Something very odd is going on if the schematic actually matches the physical circuit.

When the power switch is ON (in AC position), C21 (hidden by terminal strip) should be around 29 volts (as you measured at DC power input jack) and C22 should be around 24 volts. With the switch in the OFF position and with no external DC supply, both capacitors should drop slowly to some low voltage, and eventually to zero.

What you have observed makes no sense. How C22 gets to normal operating voltage with the switch in the DC position then decays to zero in the AC position is beyond my understanding. I'm assuming you aren't using an external DC supply, since I didn't see one mentioned.

 

I think you may be misunderstanding the power switch. When it is "off", both line and the external power source are disconnected. When it is "on" it can be powered from either the line or an external source. This switch does not select either/or.

If you are getting 21 volts at big blue, then it sounds like the circuit may be drawing a bit too much current. Lift the positive end of big blue from the terminal strip - power it on and see if the voltage is higher. If not, put BB back and try lifting C1. Does the voltage at BB get up closer to +24 volts? If not, put C1 back.

 

Right, I'm not using an external DC power supply.
Yes, no doubt that I am misunderstanding the power switch. I assumed it was on/off for AC, since it didn't make sense to me that the unit could be capable of being powered by both AC and DC at the same time. Seems strange.

When you say to "lift the positive end of big blue from the terminal strip", do you mean disconnect (unsolder) it?

 

When you say to "lift the positive end of big blue from the terminal strip", do you mean disconnect (unsolder) it?

Yes. Trying to determine if it is drawing excessive leakage currents.

 

I pulled BB (+ side) and still get 21 V across. Will put it back and try C1.

 

With BB replaced, 21 V. With C1 (+ side) removed, I still get 21 V. Ok to just put C1 back?

 

Let's repeat what you might have already done.

With everything back in place, connect a voltmeter across C22, the big blue capacitor at the top.
Apply power and record the voltage at the start and then again after 30 seconds (when the sound fades).

 

All back in place: 22 V across C22 to start....but... MrChips, your request to watch the voltage across C22 as the sound fades made me actually plug the preamp box back into the stereo system to monitor the fade audibly. The preamp works fine! I began switching between "off" and "on" during the tests (for what I assumed was battery input on/off) and found that it was fading out on "off." I guess I had assumed that when turned off, sound would cease immediately. YLLi caught it, and it didn't register with my non-electronic brain. This is somewhat embarrassing, but also a learning experience for me. Off means Off. My symptom was caused by plugging in the preamp, turning it "on" (actually turning it off) then noticing the sound fade away. I am sorry to take up you guys' time.

One last thing:
When trying to remove C22/Big Blue positive side, it was recalcitrant. Wiggling on it with a needlenose while the solder was liquid caused the cap leg to break. I did a quick fix in order to continue to troubleshoot, like this:



Since the cap leg is too short to be resoldered to the board, what do I do now? Leave it as is? Replace the cap? Where does one buy caps inexpensively?

 

Just extend the cap lead with a short piece of copper wire. The wire does not need to be larger than the cap lead. Solder the short wire to the terminal strip first, then position it next to the cap lead and flow in some solder.

 

Ok, thanks.
I should add that turning on the preamp takes around 20 seconds, too. The sound "fades in." Sort of like a tube-driven device would. This is part of the reason why I thought it wasn't working. I turned it on, nothing happened fast enough for me, and so I turned it off.

 

I am sorry to take up you guys' time.

All's well that ends well.