DDA Q-Series - Osc/Talkback Assign pops
« on: May 26, 2019, 10:47:10 AM »
Hey guys,
Brought my old Q-Series 32/8/2 Console back and am in the process of rebuilding it.
I'm hoping to get some help tracking down issues in my master section, I'm a bit of a hack and don't have a technical background, rather, I love this desk and want to bring it back to new spec.
The issue is apparent from the Oscillator/Talkback Assign section, when any of the push buttons are depressed it causes massive pops, both on the headphone and main outs. When investigating the circuit I see the caps in one of the EBO's are being damaged also (bursting).
In addition the headphone output has a constant low hum around 60Hz.
I have noticed caps C4, C8, C13, C14, C18, C24 are 22uF instead of 68uF.
I (perhaps incorrectly though?) lean to thinking the pops getting through would have something to do with pulldown resistors rather than caps, especially since those lower value caps would hold less charge?
Any chance someone could look over the schematic and give some direction?

Cheers
Cam S


Re: DDA Q-Series - Osc/Talkback Assign pops
« Reply #1 on: May 26, 2019, 10:49:08 AM »
Additional master (main) schematic attached.

cyrano

Re: DDA Q-Series - Osc/Talkback Assign pops
« Reply #2 on: May 26, 2019, 04:23:57 PM »
I'd do a complete recap first, as the capacitors are visually bad.

Those pops are probably caused by DC leaks in the signal path. Might cause other defects.
Why is it people love to believe and hate to know?

Re: DDA Q-Series - Osc/Talkback Assign pops
« Reply #3 on: May 26, 2019, 10:24:24 PM »
Cheers for the response Cyrano, the caps that are bursting are brand new (third set), all channels currently loaded in the desk have brand new caps, all polarity's have been checked visually, the EBO that is blowing matches with the other one on the master card (polarity-wise), though these EBO boards dont have any markings.

Re: DDA Q-Series - Osc/Talkback Assign pops New
« Reply #4 on: May 28, 2019, 07:19:06 AM »
Found the +18v rail of the PSU is putting out +30v, back to the drawing board on that (note; has been fully re-capped)

I have replaced the entirety of parts within the PSU, it puts out +/-30v (with an additional +50 for phantom). I was unable to find a schematic for the boards within this PSU (I will be taking it to a professional now).
« Last Edit: June 27, 2019, 09:15:14 PM by [email protected] »

cyrano

Re: DDA Q-Series - Osc/Talkback Assign pops
« Reply #5 on: May 28, 2019, 10:16:35 AM »
I was amazed, 'till I read your last post  8)
Why is it people love to believe and hate to know?

Re: DDA Q-Series - Osc/Talkback Assign pops
« Reply #6 on: May 28, 2019, 07:09:19 PM »
Haha, I'm learning  ;D
I've found a dead Transistor in the PSU, will order replacements as soon as pay day comes around.
The original Motorola transistor's appear to be obsolete, but I found an On Semiconducter equivalent 2N3055 / MJ2955.
This whole desk rebuilt has been an epic learning experience, prior to this I'd designed/built speakers with passive crossovers, that was the depth of my electronic knowledge, I'm still screwing up, but man I love this stuff.

jensenmann

Re: DDA Q-Series - Osc/Talkback Assign pops
« Reply #7 on: June 12, 2019, 04:42:55 PM »
Good luck with this one. Keep us updated. Q Series are great desks, worth the effort to keep them alive. I mixed quite a load of shows on these and always liked them.
Jens
Quote from: PRR
The tubes of course don't care what frequency they distort

Re: DDA Q-Series - Osc/Talkback Assign pops New
« Reply #8 on: June 15, 2019, 04:48:59 AM »
Will do! Currently still working on the PSU, placing an order next week for the diodes.
I'm also rebuilding the IDC cabling, brought a 100ft roll of flat cable last pay, now, finances for the connectors will slow my progress down a bit over the next month.

pics of the project here; https://imgur.com/a/0NYBKpC
« Last Edit: June 15, 2019, 06:30:57 AM by [email protected] »

 

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