DESPERATION! No heater voltage on Stellar/Alctron tube mic power supply

[xeawr]

I will do some more measurements later today. There's clearly something wrong. Does anyone worry about the 155V at pin 1 of the tube (when the PS is connected and turned on obviously)? According to Ruud it should measure around 75 or half the supply voltage.
So thankful for your input!

RuudNL wrote:
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.)

 

[Walrus]

The 155 volts at pin one indicates that no current is flowing through the tube. And that is because there is no heater voltage to heat the tube if it drops to 0.1 volt when you connect the mic.
You still have a problem in the power supply, did you try bridging the resistor again once the caps were replaced?

 

[xeawr]

Thanks walrus, I will try that asap. Can you help me understand what is the purpose of R15 is? (A guess: a LPF together with C9?)

 

[RuudNL]

I think the purpose of R15 is to create a 'slow start' for the heater voltage.
When the heater is cold, there will be a high current. This will cause a voltage drop over R15.
At the moment the heater is warmed up, the current gets lower and the voltage drop over R15 will get lower too.
It will to a certain extend also protect the 7806.

 

[Khron]

What're the odds the 7806 isn't as healthy / capable as it used to be? Let's recall it was working into a dead-short up until the cap replacement...

 

[xeawr]

that's VERY valuable information. It really helps me understanding the whole thing.

Is there a size limit for R15, say max up to X ohm?

I'm almost off to measuring! Happy weekend to all of you!!!

 

[RuudNL]

What're the odds the 7806 isn't as healthy / capable as it used to be? Let's recall it was working into a dead-short up until the cap replacement...

That's why R15 acted as a fuse!

 

[xeawr]

I haven't had much time fiddling around with the mic+ps, I just did a quick experiment:

Since I have two identical power supplies, and both had the identical failure of the 1000uF cap, both received the same repair treatment (replacing R15 with a 33 Ohm resistor and replacing the shorted cap).
When I hooked mic A up to the the power supply B I noticed a wee tiny bit of output (yeah!) at my interface/preamp (around -30 to -40dBFS after about 60dB of gain). Upon measuring the second power supply (B) I noticed that the heater voltage was up to 2.2V (loaded) instead of 0.3V with power supply A.

Maybe that tells me two things (what do you think about this?):

- The problem is the power supply - not the microphone (some of you indicated that)
- It's probably the L7806

Three simple questions here:

1. Why does it show 6.7V of output and as soon as you load it, voltage goes down? Is that a known "symptom" of a bad linear voltage regulator? yes / could be / no?
2. Can I "simulate" a 12AT7 load (around 300mA) by putting a 20 Ohm 5W resistor across the 6.7V DC output, just to see if the voltage really goes down without having to plug and unplug the mic constantly? y/n? (hey wait, or could I just connect the tube at the heater pins with alligator clips?)
3. Could bridging R15 be dangerous to the rest of the components in the PS - in case the 7806 is faulty? I don't care about completely blowing the 7806, more worried about the power tranny (and the tube - if connected).

 

[Khron]

Considering how much linear regulators cost, i'd just replace the 7806 right off the bat, and save myself further headaches by eliminating it as an unknown variable.

7805's might be more common / easier to find, though. You should then get basically the same result by also replacing the diode on its ground pin with a red LED.

Side-note: The 7806 has, as the name might suggest, a nominal 6V output. They raised its (normally) ground leg above ground by one diode drop, which makes it output about 6.6V (plus or minus some fudge-factors). Adding in the drop across the mic cable gets you "close enough" to 6.3V at the tube. The forward voltage drop of a red LED's around 1.8V, which if used in the same arrangement with a 5V regulator, would get you about 6.8V at the output, and once again, "close enough" to 6.3V at the tube. Or if you really want to tweak the value, you can add a small (suitably-calculated) resistance in the mic, in series with the heater.

 

[RuudNL]

Usually + or - 10% heater voltage doesn't make much difference.
Personally I wouldn't care if the heater voltage isn't exactly 6,3 V.
There will be a voltage drop in the cable anyway.
The best way to measure the filament voltage, is inside the microphone.

 

[Khron]

Either way, shoving in a new 7806 / 7805+LED would be loads quicker and possibly more revealing than spending who-knows-how-long speculating and stuff I guess i'm just more pragmatic(?) like that

 

[Matador]

A classic way of debugging this is to substitute a power resistor in for the tube (aka. a 'dummy load'), and connect it directly to the outputs of the PS on the heater rail (e.g., between the 6.7V output and ground).  If the tube says it flows 300mA @ 6.3V, then you need ~ 21 ohms:  22 is a standard 10% value.  It will dissipate around 2W, so you need a power resistor, or you could try tricks like four, 82 ohm resistors in parallel, each 1/2W, etc.

If the PS can't supply the dummy heater load without the microphone at all, then there's no use looker any closer inside the microphone.

You can use this same trick for the other voltages, like the tubes, if you calculate the current they draw from the supply, then you can work out a dummy resistance and plug it into the PSU and make sure the output voltages are correct.

 

[xeawr]

Ok friends, I'm back ... a couple of hours of debbuging later.

Because I didn't have a replacement 7806 available I put together a LM317 circuit and set it to 6.4V (and it works perfectly).
When I added the 22 Ohm 5W load to the PS the voltage dropped instantly from the 6.4V to the infamous 0.3V.... so - voilà - the problem is obviously somewhere in the power supply circuit...

Any idea what to test next? I mean this can't be so difficult, can it? 

 

[xeawr]

and for the record, bridging R15 doesn't make any difference, unfortunately. As soon as the load is connected, the voltage goes down to 0.30V

 

[Khron]

Well then, if it's not the bridge rectifier (very unlikely), then i'd reckon the other two electrolytics on the heater supply may well have kicked the bucket. Especially since they've been subjected to somewhat "harder work" than the one on the output of the regulator (think larger ripple).

Might be interesting to see what the loaded output looks like on an oscilloscope : Both before and after the regulator...

 

[xeawr]

I'll see if I can get a hold of an oscilloscope. Until then (tomorrow) I will unsolder C8, C9, C10 and see if anything changes. Will report back tomorrow / sunday.

I want to thank you all for your help so far!!!

 

[Khron]

You've replaced C10 (which was shorted) with a known-good one, right?

If you remove C8 and C9, the 7806 will definitely NOT be happy - at its input it'll be seeing "just" a rectified sinewave, and that's not good.

If you have any "spare" but non-dead capacitors you could replace C8 and C9 with temporarily, even if they're smaller values and/or higher voltage, it would be better than none at all, at least during testing.

 

[xeawr]

Hi Khron, hi everyone!

I've just replaced EVERY component after the rectifier diodes and the problem persists (as soon as the load is connected the voltage goes down to 0.3V. (By the way, voltage after rectification is about 12VDC)

No there is only the transformer or the rectifying diodes D7-D10 left. What would you recommend trying next? I would really really appreciate your help.

 

[Khron]

"Left" in the sense of "not replaced", right? Not "the only things left in circuit apart from the regulator".

If replacing the diodes doesn't do the trick either, then the only remaining suspect is the transformer...

 

[xeawr]

Yes right, "left" meant "the only things in the heater supply circuit that I haven't replaced" are Diodes D7 to D10 and the transformer itself. These are the only stock parts in that part of the power supply.