Help troubleshooting console power supply

[bjoneson]

I've got a linear regulated supply for an Auditronics Grandson console. I've jumpered the +15 to +15 sense, -15 to -15 sense, common to common sense, and have a 50 ohm 25W resistor from each rail to common.

I get close to 24V DC on each rail, with significant ripple, regardless of where I position the volage adjustment potentiimeter (P2).

It appears as though the pass transistors (Q8 & Q9) are running wide open, and I'm seeing the unregulated +/- 24V feed.

Looking at the schematic, it appears IC1 (LM709) provides the control voltage to the network of transistors, and IC2 (another LM709 for the negative rail) has feedback derived via a voltage divider from the sense rails. Therefore it's likely any issue with the positive rail regulation will propagate tof the negative rail.

I'm just trying to figure out where the most likely culprit is, or if I may be missing something simple. Right now I suspect IC1, but that's just an educated guess.

I've got a scope and a decent DMM if anybody has suggestions on how to narrow it down.

Any help is greatly appreciated!

 

[psych60s]

A faulty IC1 makes sense, and the regulator too. How is the adjust pot for the sense part of the circuitry? I'd quickly check that one, just in case.

 

[bjoneson]

It's in the feedback loop from the +15V sense to the inverting input of IC1. I suppose if it was open or presenting a very high resistance, IC1 would be pegged full open as well.

That's a much easier fix, and easy to test, so will definitely spot check that first. Otherwise, doesn't look like I have any spare LM709s around, so may need to order one.

 

[bjoneson]

Hmmm... after a bit of googling, looks like the LM709 is way beyond obsolete. And it's specs are horrendous to boot. I know I have a couple of spare opamp that have killer DC specs I had used in a servo circuit for some John hardy preamp clones. Maybe reworking them in would be a worthwhile endeavor anyway to improve regulation (whether or not the opamp are at fault here)?

Obviously not a drop-in, there's a compensation network in place for the 709s that would need to be removed / reworked, but seems doable.

 

[PRR]

Are the pass transistors just shorted? (Classic abuse failure.)

Is the 78L05 doing the right thing?

Old-stock '709 can be found if you hunt. Also watch circuit-sluts forums-- some folks find '709s and rush to rip them out. (They CAN be very good, and nobody actually stops to think.)

'741 will drop-in 99% of the time. You can probably leave the compensation parts connected.

TL071 is a braver adventure, "too good". Depends how much loop compensation (around the pass devices) is needed.

+/-15V 4A power boxes are not exotic items. Buy new.

 

[bjoneson]

You know, I'm sure I've got some 741s around.

You're right, at the end of the day. There are are much better supplies out there relatively inexpensive. I'm just too stubborn.

Thanks for the other tips!

 

[ricardo]

'741 will drop-in 99% of the time. You can probably leave the compensation parts connected.

+/-15V 4A power boxes are not exotic items. Buy new.

[/quote]

 

[bjoneson]

So... 741 as drop in for 709. Looking at the pinout, aren't the extra pins on the 741 for offset nulling, while on the 709 they're for frequency compensation?

Will the existing compensation network affect the offset of the 741?

It's definitely the 709. I swapped the 2, and now the negative rail is affected only.

 

[PRR]

Cut-off the extra pins on a '741. You'll never use them. That avoids conflict with '709 extra functions, and leaves the PCB "intact" as if anybody cared.

> 741 as drop in for 709

A HIGHly-compensated '709 performs very much like a '741. The designers of the '101 and '741 had '709 specs memorized, because it did sell very well and often in low-gain high-compensation circuits where a simple or no compensation part might (and did) sell even better.

 

[bjoneson]

Right on. 741 swap fixed the positive rail. Negative rail still unregulated. Measured pass transistor 5 ohms from collector to emitter. New one in route.

Thanks again for all the help!