Altec 1612A Limiting Amplifier- Help! - diyAudio
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Old 18th February 2008, 02:23 AM   #1
imix500 is offline imix500  United States
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Default Altec 1612A Limiting Amplifier- Help!

Hello all, I have this 1612A I'm repairing for a friend. I've done a complete recap job and have replaced a few resistors that were out of spec.
Does anyone have a decent schematic?
I have one but the scan is horrific and I can't make out any test voltages and some of the part numbers. I'll attempt to attach the one I have.

So, the problem right off the bat is the output power amp U1 seems to be floating. V+ is right at 5v but GND seems to be at 2.6v with respect to circuit ground.
What is the purpose of Q13?
Many more questions to follow...
Thanks!
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File Type: jpg 1612a.jpg (99.9 KB, 132 views)
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Old 18th February 2008, 11:21 AM   #2
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I cannot read the attachment but you can always buy schematics for this amp, for example here: http://www.musicparts.com/products.asp?Company=Altec

or ask here: http://www.hostboard.com/cgi-bin/ult...b=forum&f=3729

or check here: http://alteclansingunofficial.nlenet.net/

This one is slightly better than you have: http://vintageproaudio.com/Doc+Schematics/1612sch2.jpg
http://vintageproaudio.com/Doc+Schematics/1612sch1.jpg

Regards,
Marek
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Old 18th February 2008, 02:06 PM   #3
Corax is offline Corax  Germany
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Hi imix500,

I agree that the copy is really fuzzy . I guess you got that copy from the operating manual for the 1612A. In this PDF the schematic is exactly the same and not really good "readable". Unfortunately I don't have any better copy of it.
Nevertheless Q13 looks like connected to the voltage regulator, a 7805 (if I deciphered that correctly). The only thing weired about it, is the pin numbering of the 7805 regulator. The numbering supposed to be, from the left to the right of the TO-220 case with the heatsink up and the pins pointing to the bottom and you're looking onto the face where you can read the part number, is 1-2-3. In this case 1 is the input, 2 is ground and 3 is output. But in the schematic it looks like that pin 2 and 3 are mixed up a little bit. Either they count the pins in a different way or the schematic makes not really any sense. On the other hand the pinassignment would fit to a negative regulator (i.e. 7905) but again that makes no sense at all in conjunction with the rest of the schematic and the shown polarities. Pin 1 of the regulator is connected to the positive rail voltage (see also on page 3 of the operating manual, which you've not attached).
OK, but back to Q13. Assuming pin 3 of the 7805 is the GND connector there is an idle current of approx. 5mA (see also datasheets of 7805 regulators of various manufacturers) flowing through the GND pin to ground of the schematic and therefore through the emitter of Q13. Since I'm not able to decipher the values of the two involved resitors I can only guess what voltage will finally be present on pin 2 (to the left of the regulator). It might be something around 12V but that's just a guess.
So if you measure just +5V at V+ of U1 (referenced to GND) I doubt that this would be the correct voltage for a fully functional 1612A - it's more likely that the voltage regulator itself is working but Q13 has a short between collector and emitter.

Now more details about the power amp U1:
Referenced to the GND line (shown right in the middle as a horizontal line in the schematic drawing) the output voltage at pin 8(?) (or at least the pin at the far right side of the triangle) should be static at half of the power supply level for symmetric modulation. I can see no split supply (i.e. +/- xx volt - just a single positive supply voltage. That's the reason why a capacitor (possibly referenced as C21, if I deciphered that correct) is connected in series to the power amp output and the output transformer to keep any dc voltage away from the output coil.

If you can measure any larger voltage levels between GND and other "ground" levels in the real part you might have more broken/damaged parts/cables/wirings etc. in the real amplifier than you have expected so far. The shown horizontal GND line appears to me as the ground line all measured voltages are referenced to.
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Old 18th February 2008, 02:06 PM   #4
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Q13 provides a false gnd to the 7805 voltage reg, and together they create the regulated 21.3volts.
U1 should be grounded and not float.
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Old 18th February 2008, 05:00 PM   #5
imix500 is offline imix500  United States
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Ok the vreg makes more sense now. The output is indeed right at 5v so it looks like I need a new Q13. I will continue to measure voltages at the noted test points, but I get the impression that with Q13 out, voltages are going to be all over the place.

I need the test voltages to set the pots throughout, so I guess I'm buying a printed schematic.

I'll look up the 2N5367 (Q13) and see if I have anything close to get it running until I get a new part.
That schematic is much better than the one I have, thank you.
Thanks for the help so far!
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Old 19th February 2008, 11:05 PM   #6
imix500 is offline imix500  United States
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Ok, so I replaced the 2n5367 with a 2n4403 (it was the closest match I had) I now get 5.68v- .68v more than before, but 19v shy of where it should be. Can anyone tell me if the 4403 is very different in hfe? As far as I know, thats what matters here. I thought the specs looked close enough. I also have a NTE159 I can try next if that's closer. Thanks!
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Old 20th February 2008, 06:01 AM   #7
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The transistor is fine, you need to make sure the voltage divider with the two resistors are providing the correct bias for it, which looks like 15,8volts.
So the transistor collector should be at gnd, the base at 15,8volts, and the emitter should be 16,3volts, give or take +-10% or so.
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Old 20th February 2008, 12:00 PM   #8
imix500 is offline imix500  United States
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Ok, I thought something looked strange here. I get 4.91v across the 3.9k (R34), and .75v across the 11k (R35).
I checked the 11k out of circuit and it was ok. I have not checked the 3.9k
Thanks!
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Old 20th February 2008, 03:45 PM   #9
Corax is offline Corax  Germany
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Well, I'm pretty sure that the replacement (2N4403) will not work if you haven't considered the different pin assignment of both.

The 2N5367 (the original) has B-C-E while the 2N4403 (replacement) is E-B-C - if you face the flat part of the case with the pins pointing down (if I'm not terribly mistaken by interpreting the datasheets).

Either you try another type of TUP (Transistor Universal PNP) or you do the fiddling work to swap pins (B with C, risking a short if you don't isolate at least one pin) and turn the whole transistor by 180 - and I don't mean upside-down
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Old 20th February 2008, 05:18 PM   #10
imix500 is offline imix500  United States
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Doah! Pin assignment totally escaped me. I'll fix it tonight and report back.
Thanks!
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