QSC HPR 122i QUESTIONS

[Bigdrums]

UPDATE: I now have it so I can bring up the ac line power and very little current draw just like the working unit. But I have 23 VDC on the LF speaker terminals. Also the red peak light is lit. removing the 15V supply makes the red peak light go out but still have the DC offset. Chased the Speaker out circuit back to U1 pin 5 and have 17v where the good unit shows virtually none. Anyone? Need help please. Schematic in earlier post

[g1]

What are the +/-15V supplies doing?

[Bigdrums]

With the processing/input board disconnected I get +14.5v / -11.7v No load, no high voltage supply connected
Measured at the ribbon cable connector that would be going to the input board if it were attached
Still have DC offset of 2.8 V

[g1]

You are going to have offset untill all supplies are working properly and giving balanced voltages. Something is already wrong with the -15V rail without the input board even connected.
So either something is bad in the -15V supply, or something on the power board is pulling it down.

[Bigdrums]

I now have this unit back together, and seems stable. That is I can power on without having to ramp up slowly with variac. But if I ramp up the clip light will go out as it should. But as soon as I apply a signal the clip light comes on. Just powering on with the switch and the clip light stays on. The amp meter on the variac shows minimal current draw even with the clip light on so I don't think it's really cliping. On page 1 of the schematic posted below,bottom left side of the page "set clip threshold on main board for approx 2.5v peak. approx 2k-ohm". Can anyone point out where on the main board there are referring to? Also, is VR1 a bias adjustment or what? Main board is pg 2 of the schematic and I believe they are on the left side of the page, near low freq amp just below power supply
Thanks for any help here.

[g1]

So did you resolve the -15V supply problem?

[Bigdrums]

Both the working amp and the semi-working amp show +/- 11.92v after rectification, so I suppose the 15v rails are ok
At D8 the working amp has ~ minus 6v, the semi working amp has ~ minus 10v, and fluctuating a good bit, but this is from the higher voltage supply I believe.
And at the D7/D4 junction is ~ minus 3v fluctuating a whole lot, but the working unit is at 0V
Semi working amp does have output but weak and distorted and the clip light is on pretty much all the time while the signal light seems to be working properly.
I hooked up a junk speaker to the semi working amp and the DC offset did not drive the cone one way or the other. I replaced D7 & D8 prior trying to get rid of the offset to no avail

[Bigdrums]

I have verified the +&- 15v rails at 14.93v steady. Also verified the 5v and steady. Apparently my ground reference point was not the correct place for measuring these voltages
Green signal present light is operating properly as it behaves like the working amp
From all I can tell the only things left are that the red clip light stays on all the time even with no signal applied and LF output is slightly distorted
If anyone can take a look at the schematic and suggest tests or remedy I would be very grateful! Thanks

[Jazz P Bass]

Can you take a few Vdc readings around the clip/ limit circuit.
The circuit may be responding to an external fault or it may BE the fault.

[Bigdrums]

If someone might be able to tell me what the circuit around D7 does? I find that this seems to be the source of the remaining DC offset, and that even after changing D7, D8, D4, and R31 there is still a minus 23v remaining at the D7 & D4 junction showing up at the low output speaker terminals. Lifting the D7 prevents this and unconnected the output is mostly clean. Nothing else I've tried has improved the output or removed the minus 23v (which is 0v on the working amp) I would think D8 would prevent the voltage from appearing there? Schematic is linked below. Area of D& etc is on page 2, lower left.
I'll get to probing the clip light problem shortly as it is on a different board.

[g1]

D7/D4 junction is directly connected to the output. This is quite a complicated switched rail design (class G or H?). I no longer work on such things myself.
The Q22 circuitry is what switches in the higher rail, and I think when you lift D7 you are probably disabling that section of the circuit (via Q18 and 19). It's not really that the DC on the output is getting in through D7.

[Bigdrums]

OK so I find that I also have -49 v DC offset on the high freq speaker terms. I have written the various voltages on the attached schematic, Red is the working amp, and pencil is the non-working amp.
Maybe someone can look at this and tell me where the minus voltages are likely getting through.
Thanks

[The Dude]

What are the voltages on Q43 and Q51 (EBC). Please post at least to the first decimal place. A half a volt matters in this situation.

[Bigdrums]

Q43 E= 50.7 B= 50.7 C= -50.6
Q51 E= 50.4 B= 50.4 C= -52.9
Any discrepancies are due to the fact I measured them one at a time. Also both the collector values gradually were becoming more negative during measurement

[The Dude]

Neither of those transistors is biased on. Check the transistors and check associated resistors.