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OTL Asylum OTL, Output Transformerless Amplifier User Group. |
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In Reply to: RE: MP-3 posted by jumpsturdy on December 15, 2011 at 17:14:04:
You don't remove the circuit board, FWIW- instead just remove the bottom panel.
The plate resistors for the first stage are located by the second 12AT7 in each channel, followed immediately by the EQ components. The plate voltage should be near 150 Volts there and also on the 12AT7 that is the output of the phono section. There are two plate voltage per gain stage as it is a differential amplifier in each case. They may not be all that close to each other; if they are within 20 volts its working. If the plate voltage is really high, at the B+ voltage, then the tube is not conducting.
In each channel you will find pads on the board marked 'tp K1' and 'tp K2'. These are test points for the cathode circuits of each gain stage (so tp K1 occurs on the 1st 12AT7 from the rear panel). The voltages should be ~+0.3 Volts on K1 and K2 for MkIIIs and earlier. Mk.3.1s will have about 1.5V on K1.
If the preamp has these voltages then you can assume the phono section is operational.
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Follow Ups
- RE: MP-3 - Ralph 09:19:09 12/16/11 (1)
- RE: MP-3 - jumpsturdy 13:49:02 12/16/11 (0)
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