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[rdraudio]

Hi Klaus,
I've moved on to my other M269, which has a NN48hu power supply.  I am yet to go through and measure voltages with the mic hooked up, but opened the power supply to test voltage and found quite a mess.  I believe this has been discussed here before, and will search.  I believe that this still has the Stabilyt cells installed.  I would like to clean all of this corrosion off, get rid of the cells, and do anything needed to get it up to snuff.  The voltages at the supply with no mic attached currently read :

B+ 126.7
Filament 4.251

Thoughts on how to proceed?

-Mark

[klaus]

Remove the Stabilyt cells, clean up the mess, and follow Uwe Sattler's recommended solid state replacement stabilization:
https://repforums.prosoundweb.com/index.php/topic,37395.msg538543.html#msg538543

[rdraudio]

OK...so something is not right.  I removed the cells, and did the best I could to clean up the mess and corrosion.  I built Uwe's circuit, and decided before hooking all back up, to make sure that everything was still ok in the power supply. 

From the schematic I can find online (NN48a, mine is a NN48hu ) voltages are not good. I am reading 36V @ R7, as well as where the circuit would connect, and at Pin 4 of the connector ( obviously not hooking up a mic! )  B+ still at 127, unloaded.  The voltage, even without the circuit shouldn't be that high, should it?

Mark

[klaus]

Mark, I'll let Uwe chime in with his voltage calculations of an idle power supply that has been converted to his heater circuitry.The LM317 should regulate the heater voltage quite well, so something went wrong in your implementation.

In the meantime, if you look around on this forum, a while ago someone posted the dummy load resistor value that simulates the current draw of an AC701 tube. Connect that load and then measure again.

By the way, the B+ /high voltage you measured seems about right for AC701 power supplies without load.

[rdraudio]

Thanks Klaus.  Yes, the B+ didn't change, but as I posted earlier, I was close to the 4V even with no load for the filament...but this is crazy high.  Something has changed.  Would the supply voltage be that different without the stability circuit of any kind? According to the schematic, no.  Anybody have an idea what would cause this?  I hope Uwe responds.  I appreciate all the answers.

Mark

[rdraudio]

OK...So I hooked the old cells back up, and voltage at the supply is fine, filament at 4.1V unloaded.  Then I hooked up my new circuit.  Pin 4 is not 4V, but still very high 30+ V...but at test Point 2 on the circuit, I get a slow ramp up to 4.15V, so obviously the board I built is correct, but I am missing something in the hookup.  I am hooking it up like the cells were, but when I look at the schematic, I am now assuming that TP2 should go to the red where the old cells were wired, being the ramping voltage out, and I should get the input to the circuit from? 

[klaus]

I find it hard to comment without having a clearer idea how exactly you wired the work-around heater stabilization circuit.

The LM317 voltage regulator is pretty self-explanatory. There's an input, a regulating tap and an output. If you do not get the correct output, make sure you are within the voltage limits of the LM317 input.

[rdraudio]

I understand.  The LM317 is wired correctly, and working correctly.  I've used them in other applications.  If I were to wire in Uwe's citcuit, feeding input voltage from the former Red Wire position that was used by the NiCd cells, and then take the circuits output and wire to pin 4 of the tuchel, it would work, but I don't know if this is integrated correctly.  Based on Uwe's circuit, I am confused about how it integrates into the NN48.  With the cells in there, there were 2 wires.  I am assuming that this wires the same way.  My confusion is that without anything in there, the filament, at Pin 4 of the tuchel, or at any point on the board, is approx 30V.  With the cells in there, it's 4.1.  With my circuit board, it's ramping, but up to a high voltage.  I am missing something, and most likely something obvious. 

[rdraudio]

OK....been looking at this all night...and I tried this, and voltage is 4.17 unloaded at Pin 4!!!  I lifted the resistor to try it ( The one perpendicular to R8 )  The white wire bridge looks like it just connects the left tab to the right, which goes to pin 4, so I assume I could also just wire to the right tab with the yellow wire.  Is this the correct way to do this?  This is an NN48hu, which I can't find a true schematic for.  I am getting 126.7 B+ and 4.17 Filament unloaded.....

[klaus]

The hand-wound resistor on the white plastic board was the heater fine-adjustment with the Stabilyts installed. You can either leave it, and use any of its 3 tabs or fine adjustment as well, or bypass it if you just use the trim pot of the LM317 circuit.

Overall, it looks like you finally solved the problem!

[rdraudio]

Thank You so much for all of the help.   I guess if I had known what that was, (a hand-wound resistor) it might have been easier.
This does show the simplicity and how well the cells worked!   So, last question: can the hand-would resistor be removed?

[klaus]

Hand-wound resistor can be removed when installing the LM317 circuit.
Original schematic of NN48hu sent

[rdraudio]

Well...I thought this was finished.  Checked all voltages, all good unloaded.  I connected my M269....Filament only gets up to 2.8V....checked all wiring, all good.  Tried different cable between mic and supply, same result.  Tried different M269...same result.  Reconnected old cells in place of new circuit...same result.  So, any advice on where I should be looking?  Obviously the mic load is drawing down somewhere...is there an obvious place to look?  Thanks, and sorry to reopen this already too long thread.

-Mark

[klaus]

This should be easy.
Measure and note idle heater voltage at several test points: at (+) output of rectifier, after first dropping resistor R7, and so on.
Then do the same, but with the load (mic) connected.
Then note at what point/component an unusually large voltage drop occurs.
Then test that component, to confirm whether it performs as specified, and if not, substitute it.

And if there is enough voltage at the input of LM317, let us know why the trim pot that adjusts the regulator's output does not give you sufficient range.

[uwe ret]

Sorry for jumping on this late. Check for conductive contamination of the printed circuit board. It may look clean, but the alkaline electrolyte from the obviously leaked NiCd Stabilyts has probably leaked into the phenolic board. If that is the case, repeatedly neutralizing with vinegar, then rinsing with water or alcohol is recommended. The equivalent AC 701 filament load is easily simulated with a 39Ω/5W resistor (exactly 40Ω/4W derived from the 4V at 100mA heater requirement). The LM 317 solid state Stabilyt replacement circuit has been successfully deployed in dozens of old microphone power supplies. Please check the DV potential at GR2 (B30/C250). With old age and possible contamination these selenium bridge rectifiers are known to develop rather high internal resistance, which will drop the available DC-voltage under load. When replacing it with a suitable silicon bridge, make sure to add an inrush-current limiting resistor of 47Ω.

[terskelton]

Uwe,
I have a couple NKMu supplies with corrosion. I don't want to install a new regulator at this time but want to remove the Stabilyt cells and as much corrosion as I can so things don't get worse over time. You mention using vinegar on the corrosion followed by water or alcohol. Can I just immerse the entire chassis in vinegar and for how long? Maybe scrub a little with a toothbrush? And then water, etc. Maybe bake in an over at 120 degrees to dry out? I seem to remember using baking soda/water paste or is that for a different battery chemistry? Thanks. 

[uwe ret]

Do NOT immerse the entire supply in water, vinegar or any other liquid!

After removing (permanently) the Stabilyt cells, the alkaline residue needs to be scraped off and neutralized with a mild acid (like vinegar). You may have to perform the vinegar wash repeatedly, for which I suggest a stiff bristle acid brush or similar. Make certain none of the cleaning solution gets into the mains transformer, choke or rectifiers!

When everything looks clean, rinse only the affected area with clean water, but never use running water, which, again, may damage ancient insulation in the transformer and/or choke or ruin old selenium rectifiers.

You may replace Stabilyt cells with NiCd batteries, but their life expectancy as voltage stabilizers is severely limited. The suggested solution is replacement with a solid state (LM 317 or equivalent) circuit as recommended in previous threads on this and other forums.

[terskelton]

Uwe,
Thanks for the response. There are a lot of stories in the broadcast world of just hosing down equipment out in the yard, especially when it has been flooded for example.

The other day I had a new car battery installed and the tech sprayed something from a spray can on the battery connections, he said if there was any corrosion left the color of the spray would have changed and I had the impression the spray also cleaned.

I don't know if that applies to NiCd corrosion. I'm going to investigate this further. Thanks.

[Kai]

The other day I had a new car battery installed and the tech sprayed something from a spray can on the battery connections, he said if there was any corrosion left the color of the spray would have changed and I had the impression the spray also cleaned.

I don't know if that applies to NiCd corrosion.

No need to spray (never spray!) strange chemicals, probably not intended for this use which possibly make things worse.

To clean certain areas of printed circuit boards I use a thin hose, taped to a (water capable) vacuum cleaner.
While applying cleaning fluid with a small brush - usually demineralized water + isopropyl alcohol - I vacuum away the fluid from the board.

This way I can limit the cleaning fluid to the contaminated areas of the board, preventing it from spreading the contamination.
At the same time this technique removes ALL contamination, as the cleaning fluid isn‘t just drying on the board, but completely sucked away.
Always suck away the fluid from the brush before dipping it into the fluid for the next dose, to keep the fluid clean.

This works well even with very high impedance circuits, where every little bit of residue disturbs the function.

Warning: Don’t do this with microphone capsule’s diaphragms.