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I got this Theta preamp.
The tubes light up, but the only sound I get is by turning the volume sliders (!) up all the way (tuner into one of the aux inputs)
The power supply has a bunch of Mallory electrolytics strung together, interspersed with little round metal cans that appear to be transistors from before the ice age.
I am replacing the electrolytics this week, but I wonder about those little transistor thingys....the number on them 2w5448 does not ring any bells on ebay or the Mouser site.
I will know more when I get the electrolytics replaced, but I suspect that may not be my only problem.
Help!!!!
Follow Ups:
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
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
Hi
I rebuilt one of those about 2-3 years back for a customer. It had no sound because a couple of traces on the underside of the board had broken. As Jim said, those are 2N... transistors. Before you replace them you could simply measure the voltage at the plates of the tubes. If they have 120-150 volts on them then your power supply is working fine and you need to inject a signal and then use a scope to see where it vanishes. Apparently the slider volume pots are a problem in many cases as well. The transistors may well be fine. Again, I would measure that voltage at the plates of the tubes first. The tubes in the one I repaired were 6DJ8. So the plates are pins 1 and 6. See if you have high voltage there, over 120 volts DC. If so, then the power supply is working and you have a signal path problem. Good luck and post results and maybe we can help you further.
cheers,
Don
Thanks very much for your help.
I will check the high voltage today and post the results.
I am aware that some Thetas had problems with the slider volume pots. Finding exact replacements is probably a non-starter and replacing the sliders with a rotary volume pot or pots will require "re-modeling" the faceplate, which I am a little reluctant to do, at least till I hear what the old boy sounds like.
Thanks again
I get no voltage at pins one and six of any of the tubes, although they are getting something from somewhere because they light up.
I do get 380 vdc at one of the power supply caps.
It appears that the power supply has a problem
Well, then you found it. Change the caps, the transistors and possibly the diodes in the power supply. The transistors may be good, but they are so cheap you might as well replace them. Check the boards for any broken traces as well. It should work once you get B+ at the tubes. They are lighting up because the filament supply (heaters) is working fine. Those preamps aren't bad. If you can get it running for $50-75 in parts then it is worth that. I wasn't all that impressed with the one I rebuilt, but it wasn't bad by any stretch.
Good luck,
Don
Look up 2N5448, I suspect you misread the number.
If it is that number and you find you need any you can use the still available NTE159.
Jim,
Thanks...I will try that number.
This is such a great website with such helpful inmates. Without your assistance who knows how long this guy would been struggling. Hope the preamp gets up and running and he enjoys it.
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