for shorted tants with VOM...

JR

 

[musika]

Thx. Do I need to remove them (or one leg) for testing?

 

[Ike Zimbel]

Thx. Do I need to remove them (or one leg) for testing?

No. If they are shorted, they will read as a short in circuit.

 

[musika]

Great. Will check those and finish up the recap on this thing then retest.

Thanks for all the great advice.

 

[Ike Zimbel]

You should also check to make sure the IC's are in the right way around. IC-3 looks like it could be in backwards.
And, you can check the polystyrene caps for shorts in circuit while you're at it. I just found a shorted PS cap in a mic the other day.

 

[musika]

I was looking at the IC orientation earlier. Some do not have the notch and seem to have a dot on one end or the other ( or an arrow ). This unit has a load of polystyrene caps.

 

[Brian Roth]

Many/most SCAMP modules had onboard voltage regulators for the PSU rails. The frame the modules plug into has + and - 28 VDC buses feed from the external PSU unit. The regulators in the modules would typically be 7815 for + and 7915 for -. They are 3-legged devices that look like a small power transistor and usually attached to heat sinks. Those regulators are a possible failure point. So, check for correct DC rails in the module.

Bri

 

[musika]

I replaced the regulators and the caps in the power supply. But yes. This module has onboard +/-15 Volt regulators. I haven't tested outside of the rack, so I need to do that (connect up to the +/-28 Volt power supply) and verify the rails.

 

[fazer]

Rails are the first thing to test. I use a home built extender cable so unit is out of the rack open. And then test voltages and signal flow with a scope and meter. It’s Hard to find problems with the card in the rack

 

[nashkato]

my vote is on the tantalums .
if only one of them is shorted and it got a connection to DC you can
a ) read that with ohmmeter on the output of the regulators
b)it probably took down the regulators or more in the dc-circuit

before anything , remove regulators , check for shorts between gnd and regulator output .
only when you cant expect massive current its wise to think about applying power .
otherwise anything you‘ve done will be kaputt again and even more

 

[musika]

Schematic is in the Technical Documents Forum under ADR. Here are a couple of pics describing the ADT S24 Scamp module.

 

[musika]

Getting back to the 2N3820 JFET in the ADR S24 Scamp module, and, as I mentioned earlier in this post, there is a 2N5462 ( different orientation of the pins than the 2N3820 ) installed and there is a polarization knob on the socket. Have ordered the 2N3820 but need to make sure the JFET is positioned correctly. I am new to all this... so can someone help explain how to position the Source, Drain, and, Gate pins on a socket like this one? Desoldered the socket in hopes that there was a designation underneath on the board, but no labeling on the PCB. Also have been searching the web.. must not be looking in the right places for this information on these 3 pin sockets.

 

[JohnRoberts]

Everything is pretty clearly labelled... the socket and 5462 pinning is the source of weirdness.

The common triangular 3 pin footprint historically used center pin for base or gate. Looking at the photo, presumably a working unit, shows pin 3 of the 5462 the gate plugged into the center gate socket hole...

the 3820 has its gate on pin 2 so won't require as much gymnastics to plug into the socket.

JR