xeawr

Dear friends,

I have a big problem  :'( (but probably easy for you guys) and I'm pretty clueless about solving this one. In fact I'm totally desperate. The (used) power supply (for the used Alctron BV300-A) doesn't give out any heater voltage (0.1V instead of 6.7V).

By the way I have two of these microphones + PS, and both have the same problem. All voltages are fine except for the heater voltage. So far I have opened one of the two PS and checked the voltages inside. (see pic below)

When I turn the PS on it gets VERY hot at R15, in fact I think the original R15 died on me, it was all crumbly and measured 7 Ohm (instead of 20 Ohm - see schematic - I would guess?)

I have soldered out and checked three suspicious components:

- L7806CV (is ok, tested with 9V battery)
- C8 (measures fine)
- R15 (replaced by me with 35 Ohm)

Symptoms: Turn PS on, no DC voltage before and after L7806CV,  R15 gets VERY HOT

Any hints which components to check next? The other caps surrounding the L7806??

Thanks soooooo much for your help!!!!!!!!!!



RuudNL

Bridge the resistor and see if it works now.
If yes, the resistor is the problem. (Most likely.)
Maybe the resistor died because of a defective 7806?
There is a solution for every problem!

http://www.vansteenisaudio.nl

xeawr

ok, I can try that. However if I unsolder and test the L7806CV with a 9V battery it works fine, how can that be??
By the way, I've already replaced the resistor and the problem persists  :-[

xeawr

hooray, I made some progress: I did some more testing and found out that electrolytic capacitor C9 was defective (shorted). That put a 12V difference on R15 and I guess that explains the heat?

Question for the pros: What could cause C9 going bad? The 7806?

RuudNL

I suppose it was just a bad capacitor.
Replace it, and everything will work fine for years again!
There is a solution for every problem!

http://www.vansteenisaudio.nl

Khron

I'm gonna go ahead and hazard a guess that that particular capacitor was some no-name or crap brand one, wasn't it? Probably not a good Japanese one (Rubycon, Panasonic, ChemiCon, Nichicon or Elna)...  ::)
Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

xeawr

Totally spot on, Khron. A no name cap, actually the whole thing is filled up with no name components (and by the way, the PCB reads "Avantone" in the bottom left corner - funny).

Here's some more trivia about the whole issue:

These were two (used) Alctron PSUs for two (used) Alctron BV300-A microphones in mediocre condition. When they arrived, they both had exactly the same problem, which was no heater voltage, heat and burned smell (after about two to three minutes). The heat originated from R15 because it was "cooked up" (see above). As above stated, the reason in PSU number one, was a shorted C9 cap (1000uF/16V), the reason for heater voltage failure in PSU number one was exactly C10, exactly the same value/brand cap, isn't this crazy? Out of the exact same four caps two went bad in a very small time "window". That's a failure rate of 50%...

So far, the problem seems solved, the heater voltages are back up to 6,7 (at PS). Also big thanks to RuudNL!!!

Last couple of (random) questions regarding PS noise:
- Would you guys recommend upping C1-C6 from 22uF to, say, 100-330uF?
- Add a filter stage after the zeners D5, D6?
- Raise C10 (heater filter) to, say, around 5000-10.000uF?

Tomorrow I'll test the mics, hopefully everything runs smoothly, wish me luck...

Khron

Crap caps strike again... But then again, if you know what to look for, it can facilitate grabbing some good deals for nice stuff, in some cases ::)

That sounds like overkill, either way you look at it. If anything, you might want to think about adding a series inductor inside the mic (and perhaps a bit of extra capacitance too, if there isn't already some) on the two supply lines, to make sure no high-frequency stuff (that might've been caught by the psu-to-mic wiring) gets in the way.

If you do the math, the B+ supply's RC corner must be in the sub-1Hz range, and linear regs are "ok enough". All that, of course, perhaps only IF you happen to determin there IS some extraneous noise that can't come from other sources ;)
Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

xeawr

Thanks Khron for your input. I'll test the mics today and see how noisy they are as is - and then maybe calculate the RC corners and replacing caps. I'm usually pretty picky about noise, but then again, I've got a couple of those UM70S and (when I remember correctly) they have a self noise of around 18dBA - and I don't mind using those for anything.

wish me luck for testing the mics out, they look impressive - I hope they sound in some way "useable" and most importantly - I hope they work ... :)

RuudNL

I wouldn't go much higher than 47 or maybe 68 uF for C1-C6.
There are some 6.8 K resistors, so when you increase the values of the capacitors it will take a long time to reach the final output voltage. I use (more or less) the same power supply with a tube microphone, but I have absolutely no problem with hum.

(As an illustration: the Neumann U67 power supply only used 3 capacitors of 25 uF!)
There is a solution for every problem!

http://www.vansteenisaudio.nl


xeawr

Re: DESPERATION! No heater voltage on Stellar/Alctron tube mic power supply
« Reply #10 on: January 15, 2020, 01:24:56 PM »
Thanks RuudNL for the "all-clear" regarding the capacitors. But adding a filter stage after the zeners or moving the zeners before R5/C6 is still recommended, right?

Just a completely different idea that comes to mind: As I love the natural "tube compression" (think of the sag in a guitar amp - or heck - even the natural compression of a 47), is there any point in the power supply that I could alter, that would somehow support this effect?

RuudNL

Re: DESPERATION! No heater voltage on Stellar/Alctron tube mic power supply
« Reply #11 on: January 15, 2020, 03:26:18 PM »
What you could do, is to put a resistor in series with the zener(s).
This is done in several microphones as well. (To reduce the zener noise.)
It will make the zener effect a little less efficient, but it will reduce the zener noise in the output voltage significantly.
I would start with 220 ohms (or so). What you effectively would get for the zener noise, is a filter consisting of 220 ohm and (say) 47 uF.
This will kill most of the zener noise. A side effect will be that the output voltage is a fraction higher than without the series resistor, but the tube inside the microphone won't care about this...
There is a solution for every problem!

http://www.vansteenisaudio.nl

xeawr

MORE DESPERATION! Mics not working...
« Reply #12 on: January 15, 2020, 09:53:38 PM »
Thanks RuudNL, this sounds all very interesting and reasonable. I would consider the single resistor option (for a start), however I just learned today that my mics are still dead, inspite of the PS now working flawlessly.

I am very experienced with FET mics, however I don't know where to start troubleshooting a tube mic. So far I've checked and rule out:

- Tube
- 7-pin Cable
- PS obviously

Maybe this helps: When I flip the pad or filter switch I get a small signal in my preamp/interface input. That would rule out the transformer, right? (I've attached the schematics)

Any help would be extremely appreciated  :( thanks so much!!!


RuudNL

Re: DESPERATION! No heater voltage on Stellar/Alctron tube mic power supply
« Reply #13 on: January 16, 2020, 03:47:39 AM »
Tubes are Fets... "with a heater"!  :)
First check if you can see the heater glowing inside the tube (or measure the filament voltage inside the microphone).
Next measure the voltage over C1. You should get the output voltage of the power supply here.
If all this is fine, measure the voltage on pin 1 of the tube.
The voltage here should be somewhere 'in the middle' of the supply voltage. (Anything between 40 and 80 V.)
Do you hear any sound when you switch the power supply on with the microphone connected?
Usually you hear a sort of "pschchch.....woeshch" sound, followed by some noise that fades after a couple of seconds.
These sounds are typical for the heating of a tube microphone.
You should already hear some mains hum when the microphone is open on your workbench, but at the moment you touch pin 2 of the tube socket with a screwdriver, the hum should be loud(er).
If you have no sound at all, measure the resistance of the transformer windings. It could be that a transformer winding is interrupted.
And measure the voltage over C2, you should get 1/2 of the power supply voltage here. (Or a bit lower, because your voltmeter will load the circuit a little.) Because: no polarisation voltage = no sound!

Anyway, try all this first.
« Last Edit: January 16, 2020, 03:53:07 AM by RuudNL »
There is a solution for every problem!

http://www.vansteenisaudio.nl

xeawr

Re: DESPERATION! No heater voltage on Stellar/Alctron tube mic power supply
« Reply #14 on: January 16, 2020, 07:48:35 AM »
Thank you Ruud SOOOO MUCH for your precious time and help with this!

I've just finished some measurements:

- Voltage over C1 is fine (155V)
- Voltage over C2 = fine
- Voltage at pin 1 of tube = 155V
- no sound as you described when turning the PS on
- no DC hum while opened, connected and working on it
- no loud hum when touching pin 2
- heater voltage is 6.7 at the end of the cable (a bit much), but as soon as the mic is connected it drops to around 0.1V, when measured with a normal DMM
- I did get a 50 ohm reading on the secondary (preamp side - pin 5 to pin 6). I didn't get any reading on the primary side, but I don't exactly know how to measure the transformer properly (guess: section C8/R12 to ground?)


I've also re-tightend all the screws and made sure there is a good ground connection everywhere where there should be. I'm really puzzled, as you can imagine. The drop of heater voltage, as well as the voltage on pin 1 and the transformer, well basically this whole thing really confuses me...

Thanks a thousand times for your guys help!!!
« Last Edit: January 16, 2020, 08:01:23 AM by xeawr »

Khron

Re: DESPERATION! No heater voltage on Stellar/Alctron tube mic power supply
« Reply #15 on: January 16, 2020, 10:34:54 AM »
So heater supply's somehow getting overloaded.

What's the resistance of the heaters in that tube (between pin 9 and 4 and 5 respectively)?  Datasheet says that should be about 42 ohms each.

Might also be worth measuring, inside the mic, the resistance between pin 2 and ground, with the PSU connected but powered off.
Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

RuudNL

Re: DESPERATION! No heater voltage on Stellar/Alctron tube mic power supply
« Reply #16 on: January 16, 2020, 11:24:49 AM »
Measure the resistance between pin 2 and 4 (or 7) on the 7-pin XLR on the microphone, with and without the tube in place.
If there is continuity without the tube, there is a short in the heater wiring.
If there is no resistance without the tube, but (almost) zero ohms with the tube in place, the tube has an internal short circuit.
There is a solution for every problem!

http://www.vansteenisaudio.nl

xeawr

Re: DESPERATION! No heater voltage on Stellar/Alctron tube mic power supply
« Reply #17 on: January 16, 2020, 06:00:37 PM »
Hi quick reply here, with some measurements:

Tube in place, no PS connected:
7-Pin XLR: Pin 2 and 4 = ca. 3.3 ohm

No Tube, no PS connected:
7-Pin XLR: Pin 2 and 4 = infinity (bigger than 200k)

Tube in place, PS connected:
7-Pin XLR: Pin 2 and 4 = ca. 3.3 ohm
12AT7 tube: pin 2 to ground = infinity

Only the 12AT7 tube: Pin 4 to 9: 6 ohm. 5 to 9: 6 ohm (I guess that is spec because all three of the 12AT7 here measure the same)

----

Ok - I'm back, I've done some more measurements: I've checked all components that I could check with a DMM without desoldering them. So far everything seems absolutely normal. All measured components work as expected.
I couldn't however measure:

- Capacitance of caps (measured only if shorted)
- The two 51M, 200M and 1G resistor (too big for my DMM), I'm sure they are not shorted though
- I didn't measure the pad and filter components (as they are not normally in the circuit)

I have also injected a signal before C8 and the transformer works just fine.
I have also reassured the tube works by testing it out in a guitar amp (Fender Champ Clone)
I have also powered the thing up and when I amplify the weak signal I DO get a small (50Hz?) hum, however touching pin 2 of the tube still doesn't do anything.

This is so weird, I am really clueless but SO THANKFUL for your help!!!!

Khron

Re: DESPERATION! No heater voltage on Stellar/Alctron tube mic power supply
« Reply #18 on: January 16, 2020, 10:43:24 PM »
Something's not right there. 6.3V heater voltage, divided by 3.3 ohms as measured, gives 1.9A.

The datasheet says the heaters draw 300mA @ 6.3V (ie. both in parallel) or 150mA @ 12.6V (ie. both in series).

That being said though, the 3.3 ohms you measured for both heaters in parallel was cold, and there's a temperature coefficient involved (resistance goes up as the heaters heat up). According to spec, they end up around 42 ohms each (21ohms total, in parallel), but that would mean there's quite a steep slope there (a ~530% increase)...
Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

RuudNL

Re: DESPERATION! No heater voltage on Stellar/Alctron tube mic power supply
« Reply #19 on: January 17, 2020, 07:23:31 AM »
I just measured the 'cold' heater resistance here, and I got 16.3 and 16.5 ohms.
There is a solution for every problem!

http://www.vansteenisaudio.nl


 

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