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Re: What Exactly is a DVOM ??

9/26/2000 11:49 AM
Re: What Exactly is a DVOM ??
Thanks Troy just trying to learn ! I appreciate it !
9/27/2000 1:04 AM

I was recently measuring DC plate voltage on some 6L6's (I anticipated about 420 volts from the amp in question) and all the decimal points on my meter's display lit up and started flashing on and off slowly (once per sec). I had the meter on the 1000 volts, DC scale. I consulted my Wavetek manual for this 'undocumented' feature/flaw and emailed the company to no avail. I plugged in another manufacturers DMM and all the decimal point lit up but didn't flash when carrying out the same exact measurement. The voltage read about 25~30 volts lower than it should have. My Wavetek meter eventually bit it (I torched a trace on the selector wheel) when measuring this. Any idea what happened? I've never seem all the decimal points light all at once on any meter. Maybe Wavetek's got a psychic working for them and everytime the repair dept. gets a little slow!  
9/27/2000 1:29 AM
K Bull

I have a BK precision Bench 388A that starts buzzing and beeping when I make this same test and flashes a few times and then reads fine. It has only been pushes up to 560V but reads after the beeps. I also noticed that on certain model Flukes that are rated higher than 600VDC the price jumps up about $250 to the 1000vdc models. I only paid $100 for the BK so I'm waiting anytime for it to poof. BTW, Those old analog Simpsons are great meters.
9/27/2000 9:25 AM
Dave H.

"My Wavetek meter eventually bit it (I torched a trace on the selector wheel) when measuring this. Any idea what happened?"
I suspect that connecting a DMM directly to the power tube plate can make some amps oscillate at high frequency. The HF oscillation would make the DC reading incorrect. DMMs are probably not designed to withstand hundreds of volts at high frequency. The HF rips through the DMM and you end up with flashing decimal points and/or a dead meter.  
Next time a DMM acts funny when measuring plate voltage measure the DC on the OT centre tap or the filter cap. If you can read that OK try measuring the plate voltage with a 10k-100k stopper resistor in series with the probe at the plate end.  
9/28/2000 12:33 AM

Thanks, Dave.

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