N72 Rev 3.0 Now Shipping. Must read!

[tpryan]

Revision 3.0 of the N72 is now shipping with a number of new features and changes from the previous version. The updated BOM and schematic are shipping with the kits but are not yet on line. New BOM and schematic are up, revised assembly instructions are coming soon.

New components

C29, C30 100uF/63V electrolytic capacitor
D4 1N4002 diode
J5, J6 Jumpers
R41 3K9 1/2W resistor
R48 150R 1/4 resistor
T1b CMMI-2-PCA (new option, coming soon!)

Changed components

R40 is now a 47R 1W flameproof resistor
R46 and R47 are now 0.1% parts

Deleted components:

C5 1000uF/25V electrolytic capacitor
C22 82uF/25V electrolytic capacitor
C27 220uF/50V electrolytic capacitor
R18 12R 1/4W resistor
R32 51K 1/4W resistor
R41 33K 1/4W resistor

Assembly procedures are essentially unchanged, with two big exceptions:

You must insulate the bottom of the input transformer mounting bracket with electrical tape or a similar material! There are now exposed component pads under this part that may be shorted if you don't! N72KFs are now shipping with a paper insulator to be used for this purpose.

Also, in step 41, adjust the C of Q6 to 22.6V, not 21.4V. The voltages for Q4, Q5, and Q6 will differ from the table in step 42 as well, but the general troubleshooting principles are the same.

Please post all questions regarding Rev 3.0 changes in this thread. I'll post more details as soon as possible.

[Gavin]

tim,
is the on line instructions OK for the assembly of rev#3 kit?..Gavin

[Bij]

Hi Tim,

I've just started my N72 V3.0, and i think there's some mistakes on the BOM :
- The colour code for R41 isn't Yellow-Violet-Red-Gold but Orange-White-Red-Gold.
- R26 in present in the kit (and on the schem), but I think that it misses on the BOM.

Can you confirm, please ?


And how to connect J5 and J6 ?

[tpryan]

is the on line instructions OK for the assembly of rev#3 kit?

Like it says above, they're essentially unchanged except that you now must have an insulator between the input transformer mounting bracket and the top of the PC board. If you have a specific question, let us know.

[tpryan]

- The colour code for R41 isn't Yellow-Violet-Red-Gold but Orange-White-Red-Gold.
- R26 in present in the kit (and on the schem), but I think that it misses on the BOM.

New BOM and schematic are up.

And how to connect J5 and J6 ?

You can ignore J5 and J6.

[rmiller2101]

Well, I'm afraid I made it here a little late. I assembled my N72, double checked everything, hooked up my voltmeter to set the bias and turned it on. It started out at about 24 volts and I started adjusting the pot. After a few turns I heard a very quiet pop and the voltage went to 4 - 5 volts. The voltage regulator got very hot. I have retrofitted the electrical tape under the transformer but the damage is apparently done - I'm still seeing 4 - 5 volts and the voltage regulator gets very hot very fast. Any guess as to what has happened?

[tpryan]

Assuming everything else is correct, the absence of insulation under T1 won't cause the symptoms you're seeing. The fact that the regulator is hot indicates an excessive load on its output. What happens if you return R39 to the full CCW position? Is R40 very hot as well? What about R14? Are you certain that C26 and C3 are installed the right way round? Let us know what you find.

[rmiller2101]

Ah. I should have double checked C26 twice. I turned it around the right way and things are looking much better. I've got 21.4v of bias and it's passing signal.

Thanks,
Bob

[tpryan]

Also have a look here: http://www.seventhcirclestudios.com/SCA ... .php?t=951

Rev 3.0 voltages are slightly different at Q4, Q5 and Q6.

[lynyrd]

Hey Tim, (or anyone who has made both), I would like to know if Rev 3.0 sounds different at all from the previous version as I would like to have a matched pair and only got a chance to build one of the previous design. If there is a sonic difference then I guess my next question would be, do you maybe have an older kit kicking around that I might be able to get instead. Thanks for your time, the SCA stuff is fantastic.

[tpryan]

I would like to know if Rev 3.0 sounds different at all from the previous version

Rev 3.0 measures and sounds the same as previous versions. There is as much or greater variation between 2 modules of the same revision than between any rev 3.0 and rev 2.x pairing.

If you still want to make your rev 2.x modules identical to 3.0, however, you can simply snip out R32 and R41, replace R18 with a jumper, and remove C5. Optional "upgrades" would be to replace R40, R46, and R47 with the parts from the rev 3.0 BOM. You'll have to re-bias Q6 about 1V higher as well.

[lynyrd]

Thanks, looks like I will be getting a rev3.0 shortly.

[Archmart]

Hey Hey!

T1b CMMI-2-PCA (new option, coming soon!)

What's this new option about?

And the new, updated instructions... I'm thinking of holding off building the two kits I've got sitting here until those are available... or is it going to be awhile?

Archmart

[tpryan]

What's this new option about?

We'll be offering the kit with a custom wound CineMag PC mount transformer instead of the Carnhill part. It'll be much easier to assemble and offer greatly extended high frequency response (-3dB at about 120K). It has a smaller core, however, so it saturates sooner.

The new instructions won't offer anything that isn't already in the sticky except some new photos, which are still at least a week away.

[Archmart]

Hey Hey!

Interesting... So an improvement with a certain tradeoff.

And also a little less true to an original 1272, or... more true?

Archmart