lomo 19a9 psu schematic

[atastudios]

could anyone help me out, i'm trying to get hold of the schematic for the lomo19a9's psu. i am unable to get the that is needing to be done on it without this and after 3 weeks of hunting am desperate to get it back to it's original condition

[zebra50]

Hi!

Do you have a supply at all or are you planning on building something?

I traced a 19a9 microphone out one time, but I never had the power supply. I have never seen a schemo for the PSU.



The guys here might be able to reverse engineer something from the schematic. Something to note is that the plate resistor, output cap and any output transformer will be in the power supply. Also the audio effectively runs at high impedance and unbalanced, so may be prone to interference.

Looking at the rest of the design my first guess would be you could get away with modifying a common chinese supply, something with around 100 to 120V and a 6V heater. You'd need to add the resistor & cap & tranny to this.

If I were doing this I'd probably start with a 50K resistor and a 1uF output cap, and then tweak to get about 60V at the plate.

The other approach would be to pop in a simple cathode follower circuit.

[mad.ax]

There's one hand-drawed PSU schemo posted here...
http://www.electrical.com/phpBB/viewtopic.php?f=5&t=34661&start=0

Axel

[Gus]

Do you have the power supply?  If so make sure the contacts and shell are electrically clean on the cable and the contacts at the microphone and power supply are clean as well.  I "fixed" a lomo by cleaning the contacts.

I repaired a 19a9 a few years ago make sure you check the heater voltage.

[jackies]

I have a 19A9, but it has a rather crudely built psu - not original.
If you happen to have an original psu, could you possibly post some good pics of it so we can see the insides?
 
[/porn request]

[Sonic]

All hi!
To the author topic has written in private. If to whom the factory instruction, but only in Russian, in format DJVU I can send on E-mail is still necessary. About three years ago I spread images 19a-9.

[zebra50]

Hi Sonic,

If you send me the djvu file I will make it into a pdf and post it here. I can't speak russian but I'm sure someone will translate the important parts.

Stewart

[Sonic]

To Stewart, Ok!
Pleas you e-mail?

[jackies]

 
I'm russian and would be delighted to help with translation, if need be...

[zebra50]

I'm russian and would be delighted to help with translation, if need be...

That would be great.

Sonic sent the document over. Thank you!

The new pdf is here...

http://www.xaudia.com/xaudia/Documents/Entries/2010/6/13_Lomo_19a9_manual.html

I've also cut out the microphone and PSU. The components are in the manual but I'll add them to the diagrams when I get a moment.




Cheers!

Stewart
 

[zebra50]

R1 = 100Meg
R2 = 910R
C1 = 25 uF (from my microphone)

PSU
R1 =68K, 2W
R2 = 100K, 2W
R3 = 36K, 2W
R4 = 20R, 7.5 w
R5 = 43R, 10W
R6 = 20K
R7 = 56K
R8  = 51R, 15W
C1,C2,C3 = 30uF, 300V
C4 = 2000 uF, 12V
C5, C6 = 2000uF, 20V
C7, 30 uF, 160V

[Gus]

R5's setting is important.  The 19a9 I worked on had >7VDC at the heater with a 110 to 220 step up transformer with 120VAC, 124 VAC from the wall.  I used a variac to power the microphone up.  I powered down unplugged and let the supply discharge, then I moved the sliding contact to the end of its travel for the most resistance.  IIRC I ended up at about 6.1VDC still a bit high.  So we decided a variac was needed.  Yes I could have added more resistance but we wanted to keep the microphone as stock as possible.

The owner uses a variac set to 110 and I reset the heater resistor for the heater voltage I liked.  I also replaced the cathode bypass cap with a 100uf.  The original measured bad (low value, ESR) and I had a 100uf I liked so I installed it.  Once the contacts were cleaned and the heater reset I let it run for hours and it had lower noise.  The stock parts in the one I worked on were fine.

Draw out R6,R7,C7,C1,transformer and the anode connections

[zebra50]

Draw out R6,R7,C7,C1,transformer and the anode connections

Good idea Gus. It's easier to see how everything is working this way.



Heater omitted for clarity. Pentode drawn as triode. Hopefully I didn't do anything silly - let me know if you spot errors.

Looks like R6/R7 form a voltage divider that sets the capsule/plate voltage.

Does the direct connection of the capsule rear to the plate act as a feedback network?

In my microphone the grid resistor had 91 Meg written on it!

[duantro]

Hey there, I figured I'd try to post some pics of an original tube psu for the 19a9.
I had an AEG made one also. The original one I have isn't functioning. It is getting 220v in to the power switch, fuse, and light, but nothing coming out of the power transformer upon first glance. hmmm.
I really like the Lomo mic's!

edit: I've installed a new power transformer and am now getting voltage! I'm getting 14v at the heater out, unloaded, while my new psu is putting out 7v. R5 doesn't seem to alter the voltage either. hmm