Check the secondary AC voltages on the transformer. If off, will affect the plate voltages. These are determined by the line adjust circuit--which has a simplified circuit diagram in the manual. The 100-volt tap is the one used to reference the line adjust circuit.<P>Try checking the bias circuit at the tube socket. Set up for a 6L6 and measure DC between pin 8 and pin 5. After the unit is calibrated--you should get 0-5 volts on the low bias setting and 0-20 on the high bias setting.<P><P>------------------<BR>

You must always set the unit to line--first. You cannot properly measure any other voltage until this is done.<P>------------------<BR>

Refresh our memory. What parts did you replace on the tester? If you did not replace all of the electrolytics and diodes, trying to troubleshoot now won't be very productive.<P>------------------<BR>

I would replace all the diodes, and there are five. Use 1N4007's, and take care to get the cathode bands pointing the correct direction. Hard to say what the noise you heard was, but could have been a resistor overloading. A close visual inspection and keen sense of smell will usually find the problem. I would go through the calibration procedure again after you put in the diodes.<P>------------------<BR>

Switch not making contact?<P>If it calibrates, I would think that rules out the oscillator and meter circuit being faulty.<P>Is it only 6550s that don't test right? Screen and heater voltages correct? You might try a triode for testing, one less electrode to worry about.<P>------------------<BR>

<P>Hi Peter,<P>I'd have to suspect a resistor gone pop. Might not be obvious. Sometimes they crack open without showing any discoloration.<P>------------------<BR>73 de Leigh W3NLB | | Leigh@AtwaterKent.Info<BR> <A HREF="http://www.AtwaterKent.info" TARGET=_blank>http://www.AtwaterKent.info</A>

Thanks for letting us know what the problem was. So the sound you heard was the cap overheating?--or did it take out a resistor also?<P>------------------<BR>