QL600A alignment

madders

Active Member
Hi,

Does anyone know the procedure to set the idle current and offset for a QL600A?

The power amp board has VR1-4 which I assume are for the alignment, but I don't know which pins to measure, and unfortunately there's no mention of it in the service manual. There are TP1 and TP2 pins and then it looks like left and right channel out pins. There's not much else though.

Thanks,
Michael
 
It appears to be a little cap coupled amp.
Test points are for the idle. TP1 and pin 6, VR3. TP2 and pin 7, VR4. Basicly a voltage drop across the 0.5 ohm emitter resistors. What should it be? :dunno: The B+ is running at 85mA, that's 43mA's per channel. .025 volts?
Set these first before the idles:
VR1 and 2 are for setting half the voltage of the collectors.
VR1 would set half of the B+ voltage at the collector of Q11.
VR2 would set half of the B+ voltage at the collector of Q12.

You could measure the idles and report your observations.
 
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Unfortunately I already replaced all 4 trimpots. I thought I measured all the pins before replacing, but I didn't see TP1 and TP2.

Right now TP1 is 20mv and TP2 is 30mv, and adjusting VR3 and 4 changes the values, down to 0 all the way counter clockwise.
 
The idle is an ambiguous setting for us hacks on the Cap Coupled amps. The higher it is, the more heat when playing audio. If too low it sounds like crap. The 20mV's is a good spot. See how it sounds when you have it up and running.
What is the B+ volts?
 
I don't know what B+ is? Is that pin 8?

VR1 and VR2 are changing the voltage on the collector of Q11 and Q12.

On the schematic, it shows the collector should be 19.5V. When I adjust VR1 so it's 19.5V, then I can no longer adjust VR3 to 25mV - highest it goes it 7mV.

I did make a note of all the voltages before I changed anything, and originally the collectors on Q11 and Q12 were both 18.1. If I set them back to 18V, then I can set the idle as high at 14mV.
 
Pin 8, yes.
The Scat shows B+ as 42 or 36 volts, so your set must be the 36v B+, if 18v was the collector reading.
We better get a second option on the idle setting.
MTF was the guy who explained the procedure to me. I'll post his quote to me on my sx-770 in the next post.
 
The MTF post on the idle and collector voltages on my 770 amp settings:

Talk about overthinking the plumbing!!!!!!!!!!!!! :yikes:

The outputs: set one collector to HALF of the other collector's voltage with VR1(VR2) - easy to hit, it's the output transistor's CASE.
You want to play around, double checking for symmetrical clipping, go ahead, when you are done the result won't be much different than that thumbnail.
We are trying to avoid MAJOR imbalances that would reduce output swing by 50%, not less than 1%.

The scat shows 50mA idle to both main amps.
thus 25mA draw per channel.

Rather than fool around trying to figure quiescent draw of the rest of the circuit, set each channel to 25 mA which is 0.0125v across one of it's 0.5 ohm resistors using VR3(VR4).

IF you want to be picky, then measure the total B1 draw current, see how much over the current is in millivolts, divide by 2 then by 2 again, and subtract the millivolts from the starting 12.5 millivolts. It won't be much... Then dial back VR3(VR4) to the new millivolts.

OR

You could dial VR3(VR4) down to NO idle current and measure the B1 draw,
calculate IT'S difference from 50mA in amps, divide by 2 (channels) and by 2 again (0.5 ohms) and set THAT voltage using VR3(VR4).

Remember, the ONLY penalty is a bit too much idle current is some heat when NO audio is coming through.
 
It (Idle) should be around .021 volts using my maths. You have to be on full power to get the correct voltages.
 
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You have to be on full power to get the correct voltages.

Aha! This was the problem. I was on the DBT.

On full power, pin 8 is about 41V.

The schematics show 19.5V for the Q11 and Q12 collectors though. So should I set them to 19.5V, or half of pin 8 (around 20.5)? Either way, I can set the idle to 0.021V.
 
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