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Author Topic: Prinecton Reverb kit startup testing  (Read 292 times)

czgunner

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Prinecton Reverb kit startup testing
« on: December 10, 2021, 03:34:47 PM »
Got my Mojo Princeton Reverb kit assembled yesterday. I've got the instructions for startup testing and to check A/C voltage on pins 4,5 and 9 on the preamp tube socket pins. I'm getting around 3.5-3.7 VAC. Mojo doesn't include the number I should be looking for. The Stewmac kit instructions I looked at say 5-7 VAC. Is my voltage way wrong? If so, what should I be looking for to fix?
Found this: https://chuck.cranor.org/mpr/testin/xls/MOJO-PR-BIAS-4.pdf
And it seems to verify that something is wrong.
Thanks!

ETA: I was just told by Mojo I'm supposed to be reading across 4/5 and 9. When I check that way, I get 0 volts. Any ideas?

ETAA: LOL after calling Fluke and talking to the lady, I'm told my old 88 is obsolete is common for the AC side failing. I'm sending it out to be replaced with the new model at a discounted price. In the mean time I pulled out a spare Fluke 115 and my voltage is 6.8 VAC. I'm hoping this is good and I can put the tubes in and check the high voltage side. This should be DC, I believe.
« Last Edit: December 10, 2021, 05:37:45 PM by czgunner »

Enzo

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Re: Prinecton Reverb kit startup testing
« Reply #1 on: December 10, 2021, 07:22:13 PM »
Wait, your meter measures 3vAC at first, and now you get zero somewhere and want to send your meter off?  It was working when it measured 3v.  Set to AC volts, one probe on pin 9, the other probe on pins 4/5 (4 and 5 should be already wired together).  The two resistors to ground from the 6v wires are a virtual center tap, so to ground, half the voltage shows up on each side.

You now get 6-7vAC on the heater pins.  CLose enough.

czgunner

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Re: Prinecton Reverb kit startup testing
« Reply #2 on: December 11, 2021, 12:11:03 AM »
Wait, your meter measures 3vAC at first, and now you get zero somewhere and want to send your meter off?  It was working when it measured 3v.  Set to AC volts, one probe on pin 9, the other probe on pins 4/5 (4 and 5 should be already wired together).  The two resistors to ground from the 6v wires are a virtual center tap, so to ground, half the voltage shows up on each side.

You now get 6-7vAC on the heater pins.  CLose enough.

The first time I checked voltages, I was not checking across the pins, I was grounded and pin testing. The instructions weren't clear at all on that process.