DBX 166 - How to calibrate it?

[smilan]

Hi, I have an old DBX 166.
I've modified it (only one channel so far) with the following modifications:
PSU MoD: – Replace c3 ,c4, c7, and c8 with 1000uf-2200uf 35v caps
– Replace c5 and c9 with a .1uf film cap (preferably polypropelene)
– Bypass c6 and c10 with .1uf film caps
Channel 1:

IC MoD: – Replace chip OA1 with better ic (NE5532,) and install a gold plated ic socket.
– Repalce chip OA2 with better ic ( OPA134) and install a gold plated ic socket.
– Replace the VCA IC1 with a THAT 2180AL08-U (lift pin 4, pin 4 should not connect to anything)
– Add a 47pf NPO ceramic cap from pin 3 of OA2 to ground (Suggested by Jim Williams)

Capacitor/resistor MoD:
– Replace c14 with a 22uf 35v bipolar cap
– Bypass that with a .22u film cap
– Note: c14 was originally a 3.3uf cap. You could fit in a film cap in there if you are carefull.
– Replace c25 with a 22uf 35v bipolar cap
– Bypass that with a .22u film cap

– Replace r17 and r18 with 22ohm metal film resistors
– Replace r1 and r4 with 1k metal film resistors
– Replace r3 and r6 with 7.5k metal film resistors
– Replace r2 and r5 with 10k metal film resistors
– Replace r7 and r13 with 2k metal film resistors

IC Control Voltage (Optional) MoD: Turns this compressor into a beast.
– Replace chips OA3, OA4, and OA5 with gold plated ic sockets and a better ic like a LT1124.

No I'm having troubles to re-calibrate it...
I'm following the instructions of the service manual, but on the first step (slow RMS symmetry and level) my scope (Tektronix 2215) is freaking out and shows this thing (which is not even close to the screen shoots that appears on the service manual).

I'm using a Boonton1120 as my tone oscillator.
The output impedance of the 1120 is 600 Ohms instead of 50 Ohms as the service manual request, this mismatch can be the reason for what I'm seeing on the scope or there's something else that might went wrong?

 

[smilan]

* I couldn't find instructions at the service manual how to set the trim pots before starting to calibrating the unit. Maybe someone can help me with this procedure?

 

[Script]

Maybe on page 16 in pdf you linked to?

Say, what schematics do your mods refer to ? The 166 one in Tech Docs ?
Sure about this part " 47pf NPO ceramic cap from pin 3 of OA2 to ground" ?

 

[JohnRoberts]

making that many changes at the same time introduce multiple opportunities for a mistake.

600 ohm source impedance is unlikely to cause problems.

Start from the input and troubleshoot everything you changed.

JR

 

[MiamiBoy]

Hi, I have an old DBX 166.
I've modified it (only one channel so far) with the following modifications:

Have you finished both channels yet? How does your unit sound?

 

[smilan]

Have you finished both channels yet? How does your unit sound?

Yes, I finished bothe channels, here you can hear it:

And here you can listen to it against the classic 160 vu:

 

[Sebastian BX]

Hi guys, I started my mods on dbx166 following the one you shared, can you tell me :

-how to bypass ? Is it adding the film cap to both poled of the named componant ? Like to the other side of the Pcb ?
-how do you connect to ground ? Simply connect a cable to power ground ?

thanks

 

[Migs 31]

Hi guys, I started my mods on dbx166 following the one you shared, can you tell me :

-how to bypass ? Is it adding the film cap to both poled of the named componant ? Like to the other side of the Pcb ?
-how do you connect to ground ? Simply connect a cable to power ground ?

thanks

Hi @Sebastian BX , Happy New Year!

I have done a refurb/mod (read massacre-desecration) of my DBX 166.
The power supply caps were all dried up and leaky, very common apparently.
Although I didn't do all these changes numbered by Smilan in his OP.
I stopped at - IC MoD:" – Replace chip OA1 with better ic (NE5532,) and install a gold plated ic socket."

After that the sound was starting to change in a way I did not like. I llike the character/dirt of this little comp stock. So I went on a completely different route, adding more controls to the gate section and I'm working on an HPF in Sidechain at the moment. The new gate controls are great for snares etc. btw.

That silver point in the middle of circle and arrow, thats your ground plane.
If you look at the other side of PCB, that ground point or plane is easily accessible.
BTW that first pic is not mine - found it on the net. I did something a bit different
M

 

[Sebastian BX]

Hi @Sebastian BX , Happy New Year!

I have done a refurb/mod (read massacre-desecration) of my DBX 166.
The power supply caps were all dried up and leaky, very common apparently in these units.
Although I didn't do all these changes numbered by Smilan in his OP.
I stopped at - IC MoD:" – Replace chip OA1 with better ic (NE5532,) and install a gold plated ic socket."

After that the sound was starting to change in a way I did not like. I llike the character/dirt of this little comp stock. So I went on a completely different route, adding more controls to the gate section and I'm working on an HPF in Sidechain at the moment. The new gate controls are great for snares etc. btw.


M

Hi Migs 31, Happy new year,

Totally understand that you like it this way for drums/percussion purpose.

I did the PSU mod (except bypass) and Replaced the VCA IC1 with the THAT 2180AL08-U. And i can tell it was night and now it is day, completely changed the mi/high End clarity Of it. Then i would like to do the bypass thing, but my knowledge is too small. Tried to look online, i see there is bypass cap, but here it looks more like a combination, like this one :

Replace c14 with a 22uf 35v bipolar cap
– Bypass that with a .22u film cap

Any clue ?

Thanks for the picture I ll try this

 

[Sebastian BX]

@Migs 31
I think I just get it (and realize my question wasn’t accurate enough).

Bypass c6 and c10 with .1uf film caps

should I solder the .1uf film cap to + of the c6 capacitor and the other pin of the film cap to ground plane. Both of the joined pin stayed soldered to the Pcb ?

 

[MiamiBoy]

I just got mine, NOS brand new, never been used. I don't hear any noise. Complete silent!

But I noticed that both channels don't compress the same when "link" (stereo couple) is engaged for stereo compression.
Even when all knobs have identical settings on both channel.

Does it need calibration, if so, how do I calibrate it only with my MultiMeter / soundinterface?

 

[Script]

Even when all knobs have identical settings on both channel.

Does it need calibration,

Yes, sounds likely from your description.

If truly NOS, deviation might be due to capacitor ageing only, but maybe more.

If caps are/were replaced, it definitely needs recalibration. I see Rubicon in the picture, not Elna caps. Stock? Not stock? -- I wouldn't know. Either way, I'd strongly advise to NOT start turning trimmers at random !!

First need to find out what is off across channels. Deduce by operating at different settings. Inject steady tone for this and observe. That should give first clues. We do not need to know here, but you do.

The service manual gives detailed calibration procedure in correct order to follow. For RMS symmetry and level for example you'd need a scope. Other steps require only a signal generator and a meter. Could search for and install oscilloscope software on a computer with soundcard and try to work it out from there.

Or, with just a DMM, you could still follow the steps by measuring AC and/or DC at corresponding tie points in a first step, but taking only notes of measured values at first in order to deduce where the two channels deviate.

If lucky, only the thresholds and unity gains are off -- and you might get both channels to behave more similarly even without perfect calibration.

Happy proceedings and please report back

 

[MiamiBoy]

If truly NOS, deviation might be due to capacitor aging only, but maybe more.

I didn't know there were aging involved when the unit's never been used. It might only been tested, after build in the 80's / 90's.

My plan was to fully modify it, if it were noisy. Replace all caps and upgrade PSU & VCA's. But it's dead silent.
Revive Audio offers this mod for about $300. But I rather do it myself, if I choose to modify it at all.
I'd love to do it, I mean, upgrading is fun while getting my hands dirty. That's why we're all here Replacing components is easy and fun. But sourcing the correct components and calibrate it after, is a whole other thing for me.

Either way, it needs calibration.. If calibration can be done in 20 minutes with the right equipment, then a mod might not be necessary. What do you think..? Caps still need to be recapped? I mean, is that the reason it's "out of balance"?

Unfortunately I only have a DMM. Maybe if I find an oscilloscope plugin, then it can work?!

But yea, I need to figure out which of the two channels are off.
So the threshold can be off, unity gain can be off. What else can be off?
Thanks, appreciate your time & effort!

 

[JohnRoberts]

Electrolytic capacitors can age even if not powered up, due to loss of electrolyte. Heat while operating will accelerate aging.

JR

 

[MiamiBoy]

Electrolytic capacitors can age even if not powered up, due to loss of electrolyte. Heat while operating will accelerate aging.

Gotcha, thanks. So you recommend re-capping, before calibration?

I found this oscilloscope online, for about $60. Will this do the trick?
If so, I gotta see if the seller would consider to ship it to me.

 

[Script]

Why not try to figure it out with what you already have? Worst case, you have to calibrate an old oscilloscope first before you can use it to calibrate your compressor. There is oscilloscope freeware around...

 

[MiamiBoy]

You're right. Don't wanna calibrate too many machines Plus, I have to learn how to use one also..

Will a software oscilloscope really be sufficient, together with my DMM & soundinterface (Apollo 6X)?
I found a free plugin that won the tests from Oscilloscope Database. It's called Signalizer.

And will the service manual tell me what to do find which channel is off? And how to bring it back to normal?
I see from page 13 or 16 there's some "troubleshoot" steps. I can't quite figure out which one to go with

 

[Script]

For calibration proper, all steps in the listed order.

 

[MiamiBoy]

I don't see how it's possible to do these steps without a scope.
Do you think I can achieve this with DMM & Apollo interface?

A. Slow Rms Symmetry and Level:

1. Connect oscillator to sidechain Input jack.
2. Set oscillator for 50 Hz at -32.5 dBv. * (See note 1)
3. Monitor TP1 (TPS) and adjust R32 (R132) for a symmetrical 100Hz waveform on scope. See figure A.
4. Set scope for de coupling and adjust 134 (R134) such that the top of the waveform is at the 0.00 volts line on the scope. See fig B.

B. Fast Rms symmetry and level:

1. Connect oscillator to Ch.1 (Ch.2) Audio Input jack and set it for -20dBv at 1kHz.
2. Monitor TP2 (TP6) with scope and adjust R67 (R167) for best symmetry. See fig C.
3. Adjust R69 (R169) such that the tops of the waveform are 0.00v on scope.

Perhaps, will this digital scope work?

 

[Script]

I had another 166 in the storage room and just test-calibrated it.

I have a scope and only quickly looked at the RMS symmetry and level first. Not bad but I decided to ignore. Then I turned off the scope.

For the rest -- let's call it dirty calibration with random test tones and DMM -- I used non-calibrated steady signal input tone of 1.228VAC measured on DMM, sometimes higher, sometimes lower.

I followed the steps in the manual, adjusting trimmers accordingly, but double-checked gate threshold, threshold, ratio, gain and limiter with different front panel settings and different input tones to see whether it's all consistent across channels and settings (on DMM and LED gain reduction meter)

Result: Left and right channels behave the same in dual mono with equal front panel settings. It does not require adjusting knobs, as long as they are dialled in equally** and even bypassing is at the same level now when no compression or limiting is dialled in.

** The trimmers are hyper-sensitive. The tiniest nudge of a turn and the value flies off.
Also, the pots are quite coarse, so trying to dial in exactly equal settings L and R in dual mono is a game of patience cos a matter of fractions of a millimetre. -- That's what the stereo link is for.

Is it professionally calibrated this way? No, but good enough for use. It compresses a tiny tad more in stereo linked mode with heavier compression settings.

Found a broken peak stop LED... peak stop itself calibrated this way and actually works properly as scope confirmed later.