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

[xeawr]

I have just replaced the four rectifier diodes D7 to D10, they seemed working normally, but I replaced them anyway. Still nothing, heater voltage drops to 0.3V under load. Does that mean that the transformer is the culprit?

 

[Khron]

Well, since just about everything (perhaps apart from a cold solder joint?) has been ruled out, that would seem to be the case...

 

[RuudNL]

Don't you have an 8 volt transformer to (temporarily) replace the filament transformer?
(You could use your doorbell transformer... ;D)
That would show if the transformer is the problem, or not...

 

[xeawr]

Thanks RuudNL, I've considered that as well. I know I have an 9-12VAC transformer *somewhere*. I'll go later and try to find it...

Just one more question: what else do you guys think COULD it be?

 

[RuudNL]

The first thing I would like to know, is if the voltage drops before or after the regulator.
Maybe you could use a lamp to test this?
If you measure a good voltage without a load, I don't think it is very likely that the transformer is the cause of the problem.
I have never seen a transformer with an increased internal resistance of a winding.

 

[granger.frederic]

hi,
if i can help...
i think also that the transformer is the culprit
c9 has probably shorted and overloaded the transformer
no protection fuse on heather and b+ rail...
you measure a "static" voltage but a widing is probably cut
cheers
Fred

 

[xeawr]

Thanks again for your help! I've done some more measurements:

WITHOUT LOAD:

after LM317: 6.5VDC
before LM317: 12.5VDC
before rectification (after tranny): 10.23 VAC

WITH LOAD:

after LM317: 0.3VDC
before LM317: 12.2VDC
before rectification (after tranny): 10.22 VAC

I'm pretty clueless where to problem could be?!

(By the way, I've also tried a different (incompatible) tube mic ps at the same power socket that works flawlessly.)

 

[xeawr]

Also, if I hook another power transformer up it gives me the same problem.
I've connected the second tranny to after the rectifying diodes D7-D10 (before LM317).

As soon as I connect the load, the voltage drops to 0.3V. So it's somewhere in the circuit / pcb? I'm really struggling to understand this 

 

[RuudNL]

So, it is not the transformer, because both AC and the rectified DC do not change.
That means it must be the regulator, or the connections to the regulator circuit.
But hey, what can go wrong with a 7806 regulator and a diode?

 

[Khron]

The circuit board?

Shorted cap => lots of heat => potential copper trace damage (perhaps around the pads, real tricky to see)?

We've yet to see photos of the actual "patient"...

 

[RuudNL]

Maybe, as a precaution, bridge the copper tracks on the PCB with wires.
And see what happens...

 

[xeawr]

Ok, warning: here are some hi res pictures of the "patient" (best: right click and "view picture"). I have also redone suspicious looking solder joints, isolated the LM317 output wire from the PCB, checked all the copper traces with a continuity test. I've also tested with bridged and unbridged R15, tried various grounding points for measurements and have somehow hoped for a "spontaneous remission" but still nothing. As soon as I hook up the load resistor(s) the 317 output voltage goes down.

On the pictures you see details, including a picture of the pcb with explanations. Every (!) component in the heater section has been replaced by me with known working components, including the 4 rectifying diodes. One capacitor (C8, 470uF) has been mounted on the back of the pcb because of laziness. R15 is in fact two paralleled 68ohm resistors (which shouldn't make any difference).
You also see the extra LM317 pcb connected with wires to the main pcb - it was easier that way. To rule out problems cause by an incorrect wiring in the XLR jack I have removed the wire from the pcb to the jack, but - you guessed it - it doesn't make any difference.

Could it be a grounding problem in some other area of the ps? Again, I'm pretty clueless, especially now that the transformer is out of question. The "status light" on one end of the power supply is also connected to the 10VAC secondary, and it doesn't flicker or dim out when the load is being connected - I guess that emphasizes that the tranny is ok.


I'm so curious to hear some ideas and recommendations from! Thank you again so far, I'm really grateful!!!

 

[moamps]

.....As soon as I hook up the load resistor(s) the 317 output voltage goes down...

Are you sure that your 317 is implemented correctly (different pinout vs 7806, two resistors for voltage setting)?
Also 317 should be placed on a heatsink, after overheating it, the protection will kick in and it will shut down.
R15 cannot be 34 ohms. If the output current is 0,3A, it will take 0,3x34=10V and your regulator would be left without voltage.

 

[RuudNL]

You mentioned that you changed R15.
Are you 100% sure that this is a 15 (or 20) ohms resistor?
And not 150 (200) ohm or 1500 ohm...
Because that would explain a lot!

 

[moamps]

....Are you 100% sure that this is a 15 (or 20) ohms resistor?..

This value is also too high.
For a good regulation, 317 needs minimum 3V over the output voltage at its input.
So if the input voltage is 12VDC unloaded, output voltage is 6,3V, and current is 300mA, value
of  5R6 or 6R8 for 2Wmin would be correct, IMO.

 

[RuudNL]

That would mean that none of the microphones straight from the factory would work...

 

[moamps]

That means that regulator ICs don't do what they should do because of lousy engineering.  If there was engineering at all. And someone who know how to power on and use a scope near by.

Yes, they are lucky, 12ay7 tube types will not be so upset with 1V of ripple in the heater power just because they work with full AC heater voltage just fine. 

 

[moamps]

Are you sure that your 317 is implemented correctly (different pinout vs 7806, two resistors for voltage setting)?..

It looks that "adj" and "input" pin of the 317 are swapped on the proto-board.

 

[xeawr]

It looks that "adj" and "input" pin of the 317 are swapped on the proto-board.

yes indeed, I've just checked that, it could be that I have input connected to pin 1 instead of pin 3. Ironically it gave out the appropriate voltage. I will check again and flip the 317 if needed.

As for R15, I've tried it numerous times with R15 bridged and it didn't make any difference really (you get about 0.2VDC more before and after the regulator - but that seems to be the only difference.

I opted for a 32 Ohm resistor because
a) I've had enough with high enough wattage rating
b) in the attached schematic (supposedly correct for Apex 460) it looks a bit as if it were a 30 Ohm resistor as well

 

[xeawr]

You mentioned that you changed R15.
Are you 100% sure that this is a 15 (or 20) ohms resistor?
And not 150 (200) ohm or 1500 ohm...
Because that would explain a lot!

I've checked/measured the resistors a myriad of times and bridged it. I can remove it from the circuit and test it without it for a start. Also regarding the heatsink, I know I eventually need one, but for the quick voltage/load test, which doesn't take more than half a minute, everything has been "cool" (always doing the "finger test").