AER Acousticube relay not kicking in

[glebert]

Got a deal on this project Lakewood (early) AER Acousticube and since I'm a hoarder I said "sure, why not?" Preamp working, headphone out (part of preamp) getting signal. Nothing out of the power amp, no relay click, though it seemed like there was a little thump when I engaged the 48V phantom power.

I have +/-57 on main rails which seems reasonable. Everything looks symmetrical on the output devices. No DC on transistor side of the relay. +57V on both sides of the relay coil. It looks to me like the 24V relay coil is tied to +57V rail with a 680 resistor on one side and the other side is supposed to be pulled down to something like +30V though some small transistors based on power supply and output voltage. No schematic that I can find, and it sounds like AER is not helpful on tech support.

Anyone worked on any of these or seen a protect circuit that looks similar? (sorry for bad pic) I don't know if there are 4 basic designs everyone uses or if it is all secret sauce. Worse comes to worst I just bypass the relay and see what happens.

[glebert]

Anyone have a guess as to what this guy might be? Diode test on is shows 0.7V in one direction, 1.5V in the other. I guess a 1.5V zener could be a thing, but I've never seen one in a circuit. The markings on it are almost completely unreadable and the white band shows sign of flaking off, wondering if it got hot.

[Jon Snell]

You cannot accurately read the parrameters of a diode in circuit as other components may sway the result.
Lift one end if you think it is faulty.
In my experience, they go short circuit when faulty.

[glebert]

You cannot accurately read the parrameters of a diode in circuit as other components may sway the result.
Lift one end if you think it is faulty.
In my experience, they go short circuit when faulty.

This was measured out of circuit after this picture was taken. I would agree that failing to 1.5V drop would be somewhat odd, but other than a 1.5V zener (which I've never seen) I don't know what else it would be.

[g1]

I think it usually takes at least 5mA to turn on a zener of that size (reverse biased). Would your meter be capable of that? Most DMM test diodes at 1 or 2 mA.
Was the .7V reading when forward biased?

[The Dude]

Maybe ask them for a schematic and/or send them the photo and see if they will identify the diode for you.

https://www.aer-music.de/

[glebert]

I think it usually takes at least 5mA to turn on a zener of that size (reverse biased). Would your meter be capable of that? Most DMM test diodes at 1 or 2 mA.
Was the .7V reading when forward biased?

Yes, 0.7V forward biased. I never knew about the current to turn on a zener, makes sense why it usually doesn't work. On this diode thingy it goes to 1.5v reverse biased with both a Fluke 77 and a middle of the road Radio Shack DMM.


Couldn't hurt to ask AER, perhaps my german surname will predispose them to help me.

[Helmholtz]

To measure zener voltage connect it to a 9V battery with a 1k resistor in series and measure voltage across the zener.
This works if zener voltage is lower than 9V.
For higher zener voltage connect more batteries in series (or use a lab power supply).
Supply voltage must be a few volts higher than zener voltage.
Current is supply voltage minus zener voltage in mA's.

[glebert]

OK, I think I've figured out the issue with the speaker relay. Found out that I had pretty big ripple on the +57V rail. Added a smaller cap in parallel (since I didn't have a replacement 10,000uF 63V cap at hand) and now the relay kicks in as expected.

FYI for anyone else working on one of these, the preamp does need to be connected for the relay to work right. I already suspected that. It even kicks out the relay to turn off the speaker when headphones are plugged in.

Unfortunately, still not getting any sound out of the speaker, but at least now I can hopefully do regular signal tracing.

[g1]

Does that diode looking part end up with 1.5V across it (reverse biased) ?

[glebert]

Does that diode looking part end up with 1.5V across it (reverse biased) ?

Yes.:shug:

[g1]

Then I guess it is a 1.5V zener.