Late 70s MXR Digital Delay power supply and voltage regulators

[Wes]

Oof, I never really mess with stuff like this as I'm more of a tube and amp guy. A buddy has one of these that clearly needs power supply work and I'm trying to figure out the voltage regulators. Looking at the very last page of the schematic it appears that VR2 is a 12V device part number, but the voltage that is output is 15 with a 7.5 tapped off. Does this seem correct or does it seem to be a typo? Did they actually mean LM340T-15 or the output at 12v and 6v?

In selecting replacements for these, will most products work as long as they are the same voltage output, same package, same pin out (or lead gymnastics performed), and adequately suited for current?

Example- for the -15V VR1, would a ST Micro L7915ACV work?

I can't seem to figure out how to post a link or the schematic PDF.

Schematic is at:

https://hammer.ampage.org/files/Mxr_digital_delay.pdf

thanks for any help with this!

[Wes]

Oof, it's been a long time since I posted on a forum. Links worked.

[The Dude]

The gnd pin of that LM340T-12 does not tie directly to ground, so I'd expect it's output to be 12V + whatever voltage is on pin2 of the regulator. The original parts are still available, so I see no reason to use substitutes.

[g1]

I'd expect it's output to be 12V + whatever voltage is on pin2 of the regulator. The original parts are still available, so I see no reason to use substitutes.

I suppose you adjust R41 to achieve the +15V rail? Wonder why, maybe to balance perfectly with the -15V line? I'm guessing there is an alignment procedure for the unit and that tweak is one of several.

[The Dude]

Agree and I think yes. Looks like they are using the 741 op amp as a comparator to adjust how much the ground pin of the regulator is elevated making an adjustable regulator. It must be critical to some circuit, or I would think they'd just use a 15V regulator instead of all that extra circuitry.

[Wes]

Thanks very much for your replies. I will see about acquiring these VCs and give it a go.

[Wes]

VR 2 is listed as 340T-12 while VR3 is 340-12. I'm not clear on the T other than to say the packaging is TO-220. Is there some electrical difference? When I punch up the parts on mouser, everything seems to have the T in the name.

[Dave H]

I'm trying to figure out the voltage regulators. Looking at the very last page of the schematic it appears that VR2 is a 12V device part number, but the voltage that is output is 15 with a 7.5 tapped off. Does this seem correct or does it seem to be a typo? Did they actually mean LM340T-15 or the output at 12v and 6v?

The +12V regulator (VR2) maintains 12V across R9 (56k) because the output of the op-amp is at the same voltage as its +ve and -ve inputs. The current through R9 is 12/56 = 0.214 mA. If R41 is set to 5k the output voltage will be 12 + 0.214 (5 + 10) = 15.21V

[g1]

VR 2 is listed as 340T-12 while VR3 is 340-12. I'm not clear on the T other than to say the packaging is TO-220. Is there some electrical difference? When I punch up the parts on mouser, everything seems to have the T in the name.

T suffix currently means TO-220 package. If it used to mean something else, the T suffix is pretty much the only choice for that package now.
Are all the regulators in the unit TO-220 type? It could be that they just weren't uniform in their labeling on the schematic.

[Wes]

G1, yes- all the regulators are To-220 packages. I'm gonna give it a go, we'll see. I am gonna be replacing the electrolytics and the tantalums around the supplies as well. In what little experience I've had with units like this, I have always found bad tantalums that were shorted.

Again, thank you all for your help and insight.