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In Reply to: RE: Theta tube preamp posted by louie3 on September 02, 2014 at 06:40:05
One thing I forgot to ad...you would probably be best to measure the AC input to the power supply. I don't know the transformer spec, and it may be way out because it is not loaded since the power supply isn't working, but you should see 150+ volts AC or so at the input to the power supply. If you don't have that then your transformer is shorted, or the wires are broken or a board trace is bad or something. But confirm you have some AC input to the power supply before you start replacing parts.... always a good idea:)
Don
Follow Ups:
Replaced the caps in the power supply, but no joy.
I am getting voltage at the first cap in the power supply, but nothing after that.
There is a pair of 2N3440 transistor regulators before the next power supply cap.
Found some of these transistors on Ebay. I am attaching a photo of a hand drawn schematic I found in an earlier post here.
My preamp has a choke instead of the bridge rectifier, but all else is
about the same.
I get a little signal from a tuner if I turn the volume all the way up
Hi
As suggested below, measure the voltage after the cap at the resistor junction. I bet the transistor is dead. It is unlikely that the big resistors are bad, but they should be measured. At least you know the power transformer is fine. The rest of the parts are quite inexpensive. Hopefully just a bad transistor. Those things can have bad board traces....
The Theta uses capacitance multipliers instead of regulators, notice there are no voltage references, ie zeners. The combination of the two series resistors to the left of the transistors set the output voltage of each multiplier. What is the voltage at the junction of the first two resistors?
Craig
Waiting for the transistors to arrive...sometime next week. I will replace the transistors and check the resistors, but I am fast approaching the point where somebody who actually knows what they are doing should probably take over the job.
Hopefully, if the transistors don't fix the problem, I can find someone who will work on the preamp.
If you cannot find anyone local you can email me and I can probably fix it for you. Sounds pretty straightforward since we know the power transformer is OK. The rest is just working through the circuit. I suspect the transistors will fix it.
Good luck,
Don
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