onyx 1640i metering / solo issue

[Dan Mc]

Hey all,
I have a onyx 1640i metering / solo issue. The meters and solo indicator light are not functioning. The mixer is functioning other than those two features. Also, I don't see any signal input to U73 op amp - using sine wave generator. U73 pin 1 and 7 has small signal square wave output. Solo logic Q60 has negative 4.95 VDC on collector? Can anyone help me wrap my head around this circuit!
I have only attached a few relevant parts of the schematic.

[Jazz P Bass]

Cannot access the files.

[Dan Mc]

Sorry, dunno why i did not attach this morning
1640i solo etc.pdf
1640i meters.pdf

[Enzo]

The first thing I see is that the meters and the solo light both run on the +15v-M rail. Is it present?

Had you been doing other work inside this unit? ANy chance a ribbon cable has been put on the wrong pins?

[Dan Mc]

Mixer came to me with this issue and yes, the power supply rails are there. I have replaced a faulty op-amp on an input channel - #14 if i recall- but this problem seems unrelated. it is partially assembled and ribbon cables are going to correct locations. i have since tested led's (power unit off - plug 9v battery and 1k current limiter in series using probes to illuminate them) both d86 and d87 are not operating... the voltages listed in schematic for the led driver IC's are close within a few milivolts. grounding pin 1 or 2 for example U82-B pin 2 cause led to illuminate. solo logic -triggered from channel not shown in schematic gives either =14 vdc for solo or -4.95 vdc to GND-M. i removed q60 to test out of circuit and fine ( but i seem to have lost it Arghh). It is kinda driving me crazy today...

[Enzo]

Replacing an IC on a channel is electrically unrelated, but it means you were doing work in the unit, which means opportunities to break connections or if disassembled, reconnect something wrong. Who among us has never put a ribbon cable onto a dual header off one row, or off one pin sideways?

Pick a thing - the solo light. How can -5v possibly get onto the collector of Q60? Either the point you are using as ground is elevated above that collector, or some negative voltage is somehow shorted to this circuit.


DO you have the boards out of the chassis for this? If so, power off, and measure resistance from the jack sleeve of various things to other jack sleeves. IN the chassis, I expect all of them to be at ground, but if you find that not all are connected together, then the system was expecting the chassis to be the ground for at least part, and you may have to use a bunch of clip wires to get all the grounds connected if you want to run bare boards. Believe me, I have seen that enough times.


But let us look at the circuit, wherever you grounded your meter, you have +15 and -15? I assume so. So now you get -5v at Q60 collector. is there zero volts, continuity to ground at the emitter of that Q60? R781 up top connects to +15v-M. That M differentiates from some other +15 in the unit, so I can't assume that +15 on the op amps in the channel means I also have it here. So is there +15 on R781? With the LED off, you should also see +15 on R779, and also on the anode of the LED. DO you have that?


I also see GND-M, so not only do we have a separate +15 supply for the meters, we also have a separate ground for them.

[Dan Mc]

I do have boards out of chassis and before I disassembled I confirmed that theses problem existed . fortunately all of the connectors are of different sizes to not confuse but I agree, connection issues ARE SO easy to overlook.

I am measuring voltages with respect to GND-M from PSU ( if needed i can upload more info here) the power supply measures +14.8 and -14.9 .

I will measure those points tomorrow to see my readings.

incidently i made a mistake in a previos post and said collector when i meant base
The base of q60 reads -4.95 and when Q60 is removed no voltage or 0vdc at base of transistor but no change in operation I also do not have another IMBT4401 to replace

thanks Enzo!

[Enzo]

As long as you follow me that whatever rails and ground are at the power supply, they also must reach the circuits in question. Does me no good to have +15v-M at the PS if it is not reaching R781, for example.

It was hard to imagine -5 on the collector of Q60, but -5 on its base sounds OK, it just means it is OFF. We then start to look for the control signal. Picking a nearby SOLO button we want to know that the voltage at that base toggles from negative something to a positive turn-on.


But I am holding out the loss of meters along with the solo thing as a clue.

[Dan Mc]

yes, I believe they are related. Pressing a solo button brings "solo logic" voltage up to +13.95 ( should be +15)
In the morning i will run a jumper to guarantee no broken traces!?!

[Enzo]

14v is plenty close enough to 15. Those are just nominal voltages. It is just an on/off LED drive. If you have +14v on that base resistor, then it should turn on, so that points you back to checking not only for voltage down through R781 et all, but also that the emitter of Q60 is still connected to ground.



Missing connections or broken ones are common enough. And not rare is a burned off ground trace. Someone plugs something into a jack, and there is a voltage disagreement on ground, and a thin spot in a ground trace can burn open like a fuse, leaving sections of circuit ungrounded.

[Dan Mc]

As long as you follow me that whatever rails and ground are at the power supply, they also must reach the circuits in question. Does me no good to have +15v-M at the PS if it is not reaching R781, for example.

Guess what... I had -13.95 on R781 very odd... so I ran a jumper wire from + rail to R781 and boo-bam METERS are functional.
Next I had to find a replacement for Q60 since i removed it to check out of circuit and into the Abyss (my Shop floor) it fell. Luckily I had a donor board in my stash to pilfer a suitable replacement. I could not read its marking but figure NPN suitable replacement will work - TO236 is tiny. I got lucky and the first one i tacked in there works. Solo functions are back operational.

Must have been a microscopic fracture in the circuit board to cause this missing voltage. Sometimes worrying about the operatin of a circuit causes stress and you easily overlook the obvious...

Thanks ENZO I owe you one!
Dan