AKG N-SOLIDTUBE tube condenser microphone

[MartinConnolly]

I have found nothing on the net about this little doozie. A power supply and interface box for a studio microphone that uses a valve and condenser in the body to produce a 'warm' sound. Yeah right!
Any way, the box just keeps blowing its fuse. Replaced the MOVs and it made no difference. There's a high voltage and low voltage side coming out of the transformer. Its all conventional design, with no switching. Disconnected the high voltage side and the fuse doesn't blow, so.......

There's only five capacitors on the board, so I just replaced them all as it would be so cheap to do so. The box then powered up fine. I'm not sure at this stage which one was causing the problem, but I suspect the small 63v 47uF one sitting right next to four diodes that judging by the scorching on the PCB, run really hot.

Anyway has anyone seen any similar boxes like this? There appears to be very little on the net about these, and no standards fir pinouts/voltages etc

[stj]

Re: AKG N-SOLIDTUBE tube condenser microphone

there was a trend for "valve scams" a few years back,
there was even a guy who profiled the frequency response of a valve pre-amp and then wrote a "simulator" for the mac!!!!

[redwire]

Re: AKG N-SOLIDTUBE tube condenser microphone

Every microphone has colouration as far as freq. response and distortion.
But you can't measure a microphone's distortion, and in condensor mics tubes beat JFETs for gain, linearity and input capacitance. But not for microphonics, cost, reliability.
Tube condensor mics do have a place. Just do an A-B comparison with a good vocalist or instrument and you will see in general all microphones are terrible at capturing sound lol and a tube mic can sound really good. You have to have heard the difference to comment about it.

Usually these mic/psu box combos get zapped by hot-connecting or wrongly connecting i.e. phantom power on or connecrtor flipped etc. It's usually just a pentode and few transistors in the mic.
One AKG N_Solidtube repair showed a shorted MOV in the power supply. Another had a bad power transformer.
It looks like it has a couple discrete voltage regs on the board.

[MartinConnolly]

Re: AKG N-SOLIDTUBE tube condenser microphone

Thanks for the replies. There are some good photos there. You can see the two high voltage 22uF 450V smoothing caps wired I think in parallel, the low voltage 1000uF 63V smoothing cap and the two 63V 47uF caps which I'm not sure of their purpose. There appears to be one voltage regulator on the low voltage side attached to a heatsink with a variable resistor to set it, which then controls the high voltage side with a pair of high voltage transistors wired in parallel. The one I repaired had already had the transformer replaced and the links to the bridge rectifiers had been patched in on the underside of the PCB. I don't know what the four diodes near one of the small caps are for, but as I said, they looked scorched on my PCB.

[redwire]

Re: AKG N-SOLIDTUBE tube condenser microphone

There a HV regulator for the ECC83 tube, LV for the filaments (big heatsink).
23VAC (~30VDC raw) and 270VAC (~380VDC raw) secondary windings. The 22uF's 450V one is at the output of the bridge rectifier, the other at HV Vreg output.

The four glass diodes look like zeners in series, and back to back pairs. Part of the HV regulator, I can't see all the traces but it might be an AC protective clamp against overvoltage. Did this thing get wired for 120VAC and get 240VAC?
All I can see is marking "Z3" on one diode.

[MartinConnolly]

Re: AKG N-SOLIDTUBE tube condenser microphone

I hope I've worked our how to attach photos.

Here's the PCB after being removed and the MOVs removed. You can see the patching and scorching around the diodes.

[R_J]

Re: AKG N-SOLIDTUBE tube condenser microphone

The 4 diodes in series are BZY97-C62 zener diodes

[redwire]

Re: AKG N-SOLIDTUBE tube condenser microphone

That would make a 250VDC shunt-regulator. If the new transformer is a bit higher in voltage, it could stress the zeners but where's the dropping resistor. Yet you have around 380VDC raw.
I can't see what the little transistor is doing. It might be a capacitance multiplier or
the 12AX7 needs only a few mA but still a bit wee for a HV pass transistor.

I think a person would have to trace out and sketch part of the circuit around the cooked zener diodes. OP has a Rev.3 board and the link I gave is pics of a Rev. 4 board, maybe AKG lowered the zener current.

[R_J]

Re: AKG N-SOLIDTUBE tube condenser microphone

This will help him:

[redwire]

Re: AKG N-SOLIDTUBE tube condenser microphone

Thanks for the service manual find.
The four zeners make up a 250VDC shunt regulator. They are fed by a constant-current of 10mA coming from two small transistors giving 5mA each.
If one of them shorted it would burn up the zener diodes, so check both. Zetex ZTX757 is only rated 300V 1W. If 10V zener D4 went open that would also cause things to cook.
The four 62V zeners normally dissipate at most (no mic connected) 2.5W total or 0.625W each and the transistors 0.65W each so everyone runs toasty.