Neumann NN48b Help

[damianschwartz]

Hello there!
I wanted to see if someone can help me. I'm trying to fix a NN48b Neumann PSU. I have no filament voltage when i measure (without the mic connected). I'm not the best at electronics since i'm still learning lot's of things, so please bear that in mind.

I've been chasing down the issue, and apparently it is the AD130 transistor.
B to E -OL
B to C -OL
E to B -0.11v
C to B 0.11v
C+ to E- =OL
(C- to E+ = 0.15v) For what i read here https://vetco.net/blog/test-a-transistor-with-a-multimeter/2017-05-04-12-25-37-07 this might indicate the transistor is faulty?

In the case the AD140 is faulty, i have both AD149 and AD150 in stock, and for the datasheets, i assume i can use them to replace the AD130 here. I'm i right?

Both diodes seem to be ok.

 

[damianschwartz]

This happened after C2 kind of exploded, it was the original cap, i went and changed all of them, but then i did't measure the filament voltage. I checked the Mic with another nn48 psu and works fine, but i don't want to plug it to this psu till i'm sure the psu is fine.

 

[moamps]

You can use AD140 or 149 instead of AD130. Anyway, I will suggest using a robust Si transistor here, as 2N or MJ2955.
Also, your measurement results do not 100% show that the transistor is faulty. Have you check voltages on C2, C4, C5 and C7?

 

[damianschwartz]

You can use AD140 or 149 instead of AD130. Anyway, I will suggest using a robust Si transistor here, as 2N or MJ2955.
Also, your measurement results do not 100% show that the transistor is faulty. Have you check voltages on C2, C4, C5 and C7?

Hey, thanks Moamps. I was measuring voltage across C2 C4 C5 but not C7 if i remember correctly, and it was around 20v across the  +- of each cap

I don't have MJ2955 around, but i do have a bunch of 2N, which 2N would you recommend for the task?

 

[moamps]

If you already have AD types, use it. Any Si high power PNP in TO3 package can be easily used here also, one resistor should be additionally adjusted. There are lot of PNP transistors in TO 220 or similar packages like BD912, MJE2955 etc. which can be used too, but they aren't drop-in replacement. Be careful, AC701 is very sensitive to heater voltage.

 

[chops]

Hey, thanks Moamps. I was measuring voltage across C2 C4 C5 but not C7 if i remember correctly, and it was around 20v across the  +- of each cap

Do you mean you measured 0VDC at C7?  Or that you didn't measure C7?

 

[damianschwartz]

Do you mean you measured 0VDC at C7?  Or that you didn't measure C7?

Hey Chops, so i don't have voltage in C7, 0,something. i do have in the caps before, around 27v from +- in the caps.
I swapped the AD130 for a 149 but it wasn't that, still no voltage at C7. R6 120ohm that is connected between C5 and the zener gets really hot. I'm assuming some kind of problem with the diode or the resistor, the diode seems to be reading ok, i took the positive lead out of the circuit and measured, it seemed ok, though i'm not 100% sure.

Edit: i took out R2, R4, R6, all three measure correctly. I'm guessing Gr4 might be the problem, there's not many more. But i don't know what diode could i use as a substitute. To be honest all this negative stabilization circuit is pretty cryptic to me.

 

[moamps]

Remove output transistor and measure the voltage on GR4. It should be about 7V. Voltage on C7 should be about 5V.

 

[panman]

Check out R14! The trim-pots in those Neumann PSUs have more than once been the problem.

 

[damianschwartz]

Remove output transistor and measure the voltage on GR4. It should be about 7V. Voltage on C7 should be about 5V.

Will do right now.

Check out R14! The trim-pots in those Neumann PSUs have more than once been the problem.

r14 was actually one of the first things i thought about, and i couldn-t find it, following the traces i found out something i thought was a 0ohm jumper-resistor, but turn out to be r14, which is a fixed 4K7 here. I thought it was the problem, since it looks like the picture i attached here, though it measures correctly. I think i will substitute it for a new one either way. I didn't check r13 tbh, which might be a problem too.

 

[moamps]

R14 is NTC resistor, do not replace them if it has 4k7 resistance.

 

[damianschwartz]

R14 is NTC resistor, do not replace them if it has 4k7 resistance.

Ok! good to know.

 

[damianschwartz]

R14 is NTC resistor, do not replace them if it has 4k7 resistance.

Hey moamps, ok so i took AD149 out, measured GR4, -6.9v. Measured C7 across +-, and i have no voltage there... So, trying to wrap my head around that right now.

Edit: Looking at the scheme, there should be no voltage to measure with the transistor out in C7, but if i measure between C7+ and collector, or negative side of C5 i have 20v.

 

[moamps]

What's voltage on R11? If it is close to zero, check C7 for correct polarity.
Sometimes pcb tracks are damaged so there is no continuity between solder joints. Check traces with DMM in continuity mode when PS is switched off. Post  a closeup pitcures from pcb side and from component side.

 

[damianschwartz]

Ok! you did ring a bell here talking about polarity, i made sure polarity was correct on everyone, though, the original caps had 3 legs, center positive, two negatives. Traces for negative were connected on most, so i didn't check well enough. I just realized that C7 negative has one side to r13 and base, but r11 is also connected to the negative side of C7, and in this the negative track was interrupted, so i need to jumper those sides. I'm i right? here's a pic of C7 solder side, the arrow shows were both negatives are.

Edit> Alright, it was that! such stupidity, thanks moamps. I have filament voltage now, though i have 5.37v,, is that too much? not sure if that might be related to the AD149, i will try again with AD130, or maybe because i used 470uf instead of 500, which i couldn't find.
It could be that with a load (mic plugged) the voltage drops that volt?

 

[moamps]

 

[moamps]

I have filament voltage now, though i have 5.37v,, is that too much? not sure if that might be related to the AD149, i will try again with AD130, or maybe because i used 470uf instead of 500, which i couldn't find.
It could be that with a load (mic plugged) the voltage drops that volt?

Yes, it's unloaded voltage.
You can check it by connection a resistor of 420 ohms across heater output and the voltage should drop to 4,2V.

 

[damianschwartz]

Yes, it's unloaded voltage.
You can check it by connection a resistor of 420 ohms across heater output and the voltage should drop to 4,2V.

It's working!!! thanks a lot, you made my day!!! 

 

[chops]

Love this place...
Congrats!  Keep an eye on that heater voltage.  AC701 will sound just as good and last a lot longer if you keep it below 4.2.v (4.0-4.1)