Dbx 286 repair

[falcontech]

Goodmorning everyone
Today I find myself having to repair a dbx 286, the processor gives me nothing at the output when I connect the microphone, the thing I notice is that when I turn it on the De-esser and the output LEDs remain on. I don't know if it is a current problem, inside I don't have open capacitors, it is powered by a transformer with double output, after the transformer there is a small power supply circuit with a 7915cd and a 7815cd, I did a test quickly and from 7915 I have 15 volts out, from 7815 I have about 12, I don't know if that could be the problem, if you need photos of the inside of the processor I can post them.

[CapLeaker]

Re: Dbx 286 repair

The easiest way to troubleshoot this is to inject a signal on the input, then follow it through each stage with an oscilloscope and figure out where you loose the gain. Could be an opamp thats not working anymore for example.

[falcontech]

Re: Dbx 286 repair

I don't have an oscilloscope unfortunately, an alternative?

[Khron]

Re: Dbx 286 repair

An audio interface with a high-impedance (instrument) input, and some sort of makeshift probe?

[falcontech]

Re: Riparazione Dbx 286

An audio interface with a high-impedance (instrument) input, and some sort of makeshift probe?

up to the instrument instead of the microphone I had thought about it, but what do you mean by probe? can I send you some photos of the interior to guide me in the operation?

[Khron]

Re: Dbx 286 repair

I meant more like, using a high-impedance input on an audio interface as a test-input with a probe (just as you would use an oscilloscope), while looking at the signal in an oscilloscope-like plugin in your DAW of choice, while you're feeding a test signal into the mic input (as CapLeaker suggested above).

[falcontech]

Re: Dbx 286 repair

so, do I enter a signal into the dbx, exit the dbx and send the signal to an input on the sound card while running cubase (for example) with an oscilloscope plugin open on a recording track?

[Khron]

Re: Dbx 286 repair

The "exit the dbx" part is where a probe comes in. It's already clear that the signal doesn't really make it all the way through the unit, so monitoring its output wouldn't make much sense, would it?

Service manual can be found here:
https://elektrotanya.com/showresult?...a=All&kat2=All

[falcontech]

Re: Dbx 286 repair

ok i downloaded the service manual, now how should i proceed?

[madan1]

Re: Dbx 286 repair

The "probe" could be just headphones,

[falcontech]

Re: Dbx 286 repair

The "probe" could be just headphones,

ok, i suppose you say to use headphones without jack, but my problem is where i have to start following the signal, tomorrow i will post a photo of the interior if you can help me i will be grateful

[falcontech]

Re: Dbx 286 repair

these are the photos of the circuit, the only xlr present is the microphone input

[CapLeaker]

Re: Dbx 286 repair

the 7815 should have +15VDC, while the 7915 should have -15VDC. For the 7815 only having +12VDC something is already off. Either the regulator is getting loaded down, or it is bad as it cannot handle a load.
Manualslib has a block diagram for this unit within the regular manual.
https://www.manualslib.com/manual/58...S.html?page=25
Try bypassing everything processor wise and see if you get output. If no output, the problem got to be in the very early input stages, see block diagram.

[falcontech]

Re: Dbx 286 repair

the 7815 should have +15VDC, while the 7915 should have -15VDC. For the 7815 only having +12VDC something is already off. Either the regulator is getting loaded down, or it is bad as it cannot handle a load.
Manualslib has a block diagram for this unit within the regular manual.
https://www.manualslib.com/manual/58...S.html?page=25
Try bypassing everything processor wise and see if you get output. If no output, the problem got to be in the very early input stages, see block diagram.

the power supply circuit is one with the rest of the board, how do I bypass the rest of the board? what should i unplug in your opinion?

[CapLeaker]

Re: Dbx 286 repair

no unplugging anything required. Just press the process bypass button on the front panel. When pressed, a red led should turn on.
If you use a mic, make sure the 48V Phantom Power is on. Other than that you wont get anything from the mic either, if you hook the mic directly to that box.

[falcontech]

Re: Dbx 286 repair

no unplugging anything required. Just press the process bypass button on the front panel. When pressed, a red led should turn on.
If you use a mic, make sure the 48V Phantom Power is on. Other than that you wont get anything from the mic either, if you hook the mic directly to that box.

Ok thank you cap

[falcontech]

Re: Dbx 286 repair

no unplugging anything required. Just press the process bypass button on the front panel. When pressed, a red led should turn on.
If you use a mic, make sure the 48V Phantom Power is on. Other than that you wont get anything from the mic either, if you hook the mic directly to that box.

7815 and 7915 are good, I was wrong to put the test leads, now where could the problem be?

[CapLeaker]

Re: Dbx 286 repair

with the processor in bypass mode, are you getting any audio out?

[falcontech]

Re: Dbx 286 repair

with the processor in bypass mode, are you getting any audio out?

ok, without the processor inserted the microphone can be heard, even without 48 volts phantom ... so where can I find the problem?

[CapLeaker]

Re: Dbx 286 repair

so the problem isnt in the input stage. When you turn that bypass button to off and talk into the mic, are the 8 leds on the compressor moving? you may have to fiddle around with the two pots a bit.
I am thinking the problem is one of the op amps. Are you comfortable changing surface mount IC's? If yes, I guess you need to borrow an oscilloscope to figure out which IC is the culprit. Then you need to read the numbers of the IC's and find the spec sheet on the internet in order to get the pin out. Then test the inputs and outputs of each op amp and see where things go in, but nothings comes out.