Please help! Pioneer QX 949 Offset set to zero but unable to set bias appropriately

[Molnar142]

Quick question on biasing a Pioneer QX-949. I’ve searched this forum, YouTube and a general Google search and this thread was the closest I found. (Perhaps it’s too straight forward/easy. )

Just want to know, when the service manual says, “Terminate input terminals of power amplifier assembly with a 5.1K-ohm resistor” what does that mean?

Thanks for the help.

 

[rcs16]

One usually makes up special RCA plugs which contains a 5k1 resistor wired from center pins to ground (shield/shell).
Place the pre-out/power-in switch in the external pre-amp i/p position. if it has jumpers instead of the switch. you will have to remove them to insert your terminations into the power-amp i/p jacks.

 

[KeithD]

One usually makes up special RCA plugs which contains a 5k1 resistor wired from center pins to ground (shield/shell).
Place the pre-out/power-in switch in the external pre-amp i/p position. if it has jumpers instead of the switch. you will have to remove them to insert your terminations into the power-amp i/p jacks.

The QX-949 does NOT have jumpers or a switch to separate the pre-amp from the power amp.

Quick question on biasing a Pioneer QX-949. I’ve searched this forum, YouTube and a general Google search and this thread was the closest I found. (Perhaps it’s too straight forward/easy. )

Just want to know, when the service manual says, “Terminate input terminals of power amplifier assembly with a 5.1K-ohm resistor” what does that mean?

Thanks for the help.
View attachment 1152455 View attachment 1152456

It is generally accepted that you do not need the resistors specified in the service manual. Set the input selector to something other than FM/AM (such as AUX) and turn volume all the way down. Then you can make the adjustments as given on pages 21 and 22 of the service manual. It's a bit of a PIA, since you have to remove the top cover and the metal cage shield to get at the variable potentiometers to make the adjustments, but you also have to remove the bottom cover to connect your DMM to the appropriate pins to monitor voltages since those pins are only accessible from the bottom.

 

[Molnar142]

RCS16 - Thanks for the response. I’ve seen that done before but like Keith states, the QX-949 doesn’t have pre-outs.

KeithD - Thank You! That’s exactly how I was thinking of approaching it. I assume they intend you to jumper the input terminal to ground with a 5.1K resistor to essentially replicate turning the volume control all the way down just at the power amplifier section to remove any possible noise from the pre-amp/tone control section.

 

[rcs16]

I should have checked, I assumed my QX-9900 had them so why would a QX-949 not It is bad to assume sounds like Keith has worked on this model, I have not.

 

[KeithD]

RCS16 - Thanks for the response. I’ve seen that done before but like Keith states, the QX-949 doesn’t have pre-outs.

KeithD - Thank You! That’s exactly how I was thinking of approaching it. I assume they intend you to jumper the input terminal to ground with a 5.1K resistor to essentially replicate turning the volume control all the way down just at the power amplifier section to remove any possible noise from the pre-amp/tone control section.

Yes, I think the entire point is that there should be zero volts input so that you can set the "zero" point eliminating any differences from transistor output, etc. not completely matching in the circuit.

For such a high end unit as the QX-949 it is surprising there is no way to separate the pre and power amp sections, but that's how it is. Makes troubleshooting more difficult, as I have experienced!

 

[rcs16]

That is right, zero i/p DCV to set op offset, it is cap coupled ips anyways. i do not think it matters, just make sure the vol is at min.
Well you are close, do you want me to go through all the sources of DC offset in a OCL type of design
The QX-9900 is a monster, I think they just ran out of room on a qx-949, when they tried to squeeze it down in size, pre-amp/PA got tossed, sorry to say. really stupid in my opinion for a TOTL model.