Hi I’m new to the hobby and enjoy hands on learning so I took on this Grundig as a learning project.
Initially the unit had power but no audio output at all.
I removed the chassis from the console confirmed continuity in the power section and cleaned all switches and pots.
So far I was able to get, all dial lights, a strong, clear FM signal and turntable audio out however I only have sound coming from the 3 electrostatic speakers and nothing from the 2 woofers. (Note: even at full volume the output from the tweeters is pretty low but I don’t have a reference for what would be considered normal volume for them so maybe it’s fine.)
I believe speaker wiring is good. I’ve tested and have continuity through the crossover to both main drivers.
Just as another test I had 2 known working EL84s so I swapped those with no change in symptoms.
I know there could be a number of problems but my suspicions are the switches. This unit has a set of 5 tonal presets that can be switched on individually or in combination of mute and any other single preset but their wiring is a bit above my level.
On the schematic there is a switch directly wired to the two drivers (and a second switch at the top of the speaker diagram) which is where my main suspicion lies but I do not know what those refer to...? This is where I’m stuck.
I’m wondering if one or both of those switches refer back to the tone presets? There is a “mute” switch but I imagine that would apply to all speakers. There is also a “voice” preset switch which I know would attenuate the low frequencies but I don’t think that would completely kill the woofers (I could be wrong). Anyway, all of the presets do currently affect the audio coming out of the tweeters when I activate them so they do SEEM to be working normally.
Anyway, that’s as far as I’ve gotten. It’s a precarious task (for me) to remove and reinstall the chassis from the console haha so I’m just looking for something to go on before I pull it back out again.
I’m attaching the schematics. If anyone has any thoughts on the switches or if you see something that I’m overlooking I’d sincerely appreciate the help!
I would check the output transformer, these are notorious for failing after these years, Check the anode voltages on the output EL84s. Alternatively, just look at them in a darkened room to see if the screens grids are glowing red.....they will if the outputs are deprived of anode volts. With a faulty output transformer, the tweeters can still make noise because of the leakage capacitance between the windings. Also check the screen voltages, the transformer may be fine, and the screens are deprived of current, this will have a similar effect on the woofers..
Having just looked at the diagram, the btweetwers appear to be piezo devices, these are driven via the screen grids, being capacitive, they act as a screen bypass. These are working fine, so, Im putting my money on the output transformer.
Caution.... Never fire up any valve amp without speakers connected, the output transformer will arc and destroy itself in seconds
You will probably get more "views" and help on AntiqueRadioForum. ARF is more focused on restoration and repair of tube radios. https://www.antiqueradios.com/forums/viewforum.php?f=6 Rich
The schematic won’t upload but is easily found with a search under: GRUNDIG 9068 KONZERTSCHRANK SCH