Threshold FET-10e Phono

elwood625

Member
Paid Member
2013-01-30 4:15 pm
Elkhart Lake, WI
My Threshold FET-10e Phono preamp died a couple of months ago, both channels have very low gain, almost a whisper. I'm using a MM cartridge and have the dip switches set to 47k and 200pF. All four gain switches are set to MM. I lived less than a mile from Fermi Lab and have had several TV's and two channel stereo equipment fail due to transformer failures and spikes in line voltage from when Fermi Lab fires up the particle accelerator.
Now that I have moved, it's time to try to fix the FET-10e so I can listen to vinyl again. Here are pictures of the preamp.
 

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elwood625

Member
Paid Member
2013-01-30 4:15 pm
Elkhart Lake, WI
There are four gain boards as this has balanced outputs (XLR) along with RCA's. The lack of output is with both the XLR and RCA's.
First hing I did was check the remote power supply voltage, I have +27.5vdc and -27.5vdc going into the preamp. I checked the regulator section of the preamp and measured +18.0vdc and -17.99vdc at all four gain boards and the rail traces on the PC board.
Next I put a 1K sign wave at 4mV into the MM input and went searching for A/C and D/C voltage at the outputs. XLR pin 1 = 0 v A/C and D/C to ground. Pin 2 has 0.08v (L) and 0.11v (R) A/C to ground With pin 3 = 0v.
On pin 2, I found -10vdc on both the left and right channels, the odd thing was as I had the DMM hooked up to pin 2, the voltage would decrease, when I removed the DMM, the final reading was -8.5vdc. Then when I connected the DMM again to pin 2, it measured -8.5vdc and started to go down in voltage again. Every time I removed the DMM from pin 2, and then connected it again, it would measure the same D/ voltage as when I removed the DMM and start to decrease towards 0V.
 

elwood625

Member
Paid Member
2013-01-30 4:15 pm
Elkhart Lake, WI
I checked the muting relay and it was functioning correctly with the outputs shorted to ground with power off, with power on it clicks and the outputs are no longer shorted to ground.
Using my decades old Tectronix 2225 scope, I went looking for my 1K sine wave, sure enough a nice sine wave at +input to each gain boards, however when I tested - input all was not well. The scope pattern was increased voltage and what looked like D/C with the sine wave.
 

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