Power Transformer for LOMO 19A19 PSU

[mjrippe]

Hi Folks,

Have one of the original power supplies (20B99? serial 700038) with the sort of greenish-beige chassis.  It seems to have a burned out high voltage secondary on the power transformer.  Has anyone replaced one of these before?

Ideally I would like one with a 120v primary.  I know the final voltages out of the PSU are 90vdc, 24vdc, and 6.1vdc.  If anyone has a supply and can measure the secondaries I would really appreciate it.

A different supply I tested (for an earlier 19A19 with a different tube type) had secondaries of about 230v and 16v.  Seems awful high/wasteful.  That supply had no 24v output, as I believe it is derived from the 90v inside the mic. 

Yes, I could build a new PSU, but for now let's focus on repairing the original  ;D

 

[zebra50]

I've seen some weirdness with one of these power transformers. As far as we could tell, the inductance on both windings was way, way too low to make any sense. I heard some rumours of the laminations going 'bad' and losing permeability.

The owner wanted it repaired with original parts only, and in the end we sent it back unrepaired.

>A different supply I tested (for an earlier 19A19 with a different tube type) had secondaries of about 230v and 16v

Seems fair enough, with of lots of filtering!

 

[mjrippe]

Perhaps someone can translate the specs for the original?  Google translate gave me Permalloy, but nothing else useful.

 

[ungifted]

It says:

label/Name/Data
Тр. (Transformer) / Power transformer (20В.99.040) /Core Ш 16x16, 4511 - permalloy, 1st winding - 3300 turns (ПЭВ-1 - is a mark of russian wire (copper wire in enamel insulation of circular cross section) 0.08mm, 2nd winding - 3800 turns, ПЭВ-1 0.05mm, 3rd winding 260 turns ПЭВ-1 0.25mm, 4th winding 160 turns ПЭВ-1 0.1mm.

As I know audio transformer and plate resistor are in the PSU case.
Here is some 19A9 docs http://diyfactory.ru/forum/index.php?act=Attach&type=post&id=14587
and 19A19 http://www.mediafire.com/?aqad48pl23a1s7y (I see you have it)
maybe will be useful.
Let me know if you need more info, I'll try to serf russian web.

 

[mjrippe]

Thank You ungifted!  That is very interesting info, but how would it translate to voltage and current ratings?  CJ?

We know the primary is 220v and the PSU puts out 90v and 21v.  Measuring the secondary that feeds the 21v circuitry, I get about 16vac at the input of the diode bridge.  I am ASSuming that 75-80vac would be enough to get 90vdc on the other rail.  Current should be pretty darn low, so might not be a concern at all.

I'm thinking of connecting an 88vac transformer I have sitting around.  Will let you know how it goes!

 

[Gus]

I have adjusted a China tube microphone power supply for a 19A19.

 

[mjrippe]

I have adjusted a China tube microphone power supply for a 19A19.

Hi Gus, Care to share some details?

 

[Gus]

The two Lomo 19a19s I have worked on had two different circuits one was a nuvistor and the other a pencil tube both were 90VDC and 6somethingVDC IIRC
They did not have a 24V section

IIRC someone posted there was a 24VDC B+ version of the 19a19

 

[mjrippe]

Ok, things are making sense now.  The ~21vdc rail I was seeing was actually the UNLOADED filament voltage!  These particular PSUs don't use the 21v anode voltage that I was seeing in the document linked above.  Simply heater and B+/capsule polarization.  Pin 7 is not used, both higher voltages come through pin 5.

So I wired in a 220 to 88vac transformer to replace the burnt out winding, but with the trim pot wide open I only get 64vdc at the junction of C1, R3 and R4 where the voltage is picked off for pin 5.  The mic is working at this voltage and sounds good, but I'd rather be closer to 90vdc.  Perhaps with a different transformer I could get within range...

 

[mjrippe]

OK, switched to two transformers and removed the original one.  First transformer is 1:1 isolation, so 120v primary and secondary.  The other is 120v primary, 18v secondary.  Now we don't need a step up tranny for use in the USA.

With some adjustment of the variable resistors and a change of value for R3 (replaced 43K with 20k) I am able to get 6.3vdc and 90vdc with no ripple. 

 

[mjrippe]

Repaired a second supply using the same 1:1 and the transformer from a 15vdc/500ma wall wart.  Works just fine!

To simplify future repairs, I have contacted Edcor about getting a custom wound replacement.  Will let folks know what comes of it.