Man you have some of the greatest repair posts. Always so easy to read an follow along. I very much appreciate your efforts in taking the time to post the information.I have a QSC 800RB, 2008 vintage, that isnít biasing up. It came over to the shop from our rental inventory, with somebodyís label stating ďBroken Beyond RepairĒ, probably from the shop up the street. The LF ampís negative output stage had two shorted MJ15023 PNPís, and an open emitter resistor on the neg driver R53. All else looks & measures ok (semi junctions). I ordered a set of MJ15022ís & MJ15023ís, being fresh out in the shop. Replaced the open 22 ohm driver emitter resistor R53 (the 470 ohm R51/R52 are ok).
Cautiously bringing up the AC mains, monitoring the output bus voltage of both the LF and HF amp. For just checking the LF amp, Iím running without two of the three PNP outputs, while the upper half have all three outputs. I was getting +120mV DC offset on the LF amp, and less than 10mV on the HF amp. The HF amp passes signal just fine, while the LF amp, attempting to pass signal, isnít yielding anything like the input.
Checking to see if I had current flowing thru the voltage gain stage xstr Q12, no voltage drop across itís emitter resistor R37, and the voltage difference between the two driver bases is less than 10mV. Checked all of the resistors in the bias string, the collector load R40 & R41 (2.7k/1W), bias pot works, bias xstrs Q13/Q14 ok, common base xstr Q11 ok, replaced it just in case. Lifted the current limiter diodes D8/D9, no difference.
Being out of PNPís suitable to replace Q12, I rummaged thru the bone yard, came up with a back panel from a Peavey SS amp, and extracted itís 74074140 (MJ15053 -140V Vce) & 74084140 (MJ15053 140V Vce). Two measured ok, so I swapped those for the MJ15022 & MJ15023 of the HF amp. HF amp still working with these lower Vce parts, as it runs off +/-60V, so will be fine. Swapped out Q12 with the one from the HF amp, thinking perhaps Q12 measures ok but ainít. No difference. Still no bias and no current flowing thru Q12/bias circuit/R40/R41.
Looked again at the layout. I had neg supply voltage on the remaining two PNP xstrs Q21 and Q23, while Q24 and Q22 had failed shorted. I wasnít getting voltage thru R40/R41, being the collector load for Q12. R42ís negative side went to the nearest collector screw solder pad for Q22, but, isnít replicated with any of the other collector screws!
As Q12 had just been swapped out, I installed that working PNP into Q22 position. Amp now powers up, biases up fine, yields output fine. I re-biased, having already removed the bias pot to check it, put it back, set it mid-way prior to finding the reason WHY there wasnít current flowing thru the bias string/voltage gain stage.
Always the simple stupid stuff that can bewilder ya!
800RB Power Amps Schematic.pdf
Like so many others here on the forum, I too am sadden to hear of Stave A's passing. I always enjoyed following his threads and wisdom imparted to all of us.
You are one of the top people in my list.