samgraysound
Well-known member
Hey all,
I'm working on a Telefunken Hymnus Hifi Stereo. Its a console radio/amplifier. Not impressed with the build quality at all, huge rats nest.
It was blowing fuses. Found an incorrect tube in one of the sockets, pentode in a 12ax7 slot. Replaced that, filter caps, cathode caps, and the bridge rectifier. It is no longer blowing fuses although the amp draw still seems high to me. About 1A idling, with a 1.25A fuse.
I've been checking voltages through the amp. They are pretty high throughout like +25-30%. Which makes sense because it was designed for 110, and the new 1N4004s for the rectifier are probably putting out a higher voltage then what they replaced.
But one plate voltage in particular is just way high. Its V7, a 12ax7, setup in stereo, so one triode is the L and one is the R, with the exact same circuit for both. The schematic says plate voltage of 78v. One side is 140v, one is 260v!
Plate resistors tested the same but I replaced the one on the problem side anyway. Replaced the plate coupling cap. Replaced the cathode cap on both sides, they have the same resistance cathode to ground. 10M resistor grid to ground, reads as open on my multi-meter on both sides. Tried a different tube. What else is there?
I've been working off the schematics here(the 2nd page of schems is the relevant one): https://www.radiomuseum.org/r/telefunken_hymnus_hi_fi_stereo_2004.html
It is both blurry and in german.
They tube complement and general layout is the same, but there is a number of differences from the schematic. on V7 (the tube in question) plate resistors are 100k instead of 220k, and there is a cathode resistor and cap instead of the cathode being connected directly to ground, 1.2k and 47uf respectively.
What else should I try?
Sam
I'm working on a Telefunken Hymnus Hifi Stereo. Its a console radio/amplifier. Not impressed with the build quality at all, huge rats nest.
It was blowing fuses. Found an incorrect tube in one of the sockets, pentode in a 12ax7 slot. Replaced that, filter caps, cathode caps, and the bridge rectifier. It is no longer blowing fuses although the amp draw still seems high to me. About 1A idling, with a 1.25A fuse.
I've been checking voltages through the amp. They are pretty high throughout like +25-30%. Which makes sense because it was designed for 110, and the new 1N4004s for the rectifier are probably putting out a higher voltage then what they replaced.
But one plate voltage in particular is just way high. Its V7, a 12ax7, setup in stereo, so one triode is the L and one is the R, with the exact same circuit for both. The schematic says plate voltage of 78v. One side is 140v, one is 260v!
Plate resistors tested the same but I replaced the one on the problem side anyway. Replaced the plate coupling cap. Replaced the cathode cap on both sides, they have the same resistance cathode to ground. 10M resistor grid to ground, reads as open on my multi-meter on both sides. Tried a different tube. What else is there?
I've been working off the schematics here(the 2nd page of schems is the relevant one): https://www.radiomuseum.org/r/telefunken_hymnus_hi_fi_stereo_2004.html
It is both blurry and in german.
They tube complement and general layout is the same, but there is a number of differences from the schematic. on V7 (the tube in question) plate resistors are 100k instead of 220k, and there is a cathode resistor and cap instead of the cathode being connected directly to ground, 1.2k and 47uf respectively.
What else should I try?
Sam