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Thread: QSC 800RB LF Amp wonít bias up

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    Senior Member nevetslab's Avatar
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    QSC 800RB LF Amp wonít bias up

    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.

    >>>>>>>>>>>>>>>>>>>>>>>>>>

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by nevetslab View Post
    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.

    >>>>>>>>>>>>>>>>>>>>>>>>>>
    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.

    You are one of the top people in my list.

    nosaj

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    Thanks for this Nevetslab! I've been racking my brains with one of these amps for some time and will probably start my own thread soon if I can't figure it out. And it has a similar problem to yours.
    This is a GK 800RB, right? If so maybe someone can change the title of you thread.

    First I heard of Steve A. passing. So sorry to hear this. I was in contact with him over the summer about buying a board from him but he said he was having heart problems and couldn't get to his storage unit.

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    Senior Member nevetslab's Avatar
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    OPPS! I just saw I had a mental lapse while composing that thread. NOT a QSC, but INDEED a GK 800RB. nothing like seeing that after it's been running for a couple days. Let me know if you run into any problems on the GK. One of the mod's I've done on the GK is increasing a number of the inter-stage coupling caps, where the stock 800RB has too much LF rolloff, and when cranked, the Low end gets 'farty' sounding. I found all the locations where that happened, mod'd a couple of the amps of a client so they'd stop complaining. Then looked at the 1001RB circuit, and saw GK also corrected that in the same fashion. The Boost control, which is followed by a JFET....that's a weak link in the 800RB, 1001RB, 2001RB....it doesn't have the headroom that op amps have, so if you add too much 'Boost', it distorts.

    I first heard of Steve A's passing from Terry....big_teee, seeing the comments from the Members Only threads a couple days ago. He will indeed be missed by all.

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    Senior Member nevetslab's Avatar
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    OPPS! That's NOT a QSC amp, but a GK 800RB! My humble appologies....mental lapse creeps up on me sometimes

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    Quote Originally Posted by nevetslab View Post
    OPPS! One of the mod's I've done on the GK is increasing a number of the inter-stage coupling caps, where the stock 800RB has too much LF rolloff, and when cranked, the Low end gets 'farty' sounding. I found all the locations where that happened, mod'd a couple of the amps of a client so they'd stop complaining. Then looked at the 1001RB circuit, and saw GK also corrected that in the same fashion. The Boost control, which is followed by a JFET....that's a weak link in the 800RB, 1001RB, 2001RB....it doesn't have the headroom that op amps have, so if you add too much 'Boost', it distorts.
    I interested in this mod you're talking about. Which are the inter-stage coupling caps? And what value should be used?

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    Senior Member nevetslab's Avatar
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    Quote Originally Posted by pontiacpete View Post
    I interested in this mod you're talking about. Which are the inter-stage coupling caps? And what value should be used?
    I had to drill down thru my paper stack to find file. OK....looking at the preamp schematic, the first part is C42, called out as '104', which is 100nF. I put a 10uF/35V NP cap across that. Next part is C61, called out as 224 (220nF). I put a 22uF/25V NP cap across that. Next is C93, called out as 106 (10uF). I changed that to a 47uF. Next is C95, called out as 224 (220nF). I put a 22uF/25V NP across that. next is C85, following the LF Woofer Volume Pot...called out as 224 (220nF. I put a 22uF/25V NP across that.

    JFET Q89, which is a J113 JFET, tends to clip asymmetrically. You'll have to look at your amp. Patch into the Return Jack, which takes you thru the Analog Switch U5 (older amps use a TL604, newer amps use a DG419 Analog Switch. Normally no headroom problem there, but there is on the JFET stage. It usually doesn't take much to make it clip symmetrically. On the first one I did, I added 8.2k across R91 4.7k to get symmetrical clipping. The one I just finished today, I looked at it after not getting any go-ahead to mod it from our General Mgr, and it was clipping on the neg half way too soon. I DON'T know if GK had actually set them so there's asymmetrical clipping for sonic reasons. I like as much headroom as I can get, as that stage is the weak link regarding headroom thru the preamp.

    The other mod I made was doubling up on the switch contacts. In the PCB layout, I only see one of the two DPDT switches being used. I like reduncancy...so I put jumpers across the switch contacts to improve long-term reliability. Over time, I've had to replace the switches with new ones, as you just can't get contact cleaner onto the wiper/pins. I also have had to remove every pot in the older GK 800RB's to clean the contacts and remove the grunge. Yours looks like it has some age on it, and probably need cleaning (from the photo you had).

    On the power amp, I also added a 22uF/25V NP cap across C9...bottom of the feedback, in series with R33 to ground. It's called out as 106 (10uF), which yields a corner of 15.6Hz. Adding 22uF more lowers the LF corner a bit (5Hz).

    Also, make sure the output phone jacks are physically tight. G1 called my attention to those, as did Leo_Gnardo

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