Altec 1612a help

[hylaphone]

Hi folks, can anyone help clarify what is meant by the "20" written underneath R49 on this schem?
I'm working on one of these units, appears to be functioning but feeding it +4dB the meter stays about half scale when set to gain level, while VU output barely hits 0 with input and output controls dimed.

Thanks ---

 

[Whoops]

Hi folks, can anyone help clarify what is meant by the "20" written underneath R49 on this schem?

I checked the schematic but I have no idea of what the "20" means

 

[MagnetoSound]

There’s a similar figure “0” under R54 … my guess is these might be trimmers to set points on the GR meter scale.

 

[scott2000]

my guess is these might be trimmers to set points on the GR meter scale.

Would those be a percentage of trimmer or pot value? You'd think there would be an indicator of k or r if it were an actual value measured at time of readings.

 

[Khron]

... Or a dB indication, for calibrating the meter reading?

 

[MagnetoSound]

... Or a dB indication, for calibrating the meter reading?

That was my thought, yes.

 

[hylaphone]

... Or a dB indication, for calibrating the meter reading?

Seems like it, can anyone advise how to go about that? I can't find a service manual anywhere.
As mentioned, the meter hovers around -20 even with no signal applied - though the output doesnt sound particularly noisy

Thank you all for the response.

 

[MagnetoSound]

If the meter misbehaves in either mode, check the ground on the negative side of the meter, the wire from the switch to the positive side, and the integrity of the switch.

You can also check for ~21v at the junction of R48/R50 and for 0v on the ground plane. These tests should yield enough clues.

 

[hylaphone]

Thanks for the response. After replacing all electrolytics and reflowing the pin headers, VU Output mode is working correctly.
However, I'm unsure how to proceed with the meter cal in Gain Level mode. Assuming I first adjust for zero with no signal applied - I can only reach it by adjusting R49 and R54 to extreme settings, where Q11 collector reads upwards of 18VDC. Schematic suggests 8.2VDC there.
R48/R50 junction is 21.6VDC, and there is around 6.6VDC at diode CR5
Does this suggest a problem with Q11 itself? I can't find a cross reference for AL5308

 

[a soBer Newt]

When I have calibrated other compressors normally what we do is drive it so that we can measure in this case 20dB of gain reduction, and adjust that pot then drop the level so its back to 0dB and adjust 0 pot, its a back an forth kind of thing.

 

[hylaphone]

Ok that makes sense. but if I can't hit 0 even with no signal?

 

[MagnetoSound]

Yeah, Q11 might be o/c. Is there any voltage at the emitter? How about at the base?

 

[hylaphone]

Currently 12V Collector, 1.2V Base, .45V Emitter. Adjusting the trimpots for 0 brings Collector up past 18V

 

[MagnetoSound]

What’s the static voltage across CR5?

 

[hylaphone]

Pretty sure Q11 is kaput. I had left the unit on for an hour and went to measure CR5, noticed that Q11 C/B/E all read around 4 volts.
Schem says AL5308, I can't find it in the Altec transistor cross reference, nor in the Google. Any ideas what would be a good sub?

 

[hylaphone]

Ok I've got a new Q11 in there.

Just to be sure, I'm looking for approximately 8.2VDC at the collector while the meter shows 20dB GR?
I have it adjusted as such right now. Without signal the meter goes to 0, and the collector shoots up to around 18VDC
Does that sound right?