In an accident during a testing session of equipment my cheap chinese preamp had the output connected to the input for a very short time. Now the preamp does not give a lot of signal anymore. When i open the volume maximum there is still a bit signal, clean sounding (not distorted), but very weak. it!s point to point build and used to sound suprisingly good when i bought it so i would like to get it working again. If not possible, i'll do the original plan and use the enclosure with connectors and transformers for an own build (it was that cheap that it was cheaper than buying the parts seperate).
Inside nothing looks broken, no smoke or burning smell was seen during the accident. I tried to replace the tubes, but that does not make a difference. All tubes also look ok, and work in an other device without issue after being in this device. I think it also emits dc on the outpot (i used it with a solid state amp with dc blocking caps on the input to test). Could it be that the output coupling cap is gone? I don't really have test equipment to test it for the moment but a simple multimeter.
Included is the schematic and a picture of the inside in actual state. The tape thing on the side is because i modded the transformer setting from 110V to 220V and needed to fix the cables (i did not want to cut them of)
Inside nothing looks broken, no smoke or burning smell was seen during the accident. I tried to replace the tubes, but that does not make a difference. All tubes also look ok, and work in an other device without issue after being in this device. I think it also emits dc on the outpot (i used it with a solid state amp with dc blocking caps on the input to test). Could it be that the output coupling cap is gone? I don't really have test equipment to test it for the moment but a simple multimeter.
Included is the schematic and a picture of the inside in actual state. The tape thing on the side is because i modded the transformer setting from 110V to 220V and needed to fix the cables (i did not want to cut them of)
Attachments
![IMG_1968[1].jpg](/community/data/attachments/774/774808-071c44d9d1bb3805000cd505c46c886e.jpg?hash=BxxE2dG7OA)
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For a quick check in your case with only a multi-meter I would change the capacitor at the output --the feedback capacitor and the first to second stage coupling capacitor.
Thats a small and easy job otherwise like TG said check the voltages -first the anode (US=plate ) voltages then cathode voltages.
Thats a small and easy job otherwise like TG said check the voltages -first the anode (US=plate ) voltages then cathode voltages.
You mean the output of the preamp connected to the input - closed loop which may have induced oscillation, or the output of another piece equipment?
You may want to check the value of the input 470k and output 120k resistors - if the sound is not distorted you may have significant attenuation.
Thanks for the sugestions to check. I'll try to do that today and see if i can get this thing working again.
The sound is not distorted, that is the funny thing, but the amp driven seems to have difficulties to deal with the signal. That's why i think there is dc on the output. I did not have the time to measure it yet, but now i got a few days of of work so time to get to that.
The sound is not distorted, that is the funny thing, but the amp driven seems to have difficulties to deal with the signal. That's why i think there is dc on the output. I did not have the time to measure it yet, but now i got a few days of of work so time to get to that.
Check the volume pot and resistors for change in value perhaps? The positive feedback will have drawn large currents.