Heater Voltage Problem

mauster71 · Post by **mauster71** » Wed May 09, 2007 10:12 pm

Just installed a Metro PTP board and Heyboer OT in my 1987x reissue and in the initial voltage testing I'm getting heater voltages of 29vac on p2, 4vac on p8.

Any ideas?

Flames1950 · Post by **Flames1950** » Wed May 09, 2007 11:48 pm

Look at the original board from your 1987X -- does it have only one little adjustment pot or two? If it had two, you had a hum balance on the heaters which you've disturbed, and will have to work around!!!

SDM · Post by **SDM** » Thu May 10, 2007 12:03 am

Flames has likely got it here. If your 1987x did have the hum balance pot, the heater center tap was never grounded, and an artificial center tap was used on the PCB instead.

So, your heater winding likely has no ground reference right now with the turret board setup, thus throwing you off as you try to measure your heaters to ground. Your heater center tap is physically there though on your PT, just unused. So you just need to ground it out, then your readings should come out fine. You could measure from pin 2 to 7 (I'm guessing 8 was a typo above) directly to see your approx 6.3VAC now, but the amp will likely hum badly unless you ground that center tap on the PT, or alternately create an artificial center tap with two 100 ohm resistors.

mauster71 · Post by **mauster71** » Thu May 10, 2007 8:42 am

YES IT DID HAVE TWO ADJUSTMENT POTS!

So what's this thing with the two 100 ohm resistors?