Measuring bias on Marantz 2235B with VTVM

[Captn Dave]

My 2235B has a few issues and so I downloaded the service manual and read through it. To set the bias one uses a VTVM to set the bias to 8mv. I have a couple of VTVMs (Hickok 209A and a Heath IM-2B) that I use on tube gear to set bias but those settings are usually on the order of 10 to 40 volts. The lowest scale on these meters is 3 and 1.5 volts. Neither have the resolution for this job and I'm pretty sure I've never seen one that did. Do you solid state guys really have VTVMs with that kind of resolution?

I have an idea for an alternate method that I think will work but I'd like to bounce it off someone knowledgeable first. It seems like I could use a floating power supply and a 100:1 voltage divider to make myself a constant 8mv source. Then I reference the 0v lead from that source to the low voltage pin on the equipment under test. Then I take my Fluke DMM and measure the voltage difference not between the two test pins but instead between the higher voltage pin and the 8mv source. Then I adjust the bias until there is 0 volts difference between my 8mv source and the bias pin.

My thinking is that with 0 volts difference the impedence of the DMM won't matter because there is no potential. What do you think? Talk me off the ledge if need be. I usually have at least two bad ideas for every good one.

[Captn Dave]

No love for the little 2235B?

[sregor]

What's wrong with your DMM? The idle current is measured across the emitter resistors, which are usually 0.5 ohms are less and loading is not a problem. Problem was 30 plus years ago when the manual was written, VTVMs with low ranges were the only thing around which measures 8 mV reliably.

[Captn Dave]

Thank you Steve. I see what you mean. I assumed they were concerned about meter impedance loading the circuit. I'll use the DMM.