RCA BA73 Transformers

[CJ]

inner can would not come out, so we had to pull the lid off with a dental tool,

 

[CJ]

never seen this lam in a UTC transformer, this is a Neve style lam, no I bar, hi Ni,

creates a natural air gap to improve saturation level, thought i was gonna see a 24-25 EI like they use in the Ouncer units,

21 ea, so we have an over sq stack, leg is 3/16, 21 * 0.014 = 0.294,

so call it 3/16 by 9/32 inches,

 

[CJ]

ok ready to get our turns and structure,

gonna be small wire with lots of taps and 2 coils = PITA,

can we hang with it, that is the question, 

you can see the over sq stack>

 

[emrr]

now this is interesting.  I had noticed I'd never seen another UTC audio transformer in that size before, the lam explains it. 

 

[ed rees]

here is the freq plot, tight engineers, barley made the 20 hz spec,>

What tool did you use for this plot?

 

[PRR]

> i think that 26 DBM is on the sec side as 11 volts on pri would equal 11 * 7.5 = 82.5 VAC on the Sec side?

dBm is *power*, not voltage.

Power is (better be!!) the same both sides. (OK, 10% loss; TANSTAAFL.)

So it has to change with impedance.

+26dBm is 0.4 Watts.

In 600 ohms: 15.5Vrms
In 10K ohms: 63.2Vrms
In 10.8 ohms: 2.08Vrms

"dBu" is Voltage based-on dBm in 600 ohms. Hadn't been invented when this can was filled.

 

[CJ]

spaced the p-p out, me bad, 

Praise Allah for the online DB calculator! 

ok here be the goodies on the input, looked wicked at first but it's all good now,

gonna do some freq and L tests,

EMRR will be glad to know that the shields, pin 7, were shorted to pin 9, which means the input signal was shorting to ground as in the BA 73d this pin is grounded, no wonder poor Andy gave this thing after he replaced all the transistors,

 

[CJ]

carnage>

 

[CJ]

here is a shot showing the org shield wire soldered to the blk sec wirel

quality control was questionable on some of the newer vintage UTC stuff>

 

[CJ]

last of the mohicans,
there is a chunk of cardboard in between coils that you might want to add, 2 nd pic,
you can see the start/finish leads, blu and brown, that form the taps from the bifi windings>

 

[CJ]

this be Henries for RCA input,

notice drop in L at 10 Hz at 1 VAC, we are pumping 2.22 DB into a -22 DB xfmr,

this is called saturation, hey, no foolin?

 

[CJ]

here is the electrical on the input.

looks like they run a mu metal core at very low flux level for lots of Henries and little distortion, just like the output,

perm is way up there at 130,000 so this is a high quality transformer due to the lams,

it will take way more than -22 DB, they are running the core at 355 K gauss, Mu Metal core will take 5 K Gauss before saturation, we are way down on the BH near u-initial where perm is very high,

 

[emrr]

EMRR will be glad to know that the shields, pin 7, were shorted to pin 9, which means the input signal was shorting to ground as in the BA 73d this pin is grounded, no wonder poor Andy gave this thing after he replaced all the transistors,

Geez, shouldn't be connected to 8 or 9.  How did it get out of the (either) factory? 

 

[CJ]

from my experience the UTC quality goes like this:

black cans - excellent  quality
dark gray cans - great quality
light gray cans - good quality
light blue cans - (right before they went out of business) - not so good quality

i bet they checked the continuity of the pri and sec windings but did not check for shorts between shield and sec windings,

i thought i might have welded the terminals together with the heat gun while loosening up the glass tape, but i did a test with the heat gun and could not melt solder on either the low or high settings, and we used the low setting for taking off the tape,

early transformers had great taping and great processing as far as potting compound,
later stuff had wads of yellow poly and glass tape and amounts of potting compound that varied from "a little dab will do ya" to a full can of black tar,

now the black cans did have lams that were lower in perm than the dark gray cans, so the dark gray stuff is probably the best as far as electrical performance and quality control,

now how the amplifier itself made it out of the factory, that is the real mystery,

going to sweep the good one and see what it does, should be good to 5 hz with that mu metal core,

 

[CJ]

ok freq of the week chart for RCA input iron, as expected bass is strong,

limit of HP 200 is 5 hz otherwise it go to 1 hz no problem me thinks,

hi rez peak is probably due to bi fi winding which has high C,

DB graph paper would bring it down, we like to see the unweighted numbers,ok?