Yorkville MP6 Powered Mixer Repair - so close! [fixed]

[garytoosweet]

Hey there,

I'm attempting to repair a Yorkville MP6, multichannel powered mixer. The problem is that only a very faint signal passes through.

I've narrowed down the problem to the power amp section.

Schematic attached, with problematic component circled in red.

Signal is reaching the optocoupler (U3) but not passing through. The resistance of the photocell side, in circuit, is 22k when powered on, and 81k when powered off. The spec sheet for the optocoupler calls for an max resistance of 2.5k when LED is on, and min resistance of 1M when off.

The LED side of U3 has a voltage drop of 12-13V, while I believe the spec sheet calls for max 2.5V? Not sure if this indicated the problem is elsewhere.

When I jumper the resistor side, the signal passes through and there is amplification, so I don't think there are any other issues with the amp.

I pulled U3 from the circuit and my multimeter across the LED is reading OL resistance both ways. Even out of circuit, shouldn't I see around 1M ohms anode to cathode according to the spec sheet?

My questions are:

(1) Originally thought the zener diode ZD5 was to be a voltage regulator and the LED would see 4.7V, is this not the case? I replaced the diode thinking it was faulty but nothing changed.

(2) Should I go ahead and replace U3? NSL-28AA seems to be out of production, not sure if I can find a suitable replacement.

(3) What should next troubleshooting steps be?

Any help is appreciated, feel like I'm very close on this one.

Thanks!

[fzabkar]

I agree that U3 is faulty. It's voltage is far too high which suggests that it is open circuit.

[Renate]

The zener is in there to make the response a bit more "snap action" than if it were just resistors.
That protection circuit is running off the AC supply sense and (presumably) the output amplifier DC balance.
The LED drive is about 5 mA.
That U3 is not a very parameter critical device.

I know what I'd do.
I have some little 3/16" CdS photocells.
I actually bought them for a similar usage.
I have some 0805 SMD green LEDs (green is best for CdS).
Solder some leads to the LED, stick it in a piece of heat shrink with the CdS, squash the ends.

Depending on how the original U3 is put together, you might even try grinding off the LED and replacing it!

Ah, it all brings back memories of Raysistors!
http://www.technicalaudio.com/pdf/Raytheon/Raytheon_Optoelectronic_Devices_SP-1149_Raysistor.pdf

[garytoosweet]

It was U3.

I did buy some photo cells and LED's, but the ones that were available locally were too big to fit in the circuit.

Since I had to order anyways, I just got an equivalent part. Replaced it and we're all good.

@Renate Interesting about the use of the zener. I don't quite understand how a zener in circuit would accomplish the response, but hope to get there soon.

I guess I had this one figured out, but thanks for both of your responses.

[garytoosweet]

@CJay missed your post back in Nov.

Will reply now in case any one comes across this thread with a similar problem.

I replaced with NSL-32SR3 from Digi-Key. Allied Electronics also has many similar optocouplers.

https://www.digikey.ca/en/products/detail/NSL-32SR3/NSL-32SR3-ND/5039793?utm_medium=email&utm_source=oce&utm_campaign=2983_OCE19RT&utm_content=productdetail_CA&utm_cid=1100156&so=62073170&mkt_tok=eyJpIjoiTVRNek1qTTRaRGxtTVRNeiIsInQiOiJWSkp3MkMxRmxVNWw4elwvNW9EdmU0VnRFWXhCNVwvRXM5WEVmcVRpM3lDTXI3aVQ3cEFTTTJDajdTNmppWlRyem5kUnZQS1VJbXBWSFlRTytnaEhHZDZtVEJpVENHK3RHbnRUU1BHaVwvckdhR3BWeDFDVEJ3cHRoTEZLQ1VuYW0yeSJ9