Measuring Marantz Bias - Help

[KeninDC]

I need your assistance in measuring the bias on my Marantz 2238 without electrocuting myself. I had it open the other day to adjust the DC offset. The Marantz 2238 has handy DC offset and bias adjustment "trimming resistors." I was able to adjust the DC offset by measuring DC through the speaker terminals using my Fluke DMM. However, the instructions for adjusting bias to 14 mV left me befuddled since I could not find where to measure bias. Put another way, I cannot locate J714, J716, J715, and J717 (see text below). Where does one measure bias?

I have attached pages 10 and 18 from the 2238 manual that may shed some light on where to measure bias. Unfortunately, mechanical drawing in 10th grade did not prepare me to read schematic diagrams. I'm assuming the "bias measurement zone" is somewhere on the board where the trimming resistors are, but I cannot find these little buggers. What follows are the instructions for bias and offset:

Connect a VTVM between J714(+) and J716(-) and adjust the trimming resistor R733 until the VTVM reads 14 mV DC. And next, connect a VTVM between J723 and J722 (GROUND) and adjust the trimming resistor R711 until the VTVM reads 0 mV DC. Do over again. For the other channel, connect the VTVM between J715(+) and J717(-) and adjust the R734 for the same reading, and connect the VTVM between J724 and J722 and adjust the R712 for the same reading. Do over again.

Thanks,

Ken

[EchoWars]

The test points should be marked on the bottom of the board (or somewhere), but unless the amp is getting hot, I would not concern myself with it.

[Tropical]

You should be able to look at the PCBs and find out the locations. Look carefully and you will find them. It is not that difficult.

[MatthewK]

The test points should be marked on the bottom of the board (or somewhere), but unless the amp is getting hot, I would not concern myself with it.

Er - crossover distortion?

Given your level of knowledge I hesitate to argue though!

[EchoWars]

If any current is flowing at all through the output stage, the amp feedback with reduce any crossover distortion to near-zero.

I just don't wanna see you smoke the amp.

Anyway, the PC layout for the driver boards should identify which terminal is which, but you must take note of whether the PC layout is showing the component side or the foil side.

[KeninDC]

Thank you kindly. When I was adjusting the DC offset, I left the bias adjustments alone. The amp is not running hot and I do not hear any distortion. In fact, with the DC offset around 0 mV in each channel, I actually have tight bass again. My wife bought the amp in 1978 and I do NOT want to smoke it. I am going to leave it alone per Echowar's advice.

Ken