3M Model 79 Output Transformer

[CJ]

just in from across the pond,

we are going to do some "research"  on this transformer to see why it blows away Studer machines, 

Model 79 used at Motown, and supposedly has a nasty rep for stretching tape, 

 

[CJ]

heat gun and the swiss army to the rescue

 

[CJ]

don't keep diggin or you might lose the coil or the coil,

have to option out  the mu can for the sake of science,

jettison heat gun and knife - pick up Klein D228-8 and open it up like a can o sardines,

 

[gridcurrent]

we are going to do some "research"  on this transformer to see why it blows away Studer machines, 

it simply "blows", no need to compare with Studers.
maybe the legendary sonics can be attributed to those zippy 741's.

Model 79 used at Motown, and supposedly has a nasty rep for stretching tape,

and plenty of other "reps".
how about that nice wood cabinet !

 

[CJ]

the jaws of life have done their job, 

carefully pick away the rest like a case of bad acne  or poison oak scabs,

 

[QUEEF BAG]

yeow-kay...


electronic vivisection

morbid fascination


show more

 

[CJ]

de-lam on isle 5,  heat and cut, then re-heat and slice,

24 lams + 2 keepers , sq stk 375 EI,  29M6  1 x 1

turquoise material dates back to vintage Triad days,  witness HS-29 header plate

 

[CJ]

going to be rough going, broken leads, cooked wire, insulation tape all bombed up and depleted like Afghanistan, wtf?

 

[CJ]

thought we had some kind of weird winding with two coils side by side,

turns out to be the same wind,

this is what a shorted turn looks like,  current bypasses some of the coil and cooks the "new" coil it forms due to a reduction in turns which raises AC excitation current due to less inductance, and DC current due to less DCR.

the coil probably ran hot to begin with as it is direct coupled to output transistors.  Green insulation is not as tough as red, don't know why, learned this  by failures and knife stripping to tin wires.

 

[CJ]

end of the outer section,  3 layers , 165 turns of #30,

can now measure DCR, could not before as the bare wire was shorting out in may places,

run the  wire across the room so it does not short out, about 4.5 ohms,

 

[radardoug]

Wow, that got there quick! CJ, the output was coupled by a cap, 470 uF from memory.

 

[abbey road d enfer]

Green insulation is not as tough as red, don't know why,

I remember my xfmr winder offered the option of red for extra toughness or green for easy stripping ( hot tin bath).

 

[CJ]

"green for easy stripping"  that is very cool info, thought it was just me,

Radar Doug, full props for sending that! did not know it was overseas til it got here,  what a hassle shipping international eh? and mucho $$ but not bad coming this way thanks for that! 

 

[CJ]

ok we spliced together the repro path, looks like some docile TO-39 transistors in the output circuit, might have to change failure mode to over active varnish eating the magnet wire insulation instead of a shorted tun/heat scenario,  that's alright, if the NTSB can spend 2 years investigating an Amrtak engineer who blew thru 2  speed limit signs then we don't feel bad about the reversal. 

pin numbers match up with the bobbin, they mifght have used the transformer fo both input and output,

can anybody guess at the output Z on this 2N2270 - 2N4036 single supply circuit so we can put a pri Z spec on the transformer print?

Doug  is right about the output cap, good memory!  looks like they strapped the sec for series.

sorry for the weird format, should have stacked the pics,  ???

 

[abbey road d enfer]

can anybody guess at the output Z on this 2N2270 - 2N4036 single supply circuit so we can put a pri Z spec on the transformer print?

Output Z is roughly 6.8 ohms divided by the NFB factor, which is about 5000 (75dB @ 20Hz), so about 1-2 milliohm.
This xfmr is meant to be driven for near-zero impedance, but optimized for 600 ohms load at the secondary.

Still I'm puzzled since this seems like a 1:0.32 ratio, which would imply that the max output level was only 3Vrms (ca.+12dBu), which is not what I remember about this machine.
OTOH, if the xfmr is wired the other way round, as 1:3.16, that would make the max output at about +26dBu, but the reflected Z at the primary would be only 60 ohms, which the line driver can certainly do, at the cost of some stress on its output devices.
That would make the xfmr a 60:600 ohms.

 

[CJ]

that makes sense, thought that it might need to be flipped,  and one of the few leads that was not snapped belonged to pin 6, this was one of the 165 turn winds.

plus, if that cap shorted, you would have a lot of current going through that wind, which would explain the baked off insulation.

the 1000 turns and high DCR did not fit with the typical 600 ohm wind, but it is not that far off.

ok, new print makes more sense now, thanks Abby!>

 

[radardoug]

Definitely wired for step up. Quite grunty output. If you are interested, see my website for mod details to the output stage. www.djr.co.nz

 

[Monte McGuire]

3M tended to step down the signal levels to internal circuits at the input and then step up at the output. It's a good way to get something like a 741 to be able to produce 20kHz - limit the 741's voltage swing, and make it up later.