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DBX 118...something is wrong! Dynamics Processors (HW)
Old 1 week ago
  #1
Gear Head
 

DBX 118...something is wrong!

Hey People

Please help... i am trying to pinpoint the issue with my DBX 118

I have attached my readings at the power stage...and suspecting that the PSU is not really working how it should...

Included my readings in red...

Any opinion would be greatly appreciated. I have measured this against the chassis ground ( the metal shield of the PSU)


ooh... and at C4 i am getting 0,1V

Please help me out !

Thanks!
Attached Thumbnails
DBX 118...something is wrong!-dbx.jpg   DBX 118...something is wrong!-dbx118.jpg  

Last edited by mitec1602; 1 week ago at 04:36 PM.. Reason: missing info
Old 1 week ago
  #2
Gear Maniac
 

What are the DC voltage readings on the +12v and -12v points marked in the second schematic, referenced to pin 1 of OA2? Those are what I'd be looking at initially.
Old 1 week ago
  #3
It is wrong, they used an unregulated power supply so they all hum.
Old 1 week ago
  #4
Gear Head
 

Quote:
Originally Posted by WorldPowerLabs View Post
What are the DC voltage readings on the +12v and -12v points marked in the second schematic, referenced to pin 1 of OA2? Those are what I'd be looking at initially.
hey

Thanks!

Only pin 8 has voltage DC which are 8.6V at first measure and checked with my second DMM that reading was 9V

So basically nothing, only pin 8 has something measurable.
Old 1 week ago
  #5
Gear Head
 

Quote:
Originally Posted by WorldPowerLabs View Post
What are the DC voltage readings on the +12v and -12v points marked in the second schematic, referenced to pin 1 of OA2? Those are what I'd be looking at initially.
Hey, sorry posting again as I did not really reply your question, apologies.

So...did at least 10 measurements

-ve on schematic measuring 15 VDC

+on schematic measuring +9 VDC


Sorry, I was wrong in my previous post...negative voltage measurement...forgot to swap the DMM pins for that...uuuuh


Also, just spotted that R1 and R19 in my unit has the same (wrong) values! compared to the schematic..

R1 should be 20k on mine is 16k

R19 should be 150k and mine is also 16k (same as R1)

and R4 is also a lower value.... wonder if this is right?

Last edited by mitec1602; 1 week ago at 06:52 AM.. Reason: wrong . re measured
Old 1 week ago
  #6
Gear Head
 

Quote:
Originally Posted by Jim Williams View Post
It is wrong, they used an unregulated power supply so they all hum.
Thanks Jim!

Would you advise me to replace the power transformer for a better one?

a 117V / 24-24V would work in this case or shall I go with something bigger?

thanks for your advise!
Old 1 week ago
  #7
Gear Maniac
 

Interesting...

If you're measuring the actual value of the resistors in-circuit, then I wouldn't necessarily expect them to read the same as the schematic, since measuring in-circuit means that they will be connected to other components and that will affect your readings.

Having said that, if these resistors DID drift in value, that could explain why you're seeing -15/+9v rather than -12/+12v.

I wouldn't say that there's anything wrong with the transformer -- it's providing power. You just have something that's not quite right after it. I would expect to see in the range of the voltages you measured there on the secondary of a 24V transformer if there's not much load on it.

Not intending to speak for someone else, but I believe what Jim is getting at is that (by design), this power supply is not regulated. If the AC line voltage drifts, so will the DC output (although the "+" and "-" rails should still keep the same magnitude as each other -- e.g., -11/+11). I haven't studied the circuit in detail, but it is rather different from the usual topology where they'll use a center-tapped transformer and call the center tap the DC ground point, then they rectify the two "hot" sides of the transformer secondary and regulate each separately to generate the "+" and "-" voltages. Rather, this DBX design appears to be using an op-amp to create a "virtual" ground midway between the rails.

Other than the voltage reading being a bit off, what specific symptoms does the unit have?


Quote:
Originally Posted by mitec1602 View Post
Hey, sorry posting again as I did not really reply your question, apologies.

So...did at least 10 measurements

-ve on schematic measuring 15 VDC

+on schematic measuring +9 VDC


Sorry, I was wrong in my previous post...negative voltage measurement...forgot to swap the DMM pins for that...uuuuh


Also, just spotted that R1 and R19 in my unit has the same (wrong) values! compared to the schematic..

R1 should be 20k on mine is 16k

R19 should be 150k and mine is also 16k (same as R1)

and R4 is also a lower value.... wonder if this is right?
Old 6 days ago
  #8
Gear Head
 

hey...thanks for detailed reply, appreciate it.

The only difference is I see in the resistors that the values aren't a match what I see in the schematic. The 150K R19 is only 16K and the other one the 20K resistor R1 is also 16k...the color coding on them also different, does not correspond with a 150k color code

maybe someone did change these and put the wrong values? I have tried to adjust the trimmer calibration pot and while turning, no changes...the only voltage changes I am getting when I switch the only switch on the unit. The Linear / above threshold ones..

the two indicator leds are constantly on, ( one at the time depending on the switch engaged or not) , which if I understand correctly, not suppose to be on constantly at all.


PS:
the voltage regulator RCA 727 gets really hot in 20 seconds .... and some light smell of overheating too...


So far, all the electrolytic has been replaced an also replaced the IC OA5 , which if I understood correctly could be responsible for these indicators..

All other cap's are within range.

the voltage regulator gone bad?

thanks again, appreciate your time.

Last edited by mitec1602; 6 days ago at 03:21 PM.. Reason: missing info
Old 6 days ago
  #9
Quote:
Originally Posted by mitec1602 View Post
Thanks Jim!

Would you advise me to replace the power transformer for a better one?

a 117V / 24-24V would work in this case or shall I go with something bigger?

thanks for your advise!
30 to 36 volt center-tapped power transformer, a couple of 7x15 regulators and some caps will make it regulated.

Or, buy a 161 and convert that balanced.
Old 6 days ago
  #10
Gear Maniac
 

If I had the unit in front of me, it probably wouldn't take too long to troubleshoot what's wrong with it (and tweaking that adjustment pot would have been my next diagnostic test, but you've already tried that)... but troubleshooting remotely is a little more difficult. You mention the "RCA 727" becoming hot -- did you mean the 723 op-amp? If so, that suggests that there might be a short or a lower-than-normal resistance to ground across one of the power rails.

Are you certain that all electrolytics that were replaced have been installed with the correct polarity?

Assuming that you can rule out a fault after the power supply as the reason for the problem, I don't see any downside in doing as Jim suggests regarding upgrading the power supply (one potential "gotcha" is that the metal tabs on most 7xxx regulators are internally connected to the ground pin -- so be careful in how you mount them, and use insulating hardware if needed). And of course, insulate all mains voltage connections properly.


Quote:
Originally Posted by Jim Williams View Post
30 to 36 volt center-tapped power transformer, a couple of 7x15 regulators and some caps will make it regulated.

Or, buy a 161 and convert that balanced.
Old 6 days ago
  #11
Gear Head
 

thanks guys...

yes, i meant the RCA 723 sorry...

I have re checked all the electrolytics, all in order however....

thos tantalums are giving me few funny readings...i am using the "atlas ESR" peak tester unit.

C11 reand "incircuit leaky" i know that does not mean fault at first as it is in circuit but the other c13 in circuit, the same tantalum cap reads just fine. 22uf and low ESR...

shall I start replacing these tantalums? there are few on the RMS board...reads funny...maybe because there the two tantalums are reversed in paralell? "+" connected to the other cap's "-" and vica versa..?

the 1 Uf tantalum also reads 5 ohms ESR...

I do not know that this could cause some issues at the power stage as it is miles away...hmmmm
Old 6 days ago
  #12
Gear Head
 

uuuh guys...i am lost

now.. I have not changed anything...just plugged in and measure the voltages again at "+12v and -12v points marked in the second schematic"....now i am gettin

+25.6 and -25.8

the only difference is now that the while engaging the swicth, the yellow led does not come on. It did beofre, now is out...so the red led , the "above thresohold" is constantly on

Seriously... I am lost..all I have done is that I have re measured my caps and took my dmm and checked some continuity on/ short's on all the tantalum caps....(removed the RMS panel and did the same), and that was it...

what is going on on this it is a mystery...

Old 5 days ago
  #13
+ - 25 volts will most likely kill the VCA module as well as the opamps. Don't let the smoke out.
Old 5 days ago
  #14
Gear Head
 

Quote:
Originally Posted by Jim Williams View Post
+ - 25 volts will most likely kill the VCA module as well as the opamps. Don't let the smoke out.
thanks Jim..i have removed the VCAs and the RMS board for further testing...

I did a re test this time:

at C1 +ve I am measuring 70 VDC. ( wonder how? or why?)

at the diodes...

CR1 also 70 VDC/ 30VDC
CR2 also 30 VDC/ 0 VDC

At the power transformer I measure 10,5 VAC on each node to ground.

At the R45 which is the calibrating resistor, the lowest voltage DC i could dial is 20VDC. It does not go lower than that.

I think I can conclude:

PSU works fine
OA2 ( pin1 to pin 8 measuring 24VDC)
R45 resistor works

CR1 and CR2 diodes...maybe the guilty ones?
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